Wiring an 80-gal 7.5hp 230V Air Compressor

I was hoping I could get some wiring advice for my air compressor in my shop..It's an Ingersoll Rand 80-gallon 7.5hp, model 2475N7.5. I have a 200 amp breaker in the panel and the compressor is right around 55ft from it.

I am not very experienced with electrical but a DIY'er and learning, like anyone..From what I've read....I know I'll need 3-conductor wire, red and black are positive, copper or white are ground. I'm not sure if I need a 40amp breaker or 50 amp?(sticking with the cutler hammer) If 40 amp is ok, then I go with 8ga wire right? And 50A=6ga? Then I'll run that wire to a junction box right near the compressor and use the appropriate gauge extension wire to go to from the jx box the compressor box?

Any advice is much appreciated!! Pics are great too

http://www.ingersollrandproducts.com/IS/modelcomp.aspx?am_en=12878

What-is-the-amp-draw-of-the-motor??
Go with the next up even size breaker.

Example: 18.5 amp use a 20 amp breaker.

--Doozer

I just installed this same compressor in my shop, although my cable run was only about 10 ft.

Assuming you are in a location where the NEC is used, there are special rules for electric motors. Wire has to be sized to handle 125% of the current draw. So that would be 40amps x 1.25 = 50 amps. 6 gauge NM (i.e. romex) cabling can handle this, as can 8 gauge THHN, if you run conduit. As far as breaker size, code allows up to 250% of the motor current. So you can run up to 100 amp breaker if you want. For my panel the price for a 60amp breaker was 15$ and a 100amp was like 45$. I ended up going with the 60amp breaker figuring if it starts tripping on startup I can always swap it later.

I'm not sure how much voltage drop you will have at 55ft but I would think that 6 gauge would still be sufficient as well.

Yennek hit the nail on the head.

Doozer, that's just plain bad advice. It doesn't meet code either.

I would also use a disconnect next to the compressor

Cag130 is showing us pics of his main breaker, his regular breakers, his contactor and overloads, and a pic of the compressor. He does not state the amp draw of the motor. If it is 7.5 hp it is likely 230v and single phase. He gives no pertinent information with his question, only useless pics.
yennek gives good info on wire type and size, and which breakers he can use. Good info there.
I believe my recomendation of a breaker the next size up from the motor amps is a safe one. He might be over protected and quite safe. If it starts to trip, he can get a bigger breaker.
Aceman- How is this bad advise and how specifically does it not meet the national electrical code in regard to breaker size?
Bad advise? How much did you pay for it? I thought so.
I consider any information obtained from the internet as opinions only unless it cites a source. If Cag130 is really concerned about breaker size, he should ask the manufacturer or the NEC code book, or pay money to an electrician.
I believe my advise to be safe. I do not cite a source of how derived my contributed bit of information, so you are free to assume I am talking out my azz,
--Doozer

Doozer75 said:

Cag130 is showing us pics of his main breaker, his regular breakers, his contactor and overloads, and a pic of the compressor. He does not state the amp draw of the motor. If it is 7.5 hp it is likely 230v and single phase. He gives no pertinent information with his question, only useless pics.
--Doozer

Click to expand...

Actually his 4th picture shows a sticker that it needs 230volts-single pahse-60 cycles. I would not call that a usless picture.
EDIT- I do agree that nowhere did he state or show anything related to amps, and, that is a BIG part of the missing info.

you can't run #8 with a 100A CB...

although you can oversize the CB for a motor, the cable must match...
if there is no matching CB for the cable ampacity you can go up to the next size breaker...

if you ran #8 rated at say 50A, and had a fault of 80A, the 100A CB would never trip (it can carry 100A indefinitely) and eventually the wire would burn...

Doozer75 said:

He gives no pertinent information with his question, only useless pics.

Click to expand...

He did, he told us he had a 7.5hp 230 volt single phase motor on a compressor. That is all you need to know.

Doozer75 said:

Aceman- How is this bad advise and how specifically does it not meet the national electrical code in regard to breaker size?

Click to expand...

Right here:

Doozer75 said:

What-is-the-amp-draw-of-the-motor??
Go with the next up even size breaker.

Example: 18.5 amp use a 20 amp breaker.

--Doozer

Click to expand...

You don't use the motor nameplate for sizing wire or breakers.

Doozer75 said:

so you are free to assume I am talking out my azz,
--Doozer

Click to expand...

