Wire size for motor?

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trojans4

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Semi -retired master electrician. Fully retired math and physics teacher.
Guys - I am a residential electrician. I have a friend that has bought an 80 gallon air compressor. 7.5 horsepower, 240 V motor, continous duty, fla is listed as 41-43 A. He wants me to run wire for a receptacle for the compressor to plug into instead of hard wiring it because he may be moving it around to a different building depending on usage. The receptacle will only be a few feet from the breaker box. I am not experienced in wiring motors of this size and would not take the job except I am trying to help a friend and will probably only charge him for materials. The info with the compressor does not indicate wire size. So, that is my question, what wire size and what size breaker for overcurrent protection? Checking NEC code book it looks like either #10 or # 8 copper and I am not sure what breaker size, 40A or 50A? Any advice?
 
1st step: Use Table 430.248 for your calculations: 7-1/2 hp = 40 amps
2nd step: 430.22 Branch Circuit: FLA x 1.25 40 x 1.25 = 50 amp (normally #8 thhn OR 6 NM)
3rd step: 430.52 BC-GF Protection FLA x 2.5 for IT Breaker 100 max
4th step: For cord & plug disconnect: 430.109 (F) Plug rated 7.5 HP This is probably going to be you stumbling point. A plug and receptacle rated for 7-1/2 HP may be pricey.
 
Last edited:
augie47 said:
1st step: Use Table 430.248 for your calculations: 7-1/2 hp = 40 amps
2nd step: 430.22 Branch Circuit: FLA x 1.25 40 x 1.25 = 50 amp (normally #8 thhn OR 6 NM)
3rd step: 430.52 BC-GF Protection FLA x 2.5 for IT Breaker 100 max
4th step: For cord & plug disconnect: 430.109 (F) Plug rated 7.5 HP This is probably going to be you stumbling point. A plug and receptacle rated for 7-1/2 HP may be pricey.
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Augie, I read this forum daily, and have to commend you on the method in which you answer questions so thoroughly. Leaving nothing out, including code references, takes time and is appreciated!! Thanks.
 
Augie - thanks. I was on the right track for the 40 x 1.25 = 50A but did not have the 40 x 2.5 for breaker size. As far as the plug and receptacle wouldn't a 40-50 amp dryer/range - plug/receptacle work?
 
jusme123 said:
Augie, I read this forum daily, and have to commend you on the method in which you answer questions so thoroughly. Leaving nothing out, including code references, takes time and is appreciated!! Thanks.
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Thanks for your comment...

The time and effrot of others has provided me with the limited knowledge I have. It's payback time in my life :D
 
trojans4 said:
Augie - thanks. I was on the right track for the 40 x 1.25 = 50A but did not have the 40 x 2.5 for breaker size. As far as the plug and receptacle wouldn't a 40-50 amp dryer/range - plug/receptacle work?
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work ? Most likely :D
Two inherent problems:
One, if the plug/receptacles aren't properly rated (HP) it is not Code compliant = liability + possible safety violation
Second: If you use a #8 wire, feed the 60 amp receptacle with a 100 amp breaker, what happens if someone decides to connect a 60 amp load ?

The backgound of most Forumites goes beyond "what will work" :D
 
Augie - when I asked will it work I meant code compliant. I only do residential work and motors are out of my comfort range. Can you explain why you can put a 100A breaker on #8 wire for a motor? #8 wire is not rated to cary that much current. If you ran #8 for an electric stove or any non-inductive type residential load you couldn't have OC protection of 100A. Also, would the capacity/construction of a 50A dryer/range plug be any different than one rated for 7.5 horsepower? I am not arguing just asking?
 
trojans4 said:
Augie - when I asked will it work I meant code compliant. I only do residential work and motors are out of my comfort range. Can you explain why you can put a 100A breaker on #8 wire for a motor? #8 wire is not rated to cary that much current. If you ran #8 for an electric stove or any non-inductive type residential load you couldn't have OC protection of 100A. Also, would the capacity/construction of a 50A dryer/range plug be any different than one rated for 7.5 horsepower? I am not arguing just asking?
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The breaker is for short circuit and ground protection of the conductor only, overload protection for the motor should be built into the motor already, or needs to be added in a controller.

