I'm working on a job where literally nothing is grounded and the wire length is around 2.5miles. I'm trying to calculate what size ground I would need to install to meet the NEC requirements in reference to voltage drop. Breaker size is 100A, actual load of about 15A and fed by 2 3/0cu conductors @ 480v. Also, in the same conduit is another 2 conductor 1/0cu circuit, 480v. I have been searching the code book and this forum and haven't been able to come out with an answer I feel totally comfortable with. I know table 250.122 tells me that the 100A circuit should be protected by a minimum wire size of #8awg. I also know that an increase in the ungrounded conductor size requires that the EGC also be increased in size "proportionally according to the circular mill area of the ungrounded conductors" (250.122(B)). I'm confused as to how exactly I'm supposed to calculate that.. Do I need to start with the cmil of a #3awg (310.16 for 100A) and subtract it from the area of a 3/0 and then apply that percentage to the ground?? Seems like I'm missing something. Any help would be appreciated!
Voltage drop, sizing EGC based upon an existing derated for voltage drop installation
- Thread starter pippen429
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#3 = 52620cmil
#3/0 = 167800cmil
Grd for 100A is #8 at 16510cmil
167800/52620*16510 = 52659
That would push you to a #2 GRD by literal interpretation.
I disagree with the NEC proscribed method in that AWG gauge numbers are intended to represent a proportional increase or decrease in cross-sectional area. When "conventionalized" under the ANSI standard, cmil values were rounded to four significant figures, and making proportionate calculations with those rounded figures introduce errors in that may push you one or two sizes larger (in this case, one size).
If we go back to original intent of AWG size, all we would have to do is #8 - (#3 - #3/0*) = #3
* For the sake of "ought" sizes, use 0, -1, -2, and -3 as the AWG gauge number in lieu of 0, 00, 000, and 0000 (or 1/0, 2/0, 3/0, and 4/0)
luckylerado
Senior Member
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- Lake of the Ozarks
Assuming 75º wire. You would want to start with the cmil of a #1 because you would need to derate to a #1 phase conductor due to 4 CCC. That would bring your EGC down to a #4
Good point... :thumbsup:
I overlooked there being 4 conductors.
However, it could technically be derated to #2 which is 115A@75°C... so ×80% = 92A, which exceeds the 15A load and protected at 100A per 240.4(B).
petersonra
Senior Member
- Location
- Northern illinois
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- engineer
is it metal conduit? if so, no wire type egc is required at all.
luckylerado
Senior Member
- Location
- Lake of the Ozarks
Correct, but that is just wrong.
I have proposal in to permit determination by AWG size change, and also one to permit rounding to four significant figures.
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