Oh, I know it. Now why you're giving advice on here about something you don't seem to understand is the real question??

malibu101 said:

Actually his 4th picture shows a sticker that it needs 230volts-single pahse-60 cycles. I would not call that a usless picture.
EDIT- I do agree that nowhere did he state or show anything related to amps, and, that is a BIG part of the missing info.

Click to expand...

Again, you do not use the motor nameplate for sizing wire or breakers.

Schrodingers Cat said:

you can't run #8 with a 100A CB...

although you can oversize the CB for a motor, the cable must match...
if there is no matching CB for the cable ampacity you can go up to the next size breaker...

if you ran #8 rated at say 50A, and had a fault of 80A, the 100A CB would never trip (it can carry 100A indefinitely) and eventually the wire would burn...

Click to expand...

I disagree. Table 430.52 allows breakers to be sized up to 250% of the motor rating. 7.5hp=40 amps x 250%=100 amp breaker on #8 Thhn. If a 100amp breaker isn't sufficient to start the motor without tripping I can then go up to a maximum of 400% of the motor rating. Since 160 amps isn't a common breaker size, I could use a 150 amp breaker.

What you aren't realizing is there are overloads keeping the #8 thhn from ever pulling that much current. The breaker is simply for ground fault and short circuit protection, the overloads are what limit the current drawn through the wires.

Aceman said:

I disagree. Table 430.52 allows breakers to be sized up to 250% of the motor rating. 7.5hp=40 amps x 250%=100 amp breaker on #8 Thhn. If a 100amp breaker isn't sufficient to start the motor without tripping I can then go up to a maximum of 400% of the motor rating. Since 160 amps isn't a common breaker size, I could use a 150 amp breaker.

What you aren't realizing is there are overloads keeping the #8 thhn from ever pulling that much current. The breaker is simply for ground fault and short circuit protection, the overloads are what limit the current drawn through the wires.

Click to expand...

I agree, you can upsize the CB to allow the motor to start...

you can't run a CB > than the ampacity of the conductor, except as I noted, ie, no matching CB, then next size is OK...

yep, the OL's will prevent the motor from drawing the current

but if the conductor fails prior to the motor, they won't help, and you can draw 100A on a 50A wire...but not for long

Schrodingers Cat said:

you can't run #8 with a 100A CB...

although you can oversize the CB for a motor, the cable must match...
if there is no matching CB for the cable ampacity you can go up to the next size breaker...

if you ran #8 rated at say 50A, and had a fault of 80A, the 100A CB would never trip (it can carry 100A indefinitely) and eventually the wire would burn...

Click to expand...

Code allows oversized breakers on dedicated motor circuits. You size the wire to the motor loads.

Aceman is correct. Section 430 of the code pertains to motors, motor circuits and controllers, and you use this section to determine conductor size and protection. Ignore other parts of the code which might have differing conductor and protection sizing requirements.

430.22(A) says you size the conductors to 125 percent of motor's full load current rating (FLC), but then it goes on to say that you determine the FLC from the tables in the code (430.247 thru 430.250) and that the only time you use actual nameplate FLC is when both FLC and the horsepower are listed on the motor nameplate. See 430.6(A)(1) which you are referred to by 430.22(A).

Once you establish the conductor size, you establish the protection size by referring to 430.52(C) which establishes maximum breaker or fuse sizes. (250% of FLC for an inverse time breaker, or 175% of FLC for a time delay fuse). Start with the smallest current protection that will not trip in normal use.

If the compressor is more than 50 ft straight line from the panelboard, or is not in sight of the panelboard, you will need a disconnect at the compressor, which is a good idea in any case.

My Husky 7.5 hp (true, running Hp) draws 30 amps per the data plate. I have a 60 amp breaker supplying #6 THHN cu wire feeding a subpanel at the compressor with a 50 amp breaker in it and more #6 cu wire supplying the compressor.

Charles

NEC

240.4
over current protection, conductors must be protected accordinance with their ampacites Table 310.15

430.22.A
we agree, the motor conductor must NOT BE LESS than 125% of FLA

430.51
motor branch circuit overcurrent (short or ground)
the rules add to or amend 240 (not replace)

430.52.C.1
setting of said OC protection
you can use a device not exceeding the table values (1 ph inverse time CB, 250%)
it does not say you can use a conductor that violates 240

the key is to balance...an inverse time CB will take ~700% of rating for several seconds, allowing the motor to start, so size for the conductor, and set for starting, if adjustable

use the smallest CB to allow start, and a conductor a bit over 125% of FLA
that's why VFD's and soft starts are so nice
and in the old days delta-wye and auto xfmr starters (reduced voltage), and 2 speed motors

Did you receive the motor starter with the compressor? If so, the heating element in it would be correctly sized. If you procured the motor starter from other than the compressor supplier, was the heater element in it properly sized for the motor? The motor starter is the motor's overload protection, and the heater element must be sized correctly.

