Sub Panel Conductors Sizing for Residential

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dixson

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Please verify if my thinking on this subject is correct.
It is my understanding that conductor rating listed in Table 310.15(b)(6) applies to wire sizing from the weatherhead, meter, first means of disconnect and the panelboards. But does this table apply to sub panels feeding from that panelboard? My think believe I should look to table 310.16 for ampacity. I have heard a electrician feed a 100A subpanel with #4 and declare it legal. Another will say that it must be #3 and another say #1.
 
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dixson said:
Please verify if my thinking on this subject is correct.
It is my understanding that conductor rating listed in Table 310.15(b)(6) applies to wire sizing for the weatherhead, meter, first means of disconnect and the panelboards. But does this table apply to sub panels feeding from that panelboard? I have heard a electrician feed a 100A subpanel with #4 and declare it legal. Another will say that it must be #3 and another say #1.
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Table 310.15(B)(6) is only for the entire load to a dwelling. A subpanel would require you to use table 310.16 to size the feeder conductors.

310.15(B)(6) 120/240-Volt, 3-Wire, Single-Phase Dwelling Services and Feeders. For individual dwelling units of one-family, two-family, and multifamily dwellings, conductors, as listed in Table 310.15(B)(6), shall be permitted as 120/240-volt, 3-wire, single-phase service-entrance conductors, service-lateral conductors, and feeder conductors that serve as the main power feeder to each dwelling unit and are installed in raceway or cable with or without an equipment grounding conductor. For application of this section, the main power feeder shall be the feeder between the main disconnect and the panelboard that supplies, either by branch circuits or by feeders, or both, all loads that are part or associated with the dwelling unit. The feeder conductors to a dwelling unit shall not be required to have an allowable ampacity rating greater than their service-entrance conductors. The grounded conductor shall be permitted to be smaller than the ungrounded conductors, provided the requirements of 215.2, 220.61, and 230.42 are met.
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I've heard it needs to carry the whole calculated load for the main service disco.
I think part of the reason the NEC board reduced the ampacities.
Is because so many electricians we're pushing feeders to sub panels to the limit.

I have seen so many 2800 sq. ft. house sub panels feed off #2 Al. With all the entire branch circuits on it, and the dryer.

IMO that's what caused this change any way.
 
I am looking in the 2005 NEC and I don't see "all loads that are part or associated with the dwelling unit." Are you quoting from the 2008?
If connecting a 100A Sub Panel would the conductor sizing also have to comply with 110-14(c) regard equipment rating 100 amperes or less?
 
dixson said:
I am looking in the 2005 NEC and I don't see "all loads that are part or associated with the dwelling unit." Are you quoting from the 2008?
If connecting a 100A Sub Panel would the conductor sizing also have to comply with 110-14(c) regard equipment rating 100 amperes or less?
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Yes, this is new wording for the 2008 but the intent has always been there for the table to be used only for the entire dwelling load so the requirement is not new.

Yes, you would need to comply with 110.14(C) but if your conductors and terminations are all rated for 75 degrees then you may use the 75 degree column in 310.16. What is the wiring method? NM and SE cable would require the use of the 60 degree column in 310.16.
 
It does say feeder, it says "the main power feeder'

In my mind it is clear that only a conductor that supplies all the power can be a "main power feeder". A feeder to a sub panel does not usually supply all the power so the feeder to it can not be a "main power feeder"

Now that said, many smart people feel differently, enough that the CMP has now changed the wording to make it clearer. :smile:
 
... You are not alone ...

... You are not alone ...

You guys are not alone here? they have called the revised language in the 08 as a small clarification change to make the existing intent more understood? (? or something to that effect?)

It is pretty hard (for me?) to look and the previous language and see how one would actually come up with anything else? (I know many who interpreted previous language this way, including me?)

I guess, now we know for sure :) ?

mweaver
 
iaov said:
So what do you do for a 100 amp sub panel Bob?
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To be honest by the time I was making those decisions I was working commercial.

100 amp feeder = 3 AWG copper or larger
200 amp feeder = 3/0 AWG copper or larger
225 amp feeder = 4/0 AWG copper or larger
400 amp feeder = 500 AWG copper or larger
 
Those look like 310.16 numbers. I guess I won't lose to much sleep ovwer the #4 I've got out there, but I will be mending my ways. This is the point where I feel like I should start this post with "Bless me Father Iwire for I have sinned.":D Thanks for the info Bob.
 
but....................

but....................

then.... you best check to see if the lugs on your subpanel will accept the 1/0. :smile:

That is a problem we have encountered on requiring 300 kcmil AL on a 200 amp "sub" panel.
 
I encourage electricans to use the corrrect terms. If you want to find the requirements for a sub-panel, where do you look?
 
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