Subpanels and 310.15(B)(6)


  #1  
Old 04-19-05, 03:01 PM
C
Member
Thread Starter
Join Date: Nov 2004
Location: USA
Posts: 926
Upvotes: 0
Received 0 Upvotes on 0 Posts
Subpanels and 310.15(B)(6)

OK, this is mostly my rambling (and a little grumbling), but I'd appreciate comments...


Getting ready to do some remodeling, and I'll be installing a 125A 12/24 Siemens subpanel, and feeding it with a 125A breaker in my 200A main. So...I call the electrical inspector to ask a few questions.

Having read the debates over whether 310.15(B)(6) applies to subpanel feeders, and taking the view that it does (see comments below), I ask the inspector "can I feed this panel using #2 copper?". AHJ says "No, you have to use #1 copper or 1/0 Al". I think, OK, obviously he doesn't agree on this tables applicability to subs. But I'm not gonna argue. He suggests I use 1/0 AL SER. OK, fine, I can do that.


Here's where it gets weird. I check my NEC2005 Pocket Guide, and look at 310.15(B)(6). #2 copper = 125A...(which is why I had asked the inspector if I could use it). He says use #1 Cu, which, on this table is 150A. So, out of curiosity, I look down the Aluminum column where I find the 1/0 Al he suggested I use...125A. I can't use #2 copper, but I can use 1/0 Al, yet both are rated 125A!?!

I assume he is requiring use of 310.16, where 1/0Al is rated 100/120/135 at 60/75/90 degrees C respectively, #1 Cu is 110/130/150, and #2 Cu is 95/115/130. Would that seem like what he is using? If so, what temperature rating is usually used for feeders?


----My argument that 310.15(B)(6) should apply to subpanel feeders-----

Per NEC2005: "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)."
The title of table 310.15(B)(6) is: "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"

Since "panelboard" can be plural, subpanels should apply, so long as their feeder originates from the panel that contains the "main disconnect".


----OK, Now, Arguing Against myself----------
Title of 310.15(B)(6) specifically says "3-wire". Subpanel feeds must be four wire (or is the ground (EGC?) not being counted as a conductor?).

If subpanel feeders are included, type SER should be listed, since it commonly (I think) feeds subpanels.
 
  #2  
Old 04-19-05, 03:26 PM
J
Member
Join Date: Sep 2000
Location: United States
Posts: 17,733
Upvotes: 0
Received 1 Upvote on 1 Post
This is debated a lot, and I know there are people on all sides of it, but I agree with your inspector. I do not believe that this qualifies as the main power feeder to a dwelling unit.

In most cases, you can use the 75-degree rating. The 90-degree rating almost never applies because very little equipment is sold with 90-degree terminations.

Subpanel feeders don't always need to be four-wire.

The NEC is not like the constitution where scholars pick apart every word. The AHJ tries to interpret the intent without scrutinizing every word like a lawyer might. In most cases, inspectors talk to each other and try to see what interpretation other inspectors are using.
 
  #3  
Old 04-19-05, 11:53 PM
Speedy Petey's Avatar
Banned. Rule And/Or Policy Violation
Join Date: Nov 2003
Posts: 2,262
Upvotes: 0
Received 0 Upvotes on 0 Posts
I am of the opposite opinion. I think any "feeder" is a main power feeder.

Look at the definition of "feeder".
Verbatim it says:
"The conductors between the service equipment and the FINAL branch-circuit overcurrent device".
Also is says these the main power feeders are the feeders between the main disconnect and the lighting and appliance branch circuit panelboard(s).

Sounds like the cable feeding a sub-panel to me.
Wouldn't a sub-panel likely contain the final overcurrent devices?

If they didn't mean sub-panel feeders they would not have specifically applied this to "Service entrance conductors, service lateral conductors and feeder conductors. If all they intended to use this section for was only the SE conductors feeding the main disconnect they would have not included "feeder conductors" in this list.

Also it states feeders can be with or without ground. This implies 3 or 4 wire feeders so it makes the issue even more contrary. Also, like said, it is possible to have a 3-wire feeder with a ground.
Back to the issue that 12/2NM w/g has the same conductors as 12/3SO cord.

