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Posted By: shortcircuit 600 amp Residential Service - 07/20/07 01:36 AM
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
Posted By: HotLine1 Re: 600 amp Residential Service - 07/20/07 02:36 AM
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
Posted By: frenchelectrican Re: 600 amp Residential Service - 07/20/07 02:49 AM
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
Posted By: shortcircuit Re: 600 amp Residential Service - 07/20/07 09:42 AM
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
Posted By: HotLine1 Re: 600 amp Residential Service - 07/20/07 11:23 PM
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
Posted By: iwire Re: 600 amp Residential Service - 07/21/07 11:16 AM
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.
Posted By: JJM Re: 600 amp Residential Service - 07/21/07 05:42 PM
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
Posted By: resqcapt19 Re: 600 amp Residential Service - 07/21/07 06:03 PM
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)
Posted By: macmikeman Re: 600 amp Residential Service - 07/21/07 06:05 PM
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.
Posted By: George Re: 600 amp Residential Service - 07/23/07 12:24 AM
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.
Posted By: HotLine1 Re: 600 amp Residential Service - 07/23/07 12:28 AM
This subject must have been around here before; I thought I saw what Don referenced above, but could not find any written word. Bob's mention of 310.15 not being an 'ampacity chart' was debated at length locally, and the ampacities listed are to be considered 'code' for resi work.

As I said, no one has attempted this in my area.

As to using multiple conduits....my only way as an EC.

John
Posted By: shortcircuit Re: 600 amp Residential Service - 07/23/07 11:24 PM
Thankyou for the feedback guys...

It is fully agreed that table 310.15(B)(6) does NOT apply to a 600 amp service as I described...

But I can apply the table to a 400 amp service with parallel conductors in this way...

4/0 copper has a rating of 250 amps from the table so...two 4/0 conductors in parallel would give me 500amps and derated I would have a capacity of 400amps when installed in the same conduit...

Would this be code compliant?

shortcircuit
Posted By: wire_twister Re: 600 amp Residential Service - 07/28/07 12:19 AM
I have a computer program by a man named Robert J. La Capra called electric 2002, it specs. paralell 350mcm copper in seperate conduits for 240 volt single phase service
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