A pic of the motor data plate would be most useful, but I'm betting the amp draw is 29 to 32 amps.

Charles

Charles (in GA) said:

Code allows oversized breakers on dedicated motor circuits. You size the wire to the motor loads.

Aceman is correct. Section 430 of the code pertains to motors, motor circuits and controllers, and you use this section to determine conductor size and protection. Ignore other parts of the code which might have differing conductor and protection sizing requirements.

430.22(A) says you size the conductors to 125 percent of motor's full load current rating (FLC), but then it goes on to say that you determine the FLC from the tables in the code (430.247 thru 430.250) and that the only time you use actual nameplate FLC is when both FLC and the horsepower are listed on the motor nameplate. See 430.6(A)(1) which you are referred to by 430.22(A).

Once you establish the conductor size, you establish the protection size by referring to 430.52(C) which establishes maximum breaker or fuse sizes. (250% of FLC for an inverse time breaker, or 175% of FLC for a time delay fuse). Start with the smallest current protection that will not trip in normal use.

If the compressor is more than 50 ft straight line from the panelboard, or is not in sight of the panelboard, you will need a disconnect at the compressor, which is a good idea in any case.

My Husky 7.5 hp (true, running Hp) draws 30 amps per the data plate. I have a 60 amp breaker supplying #6 THHN cu wire feeding a subpanel at the compressor with a 50 amp breaker in it and more #6 cu wire supplying the compressor.

Charles

Click to expand...

the 430 section specifically states it adds to or amends 240, in effect, 240 still governs...

the table says you can use a CB up to that size...

the conductor sizing section states min 125%, it doesn't say ignore sect.240, and use a CB > than the ampacity, and the conductor can be LARGER, just not less than 125%
but it needs protected, regardless of size, in accordance with sect. 240

your install:
you have a 200% CB (60A)
motor load 30: min ampacity 37.5 (#8 would suffice correct?)
#10 would ampacity wise, but is limited by 240.4.D to 30A
but wait, 430 supercedes and invalidates 240, correct? so #10 is good for 40A
but you used #6 good for 75A...why not use the #10? or even #8?

so you complied with 240? completely, excessively and conservatively to boot...

I would do it as follows:
30 x 1.25 ~ 38 A, can't use #10 per 240.4.D becasue I need a CB > 30
CB inverse time 50A (50/30 ~ 167%, should start no issues)
conductor THHW #8 good for 50 at 75C (I like to use the middle col)
ignoring Vdrop, if less than 100 ft. you're probably OK

Schrodingers Cat said:

NEC

240.4
over current protection, conductors must be protected accordinance with their ampacites Table 310.15

Click to expand...

......unless otherwise permitted in 240.4(G). Which directs us to Art. 430 parts III-VII for overcurrent protection sizing.

Bottom line, this is not a general purpose branch circuit feeding receps in a garage. This is a motor circuit and different rules apply. Saying the breaker can go up a maximum of one size over the conductors ampacity rating is false. If I need to bump it up to a 100 amp breaker to start this compressor, you're telling me I would now need to change the wire out to #3. That's simply not the case.

Aceman said:

......unless otherwise permitted in 240.4(G). Which directs us to Art. 430 parts III-VII for overcurrent protection sizing.

Bottom line, this is not a general purpose branch circuit feeding receps in a garage. This is a motor circuit and different rules apply. Saying the breaker can go up a maximum of one size over the conductors ampacity rating is false. If I need to bump it up to a 100 amp breaker to start this compressor, you're telling me I would now need to change the wire out to #3. That's simply not the case.

