Derating SE type cables for service

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howardrichman

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On a recent meeting w/a local inspector, we were discussing a service upgrade i'de be doing in the near future, and mentioned if a sub panel is used, and installing a SER cable, i'de have to refer to table 310-16 using the 60C colum. He refered me to art. 334.80 ampacity for NM cable, but also is affected by art.338 for SE cable, but couldn't find it, only permitted and not permitted uses. I've known that Once leaving the main distribution panel; any cable; even a sub panel line when installed, table 310-15(B)(6) is not applicable, and used the 75C colum. According to the inspector, I'de have to use 4/O alum ser for a 150amp sub panel from the main disconnect. I'de thought the sub panel conductors from that main disconnect is still sized as part of the main service.
Any Help;
Howard Richman
 
infinity

infinity

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Take a look at 338.10(B)(4)(a). Once this becomes a interior feeder you need to size the cable according to the 60? column of 310.16.
 
Dennis Alwon

Dennis Alwon

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Howard I would advise you to read, very carefully, Article 310.15(B)(6). The feeder must carry the entire load of the house in order to use T. 310.15(B)(6).

IMO, the 60C rating of the feeder , in this case, would not matter. Others will disagree.
 
infinity

infinity

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Dennis Alwon said:
Howard I would advise you to read, very carefully, Article 310.15(B)(6). The feeder must carry the entire load of the house in order to use T. 310.15(B)(6).

IMO, the 60C rating of the feeder , in this case, would not matter. Others will disagree.
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I agree, and I should have included the part of the 310.15(B)(6) regarding the entire load as Dennis has mentioned.
 
augie47

augie47

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Good morning, Dennis :)
Howard, I started that way as Dennis knows I'm one that holds an opposing point of view. To some of us, the wording of 338.10 applies to ALL feeders, even "whole house feeders" and once the SE becomes an interior wiring method the 60? rating is applicable.
You can search here for "338.10" and review both sides of the argument.
I think I'm correct in stating almost all inspectors using the '08 Code hold you to 60? on interior SE installs other than whole house feeders and some hold you to that on all feeders so it can be a local issue.
(TN inspectors are about 50/50 on this..we have waited two years for an "official interpretation" from our chief AHJ)
 
C

CopperTone

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what is the difference?

what is the difference?

I understand the issue and change in 2008 code that if it doesn't feed the entire load then you must use 310.16 column C but here is my question that just seems to make too much sense in my head.

If I were doing a 100 amp service and I used 2-2-2 aluminum SEU from the meter to the panel, that is legal according to 310-15B. Now if I did a 200 amp service and then installed a 100 amp sub panel across the room feeding from the main 200amp panel I would have to use 310.16 60 degree C column to size the wire feeding the sub panel - which means I would need to use 1/0 aluminum SER.

My question is - why is # 2 good for 100amps on one but not good for 100 amps on the next installation? (other than - thats what the code says) wire size only knows at what amperage draw it will melt at - it doesn't know what application is is being used in. This seems dumb to me - can anyone explain this to me rationally that makes sense? maybe I am dumb and just don't see the obvious. And - is this going to stay in the book for the next code cycle?
 
Dennis Alwon

Dennis Alwon

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I will try and answer the above question.

The reason that we are allowed to use T. 310.15(B)(6) for residential when it carries the entire load is because there is much diversity of loads in a dwelling. What I mean is that lights are on and off and current demands are not as constant as commercial applications are. This being the case we are given a bit of a break with T. 310.15(B)(6).

Now when you change to a 200 amp service and have half the residential load taken out then the diversity changes and it will affect the load differently. Thus the need to use T310.16 for other loads.

At times it is ridiculous esp. when you take one circuit like an a/c unit out of the main feeder which then makes that feeder subject to 310.15(B)(6) rather than 310.16.

Did I make sense?
 
jumper

jumper

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Dennis Alwon said:
I will try and answer the above question.

The reason that we are allowed to use T. 310.15(B)(6) for residential when it carries the entire load is because there is much diversity of loads in a dwelling. What I mean is that lights are on and off and current demands are not as constant as commercial applications are. This being the case we are given a bit of a break with T. 310.15(B)(6).

Now when you change to a 200 amp service and have half the residential load taken out then the diversity changes and it will affect the load differently. Thus the need to use T310.16 for other loads.

At times it is ridiculous esp. when you take one circuit like an a/c unit out of the main feeder which then makes that feeder subject to T310.16 rather than T310.15(B)(6).

Did I make sense?
Click to expand...

Fixed it my way.:)

Yes, you made sense.
 