As far as the cord goes, it is either rated and listed or not. Most range cords are not listed for HP loads because ranges are mostly resistive and have no real inrush current.
 
I think there is a problem putting a 60A or larger breaker on a 50A receptacle. If you hard wire a motor, no problem putting a larger breaker than the wire size would indicate. Usually, you can't do that for receptacles except for the Welder receptacle exception. He could hope that a 50A breaker allows the compressor to start up after repeated hard use, but a 60A would be better. Not sure of the HP rating of a 60A plug/receptacle.

A 7HP 80 gallon compressor doesn't sound very portable to me. Is it on wheels?
 
Thanks Jumper. I understand the HP rating for a motor being different than amps for a range. Although, my electrical supply store tells me they sell no plug/receptacle with a HP rating and couldn't find any in their catalogues. They are all rated by amps. I get the breaker for the motor circuit is for short circuit/ground fault only and yes there is OL protection on the motor controls for this motor. So with a 100A breaker and #8 wire there would really be no OC protection for the conductors that go from the breaker box to the controls? I suppose there wouldn't have to be any since about the only way you would draw more than 50A current would be a ground fault and the GFCI should take care of that or an overload on the motor and the OL protection on the motor would take care of that?
 
trojans4 said:
Thanks Jumper. I understand the HP rating for a motor being different than amps for a range. Although, my electrical supply store tells me they sell no plug/receptacle with a HP rating and couldn't find any in their catalogues. They are all rated by amps. I get the breaker for the motor circuit is for short circuit/ground fault only and yes there is OL protection on the motor controls for this motor. So with a 100A breaker and #8 wire there would really be no OC protection for the conductors that go from the breaker box to the controls? I suppose there wouldn't have to be any since about the only way you would draw more than 50A current would be a ground fault and the GFCI should take care of that or an overload on the motor and the OL protection on the motor would take care of that?
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(minor note: may be a "typo" but you stated "GFCI".. there is no need for a GFCI. You may have referenced my SC-GF" for short-circuit/ground-fault (as in fault to ground) protection which a standard breaker will provide. You are correct in that the motor OL will prevent overloading.
That's why a receptacle rated greater than the breaker presents a problem. Other loads than the motor may not have the inherent OL protection)
 
The compressor is not very mobile. He does have it on a sturdy pallet and intends to move it with a fork lift. He owns a trucking business and has employees working on trucks in two different buildings that are close together. Problem is solved, he has decided to let me hard wire it. One of his employees told him it wasn't going to work to move it, too big of a risk for damage. Thanks for the info guys. I've never paid much attention to the requirements for wiring motors (at least this size of motor) since it doesn't come up much (if at all) in residential work. It helped a lot when you explained the "why" for the requirements.
 
trojans4 said:
The compressor is not very mobile. He does have it on a sturdy pallet and intends to move it with a fork lift. He owns a trucking business and has employees working on trucks in two different buildings that are close together. Problem is solved, he has decided to let me hard wire it. One of his employees told him it wasn't going to work to move it, too big of a risk for damage. Thanks for the info guys. I've never paid much attention to the requirements for wiring motors (at least this size of motor) since it doesn't come up much (if at all) in residential work. It helped a lot when you explained the "why" for the requirements.
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If the buildings are close together why doesn't he just run an air line between the buildings? Then both will have air at the same time. This seems like a big inconvenience to move, and when you do you don't have any air at the building it was removed from. Air is usually pretty heavily depended on service in this type of place.
 
trojans4 said:
Guys - I am a residential electrician. I have a friend that has bought an 80 gallon air compressor. 7.5 horsepower, 240 V motor, continous duty, fla is listed as 41-43 A. He wants me to run wire for a receptacle for the compressor to plug into instead of hard wiring it because he may be moving it around to a different building depending on usage. The receptacle will only be a few feet from the breaker box. I am not experienced in wiring motors of this size and would not take the job except I am trying to help a friend and will probably only charge him for materials. The info with the compressor does not indicate wire size. So, that is my question, what wire size and what size breaker for overcurrent protection? Checking NEC code book it looks like either #10 or # 8 copper and I am not sure what breaker size, 40A or 50A? Any advice?
Click to expand...
 
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