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Old 07-30-2012, 12:54 AM   #16
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Remote switch for air compressor


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Originally Posted by electures View Post
Based on the information provided the branch circuit conductors need to be sized for 50A and the branch circuit overcurrent device (breaker?) should be sized at 100A. This will allow the motor to start and run. #8 romex is rated for 40A. However, #8 THWN in conduit is rated for 50A as long as all terminations are 75 degree listed.

The feeder to the subpanel has to be sized for the largest motor FLA x 125% plus all other motors @ 100% plus 125% of all continous loads plus 100% of any noncontinous loads. So just for the compressor and dust collector = (40A x 125%) + (28A x 100%) = 78A. The feeder conductors need to be sized for 78A + all other loads as mentioned earlier. I'm guessing at a 100A to 125A feeder.

The feeder breaker has to be sized for the largest motor OCPD (compressor) + the FLA's of all other motors + other loads as mentioned earlier. 100 + 28 + all other motors + other loads. Again I'm guessing at 150A to 175A feeder breaker.

As for the control circuit conductors, they are permitted to be protected by the motor branch circuit overcurrent device in accordance with 430.72(B). #14 control conductors are rated for 45A. #12 control conductors are rated for 60A. Since the compressor breaker can be as large as 100A you can install an in line fuse or the mini breaker you mentioned earlier. If you reduce the motor compressor breaker to 45A or 60A then the control conductors mentioned earlier respectfully can be used, but this could lead to nuicance tripping.

You did not mention whether or not this is a home shop or a business. Some posters will argue that if this is a home shop that these calculations are obsurd, but the NEC does not differentiate between home shops and commercial shops. Motors are motors - code is code.

Smaller breakers can be used on the motors, but can lead to nuisance tripping. Without knowing all the motor loads and other loads, these calculations are to be considered estimates. You may want to consult a professional electrical contractor or engineer.

Good luck!!
ArTrvlr.,

Just note that I put a underline where electures posted on his quote and that will hit the spot and with compressour breaker if you are under 60 amp you can use #12 THHN as control circuit but once you use larger than 60 amp breaker for compressour circuit then you will have to add secondary fuseing or breaker to protect the conductors as Electrures mention.

I did dig up the NEC code book to make sure if there were any other items mention related and Electrures did cover it clear due mine codes ( Nomes de France ) is more stricter.

As far for the subpanel sizing it will be wise to keep it large as possible so look at the main panel to see what the main breaker rating is.

Merci,
Marc
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Old 07-30-2012, 01:10 PM   #17
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Remote switch for air compressor


Quote:
Originally Posted by electures View Post
As for the control circuit conductors, they are permitted to be protected by the motor branch circuit overcurrent device in accordance with 430.72(B). #14 control conductors are rated for 45A. #12 control conductors are rated for 60A. Since the compressor breaker can be as large as 100A you can install an in line fuse or the mini breaker you mentioned earlier. If you reduce the motor compressor breaker to 45A or 60A then the control conductors mentioned earlier respectfully can be used, but this could lead to nuicance tripping.
Good luck!!
OK, this is the answer I was looking for. The compressor runs just fine on a 40A breaker (has never tripped in 6 years), so I can use 14GA for my switch loop and they will be protected by the 40A breaker. If I understand you correctly, if I want to add the inline mini breaker for extra protection, that would not be a problem.
Thanks for all your time and help.
jp
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Old 07-30-2012, 01:38 PM   #18
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Remote switch for air compressor


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Originally Posted by ArTrvlr

OK, this is the answer I was looking for. The compressor runs just fine on a 40A breaker (has never tripped in 6 years), so I can use 14GA for my switch loop and they will be protected by the 40A breaker. If I understand you correctly, if I want to add the inline mini breaker for extra protection, that would not be a problem.
Thanks for all your time and help.
jp
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