It also specifically says; "For application of this section..." and uses the same definition as the Art. 100 definition.


Again, even most inspectors see it this way from what I hear. I know all near me do. I have never been tagged for using #2al SER for a 100a sub-panel feed. It is SOP in my area.
 
  #4  
Old 04-20-05, 12:53 AM
C
Member
Thread Starter
Join Date: Nov 2004
Location: USA
Posts: 926
Upvotes: 0
Received 0 Upvotes on 0 Posts
Thanks for the responses John & Speedy. I think this is an interesting debate. It seems like the NEC would settle this once and for all by specifying one way or another. To me though, that "(s)" on the end of "panelboard" implies more than one panelboard, and that implies subpanels.


Now, assuming my AHJ requires use of 310.16, and the 75° column, which rates 1/0 Al at 120A, why allow it for 125A? Is it just "close enough"? I believe I've read something in the code on this, but I don't recall exactly what. That same table rates #2 Cu at 115A in the 75° column. So 120A is close enough, but 115A is not? Now, I'm looking at 2005 NEC, and I'm not sure whether we are under it yet or still under 2002 (I know we are at least there as they require AFCI breakers for bedrooms). Is it possible the numbers on these tables have changed from one book to the other?


Speedy: In your use of 310.15(B)(6), would you apply it to feeders from one sub-panel to another sub-panel? I would wonder about that application since those feeders do not originate from the location of the "main disconnect". I suppose they are still "between" it and that branch-circuit panelboard, though not directly between it. Of course, the NEC doesn't specify that either.


[RAMBLING]
BTW: For me, this discussion is more academic than anything. I'm not about to debate my AHJ. I wanted to use copper because I'm more comfortable with it, but #1 copper is VERY hard to find in THHN, and almost non-existant in SER. 1/0 Al SER is just slightly more expensive than #2+#2+#4+#6 Cu THHN in conduit that I originally planned to use (which was only cheaper since I already had the conduit). I found a couple of local supply stores that had #1 Cu, and they were higher on it than HD was on 1/0 Cu (local HD only sells #1 Cu as special order, and only by 500' spool ). First elec. supply store I called didn't have #1 Cu, and wanted $1.86/ft for 1/0! (Compare to .99 at HD.) Funny thing is that they are cheapest on 1/0 Al SER. Go figure.
[/RAMBLING]
 
  #5  
Old 04-20-05, 03:02 AM
J
Member
Join Date: Sep 2000
Location: United States
Posts: 17,733
Upvotes: 0
Received 1 Upvote on 1 Post
When the ampacity of a feeder falls between two standard breaker sizes, code allows you to round up.

I don't quibble with the definition of "feeder". I quibble with the definition of "main" (which is not defined in the NEC). The NEC has more ambiguous codes than many people realize. It can take an agonizingly long time for the committee to clear them up. In the mean time, we have debates.

In another forum, Speedy had a now-famous debate about whether you could run NM in conduit. After getting vigorously and harshly beaten up over it, it appears that the clarifications in the 2005 code prove Speedy correct.

Such is life.
 
  #6  
Old 04-20-05, 03:09 AM
T
Member
Join Date: May 2001
Location: Dry Side of Washington State
Posts: 685
Upvotes: 0
Received 0 Upvotes on 0 Posts
What does it say in the 2005 NEC about this issue? Any changes, etc.?

We're still following the 2002 NEC.
 
  #7  
Old 04-20-05, 03:57 AM
C
Member
Thread Starter
Join Date: Nov 2004
Location: USA
Posts: 926
Upvotes: 0
Received 0 Upvotes on 0 Posts
Originally Posted by John Nelson
When the ampacity of a feeder falls between two standard breaker sizes, code allows you to round up.

Which makes it all the more annoying that the AHJ won't allow me to use #2 Cu. 115A rounds to 125A.
 
