ECN Forum
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

Quote
Each current-carrying conductor of a paralleled set of conductors shall be counted as a current-carrying conductor.


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 wink

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?
you raise a good point, why no derating if you ran this as three conductors of equivalent ampacity but derating for 6 conductors? the assumption is the same heat is generated by the 3 conductors as the 6 conductors or even triple and quad feeders.

Also, since your install starts in one jurisdiction and ends up in another, would you need to comply with CEC at one end and NEC at the other, with two permits, and two inspections?
Skin effect is huge as the conductors get huge. Unless spec'd the larger conductors are uneconomic.

Paralleling a fist full of 1/0s will give you maximum un-derated capacity.

The rules exist based upon heat buildup/ radiation. In underground/piped systems there is a limit as to how fast the heat can bleed off. While aboveground conduit would seem to have advantage -- remember that air temperatures of 100 F are possible. This means that worst case calculations show either route rate limited.

Since these conductors are crossing the border -- I'd hand the issue off to an EE and get him to take the exposure.

I can't see anyone paying an EC for his professional expertise in such matters. Think of yourself as a general practitioner and the EE as a surgeon. No one wants your opinion on heard operations. They certainly aren't going to pay for it.

Concentrate on what you can get paid for -- not on what piques your professional interest. Keep that as a hobby -- and don't expose yourself to professional liability.

-----

I've had instances where I was right and the 'expert' was wrong. It went his way all the way down the line. I had no business being that knowledgeable. My boss was not interested in defending my position in the slightest.

Being correct -- but beyond your credentials -- will get you killed, professionally.

Copper conductors, underground.
I think the cross section area of 500 mcm is double 250 mcm. It looks like it has half the resistance.

http://www.nexans.ca/eservice/Canada-en_CA/navigate_235212/Soft_Bare_Copper.html#top
Conductors don't cross the border. They supply power to the toll booths, shop and security building on the Canadian side. The bridge spans the border, has lights etc on it but the wires I'm dealing with are the main service wires.

Wires to first disconnect are 3 x 350MCM R90 copper ran underground to a 400amp fused disconnect. (3" rigid stubs up and comes in the building above ground and you can see 3" conduit going up a near by pole)
From there 6 x 4/0 R 90 copper conductors in a rigid 3" underground a short distance to main 400amp distribution panel. Was origionally fused at 300amps, they want more. Actually one fuse was 300amps and the other was 330amps?
Underground conductors are Diagrams B4-1 to B4-4 and Tables D8A to D15B in the Canadian Code.
Quote
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


Can someone with the NEC give me a hand on this one?
310.16 says 310 amps for copper, and 250 for aluminum reading from the 60 degree c table.
You should be able to use the 75c column.
© ECN Electrical Forums