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#166391 07/19/07 08:36 PM
Joined: Jun 2004
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600 amp service for a 10000+sqft single family dwelling.

Are 250mcm parallel copper THWN service entrance conductors installed in 4" conduit of sufficient size to feed the main 600amp switch?

shortcircuit

2017 / 2014 NEC & Related Books and Study Guides
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Short:
Are you looking at 310.15 (b) (6)?
I can't say anybody ever paralled from this table that I'm aware of.
310.16 equates to 300's using the 75 deg. col., if you can; or 350's from the 60 deg.

Last one I saw was parallel 500 Al; 3 phase.

Lets see what the guys think!

John


John
Joined: Feb 2003
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I came up with 350 twinner parllel for 600 amp and the rating is good for 625 amp but it will be ok in 4 inch PVC conduct

Merci, Marc


Pas de problme,il marche n'est-ce pas?"(No problem, it works doesn't it?)

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Hotline...I am looking at table 310.15(B)(6) and 250mcm copper is good for 300amps. But since they are in the same conduit in parllel then they must be derated to 80% of 300amps which would bring them down to 240amps...

So if I bump them up to 350mcm copper then, 350 derated 80% would give me a 280amp value and a 560amp total rating.

Next size up breaker...600amp

OK?

shortcircuit

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Short:
Yes, based on your info, math is good.
My thoughts:
There is no reference to parallel in 310.15 (b)(6); either yea or nay.
The table ampacity stops at 400 amps.
This may be another of the infamous 'gray' areas in the NEC.
As I said above, I have not seen this.

IMHO, you are OK, BUT.....anyone else care to comment??

John


John
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Originally Posted by shortcircuit
I am looking at table 310.15(B)(6) and 250mcm copper is good for 300amps.


Short, IMO that is incorrect, that table is not an ampacity chart like 310.16.

That table is a list of conductor sizes that the NEC will allow for certain size services. They have not 'changed' the ampacity of the conductors, they are simply relying on the knowledge that a service sized per the NEC will not really be loaded to that level. Much like how the power company sizes their conductors.

That said IMO if they wanted that table to carry up to 600 amps it would.

I agree with John (Hotline) that there is no reference to using that table in parallel.

However If I recall Don has a CMP statement more or less saying you can use T310.15(B)(6) for parallels but I don't see anything in the actual NEC to back that view up.

Another idea would be three 200 amp service panels and you could size the conductors as small as the calculated load.

Could be cheaper all around.


Bob Badger
Construction & Maintenance Electrician
Massachusetts
Joined: Oct 2005
Posts: 100
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If you're going to paralell, I would strongly suggest separate conduits from an "engineering" standpoint. The reason being if a fault occurs in the single conduit, now all the feeders burn up, and the building is left completely without power.

Dealt with this a little over a year ago... paralell feed, 3-phase, 500 MCM in two conduits. One of the feeders burnt up, and tripped the CB on the xfmr. POCO came out and clipped the faulty feeders, and re-energized the remaining feeders, which keep the building operational until we had time to pull the "bad" ones out. Remember that sticker shock from that one too... 500' of 500 MCM copper = $5,000... and the price couldn't be guaranteed for more than a few hours and had to be paid in green cash.

Joe

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Bob,
Yes there is a panel statement from the 95 ROP.
Proposal 6-74 in the 95ROP was to prohibit the use of the reduced wire sizes in parallel. Panel 6 rejected the Proposal with this statement: "Conductors 1/0 and larger are permitted to be paralleled by section 310-4. This would apply to Note 3."
The rule in Note 3 to Table 310.16 in the 95 code is the rule that is now found in 310.15(b)(6)


Don(resqcapt19)
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Originally Posted by JJM
If you're going to paralell, I would strongly suggest separate conduits from an "engineering" standpoint. The reason being if a fault occurs in the single conduit, now all the feeders burn up, and the building is left completely without power.


Joe, that is exactly the type of good advice that will stick to the memory pad in my brain for future reference.

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Around here 600amps would need to be engineered. The power company will do the engineering if necessary.

Separate circuits to separate panels (up to 6) makes most people happy.

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