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#29519 - 09/18/03 12:54 PM 200a conductor sizing
aldav53 Offline
Member

Registered: 08/22/01
Posts: 547
Loc: Chandler, AZ USA
Installing a 400a service with a 200a subpanel and tring to find the conductor size. Main conductors should be 3/0 (good for 215 amps). Not sure if I can down size the neutral one size or go with 3/0 also?
I believe the ground can be #4 according to table 250-66. Am I correct on this?
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#29520 - 09/18/03 01:01 PM Re: 200a conductor sizing
rmiell Offline
Member

Registered: 11/09/00
Posts: 261
Loc: La Junta, Co. USA
We will need a little more info. Is this for a residence? What type of wire? What does this "subpanel" feed? What is the supply voltages? Where do you get 215 amps for 3/0? What year of codebook are you using?

Rick

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#29521 - 09/18/03 01:27 PM Re: 200a conductor sizing
triple Offline
Member

Registered: 04/01/02
Posts: 182
Loc: Wisconsin
What do you mean by "main conductor"?

Cu or Al?

Unless you calculated the maximum possible neutral load, how can you reduce the neutral size? And, even though reducing the neutral may be possible now, a good electrician will plan ahead for future neutral loads. There are, of course, plenty of cases where a reduced neutral is perfectly fine (a motor control center for example).

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#29522 - 09/18/03 02:00 PM Re: 200a conductor sizing
aldav53 Offline
Member

Registered: 08/22/01
Posts: 547
Loc: Chandler, AZ USA
Yes it is a residence. I am installing a 400 amp panel feeding a 200 amp load center. It is single phase 240 volt. Sizing the feeders for the 200a load center, using THHN wire.
Got the sizing for the feeders from table B.310.1 in the 2002 NEC.
Will go the the same size on the neutral, as for the ground I believe it is #4.
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#29523 - 09/18/03 02:21 PM Re: 200a conductor sizing
triple Offline
Member

Registered: 04/01/02
Posts: 182
Loc: Wisconsin
3/0 Cu is what you want to use. Even though you are installing this wire in a residence, table 310.15(B)(6) cannot be used for a subpanel (otherwise 2/0 Cu would be sufficient). This was a matter of debate at one of my previous places of work so I bought two different books to clear up the confusion. Both books state that 310.15(b)(6) is not for subpanels since they don't carry the whole load of the house. In my opinion, neither the codebook nor the handbook do a very good job of describing the use of that table.

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#29524 - 09/18/03 03:14 PM Re: 200a conductor sizing
rmiell Offline
Member

Registered: 11/09/00
Posts: 261
Loc: La Junta, Co. USA
OK, but table B.310.1 is part of Annex B, and therefore not part of the requirements of the NEC.

Table 310.16 is the one to use. 3/0 cu in the 75 degree column is rated for 200amp, so 3/0 it is.

I seem to remember that the rule of thumb for reducing the neutral conductor is two sizes smaller, which would result in a 1/0 cu conductor. Take extra time in calculating this, as there are a number of rules to follow to do this.

As triple has mentioned, the use of the table 310.15(B)(6) would result in a smaller conductor, but it has some believers that it can not be used for a subpanel. Since you are installing a 400amp service, where will the other part of the load be going? If it will be for outbuildings, then perhaps the smaller wire will be justifiable, since the subpanel will indeed be the "main power feeder" to the residence.

Hope my, and the other's, information has helped.

Rick

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#29525 - 09/18/03 04:36 PM Re: 200a conductor sizing
Electric Eagle Offline
Member

Registered: 04/20/02
Posts: 928
Loc: Alpharetta, GA
310.15(b) does apply here. It is for service entrance conductors AND feeders. So 2/0 CU or 4/0 AL is acceptable. It is to(or can) be used anytime the conductor serves more than 1 load, such as a sub panel.

Technically even the main panel in a house is a sub panel if the disconnect is not in the panel. The reason you can use a smaller conductor for a subpanel is because you aren't using all the loads continuously as you might be with a dedicated load.

[This message has been edited by Electric Eagle (edited 09-18-2003).]

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#29526 - 09/18/03 07:08 PM Re: 200a conductor sizing
caselec Offline
Member

Registered: 04/14/02
Posts: 557
Loc: San Jose, CA
310.15(B)(6) says that the conductors listed in Table 310.15(B)(6) can be used for the MAIN power feed to dwelling. If a single family dwelling has multiple feeders I don't feel any of them can be considered the main feeder.

Curt
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#29527 - 09/18/03 07:43 PM Re: 200a conductor sizing
Anonymous
Unregistered


What about the heading for table 310-15(b)(6) which says "120/240 Volt, 3-WIRE, single phase Dwelling Services and Feeders"

Wouldn't the part that says "3-Wire" rule out subpanels?

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#29528 - 09/18/03 09:11 PM Re: 200a conductor sizing
u2slow Offline
Member

Registered: 09/18/03
Posts: 198
Loc: Salt Spring, BC, Canada
208/240V motor and heating loads will contribute the most to reducing the neutral conductor size. The CEC allows us to deduct these loads for determining neutral conductor ampacity - perhaps the NEC does too.

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