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?

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

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).

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.

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.

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

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).]

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

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?

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.

We run the reduced size conductors for the service panel residential 200A and under, and full size conductors for the Sub panel.
The Sub load is a load center, so I belive you could not use the feeder rule which would have a known load.

We run the reduced size conductors for the service panel residential 200A and under, and full size conductors for the Sub panel.
The Sub load is a load center, so I belive you could not use the feeder rule which would have a known load.

Looks like I will run the same size for the neutral. Is #4 sufficient for the ground with 3/0?

#4 is ok for the GEC, with a few exceptions, such as the sole connection to the ground rod(s) or plate, which does not have to be larger than a #6. Article 250.66, along with the table, will give you all this info.

Rick Miell

Electric Eagle, I am sorry but you are wrong. I purchased the 14th Edition of "Electrical Wiring Residential" from this very site. It CLEARLY explains that table 310.15(B)(6) can be used only if those conductors will carry ALL the current to the house. This text specifically states that subpanels that carry only part of the load are not covered under section 310.15(B)(6). The type of "feeders" that the code is talking about is a case such as where a fused disconnect is placed close to the meter (either inside or outside the home) and a feeder runs from it to a panel elsewhere. This panel would be a subpanel of sorts but it would carry the entire load of the building. Please tell me which reference you used to make your claim that I am mistaken and I will buy it.

I have the 2002 codebook and handbook and read that section over and over again before giving up and finding a more precise source. There are many of these types of books available that go much more in depth and explain every little detail of every little section of the code. The codebook itself cannot do it all (or it would be far more gigantic than it already is).

Caselec, as an example, if there were a 400A meter outside the home and a 400A panel inside, then table 310.15(B)(6) could be used to size the conductors between them. However, I don't know if this situation applies to aldav53's service.

[This message has been edited by triple (edited 09-19-2003).]

[This message has been edited by triple (edited 09-19-2003).]

triple, I guess you are referring to the neutral size, right? i would be safe to keep the neutral the same 3/0 size, and use (I think) a #4 for the ground.

tiple,
while i'll agree the nec does not clarify all, anything out of it is opinion and not legally binding...


HB Commentary....

The (s) as in "feeder(s)and panelboard(s)" was added in the '99 Code. In '96 it was "feeder, panelboard".
This would certainly lead me to believe that the reduced conductor sizes would apply to multiple feeders.
There is no such thing as a "subpanel" in the eyes of the NEC, and the conductors are feeders until the final branch circuit OCPD...S

310.15(B)(6) permits (for dwelling units) a main power feeder(s) to be sized according to the respective table. The conductors are smaller in size from table 310.16.

Main power feeders are the conductors that run from the residential 400 amp CT/meter enclosure to the panel(s) inside the building. They can also be the conductors that run from an outside disconnect to the panel inside the building when it is located too far inside the building and the outside disconnect is required.

Pierre

Sparky, I do realize that just because somebody writes a book about the code, that does not mean he must be an all-knowing electrical god.

I am going to try to make my point simple. The issue of table 310.15(B)(6) has come up, in some form, many times. So far, all I have heard from people on this site, as well as from fellow electricians in the field, is their own personal interpretation. Either that or the infamous "we've been doing it that way for XX years"! What I have done is to back-up my standing with an independent source. I spent some time researching this subject previously and can't find a single piece of textual evidence that supports the use of the aforementioned table for residential panels which do not carry the full load. The 14th Edition of "Electrical Wiring Residential" says that you can not.

Please, anyone, give me the name of one single book which will support your claims. I honestly want to purchase it. I must have read close to 100 opinions on this subject and, so far, I am the only one to back mine up. Like I said, I bought my residential wiring book from the bookstore on this site so it must be seen as having some merit. Since anybody can write a book and call themselves an expert in the field, surely one of you doubters can find a portion of text (among the countless in existence) that can be used to back up your statements.

Waiting...

I have always believed the (s) applies to multifamily dwelling feeders since you have more that one feeder but each feeder supplies one dwelling. If these reduced sizes apply to all feeders what is the word main for? 4/0 AL conductors would still be permitted for a 200 amp feeder if the calculated load does not exceed 180 amps but a 2/0 CU conductor would require a 175 amp overcurrent device.

aldave,
If you want to use a neutral that is smaller than your phase conductors for this installation you will need to calculate the neutral load. See 220.22. The equipment ground for this feeder would be sized according to table 250.122 which requires a minimum #6cu for a circuit protected by a 200 amp overcurrent device.

Curt

I have to go with the larger conductors. 310.15 (B)(6) says Main power feeder to a dwelling unit, once you start going to sub panels I don't think this article still pertains and you are back to 310.16.

I'm not disputing you triple, merely sitting the fence.

I'm aware the issue had been hotly debated pre-'02 , resulting in some minor typographical changes

This seems a terminology issue....

Is there anything currently* in the rop's on this?

*no pun intended.....

Hello Triple,
I find Stallcup's books, and Mike Holt's very informative in regard to Electrical wiring. They are tops in the field. I've heard them both talk, and Stallcup is a walking Code book.
I can't say enough about them.

Hello Triple,
I find Stallcup's books, and Mike Holt's very informative in regard to Electrical wiring. They are tops in the field. I've heard them both talk, and Stallcup is a walking Code book.
I can't say enough about them.