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Old 12-30-2011, 03:15 PM   #21
sberry
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Default Re: New 7.5 hp Compressor. What breaker and wire size??

Any of these comps to 7 1/2 hp will run from that circuit. 95% of the comps found in home hobby garages are smaller than that, usually 5 hp rated. Your circuit is fine for those too.
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Old 12-30-2011, 03:16 PM   #22
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Default Re: New 7.5 hp Compressor. What breaker and wire size??

Quote:
Originally Posted by Naq View Post
Probably not. Voltage drop shouldn't be a problem on a run that short. If you're using Romex (Type NM) it just won't meet the NEC in case that matters to someone in the future.

NM and it's variants are limited to the 60C column so #8=40A and #6=55A which allows you to round up to a 60A breaker since 55A is not a standard size.

Does it get hot in the attic you're running through? Are you running through insulation or with other NM cables? There are correction factors for everything. It'd be my luck that we'd go away for the weekend without turning it off and the thing would leak and run continuously for 3 days while its 130F in the attic.
Meeting code certainly does matter to me.

I bought 35 feet of #6/2 Romex, 4 feet of #6/2 S/O, a 60 Amp disconnect and a 50 Amp breaker this morning.
The compressor is in a different room than the panel so I figured the disconnect was necessary even though there is an on/off switch on compressor's box. Anyway, I think the disconnect makes it easier to wire.

Yes, I'm in Texas so the attic does get hot. But, I am pretty good about shutting the power switch off on a compressor every night.

I've got a ton of room in the attic and can run the wire above the insulation and not close to any other wires except where it will go down the wall where the breaker panel is.

Last edited by roger55; 12-30-2011 at 03:19 PM.
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Old 12-30-2011, 07:15 PM   #23
Charles (in GA)
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Default Re: New 7.5 hp Compressor. What breaker and wire size??

If you go to THIS particular post by Aceman in the thread cited earlier, it will explain in detail how to size a motor circuit. I did fix the table cite for motor ampacity, from 430.250 (which is for three phase motors) to 430.248 (which is for single phase motors). Aceman had the amps correct (17) just not the table number.

What he is saying here, is that you, by code, cannot use the motor nameplate amperage for determining the wire size and breaker size, you are supposed to, by code, reference the correct table in part 430 for the type of motor you are sizing the circuit for.

In the case of roger55's compressor table 430.248, shows, for a 7.5 hp motor, at 230 volts (they do not give 240) that the Full Load Current is 40.0 amps. You use this number, multiplied by 125% to arrive at the minimum circuit capacity for this compressor, which is exactly 50.0 amps. Because the breaker is almost certainly dual rated 60/75C then you have to use the 60 column of table 310.15(B)(16) {formerly table 310.16) and we find that #8 cu is good for 40 amps (not enough) and #6 is good for 55 amps (which is great) so we select the #6 and a 60 amp breaker.

With respect to the breaker, you are allowed to, but not required to, oversize the breaker for a motor circuit up to 250% of its Full Load Current, so this comes out to 125 amps, which you will NEVER need with this motor. The 60 amp will work just fine without tripping (and over 60 amp, breakers get REAL expensive)

With respect to the heat in the attic, indeed that could be a problem. You start derating wires for temperature when the ambient temp exceeds 86F. If you are using cable it is most likely rated to 75C. It would be best to get a cable with a 90C rating, as temp derating is calculated using the max temp the cable is rated to, and not the minimum like the other calculations. That #6 wire, if 90C rated wire, in an attic not exceeding 122F gets it ampacity reduced by multiplying by .86, so we go back to Table 310.15(B)(16) and look in the 90C column and find that the wire is good for 75 amps and then multiply by .82 and get 61.5 amps, so we are still good. Let the temp rise to no more than 131F and the correction factor becomes .76 and the amps allowed on that wire is 57 amps......


Charles

Quote:
Originally Posted by Aceman View Post
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 430.248 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.

Last edited by Charles (in GA); 12-30-2011 at 07:30 PM.
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Old 12-30-2011, 07:29 PM   #24
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Default Re: New 7.5 hp Compressor. What breaker and wire size??

Thank you for all those details Charles. At this point, should I go back to the store and exchange my 50 Amp breaker for a 60? Or as long as it doesn't pop am I fine?


Just another question for interest only.
I was always under the impression that a breaker is sized for the wire and not the device and is for protection of the wire and not for the device.
So, how can it meet code to go larger than a 60 Amp breaker on #6 wire? It would seem that the length of the wire would not be protected.
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Old 12-30-2011, 07:35 PM   #25
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Default Re: New 7.5 hp Compressor. What breaker and wire size??

