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Author Information Topic:   Wire size for 100 amp subpanel
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Name: L West
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posted May 07, 2001 at 11:50 AM       Edit/Delete Message
I'm installing a 100 amp sub-panel in my basement. The sub panel will be app. 15 ft. from the main panel. Should I use #4 or #3 copper?

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posted May 07, 2001 at 02:34 PM       Edit/Delete Message
#4 cu or #2 al ,per 310-15(b)(6), if it's a 'dwelling'

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posted May 07, 2001 at 03:51 PM       Edit/Delete Message
Thanks Steve. Just wanted to make sure before I go out and buy some cable!

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posted May 07, 2001 at 08:00 PM       Edit/Delete Message
if this is a subpanel #4 copper or #2 alum would have a rating of 90 amps.


as per 310-15(6) 120/240-Volt, 3-Wire, Single-Phase Dwelling Services and Feeders. For dwelling units, conductors, as listed in Table 310-15(b)(6), shall be permitted as 120/240-volt, 3-wire, single-phase service-entrance conductors, service lateral conductors, and

feeder conductors that serve as the main power feeder to a dwelling unit

and are installed in raceway or cable with or without an equipment grounding conductor. For application of this section, the main power feeder shall be the feeder(s) between the main disconnect and the lighting and appliance branch-circuit panelboard(s), and the feeder conductors to a dwelling unit shall not be required to be larger than their service-entrance conductors. The grounded conductor shall be permitted to be smaller than the ungrounded conductors, provided the requirements of Sections 215-2, 220-22, and 230-42 are met.

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unregistered posted May 07, 2001 at 08:26 PM           Edit/Delete Message
What is the size of the sub-feeder breaker?

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posted May 08, 2001 at 08:17 AM       Edit/Delete Message
The sub-panel has a 100 amp main. I had planned to install a 100 amp braker in the main panel as well.

Now I'm confused... is #4 copper the correct size for my application?

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posted May 08, 2001 at 08:49 AM       Edit/Delete Message
Leon,
For the purposes of this section the conductors between your main panel and the new subpanel are "main power feeder conducotrs" and the #4 copper is fine.

Sunrise,
The subpanel is a lighting and appliance branch circuit panelboard and the feeder for this subpanel is between the service disconnect and the panelboard. Table 310-15(b)(6) appies and the #4 copper is good for 100 amps.
Don(resqcapt19)

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posted May 08, 2001 at 09:40 AM       Edit/Delete Message
Don,

Doesn't 310-15(b)(6) permit the "Dwelling" to use #4 ?

As I understand, if the Dwelling is supplied with #3 Cu. or larger, then any feeders or branch circuits must follow the ampacities of the Tables such as 310-16 and not 310-15(b)(6).

Glenn

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posted May 08, 2001 at 10:49 AM       Edit/Delete Message
Glenn,
310-15(b)(6) permits the use of #4 copper for 100 amp service entrance conductors AND/OR feeders. I don't see anything that says, if the service uses full size conductors you can't use this table. If you have a reference that says otherwise, please cite it. Thanks.
Don(resqcapt19)

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posted May 08, 2001 at 11:07 AM       Edit/Delete Message
On a related subject... do you know how hard it is to find 4/3 w/ground CU romex! Seems that most folks use #2 AL for this type of application

I live in metro Atlanta and after several calls I found a supplier with a 19' pc.

Since there seems to be a bit of debate on using #4 CU for 100 amp I will probably use #4 CU and then install a 90 amp breaker in the main panel.

How does that sound to you guys?

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unregistered posted May 08, 2001 at 04:46 PM           Edit/Delete Message
What is the rating of the main panel? Is it a disconnect or is it a loadcenter w/Main Breaker?

If the main is a loadcenter then I would say that the conductors feeding the subpanel are not the main power for the house. The conductors would have originated in the main panel but be between a branch circuit overcurrent device and the subpanel. I say a #3-3G.

If the main is purely a disconnect w/no branch circuits then I think the #4-3G would be allowed.

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posted May 08, 2001 at 04:59 PM       Edit/Delete Message
The main panel is a 150 amp load center. When I went to pick up the cable the supplier made a mistake and I ended up with #2 CU instead of #4 CU. I'm sure I now have wire capable of safely handling 100 amps.

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posted May 08, 2001 at 08:34 PM       Edit/Delete Message
Don,

No, I don't have anything specific.
It's just that by adding the ", and the feeder conductors to a dwelling unit shall not be required to be larger than their service-entrance conductors." left me the impression only it only applied when all conductors were per Table 310-15(b)(6) for that particular service size.