Click to expand...

yes, the OC protection, ie, CB may be sized differently...
but not in violation of 240's requirement to protect the conductor

use logic
50A motor
#6 rated at 65A
using the table we can use a CB continuously rated at 125A (250% of 50)
if you have a wiring short in the conduit and it is impedence limited to 125A, ie, isulation breakdown, moisture, etc., what is going to protect the 65A conductor?

now there are circumstances where the conductor is considered sacrificial, carry until failure, like a fire pump..and they may not even have a CB...

a home compressor is not one of these times

the key is to select an appropriate CB...
you can find inverse time CB's that will carry 500% of rating for 6+ sec

say a 50A motor has a starting current of 700% 350A
and it's on a 125A CB ~ 250% FLA...
so to start the CB must operate at 350/125 <300% of it's rating for some duration...
a std CB can carry 300% for >20 sec...if it hasn't started by then you have a problem...

a circuit needs looked at as a system, and it needs to be coordinated
and the load characteristics are most important...
I would NEVER size a CB at ~200% (125/65) of conductor rating (50A motor/65A wire/125A CB), when it's so easy to do it right, ie, protect the conductor, motor, etc.
the WHOLE system

240.4(G) says you protect conductors for motors in accordance with 430.

240.3 says you protect equipment (motors) in accordance with 430.

Charles

Charles (in GA) said:

240.4(G) says you protect conductors for motors in accordance with 430.

240.3 says you protect equipment (motors) in accordance with 430.

Charles

Click to expand...

yes it does...and you do...
as referenced in 430
no where in 430 does it allow OC protection > ampacity
only that it may be up to 250% (or as appropriate) of the FLA
again, it does not say OC > ampacity but it does say ampacity > 125% FLA
just the opposite, 430.51 states the section is in addition to or amends 240
not superceding it...

and NO where does it state you can use a CB > than conductor, unless there is a mismatch, then 1 CB size up is OK

using your interpretation:
100A motor
125A wire (#2)
instant trip CB up to 1100% or 1100A
so a 1000A fault could be carried indefinitely (until the wire burns open)

makes no sense...

I will always protect the conductor with the CB, it's the whole reason for having one!!! and select the CB appropriately for the load, which with std breakers is usually 150% of RLA, and since the conductor is required to be 125% (min) of the FLA, I usually only have to upsize one gauge...

a 40A motor
50A wire
50 CB (which can carry 280A, 700% of FLA (or 5.6 x CB rating) for >15 sec)
usually impedence, motor design, etc. limit starting currents to 400-500%
(3.6 x CB rating) in which case the CB could carry that for 20+ sec
depending on type...buy a CB rated for motors and the match gets closer...

Charles (in GA) said:

240.4(G) says you protect conductors for motors in accordance with 430.

240.3 says you protect equipment (motors) in accordance with 430.

Charles

Click to expand...

btw:

you never did clarify why you didn't use #10 with your 30A motor and 60CB since it allowed by code, according to you...since 240 doesn't apply...

and why instead you chose to comply with 240 AND 430...

using your interpretation once more:
100A motor
125A wire (#2)
instant trip CB up to 1100% or 1100A
so a 1000A fault could be carried indefinitely (until the wire burns open)

on a 240 VAC 125A conductor circuit, with a per leg impedence of 0.11 ohm or so (quite low) you could have a leg-leg bolted fault I ~ 240/0.22 ~ 1090 A and would never trip!
870% of the conductor rating

Aceman-
"You don't use the motor nameplate for sizing wire or breakers."

Says who? Your NEC code book? Horsepower is on the nameplate I believe. Amps are also.
I know given Hp and Voltage, you can figure current based on the known efficiency of the motor.
Do you know what the efficiency is? If you knew the amp draw of the motor you could figure efficiency out. Amp draw of the motor tell you more of the requirements than horsepower. Just because you use Hp values for wire and breaker sizing does not mean it is the only way.
I just looked at my compressor. 3hp 230v 16.5 amps
I have it on a 20 amp breaker. This works using my logic.
Now check you book. Does it want me put in a larger breaker??
I am looking at table 430.52 in your book. It says Maximum Rating of motor protection device, table title is Percentage of Full load Current. Exception #1 and #2 both relate to motor current. I know it is a big book, and I am trying to relate to you on your terms, but am I missing something? Maximum means I can use a smallest breaker that will work, yes? More protection is better, yes? If I am not reading your book correctly, please point me in the right section.
--Doozer

And I thought we had problems in the steel business with MSHA, OSHA, AISC, ASTM and 100's of ways to figure load paths and connections for seismic, non seismic, wind, axial, moment and shear loading.

Electrical work is complicated.....

