Question is for Canadian code but I'm posting here because I'm curious what the NEC says. This job is in Canada but it's for a bridge going from Canada to USA.

120/240 volt service wired using parrallel 0000 conductors all in one 3" conduit. (6 wires in one 3" conduit)

Do these wires have to be derated even though they are parrallel runs of the same circuit? What's the max amps that can be run on these?

I know of no reason why you wouldn't have to derate.
315(B)(2)(a) says



I get 416a per pair for THHN/THWN-2

260a for a 4/0 @ 90c x .8 (derated for 6 in the pipe) gets 208 x 2.

That does not take into account the voltage drop which might actually be the limiting factor.

Derate in Canada, too. As with the NEC, distance is a factor.

http://www.codemath.com/cgi-bin/Run.pl?script=CecAwgRw

Neutrals don't count toward derating.

Actually the neutrals don't count exception only applies to harmonic circuits.

Todays modern electronic ballasts are non-harmonic and if they are significant then the exception cannot be used.

I've seen this the most in big box retail. The spec's call for #10 going home. The strips are factory whipped with #12 & Molex. The whole load at the smart board is held down to 10 Amps per hot.

( Big box retail now uses ranks of low-voltage switches connected by BAC back to national corporate. This way HQ has over-ride control on energy use, store by store -- all controlled from a master computer program. )

Neutrals don't count in Canada on single-phase three wire and three-phase 4 wire systems. The Canadian exception is that the neutral is included on a three phase 4 wire system if only two hots are used. I do not recall harmonics being addressed in this regard, in the Canadian code.

I'm trying to upgrade to 400amps using existing wires. Canadian code says 4/0 r90 is good for 235amps. Two runs in parrallel gives you 470amp. Because there's 6 in one pipe we derate to 80% (4 not counting the neutrals still have to derate to 80%) 470 x 80% = 376

I understand derating for multiple wires/circuits in a conduit but this is one circuit split into many wires. What would the difference be if they were 3 x 500mcm wires instead of 6 4/0's. Going from 3 to 6 wires doesn't add more circuits in the pipe or heat so why derate?

To Canadian code, install a 376 amp service, and the next fuse size is 400 amps in Table 13.

I don't understand the relationship between the size and ampacity of 500 mcm and 4/0. Isn't 500 mcm more than twice the size of 4/0 and less than double the ampacity?

500 kcmil is .519 sq/in, two 4/0 is .520 sq/in. (per table 8)

If you used 310.16 you get 380a in the 75c column for the 500 kcmil and the derated ampacity for paralleled 4/0s is 416a (80% of the 90c column 520a).

Evidently Canada sees this differently. Maybe they share Gerald's disdain for 310.16

Interesting. So the service conductors from the POCO are single 350MCM in a conduit. What amperage are they good for using the NEC?

CEC = 325amps R90

Services are usually underground or overhead. Both methods have different ampacities than conduit. Aren't they aluminum conductors from the power company?