Subpanel in detached garage

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jaggedben

Senior Member
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Northern California
Occupation
Solar and Energy Storage Installer
Since the feeder has a 100 amp breaker at each end - it will never carry more then a 100 amp device allows.

The feeder is supplying a separate building - there needs to be a main disconnect on the load side of the feeder in the separate building, main breaker in the panel is probably simplest way to do so in most cases.

The feeder conductor is only a 90 amp conductor, one could have a 100 at the supply end and that makes the conductor a feeder tap - but if outside there is no limit to length, but a 90 amp breaker would be needed on the load end. Or a 90 amp is acceptable on the supply end and it really wouldn't matter what setting is on the load end when it comes to protecting the feeder conductor.

Good points, although if the AHJ decides to enforce the 120% rule on this conductor then it's mostly moot. Not much support in the 2008 code for considering an inverter output circuit to also be a tap.
 

kwired

Electron manager
Location
NE Nebraska
Good points, although if the AHJ decides to enforce the 120% rule on this conductor then it's mostly moot. Not much support in the 2008 code for considering an inverter output circuit to also be a tap.

I have never installed PV systems, and am not up on everything involved with them - including some of the codes associated with them. In particular I am not all that familiar with this 120% rule or what/why it exists, but have a hard time seeing how a conductor with a 100 amp breaker on each end of it will ever be allowed to carry more then 100 amps outside of if you enter the trip current/time curve of either breaker you can have over 100 amps according to what the curve allows.

If you have more current on that conductor then you have gone wrong somewhere other then overcurrent protection rules IMO whether you have a PV system or not.
 

Smart $

Esteemed Member
Location
Ohio
I have never installed PV systems, and am not up on everything involved with them - including some of the codes associated with them. In particular I am not all that familiar with this 120% rule or what/why it exists, but have a hard time seeing how a conductor with a 100 amp breaker on each end of it will ever be allowed to carry more then 100 amps outside of if you enter the trip current/time curve of either breaker you can have over 100 amps according to what the curve allows.

If you have more current on that conductor then you have gone wrong somewhere other then overcurrent protection rules IMO whether you have a PV system or not.
AFAIK the 120% threshold value was "suggested" by authoritative persons in the PV industry and CMP went with it. Seems to stem from the idea that loads or faults on a feeder or bus between two sources can supply more current than either source OCPD permits.
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
I understand the concept of the 120% rule for panelboards, which have distribution spread along the busbar, and which are not necessarily tested during the listing process to examine how thermal loading affects overcurrent devices when there is a source at both ends.

I'm hopeful to see the percentage increase, maybe to 150%. (I thought SolarPro suggested this was possible in a thread somewhere here, although I can't find it right now.)

Why the 120% ever referred to conductors is indeed puzzling, and by the 2014 cycle logic prevailed and that is no longer the case.
 

kwired

Electron manager
Location
NE Nebraska
AFAIK the 120% threshold value was "suggested" by authoritative persons in the PV industry and CMP went with it. Seems to stem from the idea that loads or faults on a feeder or bus between two sources can supply more current than either source OCPD permits.
I see some logic to that, but not for a feeder with same overcurrent device on each end - it will never be loaded to more then the breaker(s) allow. If the feeder branches more then two ways then more details are necessary and there is more possibilities, but a simple feeder between two buildings like in the OP can not be overloaded if it has proper overcurrent protection on both ends, it could temporarily be overloaded until one of the devices trips though - but that is design consideration IMO and goes against 90.1(A) if we are required to consider that as well as many other things in more recent years:roll:
 
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