EGC for motor loads.

[Dsg319]

Have been pondering on this for a minute.

In the event of sizing conductors for motors, I know how, but have never had to personally size conductors for a larger moter and was curious as to how to size the EGC.

I know conductors are sized 125% of rated FLC. And EGC is based off of OCPD, but if you have a OCPD sized 250% larger than conductors, obviously your EGC need not be larger than phase conductors.

Example say I have a 75hp, 3phase 480v motor.
FLC-96amperes x 125%= 120amp conductors needed. Okay so I get some #1AWG copper rated at 130amperes.

OCPD- 96 x 250%=240amp.
Have to choose a stand size so 225amp OCPD.

Obviously I do not need an EGC that large. So how do I properly size it?

Idea#1— Do I size it to original size of FLC x125%? So in this case it would be 125ampere OCPD I would size the EGC to without the addintional 250%?

 

[Russs57]

Motors are a little different. I think most would be well served to consider the overcurrent and the short-circuit protection as separate things. The overload relay provides overcurrent protection at no more than 115% to 125% of nameplate amps. The short-circuit device can be as high as 300% of NEC book value but it could also be as low as 150%.

The short circuit should be a short duration even and the wire sized for 125% of book value motor amps shouldn't have a problem with a fuse/breaker in the 150-300% range.

 

[Dsg319]

Motors are a little different. I think most would be well served to consider the overcurrent and the short-circuit protection as separate things. The overload relay provides overcurrent protection at no more than 115% to 125% of nameplate amps. The short-circuit device can be as high as 300% of NEC book value but it could also be as low as 150%.

The short circuit should be a short duration even and the wire sized for 125% of book value motor amps shouldn't have a problem with a fuse/breaker in the 150-300% range.

So what do u think would be a considerable way to size the EGC Other than having it the same size of phase conductors on larger motors. Sizing it off of overload protection?

 

[david luchini]

So what do u think would be a considerable way to size the EGC Other than having it the same size of phase conductors on larger motors. Sizing it off of overload protection?

Size the EGC per the OCPD. So #4 for your 225A c/b.

 

[Dsg319]

Size the EGC per the OCPD. So #4 for your 225A c/b.

Thank you! I wasn’t for sure if it changed since motor OCPD can be quite large in proportion to the phase conductors. But after you pointing it out it still makes sense no matter what, the EGC is to be sized off of the OCPD size to be proportional and matching it. Being it’s main function is to have a low enough low-impedance path to operate the SC/GF device and clear the fault.

Thanks again.

 

[Dsg319]

Plot twist— given the same circumstances and facts as stated above,assume the following event happened.

Say I sized my OCPD SC/GF at only 125% of 75hp FLC.
FLC-96amperes x 125%= 120amps.
Not a standard size so I get a 125amp OCPD.
Size my EGC to it and come out with a #6awg.

All fine and dandy until the motor starts and trips OCPD due to inrush current, I than take advantage of 250% increase in size of my OCPD. And replace with a 225ampere OCPD. So that would than mean my EGC needs to be a size #4awg.

So than to be compliant I would have to replace the already pulled number #6awg sized to the first and smaller (125% of FLC)
And repull a #4awg? Correct?

 

[infinity]

Yes, use the size of the OCPD to size the EGC for a motor circuit as in (D)(1).

250.122(D) Motor Circuits.
Equipment grounding conductors for motor circuits shall be sized in accordance with (D)(1) or (D)(2).
(1) General. The equipment grounding conductor size shall not be smaller than determined by 250.122(A) based on the rating of the branch-circuit short-circuit and ground-fault protective device.
(2) Instantaneous-Trip Circuit Breaker and Motor Short-Circuit Protector. Where the overcurrent device is an instantaneous-trip circuit breaker or a motor short-circuit protector, the equipment grounding conductor shall be sized not smaller than that given by 250.122(A) using the maximum permitted rating of a dual element time-delay fuse selected for branch-circuit short-circuit and ground-fault protection in accordance with 430.52(C)(1), Exception No. 1.

 

[Dsg319]

Yes, use the size of the OCPD to size the EGC for a motor circuit as in (D)(1).

Thanks