RES Technical Corner

For this issue, I’d like to introduce and thank Tami Coghlan for contributing to the Technical Corner!

With this article we will review how to use NEC Chapter 9, Table 8 to resize an equipment grounding conductor after increasing the phase conductors to accommodate for voltage drop.

Consider the following example: You are sizing conductors for a copper feeder circuit to a panel, which requires 100 Amps, 3 phase, 208V. You originally size the conductors at (3) #1 AWG, (1) #1 AWG neutral with a minimum equipment ground size of #8 AWG. These are sized prior to applying any voltage drop calculations.

The feeder length is roughly 400 feet. As a result, voltage drop calculations are performed. It is found that the phase and neutral conductors will need to be increased to accommodate the length of the feeder, due to voltage drop requirements.

Taking into consideration voltage drop requirements for feeders, you increase the (3) #1 AWG and (1) #1 AWG neutral conductors to (3) #1/0 AWG and (1) #1/0 AWG for each ungrounded phase conductor and neutral conductor.

How do you size the Equipment Ground following phase and neutral conductor increases for voltage drop?

First, find the total circular mil value in NEC Table 8, Chapter 9 for your original conductor size:

#1 AWG = 83690 circ mils

Next, find the total circular mil value of your new (adjusted) conductor, using the same method:

#1/0 AWG = 105600 circ mils

Divide the adjusted size by the originally required size to determine the proportionate value of circular mil area adjustment. The result will then be your multiplier you use to size the equipment ground.

105600 ÷ 83690 = 1.26

The minimum size equipment grounding conductor for a 100A feeder is #8 AWG, according to NEC Table 250.122. The circular mil value of a #8 AWG conductor is 16510 as provided in NEC Table 8, Chapter 9.

Multiply the value 16510 circular mils by 1.26 (the multiplier you found in the previous step), to come up with 20802 circular mils.

With this value, once again use NEC Table 8, Chapter 9 and round up to the next higher value in the table. This will tell you the minimum ground size.

In this example, the next higher value rounded up from 20802 circ mils is 26240 circ mils, which results in a #6 AWG adjusted equipment ground size.

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 Rochester Engineer 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 , Tami Coghlan RES Director

res - technical corner AUGUST 2020 The ROCHESTER ENGINEER | 5