Wire size

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sparky

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Recently installed a 3/0 208 volt line to serve a compressor that i thought would be 15hp. Customer installed a 20 hp air compressor after job was complete. I get a call that his motor has burned out because my wire was to small. Upon investigating i find the 20 hp motor. But I have #6 copper feeding this.According to my calculations 20 hp motor is rated at 59 amps . 59 amps x125%= 74.5 amps #6 wire is rated at 75 amps this wire still should have been large enough. air compressor is in a unheated shed where temp are well below freezing i think the cold made the motor start hard any thoughts?
 
Re: Wire size

If he made the connection to compresser then blame falls all on him.It might have ran into a problem restarting if it had some pressure.Had you installed everything then you would have caught this.
 
Re: Wire size

I don't know what effect the wire had or why the motor starter overload protection did not protect the motor.
The wire size was too small.
110.14(C) More than likely the number 6 was good for 55 or maybe 65 amps. The breaker/fused disconnect and motor starter terminals would determine that.
 
Re: Wire size

Sparky, I wouldn't think that the #6 was the problem.

Why didn't the OL's (if properly sized) open the starter? In my experience compressors don't reach a FULL LOAD untill towards the end of the cycle which is usually a short (relative to the run time) duration.

Roger
 
Re: Wire size

Had a problem a year ago Installed a 208/120v service customer installed a 240/480 volt compressor. burned up the moter in a week. Look at the motor voltage rateing some will run on 208 but at a reduced HP this changes the pully sizes and even the ability to reach full pressure.
 
Re: Wire size

Here?s another possibility: A poorly made connection (i.e., the connection to the unit, as made by the owner) could cause a voltage drop, which would then raise the current going through the motor. An inspection of the terminations should confirm or refute this possibility.

But my money would be on Wayne?s comment: Check to see if the motor is rated for 240V, and not for 208V.
 
Re: Wire size

Sparky, the first question should be just what did the customer order? He surly didn't just say run me circuit for a compressor without saying how many amps or volts.
 
Re: Wire size

Like Wayne said, I would bet that it is a 240V motor. A few years ago I had to use a buck-boost transformer to run a 240V air compressor motor on a 208V circuit. The motor would not start on 208.
 
Re: Wire size

My square D slide calculator lists #4 as minimum wire size for 208 or 240 Volt 3 phase 20 HP motors. What is the length of the run (could voltage drop be a factor?).

I agree, he installed the compressor, you thought it would be 15 HP, therefore HIS fault.

I would ask for cash in advance for changing the #6 to #4.

Steve
 
Re: Wire size

Line was installed for landlord of property to be for a 15 hp compresser tenant moved and installed his 20 hp not my problem really, but just trying to figure out if the # 6 was the problem or what? Motor is dual voltage motor. Code does rate #6 for 75 amps unless I am missing something. Code also lists a 20 hp motor at 59 amps. Distance is about 50'. Not sure why motor protection didint help I only installed to disconnect.
 
Re: Wire size

Sparky Even though the motor is dual rated the compressor is set up for it to run with a certain HP. if this motor is a 240v motor and is allowed to run at 208 the HP will be less. now to run the compressor to the pressure that was set for 20 HP the pulley on the motor will now have to be smaller to reach the same pressure. It will do it but at a reduced CFM. other wise the motor will go over the FLA before it reached the full pressure. I know that a 10hp 240v motor will be 7.5 hp at 208. So a 20hp motor will have 15hp output at 208. thats quite a loss. A 208 motor is rated 200 volts so there is a difference.
 
Re: Wire size

I should have asked if what you mean by dual voltage is 240/480 with an allowance for it to run at 208? If this is the case then the above will apply. and here's the kicker it will draw more amprage than the full 20 hp would @ 240 volts.
 
Re: Wire size

On the wire size you have to go read 110.14(C).
You can not use the 90 degree coulomb for your circuit ampacity.
 
Re: Wire size

NEMA rating on motors only allows for a 10% variation on voltage. In other words you can get full hp and torque without exceeding the temp rise of the motor as long as you stay within the 10% voltage decrease. a 240 V motor at 10% is 216 volts and will overheat and burn out quickly if run at full load on a 208 volt network. I owned a motor shop for a number of years and analyzed motor failure patterns for many different manufacturers for warranty purposes. This type of failure all too common and occured many times due to the customer(or the electrician) misapplying the motor. Kept us in the rewind business also. Joe
 
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