RES Technical Corner by Brett Eliasz, PE

Technical Corner

For the article this month we will look at some discrepancies that reside within the NEC illustrated handbook and the regular black and white text which resides in section 250.122(B).

It seems as if there are only 3 reasons to increase an Equipment Grounding Conductor (EGC) when oversizing the phase conductors in a circuit:

1. If you increase the size of the current-carrying wires because you are planning surplus ampacity for future use, then you would also need to increase the size of the EGC per 250.122(B).

2. If you increase the size of the current-carrying wires because you have extra wire of a larger size than necessary, left over from a previous job, then you would also need to increase the size of the EGC per 250.122(B).

3. If you increase the size of the current-carrying wires to limit voltage drop you will need to increase the size of the EGC per 250.122(B).

The following 2 reasons do not require an increase in size for the EGC:

1. If you increase the size of the current-carrying wires due to high ambient temperature, then you DO NOT need to increase the size of the EGC per 250.122(B).

2. If you increase the size of the current-carrying wires due to the amount of current carrying conductors, then you DO NOT need to increase the size of the EGC per 250.122(B).

Let’s say you don’t have the illustrated code book (the one with blue text) and you read section 250.122(B). You will notice it certainly does not differentiate the different scenarios listed above so one might derive that this applies to ANY scenario listed above. When you read the blue text it specifically uses the voltage drop scenario as a reason to increase the EGC proportionately to the increase in phase conductors.

The rationale is that increasing an EGC for voltage drop will lower the overall impedance of the ground fault current path to ensure operation of the overcurrent protection device. What I don’t understand is why wouldn’t the impedance be lowered with a larger EGC when accounting for ambient temp or current carrying conductors…?

As I understand it, the NEC 2020 is adding the specific verbiage that alludes to the 2 scenarios listed above that do not require an increased EGC size.

I tend to lean towards increasing the EGC to match the increased phase conductors just for installation sake, but it’s your call.

Code references courtesy of NEC 2017 and 2020 Code version.

Hopefully this article finds you well and can be used as a reference for your project needs. If anyone would like to contribute to the RES magazine and add an article or would like to request information on a specific topic (not limited to Electrical) just email me at beliasz@bergmannpc.com. As always, any comments are appreciated…! Thank you for reading.

Brett Eliasz, P.E., LEED AP BD+C , RES Director