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?
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...
#96293 - 11/18/0507:56 AMRe: Smackdown: 310 vs. 334
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.
#96298 - 11/18/0510:00 PMRe: Smackdown: 310 vs. 334
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).]