  #8  
Old 04-20-05, 04:28 AM
J
Member
Join Date: Nov 2002
Location: SC
Posts: 156
Upvotes: 0
Received 0 Upvotes on 0 Posts
Originally Posted by chirkware
Which makes it all the more annoying that the AHJ won't allow me to use #2 Cu. 115A rounds to 125A.
Correct, since its less than 800A and the ampacity of the conductors does not correspond with the standard rating of a fuse or breaker. The closest standard breaker below 125A is 110 A, so I think #2 would be acceptable but ONLY if you can show the calculated load to be more no more than the ampacity of the conductor on noncontinuous loads. I read 310;15(B)(6) to use the sizes listed going to a sub panel only if the sub is rated the same as the main, such as a 200A sub fed off a 200A main, as the feeder conductors are not required to have an ampacity over that of the service conductors. Emphasis on this is the word Main disconnect, not main panel. We can use 500mcm thhn here on 400A services providing the calculated load is no more than 380A.
 
  #9  
Old 04-20-05, 06:33 AM
J
Member
Join Date: Sep 2000
Location: Fayetteville, NY, USA
Posts: 969
Upvotes: 0
Received 2 Upvotes on 2 Posts
I extend the definition of "main" disconnect to specifically imply your "service equipment". Service equipment is the box assembly containing the first disconnecting means downstream from your meter. In most cases in residential, the main disconnecting means is in the main circuit breaker panel with all the branch breakers. I think it would clarify the whole issue if it said "the main power feeder shall be the feeder(s) between the [service equipment] and the lighting and appliance branch-circuit panelboard(s)".

However, by saying "panelboard(s)" it seems implicit that you can use this table to feed more than 1 of them, in which case it is not the "main" power feeder any longer, but one of perhaps several.

Nevertheless, I agree that this is merely acedemic, and like Chirkware I would not argue with the AHJ because s/he is going to win.

It is always interesting, and also fun, to discuss Code interpretation. (Actually, I DO have a life!) Great post and great thread.

Juice
 
  #10  
Old 04-20-05, 09:01 AM
P
Member
Join Date: Feb 2002
Location: port chester n y
Posts: 1,983
Upvotes: 0
Received 0 Upvotes on 0 Posts
Please be informed that the Feeder conductors protected by a 125 amp C-B in the Service panel does not qualify for the ampacities listed in Tbl.310.15 (B)(6) because the conductors are "between" the 125 amp C-B and the Main Lugs of the S-P; they are not between the ( in your case) 200 amp Main C-B which also serves as the Service Dis-connecting Means, and the Main Lugs of the additional S-P.

The intent of the Art. is to allow interior Feeder Conductors that terminate on the Load-side of the Service Dis-connect at the "supply" end of the Feeder to conform to the same size as the Service Entrance Conductors that terminate on the Line-side of the Service-Disconnect.

Good Luck & Enjoy the Experience!!!!!!!!!!!
 
  #11  
Old 04-20-05, 09:26 AM
C
Member
Thread Starter
Join Date: Nov 2004
Location: USA
Posts: 926
Upvotes: 0
Received 0 Upvotes on 0 Posts
I could see a lawyer having a field day with this one. With what PATTBAA says, the definition of "between" comes into play. I think I could make an arguement that these feeders are indeed "between" the main and the (sub-)panelboard, but who wants to have to hire a lawyer to talk the AHJ into letting him save $0.50 a foot on feeders?


Thanks for all the comments on this. I've really enjoyed the discussion. Now, for a practical question or two:

If I go with the 1/0 Al SER as suggested by my AHJ, what steps do I need to make in assuring my terminations qualify as 75°? I know the Al needs a compound used on it, and I know the terminations are to be torqued to a certain number (I assume the breaker/lugs will specify that number).

Whats a good compound to use?

Do I simply borrow a torque wrench like a mechanic might use for the tightening, or just (as I normally do) get it as tight as I can?

Last thing that just came to mind: Sub-panel is a Siemens 125A main lug load center, with copper buss bars. I'm gonna have to look at this when I get home, but this may mean that the main lugs are copper (or copper plated maybe). What extra problems does THAT pose with Al feeders connecting to the lugs?
 
  #12  
Old 04-20-05, 09:43 AM
J
Member
Join Date: Sep 2000
Location: United States
Posts: 17,733
Upvotes: 0
Received 1 Upvote on 1 Post
The terminations at both ends must be explicitly labeled as 75 degrees. Otherwise you must use 60 degrees.
 