H

howardrichman

Senior Member
CopperTone said:
I understand the issue and change in 2008 code that if it doesn't feed the entire load then you must use 310.16 column C but here is my question that just seems to make too much sense in my head.

If I were doing a 100 amp service and I used 2-2-2 aluminum SEU from the meter to the panel, that is legal according to 310-15B. Now if I did a 200 amp service and then installed a 100 amp sub panel across the room feeding from the main 200amp panel I would have to use 310.16 60 degree C column to size the wire feeding the sub panel - which means I would need to use 1/0 aluminum SER.

My question is - why is # 2 good for 100amps on one but not good for 100 amps on the next installation? (other than - thats what the code says) wire size only knows at what amperage draw it will melt at - it doesn't know what application is is being used in. This seems dumb to me - can anyone explain this to me rationally that makes sense? maybe I am dumb and just don't see the obvious. And - is this going to stay in the book for the next code cycle?
Click to expand...
.

Well; what your saying is what I've always known and done, until recentely a local inspector informed me that the main feeder from a main disconnect to the 1st panel(main distribution panel) must ALSO be rated @ the 60C column of 310-16, and cannot use tbl 310-15(B)(6). As someone here pointed out; interior installations of SE cable- 338-10(4)(A) refers me to art. 2 of art. 334 for NM cable, that SE cable must comply to interior useage and derating as NM. I'de need a 4/o seu for a 150amp feeder to the (main)sub panel for my service upgrade. Howard Richman
 
infinity

infinity

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howardrichman said:
.

Well; what your saying is what I've always known and done, until recentely a local inspector informed me that the main feeder from a main disconnect to the 1st panel(main distribution panel) must ALSO be rated @ the 60C column of 310-16, and cannot use tbl 310-15(B)(6). As someone here pointed out; interior installations of SE cable- 338-10(4)(A) refers me to art. 2 of art. 334 for NM cable, that SE cable must comply to interior useage and derating as NM. I'de need a 4/o seu for a 150amp feeder to the (main)sub panel for my service upgrade. Howard Richman
Click to expand...

If you had 150 amp, #2/0 Al, SE cable feeding the service disconnect why would you need a larger conductor feeding the panel that is carrying the full load of the building?
 
reddy kw

reddy kw

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Mudville, upstate New York.
CopperTone said:
"... if I did a 200 amp service and then installed a 100 amp sub panel across the room feeding from the main 200amp panel I would have to use 310.16 60 degree C column to size the wire feeding the sub panel - which means I would need to use 1/0 aluminum SER."
-Assuming service entrance equipment is in a house, with cables run thru joists,etc, and pink fiberglass btwn. joists. Cables aren't in free air. No air circulation around cables means heat builds up, so we derate.
...why is # 2 good for 100amps on one but not good for 100 amps on the next installation?"
-Service entrances are separated. SE cables are often in conduit, come from outside the building, and have short run from meter to distribution panel. NO adjacent cables communicate heat to it. #2 SEC used for 100 amp service isn't subjected to ambient heating. #2 NMC used for heavy branch circuit or sub-panel feeder inside of house is.
"... wire size only knows at what amperage draw it will melt at - it doesn't know what application is is being used in."
-Exactly. When #2 wire is bundled, run thru joists up close to lots of other wires, covered with fiberglass insulation, it will melt at lower amperage because there's trapped heat.
" ...can anyone explain this to me rationally that makes sense? maybe I am dumb and just don't see the obvious."
Seriously, I don't understand all the physics and stuff. I'm sure I'm dumber then you, so I just follow the code because some smart fellas figured it out and put it in a book for me.:D
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howardrichman

Senior Member
OK; Just got pointed to 31O-15(B)(6), and what I missed is that this table also refers to the main power feeder between the main disconnect and the panelboard that provides branch circuits to a dwelling unit. 2/0 for 150 amp will fine as usuall. If a 2nd sub panel leaving the main distribution panel is installed, I'de have to refer to table 310-16 @60c, and have to make choice to either install a derated SER cable, or install smaller sized conductors from 310-15(B)(6)in a conduit. I think the inspector was just stresing out still many electricians were still using #2 SER leaving main panel going to a 100 amp sub panel, and if failed, just stick a 90amp breaker in place, but @60c, would only be good for 75amp(70A brkr).

Howard richman
 
augie47

augie47

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Howard,
You are right on top of the situation.
Some inspectors/jurisdcictions feel 310.15(B)(6) trumps 338.10 and some feel the other way.
Unfortunately one has to find out which way the wind is blowing at the job location.
 
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