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Originally Posted by browntown View Post
Sorry to interupt, but I have a similar tangental question. My house was at some point wired for a hottub, that no longer exists. The 230 wire is run through exposed conduit in my garage directly to a box on the otherside of the wall. If I cut the conduit and pull through the extra cable, then terminate it inside the garage with a switch box -- I'll have the wire in place for a 230 compressor. I'm looking to avoid added costs, and reuse the hot-tub wiring. It currently feeds off a what look like two normal breakers but have the handles joined. One breaker has the black hot and one has the red hot. The double latch shows "50" twice, but I'm pretty sure that doesn't mean 100amp, but just 50.The breakers lead to some thick 3wire (black, red, white, + copper ground, 4 wires I guess,but 3 conductors )that runs through the conduit.

What size "continuous duty" 230v motor should I be shopping for so I can reuse the hottub wiring?
The circuit is rated 50 amps for a 240 volt load = good for a 5 h.p. single phase motor
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Old 12-30-2011, 07:56 PM   #26
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Default Re: New 7.5 hp Compressor. What breaker and wire size??

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Originally Posted by roger55 View Post
Thank you for all those details Charles. At this point, should I go back to the store and exchange my 50 Amp breaker for a 60? Or as long as it doesn't pop am I fine?


Just another question for interest only.
I was always under the impression that a breaker is sized for the wire and not the device and is for protection of the wire and not for the device.
So, how can it meet code to go larger than a 60 Amp breaker on #6 wire? It would seem that the length of the wire would not be protected.
As Aceman noted, code allows you to do that for motors, simply to prevent nuisance trips of the breaker. Your 50 amp breaker will be fine, mine has never given any hint of tripping, but it is a short run and not in any hot areas like an attic. Try the 50, it is trips, a 60 is not all that expensive.

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Old 12-30-2011, 08:32 PM   #27
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Default Re: New 7.5 hp Compressor. What breaker and wire size??

So the theory here is someone may change out the motor to another one at a later time. What does exception 3 in 430.6 mean? How does 430.6 (1) apply here, it says, if the hp isn't listed we assume that it is what it is based on amps, as I read exception 3 it is a known value? Still all based on amps?
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Old 12-30-2011, 08:36 PM   #28
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Default Re: New 7.5 hp Compressor. What breaker and wire size??

Basically (1) says,,, if you don't know hp you compute it based on amps, but we do know so it fits the exception 3 correct? I noticed Ace says,,, what he does, ok, I buy that but this would be similar to making a legal welder install for particular machine and then changing machines, correct?
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Old 12-30-2011, 08:40 PM   #29
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Default Re: New 7.5 hp Compressor. What breaker and wire size??

As I recall seems I have seen these come with factory 10 cord, have seen 5's come with a 12 and some 3's come with a 14, my real 3 with a run of 18 came with a 12.
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Old 12-31-2011, 07:14 AM   #30
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Default Re: New 7.5 hp Compressor. What breaker and wire size??

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As I recall seems I have seen these come with factory 10 cord, have seen 5's come with a 12 and some 3's come with a 14, my real 3 with a run of 18 came with a 12.
NEC does not govern "cord" size. NEC is only relative to the "circuit" that supplies the appliance. The cord is governed by UL. NEC does specify "fixture wires" (Article 402) but only for 'luminaries" (lighting fixtures).

Article 430 assumes the motor could be changed in the future to a same hp rated motor but with a different FLA rating. NFPA committee has determined motors of same hp's have a range of FLA's and Article 430 "table amps" takes this into consideration.
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Old 12-31-2011, 08:04 AM   #31
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Default Re: New 7.5 hp Compressor. What breaker and wire size??

I have the same compressor and I used 6 gauge romex and a 50 amp breaker. I too thought that a 40 may work, but then I wanted to be able to plug my welder in as well.
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Old 12-31-2011, 08:12 AM   #32
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Default Re: New 7.5 hp Compressor. What breaker and wire size??

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I have the same compressor and I used 6 gauge romex and a 50 amp breaker. I too thought that a 40 may work, but then I wanted to be able to plug my welder in as well.
Did you use a 50 Amp socket and plug and then unplug the compressor to plug in your welder? Seems like that would be the only way to do it and remain within code.
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Old 12-31-2011, 08:17 AM   #33
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Default Re: New 7.5 hp Compressor. What breaker and wire size??

I don't see much in codes that "assume" anything. What I see it says is,, that if the hp isn't listed then its hp is based on amps, if it is listed then exception 3 comes in to play. Listed appliance is what this is and both factors are known, what it appears to be for is just this case, don't have to wire for inflated horsepower claims. The 6 wire certainly isnt going to hurt anything but I am not losing sleep or going int a panic attack if my bud wires his with 8 and neither him or the comp will ever notice any difference.
Quote:
So, how can it meet code to go larger than a 60 Amp breaker on #6 wire? It would seem that the length of the wire would not be protected.
I see this didnt get an answer. The wire is protected, 2, maybe 3 ways. The breaker against short circuit, the load is known and the comp has its own thermal, it will shut the motor down in the event of overheat.
The4 NEC does govern cords to some extent, it limits the size circuit a cord can be connected to.

Last edited by sberry; 12-31-2011 at 08:25 AM.
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Old 12-31-2011, 08:40 AM   #34
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Default Re: New 7.5 hp Compressor. What breaker and wire size??

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Originally Posted by Charles (in GA) View Post
As Aceman noted, code allows you to do that for motors, simply to prevent nuisance trips of the breaker. Your 50 amp breaker will be fine, mine has never given any hint of tripping, but it is a short run and not in any hot areas like an attic. Try the 50, it is trips, a 60 is not all that expensive.
Charles
Yes it is a very short run at 35 feet. But I did run the romex through the attic and I do have Texas heat.
However, the wire is run above the insulation and I have a metal roof with a bunch of soffit vents and a ridge vent. So, it doesn't get as hot as a lot of attics do.
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Old 12-31-2011, 09:24 AM   #35
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Default Re: New 7.5 hp Compressor. What breaker and wire size??

A 7.5HP motor needs:

Wire: #6 Romex or #8 THHN minimum
Breaker: 50-100 amp
Ground: #10 if breaker is 60 or less, #8 if above 60.
Cord: #6-3 SJ or SO minimum.

I would hardwire the compressor. A standard 50 amp welder plug 6-50P doesn't have the HP rating you need.

http://www.leviton.com/OA_HTML/ibeCC...minisite=10026
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Old 12-31-2011, 09:45 AM   #36
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Default Re: New 7.5 hp Compressor. What breaker and wire size??

Seems this would apply if it is a true 7.5, but if it is a listed appliance it could be wired from the data plate? If a guy builds this comp, or this is an install from fabricated components this would apply. It applies when the unit is not "listed".

Last edited by sberry; 12-31-2011 at 09:49 AM.
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Old 12-31-2011, 10:15 AM   #37
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Default Re: New 7.5 hp Compressor. What breaker and wire size??

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Seems this would apply if it is a true 7.5, but if it is a listed appliance it could be wired from the data plate? If a guy builds this comp, or this is an install from fabricated components this would apply. It applies when the unit is not "listed".
430.6(A)1 says we should take the amps off the nameplate if no HP info is given and make an "educated guess" on the actual motor HP. Since 5 HP is shown to be 28 amps, and a 7.5 HP is 40 amps according to NEC tables, I went with 7.5 HP.

The NEC from my experience usually has "worst case" motor amperages on it's tables. I know a 5 HP doesn't usually draw 28 amps, most of the time I'll see 23-25A or so. So if he has a motor that's says it draws 32-33A, I know it's most likely bigger than a 5 HP and is more likely a 7.5 HP. Plus Ingersoll markets it as such. That's good enough for me to call it a 7.5 HP and wire it accordingly.
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Old 12-31-2011, 10:46 AM   #38
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Default Re: New 7.5 hp Compressor. What breaker and wire size??

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A 7.5HP motor needs:
I would hardwire the compressor. A standard 50 amp welder plug 6-50P doesn't have the HP rating you need.
I would and did. I put in a 60 Amp disconnect and used SO from the disconnect to the compressor.
I hate working with #6 wire and those sockets and plugs anyway.

But, I used 6/2 plus a ground not 6/3. Is that what you meant?
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Old 12-31-2011, 11:27 AM   #39
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Default Re: New 7.5 hp Compressor. What breaker and wire size??

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Originally Posted by roger55 View Post
...

But, I used 6/2 plus a ground not 6/3. Is that what you meant?
Nice job on the install. Is the equipment ground conductor bare? Usually the X/3 marking counts the insulated conductors. You could go nuts and tape the white conductor red just so everyone knows its a phase conductor. I don't know if its required or not.

I agree with Aceman on this one, but I see where sberry is coming from on the motor current rating. Especially since the motor is only marked with Volts/Amps as NEC422 requires for appliances. The biggest problem I see is that the assembly is not actually UL listed as a motor operated appliance. The only UL items I could find from IR are air pressure switches. I would then have to believe that it is "just a motor" that happens to be connected to an air pump as opposed to a listed assembly like a heat pump or electric lawn mower.
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Old 12-31-2011, 11:59 AM   #40
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Default Re: New 7.5 hp Compressor. What breaker and wire size??

I am a firm believer in codes, they are there for good reason. The only reason I debate this a bit is, first i do learn, I have been proved wrong more than once and as I am not super current Ace pointed out the need to reduce number 2 alum to 90A (always thought 100 seemed high anyway) but,,, have seen enough really scary stuff and know how it is in the real world, how its used to realize when something is on the edge of a fine line and doesn't mean a pinch of shit. i am pretty certain a guy would have a hard time finding where this comp either suffered or caused a fault from being run on an 8.
There are some places where no one including me thing the minimum standard is a great idea, some of these welders are a good case, such a performance droop hooking a 225 buzzer to a 12 and the terminations are another concern as well as the ease with which the device is ran past the duty cycle, upping it to a 10 makes sense. Makes the settings on the dials more accurate, keeps people from turning them up to wring a bit more out, etc. Same with small wire feeders, they are already short on poop if anything and can use the help from a decent wire.
While the 6 wire is great don't want to leave the impression that a guy that has this on an 8 has to have a panic attack and start ripping the place up to change out the wire.
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