  #13  
Old 04-21-05, 01:36 AM
J
Member
Join Date: Nov 2002
Location: SC
Posts: 156
Upvotes: 0
Received 0 Upvotes on 0 Posts
Any compound made for the app shuld be fine. Ideal, GB, ect. I use nolox (sp). I hate running al but I would stress that you return to the job after a week or so and retighten the connections, yu should be fine then. Unless the equip is listed and marked otherwise, If the terminal is rated 100A or less, or marked for 14awg - 1awg, figure 60 C. terminals rated over 100A, or marked for conductors larger than 1awg, figure 75 C.
 
  #14  
Old 04-21-05, 01:38 AM
J
Member
Join Date: Nov 2002
Location: SC
Posts: 156
Upvotes: 0
Received 0 Upvotes on 0 Posts
Also you should have no problem with the lugs, they should be listed for al/cu use. I don't use a torque wrench but if I did I would use it on my return trip 1 week later as you may find the connection has loosened.
 
  #15  
Old 04-21-05, 12:06 PM
R
Member
Join Date: Feb 2005
Posts: 23
Upvotes: 0
Received 0 Upvotes on 0 Posts
As a sidebar to this conversation: I thought a sub panel could not exceed 50% of the rated ampacity of the main panel. Is this correct or have I been fed erroneous info???? If this is true you would be capped at 100 amps for your sub panel not the 125 amp panel cited in the original post.
 
  #16  
Old 04-21-05, 12:17 PM
J
Member
Join Date: Sep 2000
Location: United States
Posts: 17,733
Upvotes: 0
Received 1 Upvote on 1 Post
I thought a sub panel could not exceed 50% of the rated ampacity of the main panel
I know of no such rule.
 
  #17  
Old 04-23-05, 01:38 AM
C
Member
Thread Starter
Join Date: Nov 2004
Location: USA
Posts: 926
Upvotes: 0
Received 0 Upvotes on 0 Posts
Finally got a chance to check the markings...

The lugs on the sub panel do not have a rating marked directly on them, but the panel label generically says "USE COPPER OR ALUMINUM 60°/75°" without specifying which connections that refers to, so I would assume it applies to all connections. The 125A breaker that will be in the main is rated "Al/Cu 60°/75°". So, I believe I should be good using 1/0Al SER.

No problem going back in a week to retighten...It's my house.

I mentioned before that the new sub has a copper buss. Upon further review, the lugs themselves are aluminum, and are bolted to the copper buss.


On the SER cable: Any hints/suggestions/warnings regarding handling it? It'll be running through a crawl space (definately NOT a living/storage/etc area), perpendicular to floor joists. Any physical protection requirements? Any special technique removing the outer sheathing on the 2-3 feet that will come into the panel boxes?


Thanks again guys for all the comments & help.
 
  #18  
Old 04-23-05, 04:37 AM
P
Member
Join Date: Feb 2002
Location: port chester n y
Posts: 1,983
Upvotes: 0
Received 0 Upvotes on 0 Posts
C-W plans on using a Wiring Method he specifies as ( Type) "SER".

Can anyone cite the specific Code Article for this type of Wiring Method??

Cheers to All, PATTBAA
 
  #19  
Old 04-25-05, 05:33 AM
J
Member
Join Date: Sep 2000
Location: Fayetteville, NY, USA
Posts: 969
Upvotes: 0
Received 2 Upvotes on 2 Posts
Article 338 discusses service entrance conductors. Although I have found no specific application listing for SER cable 338.2(B) is for branch circuits and feeders in the same Article, and it applies to SE cable with insulated grounded conductor (neutral). In it it says that for interior installations, to follow 334, which is the Article for NM cable. So the same rules apply except that 334.15(C) says that "it shall be permitted to secure cables not smaller than two 6 AWG or three 8 AWG conductors directly to the lower edges of the joists." This says you won't have to bore through the joists and run the cable through the centers. Particularly in a crawl space, it is unlikely you would be required to protect the cable from physical damage as required by 334.15(B).

Also, 338(B)(6) prohibits installation where "subject to conductor temperatures in excess of the temperature specified for the type of insulation involved." Check the rating.

