Am I Crazy?

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mjr

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Hi-

I didn't think this was an issue for me, but I've been having a debate with a coworker about conductor sizes on subpanels. I am under the impression that I can run a #2SER (AL) to a sub from a main panel, protected by a 100A breaker in a dwelling. My reasoning is that #2SEU (AL) is used all the time on 100A services, and I felt that Table 310.15(B)(6) backs me up. My coworker insists that my subpanel feeder must be protected by no more than a 70 breaker. Who is right and why?
Thanks in advance.
 
This has been argued on this web site before; with knowledgeable people on both sides of the argument.
I'll throw in my 2 cents and say.....
In residential dwellings..... Protecting a #2 aluminum feeder (to a lighting and appliance panelboard) with a 100A breaker is allowed; as long as the feeder originates (after the main) in the main breaker panel.
That's just my opinion.
steve
 
The 70A breaker would be required if this was not a residence and if the lugs at either end were 60 degree rated only. However, since this is a residence, then 310.15(B)(6) takes over and sets the allowed ampacity for the conductor size used. IMHO anyway!
 
IMO, if the feeder to this "sub-panel" is not a " main power feeder" as defined in 310.15(B)(2)(6) it can not use the reduction and must be sized per 310.16.

Roger
 
Oops! I stand corrected! I, for some reason, thought we were talking about an apartment complex and this was a feeder to a dwelling from a meter center. Sorry I didn't read the opening post carefully! Yes, I agree with Roger. I do not agree with the logic of the whole thing, but, technically, that is how it all reads to me! Thanks for the heads up Roger!
 
Code allows it... because it is highly unlikely to have more than 70 amps continuous load... just like a residential service. (Part of the reason PoCo is allowed even smaller service wires)
Just use your head... if you're connecting several large A/Cs to it that tend to run continuous during the highest ambient temperatures... you might want to run larger wires :)
 
I agree with Roger.

This feeder to a "sub-panel" is not a main power feeder and therefore you must use 310.16 to size your conductors.

Chris
 
310.15(B)(6) ... 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 panelboards(s). ...
Click to expand...
There are only 2 conditions that have to be met for a feeder to be a "main power feeder". If the conductors meet these two conditions, then the reduced size conductor is permitted. First, is the feeder connected on the load side of the service disconnect? Second, does the feeder supply a lighting and appliance branch circuit panel? If you can answer yes to both of these questions, then you can use the reduced size. Note that it is my opinion that the code wording does not reflect the intent of the rule. I believe that the intent of the rule is to only permit the use of the reduced sizes for feeders that carry the complete dwelling unit load, but the code wording does not at this time support that intent.
Don
 
This has been debated here before, and I don?t think we should debate it again. But let me restate my opinion and the basis for my opinion. Others should feel free to restate their opinion, but let?s not restart the debate. If anyone wishes to find that earlier debate and post a link to it, that would be welcome. I haven?t the time right now to look for it myself.

I believe that the code, as worded (and as Don suggests is the intent of the authors), does not permit the use of Table 310.15(B)(6) for a feeder from the main panel to a sub-panel. Such a feeder cannot be (my emphasis here) ?THE main feeder? to a dwelling unit. The only reason that sentence (quoted by Don) uses a plural is that it applies to two-family and multi-family dwelling units. The plural is not addressing the possibility of more than one panel within any single unit.
 
DaveTap said:
Code allows it... because it is highly unlikely to have more than 70 amps continuous load. . . .
Click to expand...
I don't think that that is relevant in any way. You base the conductor size on calculated load, not on the likelyhood that the load might exceed a given value.
 
Here is a typical existing in my little burb. Single family dwelling. 100 amp meter/main combo can, with no capacity to add another breaker. 1- 100 amp main breaker installed in that can. One #2 alum ser cable on the load side serving as a feeder to the one interior sub panel serving all lighting and appliance loads in the residence. (Mostly nowadays 200 amps, but I digress) My own opinion is that this is quite acceptable per 310.15 (B) 's intent. This one #2 feeder becomes the "main" feeder for the dwelling, even though it is on the load side of the main overcurrent protection means.
 
Mike, in your example the feeder does meet the definition of a " main power feeder" and is allowed to use the reduction.

After this first "Lighting and Appliance branch circuit panel" in your example the reduction could not be used.

Roger
 
charlie b said:
This has been debated here before, and I don?t think we should debate it again. But let me restate my opinion and the basis for my opinion. Others should feel free to restate their opinion, but let?s not restart the debate.
Click to expand...

Hear! Hear!
I will state my opinion below. I will not argue for it, and won't call you wrong if your interpretation of the code section is different. I am simply presenting this as food for thought.

1) Table 310.15(B)(6) may be used to size service conductors that serve an entire residence. For example, these may be used to size the conductors from the weatherhead to the main disconnect in an overhead service.
2) Table 310.15(B)(6) may be used to size feeders that carry the main load of the residence. For example, the conductors used between a meter-main and a single loadcenter.
3) Table 310.15(B)(6) may be used to size feeders that are the same size as the feeders above. For example, if a home has a 200A service, and then has a 200A feeder to the garage, then even though 310.15(B)(6) is not normally useable for a garage, the subfeed to the garage is not required to be larger than the 'main' power feed.

310.15(B)(6) may not be used for loads that are not the bulk of a residence. A residential garage is not a home. A residential pool house is not a home. 310.15(B)(6) may not be used for these feeders, even in a residential situation.

IMHO this section of code should be entirely replaced. As written the code section says that you can use certain size conductors for certain size residential applications. This is easily misinterpreted to mean that these conductors have higher ampacity or some such.

What this section of code says to me is that residential services which must supplied using _conductors_ of ampacity X may be protected with OCPD of trip rating Y, where Y>X. IMHO it would make more logical sense if the service and feeder calculations actually gave X and Y as separate values. Then you would simply use 310.16 to size the conductors to X, and select OCPD based upon Y.

-Jon

[edited to fix spelling and formatting error]
 
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Wow! Thanks for all the great input everyone! I didn't expect this to be such a hot topic.

It is interesting to note that many of you agreed that the intent of the rule was to allow the reduced size for dwelling service entrances only. However, one has to logically assume that no matter WHAT size sub panel is installed after that main disconnect, the reduced-size service entrance cables are carrying the load anyway. You would think that the rule would be the other way around, disallowing reduction of size for service entrances...
 
charlie b said:
I believe that the code, as worded (and as Don suggests is the intent of the authors), does not permit the use of Table 310.15(B)(6) for a feeder from the main panel to a sub-panel. Such a feeder cannot be (my emphasis here) ?THE main feeder? to a dwelling unit.
Click to expand...

I agree with Charlie and the others that feel there can be only one "main feeder".

Don and I touched on this very subject the other day.....we don't see it the same.
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I don't actually see a limit of one "Main Feeder", but I think the "Main Feeder" or "Main Feeders" must originate from the Main Service Disconnect OCPD, not downstream OCPD's.

Roger
 
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