Schrodinger, maybe these links will help.

http://forums.mikeholt.com/showthread.php?t=92213&highlight=motor+breaker+fault+current
http://forums.mikeholt.com/showthread.php?t=85615&highlight=motor+breaker+fault+current

Something else to remember, the NFPA(National Fire Protection Association) publishes the NEC. Do you think they would tell us to calculate breaker and wire sizes in such a way that it would cause a fire hazard or wires to burn up?

Doozer75 said:

Aceman-
"You don't use the motor nameplate for sizing wire or breakers."

Says who? Your NEC code book? Horsepower is on the nameplate I believe. Amps are also.
I know given Hp and Voltage, you can figure current based on the known efficiency of the motor.
Do you know what the efficiency is? If you knew the amp draw of the motor you could figure efficiency out. Amp draw of the motor tell you more of the requirements than horsepower. Just because you use Hp values for wire and breaker sizing does not mean it is the only way.
I just looked at my compressor. 3hp 230v 16.5 amps
I have it on a 20 amp breaker. This works using my logic.
Now check you book. Does it want me put in a larger breaker??
I am looking at table 430.52 in your book. It says Maximum Rating of motor protection device, table title is Percentage of Full load Current. Exception #1 and #2 both relate to motor current. I know it is a big book, and I am trying to relate to you on your terms, but am I missing something? Maximum means I can use a smallest breaker that will work, yes? More protection is better, yes? If I am not reading your book correctly, please point me in the right section.
--Doozer

Click to expand...

Alright Doozer, I'm gonna give it one more go tonight before I call it quits.

I'm going to show you how to size a motor circuit right from the beginning. I'll use your 3hp 230v motor as an example, the code references shouldn't change much, if at all, for other motor sizes. This is 08 NEC.

First read 430.6(A)1 & 2

1. Motor ampacity: I go to Table 430.250 to find the FLC(full load current) rating of your motor. It shows 17 amps. The reason we use the NEC chart rather than your particular nameplate rating is the fact not all motors of the same HP draw the same current. Sometimes they're several amps less and sometimes they're more and match the tables ampacity. What happens when you decide to swap this motor out? That's why we use the tables.

2. Conductor ampacity: 430.22, 240.4(G), Table 310.16. Take 125% of 17(FLC) amps=21.25 I can use #12 Thhn or Romex minimum.

3. Breaker Size: 430.52(A), (B), (C), Table 430.52. 17(FLC) x 250% maximum breaker size=42.5 amps. 430.52(C)1 Exception 1 allows us to round up if the amperage doesn't correspond to a standard breaker size. So 42.5-->45 amp breaker per 240.6(A) standard breaker sizes.

4. Overload Size: 430.32(A)1 & 2. Sized to the nameplate. 16.5 amps in your case, lets also assume 1.15 service factor on your motor.
16.5 x 125%=20.3 amps MAX overload setting.

5. EGC(ground wire) is then sized to the breaker rating but never larger than the ungrounded(hot) wires per 250.122(A).

So to give you the extreme end of the scale on your motors breaker and wire sizing I could legally feed it with #12 on a 45 amp breaker. If it still tripped on startup, I can take it to 400% MAX per Table 430.52 Exception 2(c). 17(FLC) x 400%=68 amps. Since 400% is max, I then round down to the next standard breaker size=60.

There is no problem using a smaller breaker, as long as it can handle startup. Electricians don't typically size it as tight as you did with a 16.5 amp nameplate on a 20 amp breaker. When the motor starts wearing out and starts drawing more at startup it'll start tripping the breaker. It's easier to throw a 25-30 amp breaker on it right from the beginning and avoid the potential nuisance tripping while still complying with code.

Doozer75 said:

Aceman-
"You don't use the motor nameplate for sizing wire or breakers."

Says who? Your NEC code book?
--Doozer

Click to expand...

using 2005 code, Yes, see 430.6(A)1:

"Other than for motors built for low speeds (less than 1200 RPM) or
high torques, and for multispeed motors, the values given in Tables 430.247, 430.248, 430.249, and 430.250 shall be used to determine the ampacity of conductors or ampere ratings of switches, branch-circuit short-circuit and ground-fault protection, instead of the actual current rating marked on the motor nameplate."

The purpose of this is to allow replacing same HP motors in equipment without having to redesign the branch circuit due to slight changes in motor efficiencies.

note that 430.6(A)2:
"Separate motor overload protection shall be based on the motor nameplate current rating."

This is NOT the same as the circuit breaker in the main panel. This is the thermal overload device located in the motor control box.

