Compressor Motor Circuit Conductor Sizing - Electrical - DIY Chatroom Home Improvement Forum

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10-30-2015, 04:57 PM   #1
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## compressor motor circuit conductor sizing

Just bought a new 2 stage 5HP 60 gallon compressor. The instructions say to wire it to a 25 amp circuit with a 40 amp breaker. I've been looking at sec. 430 to try and break this down. The motor plate lists 22 amps as the full load amps (FLA). Am I correct in that the chart in sec. 430 lists motors as full load current (FLC) and that I am to use the larger of the two values? Sec. 430 list 5 HP single phase motors @ 28 amps. I am confused about fusing and if I am to do a calculation for short circuit/ground fault and overcurrent protection as a separate calculation? The way I am reading it I am to size the wire for 28 amps (#10) and the overcurrent protection @ 125% of the motor current which =35 amps (40 amp breaker)?

10-30-2015, 07:02 PM   #2
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Code overrules instructions, so you'd need wire sized at 125% of 28 which would be 35 amps. If it is wired using conduit and THHN, then 240.4(D) allows #10s. If NM cable (Romex) is used, then it'll need to be #8. If flexible cord is used, it needs to be #8.

There's no minimum breaker size (other than what the instructions say), and the maximum is 250% of 28 which would be 70 amps. A 40 amp breaker will most likely be ok.

 11-12-2015, 08:27 PM #3 Member   Join Date: Sep 2008 Location: Massachusetts Posts: 305 Rewards Points: 320 I ran #8 NM with a 40 amp two pole breaker to a 60 amp non fused disconnect. Is it acceptable to use SO or SOJ type cord to go from the compressor to the disconnect?

 11-12-2015, 11:32 PM #4 Idiot Emeritus   Join Date: Mar 2008 Location: Fernley, Nevada (near Reno) Posts: 1,849 Rewards Points: 1,492 400.7(A)(7) permits cord to be used to prevent the transmission or noise or vibration. Cord would be ok as would flex or sealtite.

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