Other than these basic rules, it is required to run non-metallic sheathed cables in dry areas per 334.10 unless specifically rated for corrosive areas, in which case it can be used in moist, damp and corrosive areas per 334.10(B). There is a big "unless", but it pertains to applications other than one and two family dwellings, which is what Chirkware's situation is I believe.

As far as stripping off the outer sheath, I do not know exactly where the Code says this, but when you use a cable-to-box connector (frequently referred to as a "romex connector"), the sheath must be fully covering the conductors where they pass through the cable clamp. Once inside the panel enclosure you are free to strip off the sheath. There may be a requirement to extend the sheath inside the box a ways, and it would be my guess that one inch will suffice.

Hope that helps.
 
  #20  
Old 04-25-05, 05:42 AM
J
Member
Join Date: Sep 2000
Location: United States
Posts: 17,733
Upvotes: 0
Received 1 Upvote on 1 Post
There may be a requirement to extend the sheath inside the box a ways, and it would be my guess that one inch will suffice.
Code minimum is 1/4".
 
  #21  
Old 04-25-05, 05:58 AM
J
Member
Join Date: Sep 2000
Location: Fayetteville, NY, USA
Posts: 969
Upvotes: 0
Received 2 Upvotes on 2 Posts
Thanks, John. Always better to exceed minimum where possible/practicable, but it is good knowing what the limit is.

Juice
 
  #22  
Old 04-25-05, 09:08 AM
P
Member
Join Date: Feb 2002
Location: port chester n y
Posts: 1,983
Upvotes: 0
Received 0 Upvotes on 0 Posts
Will the type cable he plans to use include the required EGC in the cable assembly?
 
  #23  
Old 04-25-05, 09:11 AM
C
Member
Thread Starter
Join Date: Nov 2004
Location: USA
Posts: 926
Upvotes: 0
Received 0 Upvotes on 0 Posts
Originally Posted by PATTBAA
Will the type cable he plans to use include the required EGC in the cable assembly?

The SER Cable I am planning to use is 1/0-1/0-1/0-2 Al...
 
  #24  
Old 05-19-05, 11:00 AM
J
Member
Join Date: Jan 2003
Posts: 100
Upvotes: 0
Received 0 Upvotes on 0 Posts
Originally Posted by John Nelson
I don't quibble with the definition of "feeder". I quibble with the definition of "main" (which is not defined in the NEC). The NEC has more ambiguous codes than many people realize. It can take an agonizingly long time for the committee to clear them up. In the mean time, we have debates.

In another forum, Speedy had a now-famous debate about whether you could run NM in conduit. After getting vigorously and harshly beaten up over it, it appears that the clarifications in the 2005 code prove Speedy correct.

I agree with John re: 310.15(B)(6). Petey and chirkware just mention feeder definitions and the type of panelboard served. They seem to argue that the inclusion of the term feeder implies this section covers all sub panel feeders. (If the panel is a lighting and appliance panelboard.) It seems they do not recognize the requirement that the feeder be the main power feeder to a dwelling unit. This would include feeders from a main service disconnect that is not contained in the dwelling unit’s panelboard, like in a rural setting with the main disco at the pole or in a multi family dwelling, but not feeders to sub panels in basements, garages, etc.
I understand that some inspectors will allow 310.15(B)(6) to be used for feeders to any panelboard, but in my opinion the language in that section does not.

I don’t think Petey was correct on the NM in conduit issue. Since the sentence in question, “Cables shall be permitted to be installed where such use is permitted by the respective cable articles”, was added to all the raceway articles in 2002 one can only assume there was intent for that sentence. The only intent I can see from the wording of that sentence is that permission must be granted for the use to be permitted. Therefore, the absence of permission would imply prohibition. And since, except for those portions required in 334.15(B), article 334 does not grant permission for NM to be installed in conduit that use would be prohibited.

Apparently, that was not the intent of the CMP responsible for adding that sentence. So for 2005 it was changed to “Cables shall be permitted to be installed where such use is not prohibited by the respective cable articles.” The only reason I can see for the change is that the previous wording did not convey the intent. Since wording is how intent is conveyed, Petey could not have known the intent differed from what the language stated.

