Wiring size for compressor 5hp motor.

[jimtoo]

Hi electrical know how folks.

My SIL is has purchased an air compressor and I need help on wiring size. It is 230V, 5 HP motor, SLA 21.5 SF 1.5 SFA 24.4, is on tag on motor. I have not seen it yet, but was wondering what size wire should be used to run about 65' from breaker box to compressor? What size breakers are needed?

He was told 10/3 wg Romex would be enough, but I was just wanting to check with some of the good guys we have here on the forum.

Thanks
Jim M

 

[pegmikef]

I was told to use and used 6 gauge for my boatlift (greater distance, but 65 feet is still a pretty good distance for line loss). One of the smart guys will calculate the required amperage from the horse power and come up with the breaker size.

 

[JohnDar]

First off, I am not an electrician. I had to get real familiar with wiring when I wired up my attached garage for my woodworking tools. The "professionals" wouldn't bother to give me an estimate (tried four of them), so I pulled a permit and did it myself. Passed the state electrical inspection, too.

From what I can find in my references, the #10 wire should be OK for a 65 foot run if the voltage is 230. If it were a 120V line, then the distance would be a problem. It looks like the SLA 21.5 is the service load amperage the motor will run at. The start up amperage will spike above that (the SFA 24.4?). If that is correct, a 30A breaker should suffice.

Burying the line may require it be in a conduit unless it's rated as "UF - Underground Feeder."

Best advice is to contact the local electrical inspector for guidance, if there is one in the community. Fees for a permit are nominal.

 

[jimtoo]

He is not going outside or bury the wiring. It will take about 65' from box to compressor location, up the wall, through the attic and down the wall to compressor. I just don't want to have a chance of wiring overheating.

Thanks
Jim M

 

[cookie]

A full load current for a 5hp single phase AC motor on 230v is 28 amps.
So 10ga. wire with a 30 amp breaker will be fine.
5hp....that must be a monster compressor. My 1hp running on 230 runs real efficiently.

Peace
Dave

 

[swburbguy]

AS a retired electrician a motor is supposed to be fused at 125 percent of amp rating so 28 amps x 1.25 =35 amps

 

[jimtoo]

OK,, Thanks guys. That is what I was looking for, free information from folks I trust. Not salesmen.

Jim M

 

[evolvingpowercat]

That's a BIG motor! I agree you must use wire size to carry at least 35 amps at the distance needed, to cover the 125 % engineering margin. You must use a third wire of the same bonded to the motor frame to protect against any faults from the motor windings to the motor case that is run back to the electrial service supply. A ground rod at the motor is not enough!

It may be most cost effective to use dual element time delay fuses and an appropriate enclosure to hold them.

I suggest you use a licensed electician or you make sure you do the research and know how to do this correctly if you intend to do yourself.

 

[porthole]

Grounding, bonding, earthing - know the difference?

Start with your local electrical inspector when you go for permit applications - of course you are getting permits, right?

Your code may not allow a ground rod at the equipment.

And once you figure out if 10 gauge is sufficient (probably not), stepping up a gauge is minimal if you do it now as opposed to later when you find the wires getting hot.

Someone mentioned the 125%. The other way to look at that is the circuit is de-rated for the fusing protection.

Several reference online charts quickly can point out that although 10AWG is rated for a max of 55 amps, it is only rated for 15amps power transmission.

Wire in the free air or in a conduit? Changes the ratings
THHN or other types? changes the rating.

1 phase - 3 phase? Just throwing that out there as most of the 5hp compressors around me are wired 3 phase.

According to this chart, it looks like you need 6 gauge. With appropriate circuit protection for the device.
http://www.powerstream.com/Wire_Size.htm

But- I'm no electrician and I didn't stay at any holiday Inn's for the past several years

(I did stay at that Best Western that was on 20/20 the other night though)

 

[NWILSON]

Just to throw another thought into the discussion.....most new compressors publish hp ratings like new lawnmowers. The hp on the label is PEAK and not continuous. In the case of lawnmowers if you were to run it at it's advertised hp the life expectancy is pretty short.
This may influence your choice of wire size!