The compressor in question is 7.5HP per table 430.248 shows 40 amps.

430.22 says:
"Conductors that supply a single motor used in a continuous duty application shall have an ampacity of not less than 125 percent of the motor's full-load current rating as determined by 430.6(A)(1)."

Therefore 40x1.25=50amps. So 6 gauge NM would be appropriate (unless having to de-rate due to other conditions).

430.3x deals with overload (not short circuit or ground fault) protection. This protection is usually located at the motor (i.e. the motor control box).

430.32(A) - Continous duty motors (Greater than 1 hp):
"Each motor used in a continuous duty application and rated more than 1 hp shall be protected against overload by one of the means in 430.32(A)(1) through (A)(4)."

In this case, the compressor is protected by 430.(A)(1) Seperate overload device:
"A separate overload device that is responsive to motor
current. This device shall be selected to trip or shall be rated at no more than the following percent of the motor nameplate full-load current rating:
Motors with a marked service factor 1.15 or greater 125%
Motors with a marked temperature rise 40°C or less 125%
All other motors 115%
Modification of this value shall be permitted as provided in 430.32(C). For a multispeed motor, each winding connection shall be considered separately."

I don't have the details of what is in that motor control box, but I can it does have an overload protection device, and I assume a) that is came with the compressor manufacturer, and b) Ingersoll followed the code when they chose the device.

430.5x deals with "devices intended to protect the motor branch-circuit conductors, the motor control apparatus, and the motors against overcurrent due to short circuits or grounds. These rules add to or amend the provisions of Article 240."

430.52(A)1 Rating or Setting for Individual Motor Circuit - General:
"The motor branch-circuit short-circuit and ground-fault protective device shall comply with 430.52(B) and either 430.52(C) or 430.52(D), as applicable."

So 430.52(B):
"The motor branch-circuit short-circuit and ground-fault protective device
shall be capable of carrying the starting current of the motor."

Kinda obvious I think.

430.52(C)1:
"In Accordance with Table 430.52 A protective device that has a rating or setting not exceeding the value calculated according to the values given in Table 430.52 shall be used."

Looking at 430.52 shows that single phase motors using inverse time breakers are allowed up to 250%.

430.52(C)2:
"Where maximum branch-circuit short-circuit and ground-fault protective device ratings are shown in the manufacturer's overload relay table for use with a motor controller or are otherwise marked on the equipment, they shall not be exceeded even if higher values are allowed as shown above."

I don't have the data from the manufacturer, the original poster will have to answer this.

The rest of 430.52(C) doesn't really apply in this case and 430.52(D) doesnt apply at all.

So we have for the original poster, wire that can support 50 amps (such as 6 gauge romex or 8 THHN in conduit), protected by a breaker not to exceed 2.50x40amps=100amps, as well as the factory overload protection that IR put in the control box.

EDIT: OOPS just noticed Aceman typed up something really similar to what I just typed up.

Aceman said:

Schrodinger, maybe these links will help.

http://forums.mikeholt.com/showthread.php?t=92213&highlight=motor+breaker+fault+current
http://forums.mikeholt.com/showthread.php?t=85615&highlight=motor+breaker+fault+current

Something else to remember, the NFPA(National Fire Protection Association) publishes the NEC. Do you think they would tell us to calculate breaker and wire sizes in such a way that it would cause a fire hazard or wires to burn up?

Alright Doozer, I'm gonna give it one more go tonight before I call it quits.

I'm going to show you how to size a motor circuit right from the beginning. I'll use your 3hp 230v motor as an example, the code references shouldn't change much, if at all, for other motor sizes. This is 08 NEC.

First read 430.6(A)1 & 2

1. Motor ampacity: I go to Table 430.250 to find the FLC(full load current) rating of your motor. It shows 17 amps. The reason we use the NEC chart rather than your particular nameplate rating is the fact not all motors of the same HP draw the same current. Sometimes they're several amps less and sometimes they're more and match the tables ampacity. What happens when you decide to swap this motor out? That's why we use the tables.

2. Conductor ampacity: 430.22, 240.4(G), Table 310.16. Take 125% of 17(FLC) amps=21.25 I can use #12 Thhn or Romex minimum.

3. Breaker Size: 430.52(A), (B), (C), Table 430.52. 17(FLC) x 250% maximum breaker size=42.5 amps. 430.52(C)1 Exception 1 allows us to round up if the amperage doesn't correspond to a standard breaker size. So 42.5-->45 amp breaker per 240.6(A) standard breaker sizes.

