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How do you "Proportionately Increase" an equipment grounding conductor per NEC® 250.122(B)?

+1 vote
The National Electrical Code® (NEC®) 250.122(B) requires that "Where ungrounded conductors are
increased in size from the minimum size that has sufficient ampacity for the intended installation, wire-type equipment grounding conductors, where installed, shall be increased in size proportionately according to the circular mil area of the ungrounded conductors."
The NEC® Handbook section 250.122(B) has an example of how a "Proportionately Increased" equipment grounding conductor size calculation is accomplished. This example has been in the NEC® Handbook since the '96 cycle, when the sizing equipment grounding conductors section was located in 250.95, before it got moved to 250.122 in the '99 cycle.
asked Mar 31, 2015 by David A Engelhart
edited Mar 31, 2015 by David A Engelhart
1.  By example:
     If minimum ungrounded wire from #1 to 3/0 cu, the minimum ground wire is
     #6   grd   cu = 167800 cmil.
     If ungrounded wire is increased to 500kcmil:  500,000cmil/167800cmil=2.97
        Then:  26240 cmil x 2.97 = 77932.8 cmil.
Therefore, the next standard size ground wire is 83690 cmil or #1 awg.
Follow this example and you will pass your test.
I am not following your example; you increase from #1 to 3/0 to 500; and it is NOT following the example I presented below that follows the example in the NEC® handbook. Please review the handbook example.

1 Answer

+1 vote
Did you know that if you increase a 20 amp circuit from #12 to 1/0, for what ever reason, that the proportionately increased equipment grounding conductor size works out to 1/0?

1/0=105,600 circular mills. #12=6,530 circular mills. (105,600/6,500)6,500=105600.

50 amp #8 circuit to #1. #8=16,510. #1=83,690. #10=10,380. (83,690/16,510)10,380=52,617. #3=52,620.

It works out 1 to 1 up for #14 - #10 normally sized ungrounded circuit conductors, then calculations are necessary to be conducted, accordingly.

Note: The NEC® does not require, but recommends voltage drop considerations. The Energy Conservation Codes require Voltage Drop considerations, logically so.
answered Mar 31, 2015 by David A Engelhart

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