Solar Breaker inside Sub Panel

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cmsolar

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9125 Chesapeake Drive San Diego CA
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An AHJ is requiring that we install the solar braker opposite side of the main lugs claiming this has to be opposite side of main braker. There is no Main Braker on this sub panel as its Main Lug only. We are starting to inspector that there is no main braker on the sub panel and that is why we don't need to install on opposite e side is there a code for this instance.?
 
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wwhitney

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Berkeley, CA
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If you're relying on 705.12(B)(2)(3)(b) (the 120% rule) for compliance on the subpanel busbar, then it starts off "Where two sources. . . are located at opposite ends of a busbar that contains loads . . ." So it doesn't matter whether the supply connection is to a main breaker, backfed breaker, or main lugs, your solar breaker would still need to be opposite the other source of supply.

If you're instead relying on 705.12(B)(2)(3)(a) or (c), then the breaker location doesn't matter.

For reference:


Cheers, Wayne
 
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Carultch

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Location
Massachusetts
Since you used the word subpanel, this leads me to believe that you aren't talking about a "hot bus" MLO main panel.

If there is any OCPD on the line side of the main lugs, then that acts as the main breaker as far as 705.12 and the 120% rule is concerned. The 705.12 article refers to the "overcurrent device protecting the busbar", which doesn't only mean an internal main breaker. It could be a branch breaker acting as a main, or an externalized OCPD. Such as a fused disconnect, a breaker in a meter/main combo, or a breaker from a higher ranking panel supplying the subpanel.

The reason why you need the breaker on the opposite end of the main supply to take credit for the 120% rule, is that Kirchhoff's current rule makes current subtractive at the busbar cross sections, rather than additive, when you feed the panel from opposite ends. Feed from the same end, and the current is additive. Panels are routinely populated with more load breakers than the main supply, on the underlying assumption that they won't all draw their full load at once, and the main breaker will ultimately protect the panel before it overheats. Adding a PV interconnection, allows for an overload to be in the blind spot of the main breaker.

Without putting the interconnecting breaker at the opposite end, the 120% rule becomes a 100% rule instead.
 
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cmsolar

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Location
9125 Chesapeake Drive San Diego CA
Occupation
Project manager
Carultch said:
Since you used the word subpanel, this leads me to believe that you aren't talking about a "hot bus" MLO main panel.

If there is any OCPD on the line side of the main lugs, then that acts as the main breaker as far as 705.12 and the 120% rule is concerned. The 705.12 article refers to the "overcurrent device protecting the busbar", which doesn't only mean an internal main breaker. It could be a branch breaker acting as a main, or an externalized OCPD. Such as a fused disconnect, a breaker in a meter/main combo, or a breaker from a higher ranking panel supplying the subpanel.

The reason why you need the breaker on the opposite end of the main supply to take credit for the 120% rule, is that Kirchhoff's current rule makes current subtractive at the busbar cross sections, rather than additive, when you feed the panel from opposite ends. Feed from the same end, and the current is additive. Panels are routinely populated with more load breakers than the main supply, on the underlying assumption that they won't all draw their full load at once, and the main breaker will ultimately protect the panel before it overheats. Adding a PV interconnection, allows for an overload to be in the blind spot of the main breaker.

Without putting the interconnecting breaker at the opposite end, the 120% rule becomes a 100% rule instead.
Click to expand...
Thank you very much. This makes total sense.
 
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cmsolar

Member
Location
9125 Chesapeake Drive San Diego CA
Occupation
Project manager
wwhitney said:
If you're relying on 705.12(B)(2)(3)(b) (the 120% rule) for compliance on the subpanel busbar, then it starts off "Where two sources. . . are located at opposite ends of a busbar that contains loads . . ." So it doesn't matter whether the supply connection is to a main breaker, backfed breaker, or main lugs, your solar breaker would still need to be opposite the other source of supply.

If you're instead relying on 705.12(B)(2)(3)(a) or (c), then the breaker location doesn't matter.

For reference:


Cheers, Wayne
Click to expand...
Thank you very much this is very helpfull.
 
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