4. Overload Size: 430.32(A)1 & 2. Sized to the nameplate. 16.5 amps in your case, lets also assume 1.15 service factor on your motor.
16.5 x 125%=20.3 amps MAX overload setting.

5. EGC(ground wire) is then sized to the breaker rating but never larger than the ungrounded(hot) wires per 250.122(A).

So to give you the extreme end of the scale on your motors breaker and wire sizing I could legally feed it with #12 on a 45 amp breaker. If it still tripped on startup, I can take it to 400% MAX per Table 430.52 Exception 2(c). 17(FLC) x 400%=68 amps. Since 400% is max, I then round down to the next standard breaker size=60.

There is no problem using a smaller breaker, as long as it can handle startup. Electricians don't typically size it as tight as you did with a 16.5 amp nameplate on a 20 amp breaker. When the motor starts wearing out and starts drawing more at startup it'll start tripping the breaker. It's easier to throw a 25-30 amp breaker on it right from the beginning and avoid the potential nuisance tripping while still complying with code.

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the NEC is NFPA Vol. 70
yes, they would not allow a condition that would allow a fire, hence conductor ampacity must be > CB rating...

please explain my example of 100A FLA/125A wire/1100A CB instant trip)
and the resultant bolted fault of >1000A sustained...you say i's allowed and is good practice..I say it's an accident waiting to happen...

you can't use #12 with larger than a 20A CB per 240.4.D

you are misinterpreting the code...if you select a 45A CB you may use #10, but that would appear to violate 240.4.D, so it would need to be #8

I would size as follows:
FLA 17A
sizing conductor 21.25A, #12 would do, but reuires a 20A CB, too small
go with #10 and a 30A CB (inverse time)
this will allow LRA (assume ~ 85A or 280% of the CB rating to flow for >10 sec while starting, which usually takes 5 sec, and decreases logarithmatically)

so 3#10, 1#12 and 30A CB

and you are allowed to size to name plate rating

Schrod and Aceman:

Lets first all agree that the "code" is the "minimum" specification for application of ampacities. It then stands to reason that you both are right. While both of your approaches are "interpertations" of the code, both would pass an inspection and operation with Schrod's costing more and being extremely conservative.

The code does take into account changes in the technology of materials and the application of advances in testing and manufacturing resulting in higher quality products. This equates to a tighter application of the principles. Refer to page 70-22 (2005 code, looseleaf version) and the scope of the committee (guide) is stated.

This is a perfect example of why electrical questions do not always have simple answers...

By the way, did the compressor come with an owner's manual? Chances are there's some information in there on the manufacturer's recommended installation.

I've spent enough time trying to explain this so I'm going to make this short and to the point.

Schrodingers Cat said:

you can't use #12 with larger than a 20A CB per 240.4.D

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You keep referencing 240.4(D). The first two lines state "unless specifically permitted in 240.4(G)...." Which tells us overcurrent protection for specific conductor applications shall be permitted as provided in Table 240.4(G). Table 240.4(G) then lists motors and motor control circuit conductor as being one of the applications permitted. Everything else after this has already been explained to you numerous times.

Schrodingers Cat said:

you are misinterpreting the code.

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I am a licensed electrician and was top of my class. I guarantee you I am not the one misinterpreting the code.

Schrodingers Cat said:

and you are allowed to size to name plate rating

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Here's a little snippet of 430.6(A)1

"the values given in Tables 430.247-430.250 shall be used to determine the ampacity of conductors or ampere ratings of switches, branch circuit short circuit and ground fault protection, instead of the actual current marked on the motor nameplate."

Schrodingers Cat said:

btw:

you never did clarify why you didn't use #10 with your 30A motor and 60CB since it allowed by code, according to you...since 240 doesn't apply...

and why instead you chose to comply with 240 AND 430...

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Charles (in GA) said:

My Husky 7.5 hp (true, running Hp) draws 30 amps per the data plate. I have a 60 amp breaker supplying #6 THHN cu wire feeding a subpanel at the compressor with a 50 amp breaker in it and more #6 cu wire supplying the compressor.

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Charles (in GA) said:

Once you establish the conductor size, you establish the protection size by referring to 430.52(C) which establishes maximum breaker or fuse sizes. (250% of FLC for an inverse time breaker, or 175% of FLC for a time delay fuse). Start with the smallest current protection that will not trip in normal use.

