Im moving my service entrance from my house to a shop 75' away (in Tucson AZ, with TEP). The new service will be 400A (well 320/400) with the shop eating 200A and a backfeed to the house underground to its original panel (but moving the feed from overhead wire to underground). This panel will have the ground bus and neutrals separated with 4 wires feeding the unit: 350 KCMIL AL for the HOTs, a 4/0 AL for the Neutral (using the 70% rule) and a #6 cu ground wire. The wires are suitable for direct burial, but i'm encasing them in conduit anyways and burying at 36" depth.

So the question is: the 6AWG Cu looks tiny compared to the massive current carrying conductors. Did I interpret the NEC code incorrectly?

Here is the Result: main Box with 4 wires for each feeder 350 350 4/0 and #6 enter image description here

  • For the new 400A main panel, I am using #2 Cu to tie into several Grounding Rods. based on the size of the 400A feeds. (which are 600KCMIL x3 AL) – mark f Sep 17 '20 at 23:26
  • I'm not sure why you need 250 kcmil. Sine the now-backfeed line to the house serves an entire domicile, I would expect 310.15(B)(7) to kick in. Now on that line from service to house, is there a ground wire? – Harper - Reinstate Monica Sep 18 '20 at 0:33
  • The new feed will contain a ground in addition to the three power conductors. The lugs arw only 75C lugs. Ambient can top 100f here, so I vacated the feed based on those factors... what I didnt do was run the normal residential. 83 multiplier because I thought feeder had to be rated to 125% of peak load? – mark f Sep 18 '20 at 2:09

No, you're interpreting the Code correctly

Table 250.122 in the NEC is what governs equipment grounding conductors, and for a feeder with a 200A maximum breaker ahead of it, it requires a 6AWG copper wire, which is what you are installing. This is because the grounding conductor only has to carry current long enough for the breaker to trip; in normal operation, there should be 0 current on it, as opposed to the hots and neutral, which have to sing for their supper, 24/7/365.

Note that this is a different table from the table that's used to size grounding electrode conductors such as those going off to your ground rods. Your 2AWG Cu GEC is a trifle undersized for 600 kcmil Al service conductors according to Table 250.66, which'd require 1/0AWG Cu for that application; however, since you're dealing only with ground rods and not a water pipe or miscellaneous electrode, NEC 250.66(A) kicks in and says that a 6AWG conductor is all that's needed. (A concrete-encased electrode would fall under 250.66(B) which only requires a 4AWG conductor, and ground rings require no more than a 2AWG as NEC 250.66(C) requires the GEC to be no larger than the ring conductor, which is required to be 2AWG minimum by 250.52(A)(4).)

  • Thanks, As to the Ground rods I've got two each with 2AWG if that gets to the intent of 1/0 for grounding electrodes... That #6 just looks out of place in the conduit with the giant Al wires. Im glad I interrupted it correctly. – mark f Sep 17 '20 at 23:42
  • @markf -- well, the Code would have let you run 6AWG to those rods, so the 2AWG is fine :) – ThreePhaseEel Sep 17 '20 at 23:45

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