310.15(B)(6) Main Power Feeder.

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charlie b

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don_resqcapt19 said:
If a feeder is not part of the supply to the building, why was it installed?
It's a supply within, and not to, the building. The supply to the building comes from outside the building.
 

don_resqcapt19

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Charlie,
It's a supply within, and not to, the building. The supply to the building comes from outside the building.
So this table can never be used if the service disconnect is in the building. Also even where the service disconnect is installed out side on the building, the service is already "to" the building, so we can never use this table unless the service disconnect is remote from the building.
Don
 

suemarkp

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charlie b said:
No feeder that begins at a panel within the dwelling unit and that serves another panel in the same dwelling unit could be considered a "feeder to" the dwelling unit. Rather, it's a "feeder within" the dwelling unit.

So what you're saying is this:

I can have a 400A meterbase on the outside of the house feeding two 200A panels inside the house. Since these are all service conductors we can use 310.15(B)(6) (400KCMIL cu to meter, 2/0 cu from meter to each panel).

Now lets put in a 400A metermain instead of just a meterbase. This meter main will have two 200A breakers and will feed two 200A MLO panels inside the house. Now 310.15(B)(6) could not be used because the wires to the panels are feeders, but they are not feeders TO a dwelling just feeders IN a dwelling.

Seems kind of strange to allow the first case but not the second. All that changed is where the breakers were located.
 

charlie b

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I'm not being clear. It does not matter where the disconnect is. If power is coming from the utility (via the disconnect) then it is power to the building. If power is coming from one panel within the building to another panel within the building then it is not power to the building. It has to be power to the building as a whole. It has to be the main power feeder to the building, not a sub power feeder within the building.

If you go from the main disconnect to one panel, then the feeder to that panel carries all the loads in the building. If you then go from the first panel to the second panel, then the feeder to the first panel will carry some load that the feeder to the second panel will not carry. There is no way you can call the second feeder the "main power feeder" to the building.
 

charlie b

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suemarkp said:
Seems kind of strange to allow the first case but not the second.
I think the wording, as written, allows the use of the table for both of your cases. That is because in both cases power is coming from the utility to the building. You are not yet past the "main power feeder" (i.e., you have not yet transitioned from "to the building" to "within the building") until you land on the first panel. In your two cases, the "first panel" consists of two panels that collectively carry all the load in the building.

If you then went from each of your two panels to another panel downstream (now you have a total of four panels), then the feeders to the two sub-panels cannot be sized per 310.15(B)(6). That is because the feeders to the two sub-panels are not "main power feeders" to the building.
 

don_resqcapt19

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Charlie,
It has to be power to the building as a whole. It has to be the main power feeder to the building, not a sub power feeder within the building.
Yes, you might think that is what the rule is saying because of the use of the word "main", but its use in this code section is not within the common meaning of that word. As long as the feeder originates on the load side of the service disconnect and it feeds a lighting and appliance branch circuit panel, it is a "main power feeder". There is nothing in the section that requires a "main power feeder" to supply most of the building load, or even to supply a majority of the load. I agree, that was most likely the intent of the code rule, I don't agree that the code words support this intent.
Don
 

charlie b

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Let me see if I have this right:
  • "Main" doesn't mean "main,"
  • "To" doesn't mean "to," and
  • "Between" doesn't have to mean "directly between."
Sounds to me like a shaky basis for an interpretation.

We're not likely to come to an agreement here. But for my money, if a conductor doesn't touch the main disconnect, you can't use 310.15(B)(6) to size it.
 

don_resqcapt19

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Charlie,
"Main" doesn't mean "main,"
The term is defined in the code section and that is the only definition that can be used for this application. I think that this code section would be much better if the definition of main power feeder was left out, because then "main" would mean "main".
But for my money, if a conductor doesn't touch the main disconnect, you can't use 310.15(B)(6) to size it.
Can you expand on that? Are the termination points on a disconnect part of the main disconnect? Are the busbars in the panel that contains the service disconnect part of the main disconnect?
Don
 

jwelectric

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don_resqcapt19 said:
Are the busbars in the panel that contains the service disconnect part of the main disconnect?
Don

NO

When the breaker is off is the bus bars still entergized?
If they are no then the "MAIN DISCONNECT" must have turned them off.
 

roger

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Hey, this might be a good time for another poll. :D

Does the new software have the capabilities?

Roger
 

don_resqcapt19

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Mike,
NO
When the breaker is off is the bus bars still entergized?
If they are no then the "MAIN DISCONNECT" must have turned them off.
How is that different from the lugs in a disconnect? If you apply what you say, you can never use this table.
Don
 

jwelectric

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don_resqcapt19 said:
How is that different from the lugs in a disconnect? If you apply what you say, you can never use this table.
Don

The lugs in the disconnect will not accept overcurrent devices rated at 30 amps or less and utilize the grounded (neutral) conductor.
The bus bars of the lighting panel can and does thus Table 310.15(B)(6) can no longer be used to supply another panel.
:)
 

don_resqcapt19

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Mike,
The lugs in the disconnect will not accept overcurrent devices rated at 30 amps or less and utilize the grounded (neutral) conductor. The bus bars of the lighting panel can and does thus Table 310.15(B)(6) can no longer be used to supply another panel.
What words in the code support that statement? I don't find any. A feeder that originates at a panel that contains the service OCPD and feeds a lighting and appliance panel is most certainly "between the main disconnect and the lighting and appliance branch-circuit panelboard" and is therefore a "main power feeder".
Don
 

jwelectric

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don_resqcapt19 said:
What words in the code support that statement? I don't find any. A feeder that originates at a panel that contains the service OCPD and feeds a lighting and appliance panel is most certainly "between the main disconnect and the lighting and appliance branch-circuit panelboard" and is therefore a "main power feeder".
Don



I agree with the words that you have stated as long as it happens the way that you stated them.