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Read what I wrote.

Charles

That particular comp runs from an 8 wire and a 50 brkr, at most a 60. I didn't read all the rest of the details posted here but Ace is probably one of the code guru's here, about as knowledgeable as anyone but a manners class along with the code class would benefit a guy with a long career ahead of him.

So to give you the extreme end of the scale on your motors breaker and wire sizing I could legally feed it with #12 on a 45 amp breaker. If it still tripped on startup, I can take it to 400% MAX per Table 430.52 Exception 2(c). 17(FLC) x 400%=68 amps. Since 400% is max, I then round down to the next standard breaker size=60.

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I have a question here, wouldnt we be limited to 50A OCPD on a 12 wire? Have to be a 10 to be on 60A?

Doesnt this 250% factor say FLC, doesnt say 250% of the amp rating of the conductor size. Hence, in simple forms, 12 per 50A, 10 per 60, 8 per 100 and 6 to 200A

All that is some code ******* stuff they got other forums for. For the DIY garage types,,,,, yes they often put breakers larger than 20 on a 12 wire, not common to require as radical as a 50 but often a 30, a well is a good example, a compressor, saws, welders. In some cases there could be multiple motors or tools. Good example would be 2 small welders connected by 12 feeders, both connected to 50A ocpd.
For the most part here on this forum though we talking 1 appliance to one breaker circuits. As the man said, often later start ups become more of a problem, hence about 10 yrs and a couple of trips I change my well out to a slightly larger breaker, hence sparky just puts a 30 on it right from the start. Well installers put them in by the 1000's, a 1 hp motor with its own thermal, a 12 romex to a couple 30A fuses. (its why they make those 30A fuses you seem to find everywhere) I assume below a HP they toss 20's in.
For this crowd, common to use breakers 1 size above the wire required. Some times 2, example is Lincoln buzz box. In barns and garages millions of them running from a 50 with a 10 romex.
Tools like 120V comps and chopsaws are notorious, they come with 15A plugs but will routinely trip 20A, should have its own circuit.

Maybe this will help.

Per 2005 NEC Article 430 which supersedes Article 240.

For continous duty motors greater than 1 hp:

1. Conductor size = 125% of the motor amp rating ("FLC") per nec table(not name plate)

2. Overload (starter with thermal overload)= 125% of motor name plate (FLC) (or table if name plate is not supplied)

3. Short circuit protection (breaker) = 250 % of FLC or up to 400% of FLC

I believe this is a good interpertation of what is being said. There are exceptions but this is based on the compressor in question.

sberry...Thanks for the insight and help on this issue. I understand code is important but bottom line, the guy wants to know wire size and breaker size.

Thanks!!

Sberry, your well example falls within the code as you described. Your welder example also follows the code. Lets remember that the code is the minimum requirements. However, these two examples are not the norm for general lighting and cord-and-plug outlets. With motor applications the wire is sized to the "running" load, the breaker is sized to the starting current. With welders the wire is sized based on the primary supply current times the duty factor of the welder. The breaker can be sized up to 200% of the primary current requirement. A smaller breaker is acceptical (and more conservative) if it does not trip when in service.

Just to throw in my 2 cents and what my EE firm would do:

A single-phase 240V 7.5HP compressor = 40A

40A x %125 = 50A, use #6's(but MAY use #8's depending on the wire type used) Based on 2008 NEC, 310.16

However, these two examples are not the norm for general lighting and cord-and-plug outlets.

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Correct. These are for specialty circuits.
As a side note, I did a sloppy job on hookup to test something, 1 strand from a 16Ga cord wire snuck out and manage to hit metal electric cover, not 1 16ga wire but 1 strand, knocked the 20 branch brkr and the 2P 60 feeding the panel back at the main.

I am often amazed but never surprised at how far some people will go on trying to defend their position when they are so wrong.

Aceman and Charles are 100% correct in their understanding of the application of the NEC in reference to the compressor installation.

Schrodengers Cat, Yennek, and Doozer are perfect examples of the unfortunate few who literally define the phrase "know enough to be dangerous".

The shame of it all is that the people asking for help, don't really know any better and believe everything these "experts" tell them. And if not for the comments of a few like Aceman and Charles , people could get seriously hurt or even killed by trying to do their own electrical work according to the Cat, Yennek, & Doozer Code Book!

These folks are convinced they are right and everyone else in the world is wrong. Amazing concept to live by.

JP