EGC for motor feeders

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mshields

mshields

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Boston, MA
I believe that the EGC for a motor feeder needs be sized based on the upstream OCPD and in accordance with Table 250.122. This means for example if you had a motor with FLA of say 50A, the wire size would have to be 125% of that (let's say based on 70Amps) and the OCPD could be as large as 125A. The EGC would need to be #6 to mach the OCPD. Is that correct or am I oversizing the EGC?

Thanks,

Mike
 
You are mostly correct.
The EGC is based on the OCPD with some variations when using Instantaneous Trip devices.
(that is based on '08.... In case you are lookng at an existing install, I may be wrong, but I think in some earlier Codes the OL device rating might have been permissible)
 
I would bet that this is overlooked alot. Think of a motor where the conductors are rated 60 amps and the OCPD is 100 amp. Unless metal conduit is used the egc would have to be based on 100 amps, not 60.
 
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The Code reference for all this is 250.122.
I might add there is a provision that the EGC need not be larger than the circuit conductors so in some cases with motors you might get by smaller than normally required by the OCP device.
 
mshields said:
I believe that the EGC for a motor feeder needs be sized based on the upstream OCPD and in accordance with Table 250.122. This means for example if you had a motor with FLA of say 50A, the wire size would have to be 125% of that (let's say based on 70Amps) and the OCPD could be as large as 125A. The EGC would need to be #6 to mach the OCPD. Is that correct or am I oversizing the EGC?

Thanks,

Mike
Click to expand...

I don't see there is any argument that the size of the EGC is always based on the size of the BC OCPD. OTOH, a lot of times it is a moot point as the conduit is used as the EGC on motor feeders.
 
augie47 said:
The Code reference for all this is 250.122.
I might add there is a provision that the EGC need not be larger than the circuit conductors so in some cases with motors you might get by smaller than normally required by the OCP device.
Click to expand...

Yep, I've had that happen a time or two...
 
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