310.15 (B) (7)

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zcanyonboltz

Senior Member
Location
denver
I searched on here and found most info was from pre 2014 NEC.
My question is if a 150A meter/main breaker combo panel is mounted on a detached garage and the garage branch circuits are powered from this panel, then a feeder runs to the house and feeds a 150A main breaker panel can the 83% adjustment factor be used when sizing the service entrance conductors? I am not trying to apply the 83% to the feeder to the house I'm just wondering if the main on the garage then feeding the house makes 310.15 (B) (7) 83% size of main breaker not applicable in this case?
If so the feeders to the house may be larger than the service entrance conductors. Thanks.
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
Yes you can use 83% to the house but you cannot use it to the separate structure (detached garage)

May I respectfully disagree,
It seems to me, with the combo mounted on the garage, and a feeder run to the house, the 83% could be applied to the service conductors (at the garage) since they are carrying the entire load associated with the dwelling and the 83% could not be applied to the feeder
 

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
May I respectfully disagree,
It seems to me, with the combo mounted on the garage, and a feeder run to the house, the 83% could be applied to the service conductors (at the garage) since they are carrying the entire load associated with the dwelling and the 83% could not be applied to the feeder


I agree but I was basing my answer on the power at the house. My bad... I read right thru the service being at the garage.
 

texie

Senior Member
Location
Fort Collins, Colorado
Occupation
Electrician, Contractor, Inspector
I searched on here and found most info was from pre 2014 NEC.
My question is if a 150A meter/main breaker combo panel is mounted on a detached garage and the garage branch circuits are powered from this panel, then a feeder runs to the house and feeds a 150A main breaker panel can the 83% adjustment factor be used when sizing the service entrance conductors? I am not trying to apply the 83% to the feeder to the house I'm just wondering if the main on the garage then feeding the house makes 310.15 (B) (7) 83% size of main breaker not applicable in this case?
If so the feeders to the house may be larger than the service entrance conductors. Thanks.

The code recognizes the possibility of this with 310.15(B)(7)(3) so you don't end up with a feeder larger than the service conductors that are feeding it. While your situation is slightly different this occurs all the time when you use a feed thru meter/main panel on the outside of a house but you use some of the spaces in the meter/main panel to feed , say, the condensing unit or any other equipment that may be more conveniently connected there instead of running the circuits all the way to the inside panel.
 

kwired

Electron manager
Location
NE Nebraska
May I respectfully disagree,
It seems to me, with the combo mounted on the garage, and a feeder run to the house, the 83% could be applied to the service conductors (at the garage) since they are carrying the entire load associated with the dwelling and the 83% could not be applied to the feeder
If anything I think the service conductor may need to be sized directly from table and not from (B)(7) because it feeds more then just the dwelling.

Question becomes is the garage load part of the load "associated with the dwelling"? If not then the service conductors can't use (B)(7), yet if we were supplying the house first most here would have no problem with using (B)(7) to size the service conductors.

It is the same load on service conductors no matter which structure gets hit first.
 

zcanyonboltz

Senior Member
Location
denver
If anything I think the service conductor may need to be sized directly from table and not from (B)(7) because it feeds more then just the dwelling.

Question becomes is the garage load part of the load "associated with the dwelling"? If not then the service conductors can't use (B)(7), yet if we were supplying the house first most here would have no problem with using (B)(7) to size the service conductors.

It is the same load on service conductors no matter which structure gets hit first.

This is what I was thinking since the garage is fed first and power comes out of the garage panel to feed garage (B) (7) may not be okay to use? Then feed from garage to house may be bigger than service entrance conductors. Another way to use (B) (7) would be to have a disconnect on garage and then feed garage from house.
 
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zcanyonboltz

Senior Member
Location
denver
The code recognizes the possibility of this with 310.15(B)(7)(3) so you don't end up with a feeder larger than the service conductors that are feeding it. While your situation is slightly different this occurs all the time when you use a feed thru meter/main panel on the outside of a house but you use some of the spaces in the meter/main panel to feed , say, the condensing unit or any other equipment that may be more conveniently connected there instead of running the circuits all the way to the inside panel.


This is what we were doing when I was on this job, we'd power garage out garage meter/combo panel then use a feed thru to feed house. Does the feed thru allow (B) (7) to be used to size service entrance conductors and feed thru to house to be the same size conductor? The garage had 2 15A circuits in it.
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
If anything I think the service conductor may need to be sized directly from table and not from (B)(7) because it feeds more then just the dwelling.

Question becomes is the garage load part of the load "associated with the dwelling"? If not then the service conductors can't use (B)(7), yet if we were supplying the house first most here would have no problem with using (B)(7) to size the service conductors.

It is the same load on service conductors no matter which structure gets hit first.

Judgement call, IMO. IF the garage was attached to the house we would be using (B)(7) for our service. The simple fact that you move it away from the house does not alter it's association and its being part of the entire load "associated with the dwelling".
In either event the (attached or detached) "house" panel would be a subpanel with the feeder not carrying the entire load so (B)(7) would not be allowed for the feeder.
 

kwired

Electron manager
Location
NE Nebraska
Even if not using (B)(7), you potentially can run same size conductor depending on actual load calculation.

150 x .83 = 124.5

Using (B)(7) we can run 1 Cu (130 A) or 2/0 Al (135 A)

If not able to use (B)(7) next standard size overcurrent device for either is 150 amps - and can be used as long as load calculation is not over 130 for copper or 135 for aluminum conductors.
 

zcanyonboltz

Senior Member
Location
denver
Even if not using (B)(7), you potentially can run same size conductor depending on actual load calculation.

150 x .83 = 124.5

Using (B)(7) we can run 1 Cu (130 A) or 2/0 Al (135 A)

If not able to use (B)(7) next standard size overcurrent device for either is 150 amps - and can be used as long as load calculation is not over 130 for copper or 135 for aluminum conductors.

Thanks
 
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