Interpretation of 310.15(B)(6)

dema

Senior Member
This section states that conductors as listed in the table shall be allowed for service conductors that act as the main power feeder. But then they define the main power feeder to be on the load side of the disconnect. Has anybody had an issue with this? What happened?
 

charlie b

Moderator
Staff member
I don't understand why you think there might be an issue. You can use that article and its associated table for the service conductors (i.e., upstream of the main disconnect) or for the main feeder (i.e., downstream of the main disconnect).

By the way, it would help if you let us know what NEC edition you are using. I use the 2014, and that article has been renumbered. You must be on 2008 or earlier.
 

George Stolz

Moderator
Staff member
This section states that conductors as listed in the table shall be allowed for service conductors that act as the main power feeder. But then they define the main power feeder to be on the load side of the disconnect. Has anybody had an issue with this? What happened?
"Acting as" is not the same as "is". A feeder cannot exist on the line side of a service disconnect.
 

Little Bill

Moderator
Staff member
This section states that conductors as listed in the table shall be allowed for service conductors that act as the main power feeder. But then they define the main power feeder to be on the load side of the disconnect. Has anybody had an issue with this? What happened?
Not sure what you are asking but this covers both service wires and feeder wires such as a subpanel. The sub would have to supply the entire dwelling to be able to use the table though.
In short, to use the table, the wires (service or feeder) have to serve the entire load.
 

charlie b

Moderator
Staff member
Actually George, he misquoted the article.
This section states that conductors as listed in the table shall be allowed for service conductors that act as the main power feeder.
No it doesn't. It states that conductors, as shown in the table, are "permitted as . . . service-entrance conductors, service-lateral conductors, and feeder conductors that serve as the main power feeder. . . ." This quote is from the 2008 NEC.

 

augie47

Moderator
Staff member
The only "issue" that I have encountered is one where the E/C does not understand "main power feeder" and attempts to use this Table for situations where the feeder does not supply "all loads associated with the dwelling unit".
 

kwired

Electron manager
The only "issue" that I have encountered is one where the E/C does not understand "main power feeder" and attempts to use this Table for situations where the feeder does not supply "all loads associated with the dwelling unit".
That and it is easy to assume that it applies to all dwellings - when it in fact says "120/240 volt", so a typical multi-family application supplied by 208/120 can't use it either.
 

dema

Senior Member
Thank you

Thank you

The trouble arose because main power feeder is defined within the verbiage as being between the disconnect and the panel. Therefore, if there is no disconnect, does it still apply?

I think this code section is worded badly because of that. It states that it applies only for the main power feeder, refers to the service feeders as being among other things that constitute a main power feeder, and then says "For application of this section the main power feeder shall be between the main power disconnect and the panel it supplies" So they took back what they just said. Very clearly in fact.

My understanding is that in practice you have used that same wire size for the service feeder and had it approved and never thought about it again. That says a lot. Thank you very much.
 

kwired

Electron manager
The trouble arose because main power feeder is defined within the verbiage as being between the disconnect and the panel. Therefore, if there is no disconnect, does it still apply?

I think this code section is worded badly because of that. It states that it applies only for the main power feeder, refers to the service feeders as being among other things that constitute a main power feeder, and then says "For application of this section the main power feeder shall be between the main power disconnect and the panel it supplies" So they took back what they just said. Very clearly in fact.

My understanding is that in practice you have used that same wire size for the service feeder and had it approved and never thought about it again. That says a lot. Thank you very much.
When ever NEC says "for application of this section" you can disregard any general purpose or even art 100 definitions and go with whatever information that follows, but that statement is limited to the section it is a part of or refers to.

It does complicate things a little when they mention "power feeder" when "feeder" is a NEC defined term though.
 

roger

Moderator
Staff member
The trouble arose because main power feeder is defined within the verbiage as being between the disconnect and the panel. Therefore, if there is no disconnect, does it still apply?
Yes, because the disconnect would be the Main Breaker at the Service Point and the table would apply to the Service Entrance or Service-Lateral Conductors. If there is only one feeder leaving this panel and feeds a downstream panel that serves all the loads it could also use the table to size this feeder.

