Wiring size for compressor

[Jwbfx4]

I’ve searched and seen a lot on here about this. Some threads I read say don’t go by the hp rating but the information that’s on the tag, then I’ve seen some that says this up rating= this size. Figured I would come here and get the answer for myself.

Buddy just won a 7.5hp compressor from his work that we are putting in his shop. He originally had a 3hp compressor in there wired up with #10 thhn. This compressor tag reads. 208-230v, 7.5hp, 1 phase, 60hz. Running amps 32-30 and recommended breaker 35amp.

What size wire and breaker would be needed for this setup. Thanks for the help.

 

[pattenp]

By NEC 7.5 HP is 40A, so 40A X 1.25 = 50A, so #6 NM or #8 THHN in pipe. This is if the data plate on the motor states 7.5HP, not some label on the tank.

 

[wyliesdiesels]

The correct way to size motor circuits is by the HP rating on the motor nameplate.

Labels on the tanks and other places are often misleading marketing ploys.

If the motor nameplate HP rating is unlisted or says spl, then use the listed FLA and the FLC chart in the NEC to determine the approx HP rating.

 

[TRWham]

The tables in NFPA 70 (aka the NEC) Article 430 tell you what has been correctly posted above. If the conduit is 1/2" and carrying no other circuits it should be big enough for 8 AWG THHN wire, but NOT for XHHW if he were so inclined (thicker insulation on the XHHW). 1/2 EMT trade size is over 0.700" outside diameter and 3/4 is nearly 1". Rigid conduit (RMC or IMC) is even larger OD, but it's probably EMT.

I would not bother with the 35 amp breaker, but run a 40 or 50 and size the overloads for the 35 amps. My 7.5 HP compressor motor is happy on a 40A breaker, but only pulls 28A when fully loaded because the compressor is really a big 5 horsepower (just over 21 CFM at a little over 600 rpm) and I have almost 250V service.

If the unit comes with a starter, double check what overloads are in it. The motor might be internally protected, but probably not. It will say so on the nameplate if it is. If so, overloads are redundant but harmless so long as not undersized for the load. Properly sized overloads can save a motor that is overloaded by some mechanical fault in the compressor or drive, so avoid oversizing them.

 

[Jwbfx4]

Thanks for the responses. Not sure what the size conduit he has but we will dig into today and hopefully get it finished up.

 

[Chuckster in NJ]

746 watts per horse power...... Then divide the watts by your volts to get the amperage.
I suggest running the next size up wire on all motor loads because of starting amps.

 

[alfredeneuman]

746 watts per horse power...... Then divide the watts by your volts to get the amperage.
I suggest running the next size up wire on all motor loads because of starting amps.

That assumes the motor is 100% efficient, which none of them are.

 

[TRWham]

746 watts per horse power...... Then divide the watts by your volts to get the amperage.
I suggest running the next size up wire on all motor loads because of starting amps.

The nameplate shows the amperage as 32A, so it is not necessary to estimate it. The previously suggested wire size that satisfies NEC is already 50A for that 32A load. Do you think it should be even larger?

 

[Aceman]

746 watts per horse power...... Then divide the watts by your volts to get the amperage.
I suggest running the next size up wire on all motor loads because of starting amps.

You size the wire to the NEC tables for the given HP, not the motor nameplate amps or some calculation.

You size the overloads to the nameplate amps.