310.15(B)(6) and NM

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Re: 310.15(B)(6) and NM

Could I ask a couple of questions before I answer?

Is this a 100 amp 240 volt panel fed from the main?
You will need to install 2 ungrounded, 1 grounded and 1 grounding conductor.


Are you using a copper conductor?
If so then no problem but is using aluminum or copper-clad aluminum then you will need a #1.

Is this sub panel used as the lighting and appliance branch-circuit panelboard?
If it is not then table 310.15(B)(6) can not be used.
 
Re: 310.15(B)(6) and NM

The situation is I need to feed a main lug panel off of the service panel in a residence for lighting, appliances and receptacles. Normally this is trivial as I would run #2 SER (table 310.15(B)(6). However the homeowner refuses to have any aluminum cable in the house. Rigid onduit is not an option, ent could be used I suppose. Anyway, when referencing the dwelling service and feeder conductor table, 310.15(B)(6), does this table apply to NM-B? If it does i could even use #4-3 NM, if not then even number two would not work because the ampacity of #2 NM is 95 by 310.16. In the heading where the conductor types are listed for table 310.15(B)(6), there is no NM listed, but then again i have never seen a insulation classification for NM. I would say this table does not apply to NM because article 334 states the ampacity is to be determined by the 60 degree collumn which contains types TW and UF, both of which are not listed as conductor types in table 310.15(B)(6). Maybe I just answered my own question, but my reasoning could be flawed, just wanted to know what other people think on this. The cable in question is 2-3 NM, so it has the required three insulated conductors and EGC.

Regards, EThan.
 
Re: 310.15(B)(6) and NM

Ethan

Table 310.16(B)(6) can NOT be used for type NM cable. If the load on this feeder is 95 amps or less use #2 CU type NM cable protected by a 100 amp CB.

Curt
 
Re: 310.15(B)(6) and NM

Curt, I think I'm with you here. Are you using 240.4(B) to allow the feeder overcurrent device setting to be higher than the feeder ampacity? I don't believe calculated load can be used to size feeders protected by a given OCD??

Ethan.
 
Re: 310.15(B)(6) and NM

Curt...why can't we use table 310.15(B)(6)for ampacity selection of NM cable used as a main feeder to a dwelling unit? See exhibit 310.8 on page 274 of the 2002NEC handbook. If I were to use NM cable between the service disconnecting means and the sub-panels in the apartments wouldn't I apply the allowable ampacitys of table 310.15(B)(6)?

electrofelon...table 310.15(B)(6) can only be used for the 3wire service entrance conductors, service lateral conductors, or feeder conductors that serve as the main power feeder for a dwelling unit. If this is for that garage sub-panel you have been posting, select conductors from table 310.16...so, #2 copper NM has a ampacity of 95 amps from the 60 degree column and a 100 breaker is allowed according to 240.4(B) as the next available higher size overcurrent protective device.

This post sparks another question...shouldn't the heading of 310.15(B)(6) and table 310.15(B)(6) read like this...

120/240,3-wire and 4-wire,Single-Phase Dwelling Services and Feeders.

Considering a feeder consists of 2 ungrounded conductors, 1 grounded conductor and 1 grounding conductor when useing a cable for a 120/240 volt dwelling unit feeder?

shortcircuit2

[ March 11, 2005, 10:55 AM: Message edited by: shortcircuit2 ]
 
Re: 310.15(B)(6) and NM

Correct me if I am wrong but I think that is both terminations are rated at 75 degrees then the conductors can be selected from the 75 degree column. ?110.14 (C) (1) (3) Conductors with higher temperature ratings if the equipment is listed and identified for use with such conductors?
Unless these conductors are feeding the lighting and appliance panel board such as is used to feed a mobile home or from the main on an apartment building then 310.15 cannot be used.
 
Re: 310.15(B)(6) and NM

For the 2002 NEC, there was an errata to 310.15 B 6. but the end result is NM can not be used at the increased ampacity, its not one of the allowed cable types
Per 334.80, the 90 deg c rating can be used for derating, but can't exceed the 60 c rating. So 12-2 is rated at 30 amps for derating for temp, but cant exceed 20 amps
 
Re: 310.15(B)(6) and NM

Now this is getting interesting, I never really thought much about the "3-wire" wording in 310.15(B)(6). Shortcircuit2, you brought up an interesting point about the wording. What is the consensus on this applying to INDOOR feeders? Are they 4-wire or are they 3-wire with an equipment grounding conductor? The Code states: "...are installed in raceway or cable with or without an equimpment grounding conductor.." Not only that but the next sentance is: "For the 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)" I would argue this applies to sub-panels. What is the intent here?

