Clarification on 310.15(B)(7)(3)

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Leespark57

Senior Member
Location
Boston, MA, USA
Here is the scenario. One unit of a 4 family dwelling is fed by #2 SE aluminum from a multi-gang meter socket to a 100 amp main breaker panel (one of the < 6 service disconnects). The heating and air-conditioning equipment are fed from this panel. A 2-3 SER aluminum leaves this main panel and feeds a sub-panel within the unit for the remaining branch circuits.
In this situation it ok to protect the 2-3 SER with a 100 amp breaker? Is this scenario what 310.15(B)(7)(3) is referring to?
Thanks
 

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
Yes, that section is very confusing. Why mention that a feeder must control all loads to use the 83% rule then state that no feeder ever has to have an ampacity higher than 83%.
 

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
Lee, I think what is happening here is that in your case you can use the 83% rule. (2) says it must carry the total load but (3) says it does not have to be bigger than the service conductors. Basically, you cannot use the 83% rule on any feeder unless the feeder has the same ampacity as the service conductors.

That seems to be what is going on here
 

jumper

Senior Member
Lee, I think what is happening here is that in your case you can use the 83% rule. (2) says it must carry the total load but (3) says it does not have to be bigger than the service conductors. Basically, you cannot use the 83% rule on any feeder unless the feeder has the same ampacity as the service conductors.

That seems to be what is going on here

Where is George? He rewrote the darn section.
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
Lee, I think what is happening here is that in your case you can use the 83% rule. (2) says it must carry the total load but (3) says it does not have to be bigger than the service conductors. Basically, you cannot use the 83% rule on any feeder unless the feeder has the same ampacity as the service conductors.

That seems to be what is going on here
And depending on what the service conductors may supply other than a single dwelling unit, you will not necessarily be able to apply the 83% allowance to the service conductors in the first place.

I can see this wording being added to deal with the annoying paradox of the original formulation that if, for example, an A/C was fed directly from the service disconnect while the rest of the dwelling goes through a feeder and subpanel, then the reduction of load on the feeder prevented the 83% allowance from being taken.
 

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
And depending on what the service conductors may supply other than a single dwelling unit, you will not necessarily be able to apply the 83% allowance to the service conductors in the first place.

I can see this wording being added to deal with the annoying paradox of the original formulation that if, for example, an A/C was fed directly from the service disconnect while the rest of the dwelling goes through a feeder and subpanel, then the reduction of load on the feeder prevented the 83% allowance from being taken.


Interestingly I just receive a pdf from our head authority having jurisdiction with all the rule interpretations that have been issued.

For instance, if you had a meter main - no service conductors then you would not be allowed to use the 83% for the feeder that does not carry the full load. Otherwise it is as I stated above

The electrical code recognizes that there is no logical reason to require a feeder to be larger than the rating of the service-entrance conductors (present or not) or when portions of the dwelling’s load are taken off of the feeder that is essentially an extension of the service-entrance conductors, except the feeder possesses overcurrent protection. Therefore, the feeder from the busbar lugs, protected by the main breaker, and feeds a subpanel in the house is not required to be larger than 2/0 AWG copper in accordance with section 310.15(B)(7)(3).
 

kwired

Electron manager
Location
NE Nebraska
Interestingly I just receive a pdf from our head authority having jurisdiction with all the rule interpretations that have been issued.

For instance, if you had a meter main - no service conductors then you would not be allowed to use the 83% for the feeder that does not carry the full load. Otherwise it is as I stated above
Is that how it is being interpreted on a national basis or just how that particular AHJ has decided how to interpret it? Seems to me NEC wording can be taken either way.
 

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
Is that how it is being interpreted on a national basis or just how that particular AHJ has decided how to interpret it? Seems to me NEC wording can be taken either way.


This is definitely NC's interpretation but I think that is the intent
 
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