sub-panel feeder code section?

Status
Not open for further replies.

charlie b

Moderator
Staff member
Location
Lockport, IL
Occupation
Retired Electrical Engineer
There are no special rules for sub panels. Use whatever table applies to the method of installation. Most of the time, that will be 310.16, but you didn't say whether the wires will go underground, or in cable tray, or in conduit.
 

cowboyjwc

Moderator
Staff member
Location
Simi Valley, CA
There are no special rules for sub panels. Use whatever table applies to the method of installation. Most of the time, that will be 310.16, but you didn't say whether the wires will go underground, or in cable tray, or in conduit.

And I didn't feel like asking.:grin: I also didn't ask if it was resi or commercial. If it's resi, depending on what you're doing, you may be able to use table 310.15(B)(6), but I doubt that's the answer.
 
The NEC does not recognize the term subpanel.

See: Article 408 Switchboards and Panelboards
Part III. Panelboards

408.36 Overcurrent Protection ('08 NEC)

240.4 Protection of Conductors
240.6 Standard Ampere Ratings
310.15 Ampacities for Conductors Rated 0-2000 Volts.
pay close attention to 310.15(B)(2) - and all of the (B) subsections

This should get you to Table 310.15 (B)(2)(c), 310.15(B)(6) or 310.16

Be aware that different wiring methods in Chapter 3 have certain restrictions when applying Conductor ampacity with the specific wiring method and location where it may be installed. Such as: Art 320, 334, and 338 wiring methods.
[see 320.80(A)- this has could have a bearing on feeders]


Yes, I have a break with all the madness today........;)
 

cowboyjwc

Moderator
Staff member
Location
Simi Valley, CA
John
I am not sure if I understand what you posted.
Would you mind teaching an old guy a new trick?

Sorry, I was just rereading that and I left our the dwelling part.

Let's say you have a 100 amp service and you want to ad AC and need to do an upgrade. You could put a 200 amp service next to the old 100 amp service and if all you do is refeed the 100 amp panel from the new 200 amp service, you can use table 310.15(B)(6) to size the conductors.

Make sense?
 
John
I am still confused by your post(s).

310.15(B)(6)
"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."

How are you applying the A/C to this Section?
 

iwire

Moderator
Staff member
Location
Massachusetts
Sorry, I was just rereading that and I left our the dwelling part.

Let's say you have a 100 amp service and you want to ad AC and need to do an upgrade. You could put a 200 amp service next to the old 100 amp service and if all you do is refeed the 100 amp panel from the new 200 amp service, you can use table 310.15(B)(6) to size the conductors.

Make sense?

No, if you are saying the new AC is coming out of the new 200 amp service you would have to use 310.16 for the conductors to the old panel.
 

cowboyjwc

Moderator
Staff member
Location
Simi Valley, CA
"....for the 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). The feeder conductors to a dwelling unit shall not be required to have an allowable ampacity rating greater than their service entrance conductors."

As long as the major load for the house still is on the subpanel, you can apply this section.
 

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
John, perhaps your area has an amendment to this section. I heard there were proposals for the 2011 code to do exactly as you mentioned but as of yet it is not code.
 

iwire

Moderator
Staff member
Location
Massachusetts
As long as the major load for the house still is on the subpanel, you can apply this section.

No, not the major portion, the entire load.

Not even one 15 amp circuit to a night light can be pulled off ahead of the feeder using 310.15.

This has been the case for many code cycles however in the 2008 NEC they made it clearer.:)
 

cowboyjwc

Moderator
Staff member
Location
Simi Valley, CA
The phrase "total current supplied by that service was removed in the 1990 NEC, which has the effect of permitting the use of the increased ampacitites for feeders.

Prior to the 1990 NEC, in Note 3, it was clearly indicated that the higher allowable ampacitites for residential occupancies using 3 wire single phase services also may be applied to feeders in those cases where the feeder conductors from the service equipment to the subpanel or other distribution point carry the total current supplied by the service conductors. Inclusion of the prhrase "that supply the total load to a dwelling unit" in the 1993 Code would not recognize use of the increased ampacitite given in Note 3 for a feeder, if, say, swimming pool circuits or other loads are supplied from the service, but are not carried by the feeder. Now, the revision in the 1996 NEC of that portion in Note 3 which now reads "the main power feeder to a dwelling unit", recognizes application of Note 3 for just that type of feeder.

Been in the code for a long time.
 

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
Now, the revision in the 1996 NEC of that portion in Note 3 which now reads "the main power feeder to a dwelling unit", recognizes application of Note 3 for just that type of feeder.
Been in the code for a long time.

I disagree John. Read the highlighted section. The main power feeder must carry all the loads associated with the dwelling. Thus you can have a main disconnect and feed an interior panel using T. 310.15(B)(6) as long as that feeder carries the entire load. Once an a/c circuit is installed in the main disco panel then that table does not apply. Bunch of crap but that is what it states. IMO

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.
 

iwire

Moderator
Staff member
Location
Massachusetts
John, I am not budging on this one, I can dig up the CMP statements if you would like. :)

The idea is to ensure the load diversity that is expected for an entire dwelling unit.
 

cowboyjwc

Moderator
Staff member
Location
Simi Valley, CA
John, I am not budging on this one, I can dig up the CMP statements if you would like. :)

The idea is to ensure the load diversity that is expected for an entire dwelling unit.

Oh I don't mind, I'm actually having fun with this one, no name calling so far.:) Right and also because the coductors almost never see full demand load approaching their ampacity

Dennis, that's what the change was supposed to cover in the '96 Code. Like I wrote "...If, say, swimming pool circuits or other loads are supplied from the service, but are not carried by the feeder."

"Revised in the '96 code, Note 3 now permits the main feeder to a dwelling unit to be sized based on the conductor sizes in the the Table for Note 3, even if other loads, such as ac units and pool loads, are fed from the same service." That's from the "96 handbook.
 
Status
Not open for further replies.
Top