So now we are back to where we were in 1999 and earlier.
Is NM intended to be installed in conduit?
Would any professional install it that way? (Petey has said he would not)
Can NM be installed in a conduit system without violating any of the rules in article 334?
 
  #25  
Old 05-19-05, 01:10 PM
Speedy Petey's Avatar
Banned. Rule And/Or Policy Violation
Join Date: Nov 2003
Posts: 2,262
Upvotes: 0
Received 0 Upvotes on 0 Posts
You can disagree with me all you like. That is what makes this country great.

Originally Posted by Juhl
It seems they do not recognize the requirement that the feeder be the main power feeder to a dwelling unit.
Where is this requirement written that the "main" power feeders are only the one set between the meter and main disconnect/panel?


I stand by the statements in my prior post:

"Look at the definition of "feeder".
Verbatim it says:
"The conductors between the service equipment and the FINAL branch-circuit overcurrent device".
Also is says these the main power feeders are the feeders between the main disconnect and the lighting and appliance branch circuit panelboard(s).

Sounds like the cable feeding a sub-panel to me.
Wouldn't a sub-panel likely contain the final overcurrent devices?


All inspectors in my area, as well as many others, agree so I have no reason to question this issue.


As far as the NM in conduit issue, I unequivocally stated I never use this practice. The discussion was purely a code discussion. If it is even suggested, I say it is an extremely contentious code issue and that no self respecting electrician would use this practice.
That is the last I will say on that.
 
  #26  
Old 05-19-05, 06:38 PM
J
Member
Join Date: Sep 2000
Location: United States
Posts: 17,733
Upvotes: 0
Received 1 Upvote on 1 Post
Since the sentence in question, “Cables shall be permitted to be installed where such use is permitted by the respective cable articles”, was added to all the raceway articles in 2002 one can only assume there was intent for that sentence. The only intent I can see from the wording of that sentence is that permission must be granted for the use to be permitted. Therefore, the absence of permission would imply prohibition. And since, except for those portions required in 334.15(B), article 334 does not grant permission for NM to be installed in conduit that use would be prohibited.
Just for fun, when you get a chance, read how this wording was changed in the 2005 code. It seems to me that the code committee was trying to clean up the mess they made with slopping wording in the 2002.

Where the 2002 says:
Cables shall be permitted to be installed where such use is permitted by the respective cable articles.
The 2005 says:
Cables shall be permitted to be installed where such use is not prohibited by the respective cable articles.
An interesting editorial change, wouldn't you say? I don't think they were intending to change the rule, but prevent the rampant misunderstanding that occurred with their poor choice of words in the 2002 code.

Draw your own conclusions.
 
  #27  
Old 05-20-05, 11:44 AM
J
Member
Join Date: Jan 2003
Posts: 100
Upvotes: 0
Received 0 Upvotes on 0 Posts
Originally Posted by Speedy Petey

Where is this requirement written that the "main" power feeders are only the one set between the meter and main disconnect/panel?

I never said that. In fact, the conductors you described, those between the meter and service disconnect, would not be feeders but service conductors. 310.15(B)(6) covers service conductors and feeder conductors that serve as the main power feeder to a dwelling unit. Again, it is not the definition of a feeder that I am arguing but what the feeder serves.
"To a" dwelling unit is not the same as "in a" dwelling unit. A basement (in a single family dwelling) or garage is not a dwelling unit.
 
  #28  
Old 05-20-05, 11:47 AM
J
Member
Join Date: Jan 2003
Posts: 100
Upvotes: 0
Received 0 Upvotes on 0 Posts
Originally Posted by John Nelson
Just for fun, when you get a chance, read how this wording was changed in the 2005 code. It seems to me that the code committee was trying to clean up the mess they made with slopping wording in the 2002.

I have read how the wording was changed for 2005, I included the new wording in my first post.

The wording in 2002 was conditional, it required permission to be granted for the use to be permitted.

The wording in 2005 permits unless prohibition is expressed.

Unless a conditional statement, like the sentence in 2002, is made the absence of prohibition implies permission. The change in 2005 nullified the sentence in 2002, I wonder why it was ever added.
 
 

Thread Tools
Search this Thread
 
Ask a Question
Question Title:
Description:
Your question will be posted in: