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240v compressor receptacle - any reason to go over 10/2?

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5mall5nail5

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Hey guys - so I have a 120v compressor now, but I am going to upgrade in the future. I looked at some big 80 - 100 gal units and it looks like they draw about 22A on average. I am thinking of pre-wiring for a 10/2 drop which will hold 30A. It'd be the only thing on the circuit. Any reason to go with larger wire? I already have a 6/2 240v drop on the opposite wall for 50A welder/etc. in case I ever need. Since this is inside a large closet it will be dedicated to the compressor only.

Thanks
Jon
 
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soj

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How long is the wire run?

I have a 7.5HP compressor, 32 full load amps. I wired it with #8 wire and a 40A breaker, with about 40 feet of wire. Never had any trouble with breaker tripping on startup.

I am not an electrician, but some here will help with motor wire sizing requirements, which are different than other loads.
 
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5mall5nail5

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sands35

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Square D has a NEC motor calculator. You can find it through a web search. For motors wire awg and breaker size are not calculated the same as "standard" outlet loads. Basically because of startup draw vs running draw.

You will need some sort of disconect as well (which can be a plug on a cord). I grabbed a safety switch off Ebay for 40ish.

Conduit size is based on fill charts. Google "conduit fill" and look for the type of conduit you need.
 
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5mall5nail5

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sands35 said:
Square D has a NEC motor calculator. You can find it through a web search. For motors wire awg and breaker size are not calculated the same as "standard" outlet loads. Basically because of startup draw vs running draw.

You will need some sort of disconect as well (which can be a plug on a cord). I grabbed a safety switch off Ebay for 40ish.

Conduit size is based on fill charts. Google "conduit fill" and look for the type of conduit you need.
Click to expand...

Thank you - so I would run EMT off of my panel on the opposite wall?

Found the 2008 NEMA motor calculator. Says 40A on #8. I assume #8 THHN? Not #8 NM-B?
 
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sands35

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I'd use pvc. Easier and meets code. You will likely be OK with 1" conduit for anything 7hp and under. (Don't have fill charts in front of me, but I did the math for a 7hp compressor and that's what I ended up at.)
 
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5mall5nail5

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sands35 said:
I'd use pvc. Easier and meets code. You will likely be OK with 1" conduit for anything 7hp and under. (Don't have fill charts in front of me, but I did the math for a 7hp compressor and that's what I ended up at.)
Click to expand...

Wow wait a sec. Second look at this calculator says 7.5HP @ 230v (1ph) is #8 wire, 40 full load amps, circuit breaker size 80 :shocking: WTF.

Further, if #8 is true with THHN then it looks like a simple 1/2" or 3/4" conduit will work. It doesn't list PVC as an acceptable material.
 
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soj

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5mall5nail5 said:
Wow wait a sec. Second look at this calculator says 7.5HP @ 230v (1ph) is #8 wire, 40 full load amps, circuit breaker size 80 :shocking: WTF.

Further, if #8 is true with THHN then it looks like a simple 1/2" or 3/4" conduit will work. It doesn't list PVC as an acceptable material.
Click to expand...

Like I said before, I have a 7.5HP on a 40A breaker, but have been told I need higher amperage to avoid "nuisance" tripping. But I have had it this way for about 10 years and it has never tripped. YMMV.:dunno:

I also have a disconnect switch. It is a plastic box with a switch inside that looks like a circuit breaker, but is only a switch. Made by Square D. You can also use a pullout type disconnect.
 
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sands35

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Yup.

Explanations if you are curious:
http://www.mikeholt.com/mojonewsarc...s-Motor-Circuits-and-Controllers~20031126.htm

overload != ground fault or short circuit.

The breaker is sized to prevent nuisance trips on start-up but still protect against overload.

Starting out with a lower amp breaker to see what happens is OK too. (as soj suggests) In the typical residential man-cave use of compressors, rating the breaker for the wire would be just fine.

You also notice that the calculator suggests a 60 amp time delay fuse?

There isn't enough of a cost difference between 3/4" and 1" to make a difference for reasonable run. 1" will be MUCH easier to pull.
 
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pattenp

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By NEC 7.5 HP is 40A, so 40A X 1.25 = 50A. So, a 50A circuit is what you are to have by NEC for 7.5HP. 5HP is 28A X 1.25 = 35A, so 40A circuit will do for 5HP. #8 NM-b is good for 40A and #8 THHN is good for 50A.
 
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5mall5nail5

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Thanks guys very helpful. I was just talking to our facilities guy here and we have a TON of 1" (ID) Liquid-Tight conduit and fittings. We don't use it here any more for various reasons so its all surplus he said I could have whatever I need. That said - could I run (3) #8 THHN in liquid-tight conduit (1") to a 60A AC pull-type disconnect inside my closet on the wall? Trying to do this as cost effective as possible and the flexible Liquid-Tight would be nice to route in the attic/etc since I don't have to mess with rigid. Not sure if it's to code though.
 
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pattenp

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It can be installed in the wall or ceiling. But lengths greater than 6ft need to be secured every 3ft and within 12 inches of connection to a box.

5mall5nail5 said:
Is Liquid-Tight non-metallic conduit permitted in-wall and in-ceiling?

That switch setup will work well.
Click to expand...
 
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SiGmA_X

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Pattenp or other electricians, can you confirm/deny the requirement for a non-receptacle wiring for a 7.5hp motor? Jon and I were talking about this today and I never realized that requirement existed. I had been suggesting to him perhaps to use a contactor relay in a box and a 120v circuit to switch both the contactor for the compressor and the drain too, and then put a 50amp receptacle on the box or in the wall next to the box, or something. Jon said he didn't think it was code legal for loads of over 3hp to be disconnectable like that.

Insight plz? :D
 
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Jawn

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soj said:
+1 on the switch over a pull out. I use mine to turn off the compressor at the end of the day, that's just easier to do with a switch.
Click to expand...
Do you have a particular reason to use that instead of the off/auto lever on the compressor's pressure switch?
 
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pattenp

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To be code compliant for a 7.5hp motor you'd need to hardwire it. I'm not aware of any 50A plug and receptacle that is rated more than 3hp.

SiGmA_X said:
Pattenp or other electricians, can you confirm/deny the requirement for a non-receptacle wiring for a 7.5hp motor? Jon and I were talking about this today and I never realized that requirement existed. I had been suggesting to him perhaps to use a contactor relay in a box and a 120v circuit to switch both the contactor for the compressor and the drain too, and then put a 50amp receptacle on the box or in the wall next to the box, or something. Jon said he didn't think it was code legal for loads of over 3hp to be disconnectable like that.

Insight plz? :D
Click to expand...
 
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pattenp

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In my case I use the Auto/Off switch on the compressor to turn it on and off. The disconnect switch on the wall is not as convenient. I see no reason to use the disconnect switch over the auto switch unless it's more convenient.

Jawn said:
Do you have a particular reason to use that instead of the off/auto lever on the compressor's pressure switch?
Click to expand...
 
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wyliesdiesels

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5mall5nail5 said:
Wow wait a sec. Second look at this calculator says 7.5HP @ 230v (1ph) is #8 wire, 40 full load amps, circuit breaker size 80 :shocking: WTF.

Further, if #8 is true with THHN then it looks like a simple 1/2" or 3/4" conduit will work. It doesn't list PVC as an acceptable material.
Click to expand...

In a motor circuit, a breaker or fuse is sized much larger than the ampacity of the wire connected to it, which is how breakers and fuses are sized for normal branch circuits. This is to prevent nuissance tripping as mentioned by SOJ. Motors have high 'in-rush' currents that can be 5-8x FLC currents and if fuses/breakers were sized based on wire ampacity the breaker MIGHT trip!

The breaker is allowed to be oversized because it is ONLY providing ground fault and short circuit protection. Overload protection for the wire and motor is done by overloads built into the motor(on smaller motors) or heaters or overloads connected to the motor starter.

EDIT: NEC code for sizing fuses/breakers for motor circuits, is 175%/300% FLC for time delay/non time delay fuses and 250% FLC for breakers.

sands35 said:
Yup.

Explanations if you are curious:
http://www.mikeholt.com/mojonewsarc...s-Motor-Circuits-and-Controllers~20031126.htm

overload != ground fault or short circuit.

The breaker is sized to prevent nuisance trips on start-up but still protect against overload.

Starting out with a lower amp breaker to see what happens is OK too. (as soj suggests) In the typical residential man-cave use of compressors, rating the breaker for the wire would be just fine.

You also notice that the calculator suggests a 60 amp time delay fuse?

There isn't enough of a cost difference between 3/4" and 1" to make a difference for reasonable run. 1" will be MUCH easier to pull.
Click to expand...

No, a breaker for a motor circuit does NOT provide OVERLOAD protection. Using 5mail5snail5's example above, how would an 80a breaker be providing overload protection for #8 THHN wire rated at 50a? Read what i said above!!
 
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pattenp

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I think you're confusing welder circuits with motor circuits. It's 125% for conductors supplying a single motor. A 7.5HP motor will take a 50A circuit (40AX1.25=50A).

Edit: Never mind, you're talking breaker sizing and I'm thinking conductor sizing. Duh..

Edit Edit: That's what happens when you challenge me to correct you. :bowdown:

wyliesdiesels said:
In a motor circuit, a breaker or fuse is sized much larger than the ampacity of the wire connected to it, which is how breakers and fuses are sized for normal branch circuits. This is to prevent nuissance tripping as mentioned by SOJ. Motors have high 'in-rush' currents that can be 5-8x FLC currents and if fuses/breakers were sized based on wire ampacity the breaker MIGHT trip!

The breaker is allowed to be oversized because it is ONLY providing ground fault and short circuit protection. Overload protection for the wire and motor is done by overloads built into the motor(on smaller motors) or heaters or overloads connected to the motor starter. NEC code for sizing fuses/breakers for motor circuits, IIRCC, is 175% FLC and 300% FLC, respectively.(I dont have my NEC book or Uglys book in front of me, so I'm sure someone like pattenp will correct me if im wrong!)




No, a breaker for a motor circuit does NOT provide OVERLOAD protection. Using 5mail5snail5's example above, how would an 80a breaker be providing overload protection for #8 THHN wire rated at 50a? Read what i said above!!
Click to expand...
 
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wyliesdiesels

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pattenp said:
I think you're confusing welder circuits with motor circuits. It's 125% for conductors supplying a single motor. A 7.5HP motor will take a 50A circuit (40AX1.25=50A).

Edit: Never mind, you're talking breaker sizing and I'm thinking conductor sizing. Duh..

Edit Edit: That's what happens when you challenge me to correct you. :bowdown:
Click to expand...

I actually got it slightly wrong, now that I have my book in front of me.
T430.52

these are maximum ratings for single phase:

percent of FLC- type-

300% Nontime delay fuse
175% Dual Element(time delay) fuse
800% Instantaneous trip breaker
250% Inverse time breaker

I usually size motor SC/GF breakers @ 200%. If nussance trips occur, then i go from there.
 
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SiGmA_X

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Thanks for the future education Wylies and Pat!
Jawn said:
Do you have a particular reason to use that instead of the off/auto lever on the compressor's pressure switch?
Click to expand...
Yes, because its a PITA to go into a closet many feet away to turn your compressor off. My closet is inside a storage room. It makes it so the compressor can barely be heard in the shop. SO nice.
pattenp said:
To be code compliant for a 7.5hp motor you'd need to hardwire it. I'm not aware of any 50A plug and receptacle that is rated more than 3hp.
Click to expand...
Gotcha! Thank you sir, as always! :beer:
pattenp said:
In my case I use the Auto/Off switch on the compressor to turn it on and off. The disconnect switch on the wall is not as convenient. I see no reason to use the disconnect switch over the auto switch unless it's more convenient.
Click to expand...
Agreed - depends on your situation! For me, the Auto/Off is not convenient, but for some its easier than the disconnect. All depends on how you have your compressor/shop configured!
 
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