Here is one thing I've been wondering about for awhile:

The 75 degree column of Table 310.16 says #4 copper is good for 85A. If the conductor is used in a 120/240V, 3-wire service or feeder for an individual dwelling unit, Table 310.15(B)(6) trumps 310.16, and we may use #4 copper for up to 100A.

But suppose we use 4/3 Type NM cable? 334.80 (2005) says that the ampacity of conductors in Type NM cable "shall be in accordance with the 60 C (140 F) conductor temperature rating" of 310.15.

Does 334.80 trump 310.15(B)(6)? If so, I can use #4 Cu for a residential feeder if I am using SER or TC, but I must use #2 if using NM.

I suspect there are differing opinions about this, including among AHJs. Ideas?

You can use any of the wiring methods listed in the Table at the ampacities shown in the table.

Hmmm... I see conductor types, but not wiring methods, other than the very general "raceway or cable"...

334.80 also says that the conductors of Type NM cable must have 90 C insulation, although the ampacity must be calculated at 60 C. No conductor type is ever identified on either the insulation or on the sheath, but it sure looks like THHN...

I agree that 334.80 would apply and #2 copper NM has to be used.

But, in Massachusetts we have rewritten 334.80 and allow the 75 degree column to be used for NM not installed in thermal insulation , so #3 copper NM would be allowed.

shortcircuit

I don't know how you can say 334.80 trumps 310.15(B)(6), because I don't see how 310.15(B)(6) can apply to NM cable.

If you want to use cable with 310.15(B)(6), I think SE is your only choice.

Mark, you are correct, table 310.15(B)(6) is very specific in the conductors and cables allowed.


Roger

yaktx, something to keep in mind is that Table 310.15(B)(6) is not an conductor ampacity table at all. It is not at all the same as 310.16 etc.

It is simply a section of code that allows ignoring the usually conductor ampacity requirements for certain applications (dwelling units) and certain wire types.

Table 310.15(B)(6) allows the wire to be protected above its ratting as the NEC is well aware a dwelling unit service does not draw the full rating of the service.

I am aware of this fact.

What I'm hearing is that one of the conductor types listed in the heading of the table must be identified either on the conductor or on the cable sheath. Is this the case with SE? I don't have a sample handy to look at, and I never noticed this before.

310.15(B)(6) brings up a whole host of gray areas. What exactly does "main power feeder" mean? I've always assumed that in a dwelling unit, this table can be used with all feeders 100A and up that have load diversity (i.e. not all air conditioning units).

What if the customer wants a 100A feeder to a detached woodworking shop? Is that a dwelling unit load? I asked this question once of a continuing ed instructor who is a master electrician and EE, and he said that 310.15(B)(6) applies here. I went ahead and applied it (THWN in EMT and overhead span of quadruplex), and the AHJ approved it.

Somewhere there must be a judgement call of where 310.15(B)(6) does and does not apply.

Any thoughts?

Any SE cable I have looked at says XHHW on the sheath.



It does leave some gray area, in my opinion it means the service conductors or "the feeder" to a dwelling unit.



I do not agree with the above.


No, a garage is not a dwelling unit, check article 100.



I do not think it is anywhere near as 'gray' as you make it sound.

Iwire and all: Thank you for your opinions.

This basically concurs with what I have generally been practicing. I have just wondered what the limits are to this application.

There are some areas of the NEC that are pretty crystal-clear, and others not so much. Most of my questions here are "devil's advocate" questions (and I do see now that my earlier reading was a little careless and that "SE" appears in the heading of the table).

[This message has been edited by yaktx (edited 11-18-2005).]

It does seem to cause a lot of confusion. Some places allow 310.15(B)(6) for most residential things, others do not. The argument I'd make for a garage or dwelling accessory building is what type of application do you think this is? Certainly, when you install a branch circuit in a garage or shed, you're not under commercial rules. No continuous lighting rules, no 180VA per receptacle. Many parts of NEC 220 spill dwelling rules over to attached/detached garages and accessory buildings on residential property.

The other part of 310.15(B)(6) that seems to cause questions is what is the main disconnect? If you have a 400A main disconnect, I would expect that you could run as many 100A or larger feeders out of that to various locations in a single dwelling and use table 310.15(B)(6) for each of them (as long as each panel was a lighting and appliance panelboard). I question how this is any different than having a 400A panel with 40 slots and feeders and branch circuits leaving it compared to the 400A disconnect that only has a single main breaker. To me, these feeders are still between the main disconnect and the L&A panelboards, there's just a lot of parallel overcurrent devices in the enclosure with the main disconnect.