Roger
 

George Stolz

Moderator
Staff member
I'm not seeing the confusion. Service conductors exist on the line side of the service disconnect, and feeders exist on the load side. There is no such thing as a "feeder" that has no disconnect on its line side.
 

Dennis Alwon

Moderator
Staff member
Look at the definition of feeder in article 100. Now if that feeder feeds the entire load of the dwelling then it may use the table otherwise it must use the standard tables

Feeder. All circuit conductors between the service equipment,
the source of a separately derived system, or other
power supply source and the final branch-circuit overcurrent
device.
 

dema

Senior Member
The confusion is in the fact that they say that it applies to the service entrance conductors that serve as a main power feeder and then they say that for application of this section the main power feeder is between the disconnect and the panel - well, that is contradicting what they just said. The service conductor is NOT between the main disconnect and and the panel, it is on the line side. So first they say that the line side counts, and then they say that the table only applies to the main power feeder which is only on the load side.

If you read what they say literally, it makes your head spin off like those little toys that launch a little helicopter blade into the air.

Read what I mean not what I say?
 

Smart $

Esteemed Member
The confusion is in the fact that they say that it applies to the service entrance conductors that serve as a main power feeder and then they say that for application of this section the main power feeder is between the disconnect and the panel - well, that is contradicting what they just said. The service conductor is NOT between the main disconnect and and the panel, it is on the line side. So first they say that the line side counts, and then they say that the table only applies to the main power feeder which is only on the load side.

If you read what they say literally, it makes your head spin off like those little toys that launch a little helicopter blade into the air.

Read what I mean not what I say?
This is how the first part is broken down properly:

...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...
The main power feeder portion only applies to feeder conductors.
 

Dennis Alwon

Moderator
Staff member
Service conductor are not feeders. That is why I posted the definition of feeder. The service conductors are the conductors that go from the service to the first disconnecting means. From there there are branch circuits and feeders.

Think of it this way. If you have a meter with service conductors that go to a main lug panel (using the 6 handle rule) then those conductors can take advantage of the residential table. Now if you have one dp breaker in that panel and it feeds another panel that has all the loads in the dwelling then you can use the table for those feeder conductors also. However, if I add another breaker in the main panel , say for an a/c, then the feeder going to the sub panel cannot take advantage of the residential panel. Same is true if the service panel had a main.

We often have a feed thru main breaker panel 200 amp but we cannot take advantage of the resi table if we install any other breakers in the panel
 

kwired

Electron manager
Which NEC edition are you trying to figure out? At some point this topic moved to (B)(7) but I'm not sure which year or how much the wording had changed.

2014 has been completely re-written, the table is gone and the conductors are sized pretty much to 83% of what they would be in most other situations. It still gives you same conductor size as the table gave you in most instances.

Basic intentions of what is allowed have not really changed, just how they wrote it up is what has changed. It has always and still is intended to only apply to the main supply conductors of a dwelling unit, that carry the entire load of the dwelling, whether they be feeder or service conductors.
 
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George Stolz

Moderator
Staff member
The confusion is in the fact that they say that it applies to the service entrance conductors that serve as a main power feeder and then they say that for application of this section the main power feeder is between the disconnect and the panel - well, that is contradicting what they just said. The service conductor is NOT between the main disconnect and and the panel, it is on the line side. So first they say that the line side counts, and then they say that the table only applies to the main power feeder which is only on the load side.

If you read what they say literally, it makes your head spin off like those little toys that launch a little helicopter blade into the air.

Read what I mean not what I say?
(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.
I really don't get your confusion - it spells out what a feeder is in the section itself. It can apply to service conductors. It can also apply to main power feeder conductors. It never says the two are the same thing.
 

GoldDigger

Moderator
Staff member
I really don't get your confusion - it spells out what a feeder is in the section itself. It can apply to service conductors. It can also apply to main power feeder conductors. It never says the two are the same thing.
The only moderately reasonable alternate reading I can see is
"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 that does not change what applies to feeder conductors.
 

infinity

Moderator
Staff member
A related question, are you required to use that table or is it optional? Would #3/0 copper MC cable for a 200 amp service need a larger EGC since it's increased in size from the #2/0 listed in that table?
 
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