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Conductors in Parallel #39739
06/28/04 09:31 PM
06/28/04 09:31 PM
D
Dave55  Offline OP
Member
Joined: May 2004
Posts: 697
Crystal Lake, Illinois, USA
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

Work Gear for Electricians and the Trades
Re: Conductors in Parallel #39740
06/28/04 09:53 PM
06/28/04 09:53 PM
N
Nick  Offline
Member
Joined: Aug 2001
Posts: 599
Riverside, CA
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.

Re: Conductors in Parallel #39741
06/29/04 08:33 AM
06/29/04 08:33 AM
E
earlydean  Offline
Member
Joined: Dec 2003
Posts: 751
Griswold, CT, USA
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


Earl
Re: Conductors in Parallel #39742
06/29/04 11:04 AM
06/29/04 11:04 AM
J
JBD  Offline
Member
Joined: Jul 2001
Posts: 599
WI, USA
The 75C column must be used because UL does not list Overcurrent Protective Devices for use with conductors based on 90C ampacities.

Re: Conductors in Parallel #39743
06/29/04 04:24 PM
06/29/04 04:24 PM
Tom  Offline
Member
Joined: Jan 2001
Posts: 1,044
Shinnston, WV USA
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


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