Ethan.
 
Re: 310.15(B)(6) and NM

I think it's 3 wires, regardless of the presence of an equipment grounding conductor.

We could whittle this to death, but I believe that the grounded and ungrounded conductors are addressed here, and the wording "with or without an EGC" is attempting to state that.

14-3 romex is referred to as "three-wire" despite bearing four conductors.

My opinion.
 
Re: 310.15(B)(6) and NM

I am not sure you can even use this table at all for this application regardless of what wiring method you are using. This does not sound like the main power feeder to the dwelling unit.
 
Re: 310.15(B)(6) and NM

When I have a home owner squirm when i say Al I tell them well what do you think is in that meter from the poco :D You insist on Cu well here is the difference.Several years ago I did an 11,000 sq ft. home for the Yankees owners son Hal 600 amp K based meter with 3 sub feeds in 4/0 al well the contrator flipped when he saw AL and wanted it changed to CU (AL was in the bid) Sign this $3,200.00 change order and you got it :D
 
Re: 310.15(B)(6) and NM

Originally posted by electricmanscott:
I am not sure you can even use this table at all for this application regardless of what wiring method you are using. This does not sound like the main power feeder to the dwelling unit.
Click to expand...
Scott, it doesn't have to be a main feeder. Just a service or a feeder in a dwelling unit using 120/240 1? power. ;)
 
Re: 310.15(B)(6) and NM

2005 - 310.15 (B) (6) 120/240-Volt, 3-Wire, Single-Phase Dwelling Services and Feeders. For individual dwelling units ----Table 310.15(B)(6), shall be permitted ---- feeder conductors that serve as the main power feeder to each ----
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).

2002 - 310.15 (B) (6) 120/240-Volt, 3-Wire, Single-Phase Dwelling Services and Feeders. For dwelling units, conductors, --- Table 310.15(B)(6), shall be permitted as --- feeder conductors that serve as the main power feeder to a dwelling unit ----
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) .

I read both of these to say about the same thing, ?AND FEEDER CONDUCTORS THAT SERVE AS THE MAIN POWER FEEDERS.?

I think this means that Table 310.15 (B) (6) can?t be used to size conductors for any sub panel from a lighting and appliance panel board as defined by ?02 in 408.14 (A) and 408.34 (A) in the ?05 code. (10 percent of its overcurrent devices protecting lighting and appliance branch circuits.)

Table 310.15(B)(6) Conductor Types and Sizes for 120/240-Volt, 3-Wire, Single-Phase Dwelling Services and Feeders. Conductor Types RHH, RHW, RHW-2, THHN, THHW, THW, THW-2, THWN, THWN-2, XHHW, XHHW-2, SE, USE, USE-2
NM cable is not listed above so if this is the main power feeder NM cable would not be allowed.

If this is a sub panel feed from a lighting appliance panelboard then three conductor with ground NM cable will be fine if sized from Table 310.16.
110.14 (C) (1) (2) both codes allow the use of the 75 degree column

[ March 12, 2005, 10:53 PM: Message edited by: jwelectric ]
 
Re: 310.15(B)(6) and NM

JW, even if the terminals are rated for 75C you can not exceed the ampacity listed in the 60C column if type NM cable is used.
 
Re: 310.15(B)(6) and NM

We might as well keep beating this one to death. But really, this is a very important question and I'm surprised we seem to be split pretty even so far (The Nm issue has been resolved, talking about he 310.15(B)(6) applying to indoor feeders/subpanels).

In many cases it probably doesn't matter whether 310.15(B)(6) applies because even if it doesn't, you get to use the next highest OCD for the feeder which often gets you back up to where 310.15(B)(6) would put you. However consider a 100A sub, fed with #2 SER. 310.16 gives the ampacity of 90A which is a standard size. There are millions of 100A subs in the US, what are people feeding these with? I have always seen and used #2SER but that may be undersized.

"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)"

When I see the phrase:

'For application of this section A shall be B.'

I read that as:

'for this section only, forget everything you know or think you know about A, and consider it B.'

n other words, forget what you think a main power feeder is, because (for this section) it is the feeder(s) between the main disconnect and the lighting and appliance branch-circuit panelboard(s).

Ok maybe I am totally over-analyzing this but it does seem to define 'main power feeder' for us.
 
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