How many amps do two parallel 4/0 copper conductors carry?

460 amperes at 75 degrees C in separate raceways. In one raceway with four current carrying conductors, the 80 percent adjustment drops that to 368 amperes.

Does the neutral count as a current carrying conductor?

Not on a 3 wire single phase circuit where it carries only unbalanced current, per NEC 310.15(E)(1).

What is the 83 percent service rule?

NEC 310.12(A) allows one family dwelling service conductors to be rated at not less than 83 percent of the service rating on 120/240 volt single phase installations.

Are two parallel runs of 4/0 copper in the same conduit enough to feed a 400 amp meter base?

Two parallel sets of 4/0 copper look like an easy yes at first glance. Table 310.16 gives 4/0 copper 230 amperes in the 75 degree C column, and two in parallel is 460 amperes, comfortably above 400. The problem is the words in the same conduit. Once you put both sets in one raceway you have six current carrying conductors on a single phase 120/240 volt service, or more if the neutral counts, and Table 310.15(C)(1) applies.

Running the derate

Table 310.15(C)(1) requires an 80 percent adjustment factor for 4 through 6 current carrying conductors and 70 percent for 7 through 9. On a single phase 120/240 volt service with two parallel sets, you have four ungrounded conductors plus two neutrals. Section 310.15(E)(1) says the neutral of a 3 wire single phase circuit is not counted as a current carrying conductor because it carries only the unbalanced current. That leaves four current carrying conductors and an 80 percent factor.

230 amperes times 0.80 is 184 amperes per conductor, and two in parallel gives 368 amperes. That is short of 400. Two parallel 4/0 copper sets in one conduit do not make a 400 ampere service.

Run the two sets in two separate conduits and the derate disappears. Each raceway carries two hots and a neutral, which is three conductors, two of them current carrying, and no adjustment applies. 230 plus 230 gives 460 amperes, and the service is good. Section 310.10(G) permits paralleling in 1/0 and larger, and requires the parallel conductors to match in length, material, size, insulation, and termination method.

The 83 percent service rule

Section 310.12(A) allows service conductors for a one family dwelling and the feeder conductors to the main disconnect to have an ampacity not less than 83 percent of the service rating for 120/240 volt single phase installations. 83 percent of 400 amperes is 332 amperes. Two parallel 4/0 copper in a single conduit at 368 amperes clears 332, so the single conduit build can be legal on a one family dwelling under 310.12. It is not legal for a commercial 400 ampere service, and it is not legal if the load calculation exceeds 332 amperes.

Neutral, ground, and terminations

Section 220.61 sizes the service neutral for the maximum unbalanced load. Table 250.66 sizes the grounding electrode conductor from the largest ungrounded service conductor, and for parallel conductors you use the equivalent area. Two 4/0 copper conductors equal 2 times 211,600, or 423,200 circular mils, which lands in the over 350 kcmil through 600 kcmil row and calls for 1/0 copper.

Check the meter base and disconnect lugs. Many 400 ampere meter mains accept only one conductor per phase unless a listed dual lug kit is installed, and 110.14(A) prohibits connecting more than one conductor to a terminal unless the terminal is identified for that purpose.

Field takeaway

Two 4/0 copper sets in two conduits gives a clean 460 amperes for a 400 ampere service. Both sets in one conduit derates to 368 amperes, which only works on a one family dwelling under the 83 percent rule in 310.12 and only if the calculated load fits. Pull them in separate raceways and skip the argument with the inspector.