Art. 250.122(D)

[KP2]

When sizing an EGC I use T250.122 and the OCPD for the branch circuit.

Option 1 : So a 80 amp OCPD needs a #8 EGC.

I was recently pointed to art. 25.122(D) in the last CEU class I instructed and was conviced that day that I was to use the overload sizing to calculate my EGC.

Option 2 : The overload neened is 51.5 amps and a #10 EGC.

After looking back today I read the part in the article; "but shall not be less than the size shown in table 250.122".

Option 3 : Now do I take the 51.5 amps and use T310.16 to determin a # 6 EGC and compare that to T250.122.



I like option 1.

 

[charlie b]

The overloads are not relevant to the decision. The starting point is the overcurrent (not "overload") protective device. If that is the commonly used inverse time circuit breaker, then you use table 250.122 based on the rating of that breaker. I think that is what you have. So my answer is option 1.

If, however, you are using an instantaneous trip breaker and motor overload devices, then the EGC should be sized on the basis of 175% of the motor's full load current. With that type of short circuit device, we are told to apply the max rating that would have been based on the dual element fuse. You need to go back and forth between 250.122(D) and 430.52(C)(1) exception 1.