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#96290 - 11/17/05 11:37 PM Smackdown: 310 vs. 334  
yaktx  Offline
Member
Joined: Feb 2003
Posts: 288
Austin, Texas, USA
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?


2017 / 2014 NEC & Related Books and Study Guides

#96291 - 11/17/05 11:45 PM Re: Smackdown: 310 vs. 334  
iwire  Offline
Moderator
Joined: Jan 2003
Posts: 4,391
North Attleboro, MA USA
You can use any of the wiring methods listed in the Table at the ampacities shown in the table. [Linked Image]


Bob Badger
Construction & Maintenance Electrician
Massachusetts

#96292 - 11/18/05 12:54 AM Re: Smackdown: 310 vs. 334  
yaktx  Offline
Member
Joined: Feb 2003
Posts: 288
Austin, Texas, USA
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 - 11/18/05 07:56 AM Re: Smackdown: 310 vs. 334  
shortcircuit  Offline
Member
Joined: Jun 2004
Posts: 608
massachusetts
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


#96294 - 11/18/05 03:50 PM Re: Smackdown: 310 vs. 334  
markp  Offline
Member
Joined: Apr 2004
Posts: 60
Kent, WA, USA
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
Kent, WA

#96295 - 11/18/05 04:06 PM Re: Smackdown: 310 vs. 334  
Roger  Offline
Member
Joined: May 2002
Posts: 1,716
N.C.
Mark, you are correct, table 310.15(B)(6) is very specific in the conductors and cables allowed.

Quote
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


Roger


#96296 - 11/18/05 05:57 PM Re: Smackdown: 310 vs. 334  
iwire  Offline
Moderator
Joined: Jan 2003
Posts: 4,391
North Attleboro, MA USA
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.


Bob Badger
Construction & Maintenance Electrician
Massachusetts

#96297 - 11/18/05 09:47 PM Re: Smackdown: 310 vs. 334  
yaktx  Offline
Member
Joined: Feb 2003
Posts: 288
Austin, Texas, USA
Quote
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?


#96298 - 11/18/05 10:00 PM Re: Smackdown: 310 vs. 334  
iwire  Offline
Moderator
Joined: Jan 2003
Posts: 4,391
North Attleboro, MA USA
Quote
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.


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


Quote
310.15(B)(6) brings up a whole host of gray areas. What exactly does "main power feeder" mean?


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


Quote
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).


I do not agree with the above.

Quote
What if the customer wants a 100A feeder to a detached woodworking shop? Is that a dwelling unit load?


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


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

Any thoughts?


I do not think it is anywhere near as 'gray' as you make it sound. [Linked Image]


Bob Badger
Construction & Maintenance Electrician
Massachusetts

#96299 - 11/18/05 10:54 PM Re: Smackdown: 310 vs. 334  
yaktx  Offline
Member
Joined: Feb 2003
Posts: 288
Austin, Texas, USA
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). [Linked Image]

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


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