"between the main disconnect and the lighting and appliance branch-circuit panelboard"

Should there be any other overcurrent device between the main overcurrent device and the next panel then the feeders couldn?t be between the main disconnect and the panel. There would be a separate overcurrent device between the main and the feeders.

As long as that second overcurrent device is located in a power panel then there is no problem but if that second overcurrent device is located in a lighting and appliance panel then it is impossible for it to be between the main and the second panel.
:)
 

don_resqcapt19

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Mike,
As long as that second overcurrent device is located in a power panel then there is no problem but if that second overcurrent device is located in a lighting and appliance panel then it is impossible for it to be between the main and the second panel.
Here is where we do not agree. I see no difference between the feeder breaker being in a power panel or the lighting panel that has the service disconnect in it. As long as the service disconnect is the source of power all feeder breakers are between the service disconnect and the panel.

We will have to agree to disagree on this. As i said before, the intent of the rule is most likely they way you read it and I agree with that. I just don't think that the words fully support the intent.
Don
 

don_resqcapt19

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This section was very clear in the 1987 code. At that time the rule was found in Note 3 to Tables 310-16 through 310-31. The note said that the reduced conductor sizes could be used for a "... feeder that carries the total current supplied by the service". Very clear and concise code writing.

In the 1990 code the wording for Note 3 was changed to read; " ... single phase service entrance conductors and feeder conductors ...." There was no restriction on the use of the reduced conductor sizes for feeder conductors in the 1990 code. The proposal (TCR89A 6-83) stated that "Note 3 only applies to feeders of 100 amps or more, so the diversity contemplated in the rule is going to be present whether some additional load is taken off ahead of the feeder or not". The submitter also stated that often a second feeder is used for loads such as swimming pools or air conditioners. The panel accepted the proposal in principle and deleted the words "that carries the total current supplied by the service".

In the 1993 code the panel reversed itself. Panel action on comment (92TCD 6-49) restored wording that required a "feeder to supply the total load to the dwelling unit" in order to use the reduced conductor sizes in the table. This comment suggested use of the words "that serve as the main power supply to a dwelling unit", but the panel replaced those words with the words shown above.

In the 1996 code the panel introduced the words "main power feeder". This code cycle had a number of proposals to change Note 3, and the panel combined them into their own proposal (95ROP 6-73a). The panel comments indicated that they intend that a "main power feeder" supply the total dwelling unit load, however they also added wording that said a feeder would never have to use conductors larger than the service conductors. They added this to permit an outside service panel to feed loads such as air conditioning and still permit a feeder to use the reduced size conductors. There was a comment (95ROC 6-31) that recommended that the words "feeder conductors that serve as the main power feeder to a dwelling unit", be replaced by the words "feeder conductors that carry the load to the dwelling unit". The commentator suggested that the words "main power feeder" would lead to interpretation problems and confusion as to what is a "main power feeder'. The panel rejected the comment with a panel statement that read: "The use of Note 3 is based on load diversity and if the user does not have load diversity, Note 3 should not be used. The panel does not believe that the recommended wording provides additional clarification."

In the 1999 code the Notes were moved into code text as a part of 310-15(b)(6). The "definition" of main power feeder was added in this code cycle. The substantiation said the change was to clarify that the "main power feeder" is/are the feeder(s) that carries diversified loads. The panel accepted in principal. I really don't see how this wording clarifies the section. Why can't it just say a "feeder that carries the total dwelling unit load"? There would be no question as to the meaning then.
In the 2002 code cycle there was an attempt to clarify this section with words that would have clearly permitted the reduced size conductors to be used for any feeder that supplies at least 40% of the line to neutral load of the dwelling unit. This was a proposal that would have permitted a single 400 amp service to have two 200 amp "main power feeders". The proposal was rejected with a panel statement that said: "The present wording adequately describes the requirement." Like one of my favorite code instructors used to say, "boy, I'd like a puff of what they have been smoking". (not Mike Holt, you would have to be an old guy like me to know who often said this at his code classes)

As I said previously in this thread, I know what the intent of the rule is, I just don't agree that the current wording fully supports that intent. After researching the ROPs and ROCs tonight, I find that I am in good company with that thought. Some of the heavy hitters in the NEC world have made proposals and comments to clear up the wording. We need to go back to the 1987 wording as that is CMP 6's stated intent of the section. This would prohibit the common practice of using two 200 amp "main power
feeders" on a 400 amp service.

Don
 

jwelectric

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Location
North Carolina
I agree with you Bryan it is good to have someone that will take the time to do this type of research.

With your permission Don I will use this information is the classroom. I have gone back through older cycles of the code and read the section but I have never researched the ROPs.

Thank You
:)
 
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