Determining awg for branch / feeders

Specific Inspection Topics Electrical Inspections
1

How do other inspectors look at the electrical panel?

New construction home, 200 Amp Cutler Hammer main service panel,
125 Amp Cutler Hammer distribution panel in garage,
and breaker disconnect at the furnace.
The 100 Amp breaker in the main panel labeled ‘furnace’ was wired with 2 awg Aluminum,
I questioned it because the Electrical 2005 Code Check lumps branch and feeder together on Table 10, and calls for 1/0 awg AL.

The electrical inspector for the city proceeded to tell me this was a feeder (because it supplies power to another breaker panel at the furnace) and as a feeder it can be wired with 2 awg, NEC 310.15 (B) 6. If it was a branch circuit, it would be wired with 1/0 awg. NEC 310.16

Are you using 310.15(B)6 for service and/or feeders, and Table 10 (310.16) for branch???

Thanks for your help,
Linda

2

Table 310.15(B)(6) is for service and feeders that carry the entire dwelling unit.

You are correct that #2 AL can not be protected with a 100 amp breaker in the installation you have described.

Edited to add;

here is the text in the NEC;
(6) 120/240-Volt, 3-Wire, Single-Phase Dwelling Services and Feeders. For individual dwelling units of one family, two-family, and multifamily dwellings, 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 each 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 panelboards(s).

3

I have to take the oposing view.
In my area we DO use T310.15(B)(6) for any feeder, inlcluding to a sub-panel.
IMO this only makes sense. What different is a main panel from a sub-panel as far as load diversity goes? Not much.

Now if this is a branch circuit all bets are off.

4

Diversity is the issue.

With a service calculation it would be unlikely that everything in that dwelling to be in use at once but a sub panel is a different story.

I have seen subpanels supplied with a #2 and protected by a 100 amp breaker that was drawing close to 100 amps continuously.

I just redid a panel in a storage building that was supplied with #2 AL under a 100 amp breaker that had been turned into an apartment for the eldest son that had a load that calculated at 150 amps.

This was cleared up with a change in the text in the 2008 code cycle and is very clear that subpanels are not allowed to use Table 310.15(B)(6)

5

I understand that philosophy, but I still don’t feel the same way.
A sub-panel in a residence is no different than a main as far as load diversity goes IMHO. As long as a proper load calc is done and the panel is sized properly. 100 amps in a residence is a LOT. This is the only size that is even an issue any way.
Key word here being opinion.

I realize they did change the text for 2008.
As with MANY of the changes in this new edition, I don’t agree with it.

6

This one has always been a nice debate…the NEC’s intent is this…

It has to be for one a “single” dwelling unit…and can’t be used lets say to size service conductors to a service for a multi-family dwelling…now it can for each individual dwelling but not in combination.

Now it is my opinion that lets say you supply to a multi-family dwelling with properly sized conductors via 310.16 as we discussed before that you can indeed go from the exterior disconnection means for that single unit to the “remote distribution panel” which is now a feeder and still be able to use 310.15(B)(6)…but only in respect to it being the " main power feeder "…simply going to a “subpanel” would not make it a "main power feeder "…

This area still needs work and we have some proposals coming for 2011 on this I have spoken to Mike Holt about it and he is on the same page and will submit them…

Generally it becomes an issue of is it a “Main Power Feeder” to the individual dwelling…going to barn, detached so on begs us to say it is not a dwelling and the allowance of this reduction says very clear…

(6) 120/240-Volt, 3-Wire, Single-Phase Dwelling Services and Feeders. For individual dwelling units of one family, two-family, and multifamily dwellings, 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 each 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 panelboards(s).

Those words in RED have strong meaning…:slight_smile: just chiming in fellas…maybe I am wrong…thehehehe

Now I had to add…the 2008 change on this refers to the following:

"The reference to “lighting and appliance branch circuit panelboard” has been deleted to correlate with the changes in Article 408. Table 310.15(B)(6) has been rearranged to provide the ampacity values in the left column as a result of CMP-6 action on Proposal 6-66. The second sentence as revised NOW requires a main power feeder to supply all loads associated with the dwelling unit.

Basically…if you do size this using an example where you have an exterior disconnect and this making that the Main Disconnection means and then you feed it throught the house to a "remote distribution panel " or because of the nature of the bonding process a " subpanel " then 310.15(B)(6) would apply…but only loads associated with the dwelling itself can come out of the panel…WOW…that gets wordy…lol

7

Here is a slide from the Analysis of Changes 2008 by the IAEI

As can be seen it must carry the entire load of the dwelling unit and can not be a remote panel somewhere in the dwelling for convenience such as a subpanel to supply part of the house or a couple of appliances.

8

In my interpretation what if the service disconnect in that graphic was also the main panel? Then that main power feeder would be feeding a sub-panel.
I still say the words “main power feeder” are subject to scrutiny. Who is to say a “main feeder” is not ALL of the feeders in a residence?

This is where my argument looses validity. The word “to” in the quote below. If that word were “for”, then my stance would be much more stable.
"serve as the main power feeder to each dwelling unit"

Again, local allowances are the key here. In my area we cannot even get 1/0al SER, or #2cu SER. At least without a special order.
EVERYONE uses #2AL for 100A sub-panels as it is the locally accepted method.
Will this change? Who knows. Will I special order 1/0al SER? No.

9

It is not real clear in the OP what the #2 AL is feeding. The OP states that there is a main panel, a sub panel, and a disconnect at the furnace. She says the 100 amp breaker is in the main panel, and labeled “furnace”. This does not sound like it is the sub panel feeder. It sounds like it is the circuit that feeds the furnace disconnect, and might be protected against overload at the disconnect end?

10

I hear ya brother…basically it is the term " sub-panel " that messes things up. It appears no where in the code but is used ALOT…and i mean ALOT…lol

In the image Mike posted we do indeed see the Main Disconnection Means as the disconnect below the meter…this is where the bonding to the case takes place, the GEC leave to the GES and so on…so it is the main disconnection means as defined in the NEC.

Now…he “main power feeder” as allowed from in 310.15(B)(6) is the portion between the now explained “main disconnection means” and the panelboard for the actual dwelling…

and SINCE all dwelling related loads must come from this panel to meet the allowance for the reduction of 310.15(B)(6) it would make it impossible to run another lets say feeder to a detached garage from this panelboard and use 310.15(B)(6)…the reduction allowances has already taken place…and the one going to the detached remote building is not a “main power feeder” for the individual dwelling…the verbiage does help clear it up for HI’s which is a GREAT thing…atleast they are starting to get the wording right in the NEC now on these things…

While it is indeed in THEORY a “sub-panel” in this picture because we know that the case to neutral bonding cant take place AFTER that main disconnection means…it would indeed be in theory a SUB panel that is fed from the MAIN POWER FEEDER…because it is a feeder now…

God i love the code…

Yep in the end…the AHJ will make the call but hopefully as the NEC gets easier to understand ( eck…did I say easier…) it will change.

11

Sorry. I only included the term ‘sub-panel’ in the electrical layout because there is one in the garage. It has nothing to do with the rest of the question. :wink: Forget that part.

The 100 amp breaker labeled ‘furnace’ in the main panel using 2 awg AL feeds the two-breaker panel ahead of the furnace.

The ahj tells me that 310.15 applies. I didn’t necessarily trust his opinion. I think you guys have a better handle on it !! :roll:

12

The short answer is this…310.15(B)(6) would not apply to this 2 breaker panel feeding the furnace.

it would have to be sized based on 310.16 which if we are dealing with the panel feeder…using SE cable @ 75 degree - 1 AWG if we are speaking of Al conductors…nope 310.15(B)(6) would not apply.

We just figured you all would like the understanding of HOW the NEC comes to this explanation…sorry if we long winded it.

13

Does this apply to both 2005 and 2008 NEC?, or 2008 only.?

I don’t mind the long winded, look how much knowledge is there.!! That’s what it’s all about !

14

Yes…the intent did not change. Just some of the way it is said changed.

15

Had a post ready to go about noon my time but then scrapped it as it wasn’t clear if the panel near the furnace was simply a 2-pole 240V breaker disconnect only…in which case, it was not a disribution sub-panel and the diversity issue/theory is mute. This would then be a branch circuit for the furnace only and rules for branch circuits would apply.