ECN Forum Forums General Topics of the Trade General Discussion Area Conductors in Parallel

I have seen 800 amp services using two 500 kcmil conductors for each phase. I'm assuming that the ampacity is figured as 430 amps(THHN) X 2 = 860 amps, rather than as being equivalent to one 1000 kcmil conductor at 615 amps. Would three 250 kcmil parallel conductors be 290 amps X 3 = 870 amps?

Dave

The ampacity is figured using the 75 deg table (380A). Times two conductors is 760A. Then article 240.4 is applied allowing the next higher standard fuse size or 800A.

The 75 degree section is used because of the rating of the termination lugs. See 110.14(C).

Remember also to apply 310.15(B)(2)(a) and derate the conductors if you pull all of the paralled conductors in one conduit.

Earl

The 75C column must be used because UL does not list Overcurrent Protective Devices for use with conductors based on 90C ampacities.

One little detail to watch out for is to be sure that the noncontinuous load plus 125% of the continuous load does not exceed 760 amps for parallel 500kcmil. This is usually not the case since most of us don't cut these things real close to the bone & leave some room for future growth.

Tom