I did, just now, look up Proposal 6-73 of the 1995 ROP which CMP accepted the above text.

The Substantiation is not what I thought it was when I made my post of #3 or larger.

Glenn

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posted May 09, 2001 at 07:22 AM       Edit/Delete Message
Glenn;
could you let us in on the rationale?

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posted May 12, 2001 at 07:50 PM       Edit/Delete Message
I see there are ROP's intended to clarify this issue. The code loop i'm stuck includes T310-15(b)(6) * 338-4(a) *336 A&B *336-26 110-14(c)(1) and T310-16

consider #2 Al run to a subpanel

338-4(a) ref's to 336 A&B

336-26 wants that 60 deg rating used

110-14(c)(1) & 310-16 would seem to go along

T310-15(b)(6) overrides all? what is the 99' change all about anyway?

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unregistered posted May 13, 2001 at 12:03 AM           Edit/Delete Message
YOU CLEARLY HAVE A "SUB-PANEL" AND SECT. 310-15(B)(6)IS NO LONGER AN OPTION. YOU MUST SIZE THE WIRE FOR THE LOAD TO BE SERVED. THE RATING OF THE PANEL DOES NOT DETERMINE THE WIRE SIZE OR BREAKER SIZE.
EXAMPLE; IF YOUR CALCULATED LOAD IS 67A THEN YOU MUST USE #4CU AND A 70A CB.
HOWEVER YOU MAY CHOOSE TO INSTALL A 100A PANEL PROTECTED BY A 100A CB BUT YOUR CONDUCTOR MAY NOT BE LESS THAN #3CU.

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posted May 13, 2001 at 01:52 AM       Edit/Delete Message
EC0000648
The section, by the use of the word 'panelboard(s)" clearly allows multiple panels to use Table 310-15(b)(6). All such panels are sub-panles because they are on the load side of the service disconnect.
don(resqcapt19)

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posted May 13, 2001 at 08:03 AM       Edit/Delete Message
Does 310-15(b) overrule all other codes mentioned ;

Ampacities for conductors rated 0-2000 volts shall be as specified in the Allowable Ampacity Tables 31-16 through 310-19 and Ampacity Tables 310-20 and 310-21 as modified by (1) through (7)

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posted May 13, 2001 at 09:33 AM       Edit/Delete Message
While all such panels are (sub panels), all sub-panel feeder conductors may not serve as "the main power feeder" to the dwelling, which is what would allow the use of table 310-15(b)(6)to size the conductor. A determination by the AHJ would be required to determine if this is the case.
example) You have a 150 amp service to a dwelling unit, service entrance conductors are sized per the table in 310-15(b)(6) feeding a 150 amp main breaker panelboard. From this panelboard you install a 100 amp breaker and run a set of feeders to a (sub panel)(is there a definition of sub-panel in the NEC?)the loads from this panelboard will determine if the feeder can be sized utilizing 310-15(b)(6). If this panelboard serves say various equipment in a work shop area in the basement would these conductors be considered the "main power feeder to the dwelling unit"? The wording in this section could be a little more refined.

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posted May 13, 2001 at 10:08 AM       Edit/Delete Message
This topic causes a lot of confusion by a lot of people. The intent of this article is very clearly defined, even if I do not personally agree with it.

310-15(b)(6) is allowed to be used by dwelling units for conductors used as "service-entrance conductors, service lateral conductors, and feeder conductors that serve as the main power feeder to a dwelling unit". The main change to this section from 1996 is as follows: "For application of this section, the main power feeder shall be the feeder(s) between the main disconnect and the lighting and appliance branch-circuit panelboard(s)".

This is a very specific limitation. Not all feeders in a residence meet the criteria of being a 'main power feeder' by the new definition. It would be allowable to have from 1 to 6 main power feeders in a residence, the number will be determined by the number of main service disconnects present at the residence. This would have to be a one to one ratio as a main power feeder can only originate in a main disconnect.

For this reason, very few residential sub-panels can utilize section 310-15(b)(6) to size the feeder wire. Most residential panels have a main disconnect installed in them. This means that service conductors are installed to the line side of this disconnect, and the lighting and appliance panelboard bus bars are installed on the load side. In order to feed a sub panel in this scenario, a feeder breaker must be installed on the lighting and appliance panel board bus. This is now a standard feeder, not a main power feeder, as the feeder is not installed from a main service disconnect. The only exception to this scenario is "the feeder conductors to a dwelling unit shall not be required to be larger than their service-entrance conductors".

If any doubt the limiting intent of this article, then read the explanatory material in the 99' handbook under 310-15(b)(6). Figure 310.7 shows an example of multiple feeders, and clearly states it is allowed "because these feeders carry the entire load".

All of this says nothing to the fact of the huge change made in article 336 regarding ampacity of NM cable (which is also referred to by article 338 as being applicable to SE cable). The shifting of 336-26 to section B now means that SE cable for interior installations is now limited to the 60C column.

So, in December 1998, I could have installed #2 al SE-R cable from a 100A feeder breaker in a residential service panel to a sub panel and been code compliant.

Today, that same installation has a feeder rating of 75 amps. That is a 25% reduction in ampacity for the same usage of conductor. Is this an uninteneded byproduct of an organizational mistake? Did the code panel base these significant changes on numerous incidents of failure? I cannot tell you, but even more important to me is the fact that I am the one responsible for the installation. "That's the way I've always done it" is a poor defense in court, and the inspector that allows the installation to be done the old way is not the one responsible in the end (no inspector is responsible for the work they inspect, the contractor is the responsible entity for the work performed). As a contractor I am legally bound to follow the letter of the code, even if I don't agree with it.

Enjoy your day!

[This message has been edited by slkrening@home.com (edited May 13, 2001).]

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posted May 13, 2001 at 03:50 PM       Edit/Delete Message
Scott,
You are correct that not all feeders meet the rule in 310-15(b)(6), but in my opinion all feeders that orignate in the panel that contains the service diconnect and feed a lighting and appliance branch circuit panelboard do. Nothing in this section says that the feeder must directly conenct to the service disconnect means, it only says that the feeder must be between the service disconect and the lighting and appliance branch circuit panelboard(s). The proposal (6-81) that changed this section said that "the main power feeder shall be the feeder(s) that supply the lighting and appliance branch circuit panelboard(s). CMP 6 added the "service disconnect" wording to make the statement clear. The intent is that any panelboard serving "diverse" loads in a dwelling unit be able to use this table.
As far as the handbook goes, their opinion is no more vaild than yours or mine. It is not an official interpretation of the NEC.
The change in 336 that affects SE cable in 338 does not affect this section. 310-15 is a modification to Table 310-16 and 310-15 lists SE as one of the wire types that can use Table 310-15(b)(6). If the CMP wants to apply the 60C ampacity to this table they will have to make that change in 310-15(b)(6) as that section modifies the change made in 336.
Don(resqcapt19)

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posted May 13, 2001 at 04:36 PM       Edit/Delete Message
Don - Thanks for your opinion. It sounds like you have a little more 'insider' info than I do. It still seems to me that the code panels are trying to restrict the use of 'undersized' residential feeders.

I guess I don't see a difference between a feeder that originates in a service disconnect panel, and a feeder that originates from a lighting and appliance panelboard in the terms of your explanation. All feeders are still in between the service disconnect and their supplied panelboards. Q: Why bother creating and defining the term 'main power feeder' at all? A: To give me a headache!

Enjoy your day!

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unregistered posted May 13, 2001 at 08:28 PM           Edit/Delete Message
Scott,

As per Article 100 the definition of a feeder is "All circuit conductors between the service equipment, the source of a separately derived system, or other power supply source and the final branch-circuit overcurrent device.

In my opinion the conductors between the lighting and appliance panelboard and the other (sub)panelboard is a feeder. The definition of a subpanel is not defined in the Code. If a 100A DP C/B is installed in the L & A Panel to feed the othe (sub)panel then the conductors between these two panels s/b rated at 100A. If you are using conduit and type THHN CU wire then the size of this wire per T310-16 75 CU is #3. If you are using Al THW wire in the conduit the size would be #1. If you are going to use CU NM-B (Romex) the wire size would be #1 (60 Column).

On the subject of 1-6 disconnecting means for a residential dwell: 384-16 Overcurrent Devices: (a) Lighting and Appliance Branch-Circuit Panelboard Individually Protected. Each L & A branch-circuit panelboard shall be individually protected on the supply side by not more than TWO main circuit breakers or TWO sets of fuses having a combined rating not greater than that of the panelboard.

"So, in December 1998, I could have installed #2 al SE-R cable from a 100A feeder breaker in a residential service panel to a sub panel and been code compliant."

I disagree! In December of 1998 the Ampacity of #2 AL SE-R was 90A under the 75 column. Why the 75 when the conductor says XHHW-2? Because the terminations (lugs) are not rate 90, therefore you have to use the 75 column.

There has been no change in the ampacity of NM cable. I looked all the way back through the 1987 NEC and it has remained at 60 since then. Type NM-B cable is rated at 90 C for derating purposes, but in looking up the ampacity of type NM-B cable use the 60 column.

In reference to 338-4(a), this section does make reference to 336 A & B. If you read 336-26, it makes no reference to SE or USE cable. I am of the opinion that Type SE-U and SE-R cable with a conductor type of XHHW should still be looked up under the 75 column.

Don

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posted May 13, 2001 at 10:18 PM       Edit/Delete Message
Don,
I agree with Scott as to the ampacity for SE cable used for interior wiring. 338-4(a) requires compliance with Parts A and B of Article 336. 336-26 requires the use of the 60C column for type NM cable. 338-4(a) requires that you follow 336-26 for SE cable installed on the interior of a structure. 336-26 does not say SE because it is the NM article, but 338-4 requires you to treat SE cable as NM cable for interior applications.

It is still my opinion that Table 310-15(b)(6) can be used for all feeders that serve lighting and appliance branch circuit panelboards in dwelling units.
Don(resqcapt19)

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posted May 15, 2001 at 10:38 PM       Edit/Delete Message
Don:
Here is my two cents wroth. You can't use 310-15(b)(6)for all the reasons given in this thread. You must go to 240-3 protection of conductor for the answer . therefore a #3thwn conductor is required to supply the 100 amp. subpanel.s

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posted May 15, 2001 at 11:24 PM       Edit/Delete Message
Phil,
240-3 is satisified because Table 310-15(b)(6) changes the ampacity of the #4 to 100 amps for dwelling unit main power feeders.
Don(resqcapt19)

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posted May 16, 2001 at 07:22 AM       Edit/Delete Message
so here we are .all good opions but three differate ones. what's a contractor to do? feed that subpanel with a 100 amp c/b,90 amp c/b or a 75 amp c/b ? as a ahj I would say 90 amps. sure hope this is not a 120/208 single phase service like they have in some parts of my area .how about a multi use building 120/240 single phase service ,1st fl a store 2nd fl 2 apartments,all mains outside 2 apt. feed with ser on 100 amp c/d
is a dwelling unit right? store feed with 4/0 alum. on 200 amp main .

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posted May 16, 2001 at 08:17 AM       Edit/Delete Message
Sunrise,
Table 310-15(b)(6) can only be used for 120/240 volt single phase systems, it can't be used for 120/208 volt single phase systems. It can't be used to feed the store. It could be used in your example for the feeders from the outside disconnects to the apartments.
Don(resqcapt19)

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unregistered posted May 16, 2001 at 01:54 PM           Edit/Delete Message
I agree with Don. If the subpanel is a lighting and appliance branch-circuit panelboard then you can use Table 310-15(b)(6). If the intent of this section is otherwise - then it needs to be reworded.
Mike

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posted May 16, 2001 at 11:31 PM       Edit/Delete Message
Don:
I respect your opinon , but I must disgree. The intend of the code is the subpanel does not carry the same load as the main power feeder therefore 310-15(b)(6) does not apply.The subpanel is a seperate indenity in its self.

Thanks for hearing me out.

Phil C

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unregistered posted May 17, 2001 at 09:04 AM           Edit/Delete Message
What is the difference between "intent" and "interpretation"?
Mike

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posted May 17, 2001 at 12:12 PM       Edit/Delete Message
Phil,
The intent of the code is load diversity. In my opinion any lighting and appliance branch circuit panelboard in a dwelling unit will have similar diversity.
Why does 310-15(b)(6) use the words "feeder(s)" and "panelboard(s)"?
Don(resqcapt19)

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posted May 18, 2001 at 05:07 PM       Edit/Delete Message
Don;
i have to side with your responses in this, and other BB's. The only point i could possible forward is that it reads hard, evident by the ROP's

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posted May 18, 2001 at 06:50 PM       Edit/Delete Message
Steve,
If the code wasn't hard to read, we wouldn't need all the books and classes.
Don

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posted May 20, 2001 at 06:03 PM       Edit/Delete Message
Nor would the entire NEC need a top end job every 3 yrs, my point is the technical intent is poorly translated in this code.

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