Interpretation of 310.15(B)(6)

[Carultch]

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?

I wonder that answer myself. I don't even know why we are allowed to reduce the size of service conductors in the first place, in contrast from what it would need to be for a feeder conductor in the same situation.

 

[Little Bill]

I wonder that answer myself. I don't even know why we are allowed to reduce the size of service conductors in the first place, in contrast from what it would need to be for a feeder conductor in the same situation.

A feeder can be reduced as well as long as it serves the load on the entire dwelling.

 

[Carultch]

A feeder can be reduced as well as long as it serves the load on the entire dwelling.

Understood. It makes sense that if there is no other current on the feeder, than the current on the service conductors, that they should not need to be any larger than the service conductors for ampacity reasons.

My point is the entire concept is mysterious that services have the 83% reduction factor in the first place, compared to sizing feeders in general. Such that a 200A service only needs 166A worth of conductor ampacity, while a 200A feeder originating from a much larger service would need the full 200A worth of conductor ampacity.

 

[Smart $]

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?

It is optional, as "shall be permitted" means in addition to being allowed, it is not required.

Given that it is optional, I do not consider a 3/0 Cu conductor as upsized... unless I would have made it 2/0 if not for a voltage drop issue. This would be one of those times where you must question your own integrity... or not.
:happyyes:

 

[infinity]

I could see it being argued that with the new wording in 250.122(B) of the 2014 NEC "from the minimum size that has sufficient
ampacity for the intended installation" that T310.15(B)(6) could be considered the starting point for the minimum ampacity.

 

[kwired]

Understood. It makes sense that if there is no other current on the feeder, than the current on the service conductors, that they should not need to be any larger than the service conductors for ampacity reasons.

My point is the entire concept is mysterious that services have the 83% reduction factor in the first place, compared to sizing feeders in general. Such that a 200A service only needs 166A worth of conductor ampacity, while a 200A feeder originating from a much larger service would need the full 200A worth of conductor ampacity.

At some point someone has determined that the load diversity on a dwelling is great enough that the supply conductors never see more then 83% of their capacity - but this is for supply conductors to the entire dwelling. Don't ask me how they determined it. From my experiences we can often get by with even smaller conductors then that, but we do need a 'what if' factor in there as well.

If you do have a 100 amp dwelling supply (sized per art 220 calculations) loaded to 83 amps or more - it usually doesn't last for too long.

 

[jumper]

At some point someone has determined that the load diversity on a dwelling is great enough that the supply conductors never see more then 83% of their capacity - but this is for supply conductors to the entire dwelling. Don't ask me how they determined it. From my experiences we can often get by with even smaller conductors then that, but we do need a 'what if' factor in there as well.

If you do have a 100 amp dwelling supply (sized per art 220 calculations) loaded to 83 amps or more - it usually doesn't last for too long.

The basis for the 120/240V residential calculations/table come from historical data provided by POCO's on their customers usage.

 

[Dennis Alwon]

At some point someone has determined that the load diversity on a dwelling is great enough that the supply conductors never see more then 83% of their capacity - but this is for supply conductors to the entire dwelling. Don't ask me how they determined it. From my experiences we can often get by with even smaller conductors then that, but we do need a 'what if' factor in there as well.

If you do have a 100 amp dwelling supply (sized per art 220 calculations) loaded to 83 amps or more - it usually doesn't last for too long.

Basically 83% results in the same size as the tables so the 83% is easy. The question is how did he table come about originally and who decided one size down is good enough. I guess it is possible that the table was orig. based on 83% IDK

 

[kwired]

Basically 83% results in the same size as the tables so the 83% is easy. The question is how did he table come about originally and who decided one size down is good enough. I guess it is possible that the table was orig. based on 83% IDK

So is it 83% or one size down? Nearly all cases though it is both, and I'd guess also ties in somehow with values in the table used in older codes.

The thing that has changed is it is now more clear how to do ampacity adjustments if needed to these conductors then it was with the table, otherwise if you leave adjustments out of the picture 83% or one size down will almost always give you same size conductor as the old table did.

 

[charlie b]

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?

If there is no disconnect, then you are still dealing with the service conductors, not a feeder.

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.

This is where you are reading the article wrong. They revised the wording in the 2014 NEC, and I think that made it more clear. But in 2011, it says that you can use the table for three things:

• Service entrance conductors,
• Service lateral conductors, and
• Feeder conductors that serve as the main power feeder. . . .

The word "that" only applies to the words immediately preceding it, not to the entire string of three things (separated by commas) that preceded it. To be explicitly clear, the NEC is NOT saying that we can use the table for these three things:

• Service entrance conductors that serve as the main power feeder,
• Service lateral conductors that serve as the main power feeder, and
• Feeder conductors that serve as the main power feeder. . . .

 

[charlie b]

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"...

As I mention in my previous post, that interpretation would be a violation of the rules of English grammar. But I do believe that that is the basis for the confusion we are seeing in this thread.

 

[jumper]

Nice explanation Mr.B:thumbsup:

 

[Smart $]

I could see it being argued that with the new wording in 250.122(B) of the 2014 NEC "from the minimum size that has sufficient
ampacity for the intended installation" that T310.15(B)(6) could be considered the starting point for the minimum ampacity.

Argued, of course... the argument being minimum required size vs. minimum permitted size. With Code not specific to that degree, there's no resolution without opinion or agenda entering the decision.