# Thread: Derating Main Panel Breaker When Installing PV System

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## Derating Main Panel Breaker When Installing PV System

Hello,

This is a very basic question, but I'm a basic guy.

Can someone explain the reasoning behind derating the main panel breaker when installing a PV system?

Thank you for entertaining such a basic education question.
Mark

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Per NEC 2011, which applies in CA, the sum of the breakers feeding the busbar of a panel—i.e., the main breaker and the PV-connected breaker—cannot exceed 120% of the busbar rating. The reason people will downsize the main breaker rating—if permissible after performing a load analysis—is to create more capacity for the PV-connected breaker. While the 120% rule itself is pretty basic, downsizing the main breaker is a more advanced PV interconnection technique.

As an example, if you have a 125 A panel w/ a 125 A main breaker, you'd be limited to adding a 25 A circuit breaker for PV (125 A busbar x 120% = 150 A total - 125 A main = 25 A for PV). But if you were able to downsize the main to a 100 A breaker, based on the results of a load analysis, then you could now interconnect 50 A circuit breaker for the PV system under the 120% rule (125 A busbar x 120% = 150 A total - 100 A main = 50 A for PV).
Last edited by SolarPro; 03-13-15 at 06:08 PM.

3. Did you mean derating as SolarPro covered, or did you mean lowering the breaker nominal rating because it is in an enclosure on a roof exposed to sunlight (e.g. derating a 100A breaker to 95A)... not changing it out with a lower nominal rated breaker...???

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Originally Posted by mfountai
Hello,

This is a very basic question, but I'm a basic guy.

Can someone explain the reasoning behind derating the main panel breaker when installing a PV system?

Thank you for entertaining such a basic education question.
Mark
When you backfeed a panelboard (or analogous equipment) from the opposite side of the bus, NEC705.12(D) limits you to the famous "120% rule". The sum of the PV breaker and the main breaker cannot exceed 120% of the busbar ampacity. As an example, consider a 200A panel with a 200A main, and a 40A breaker for PV interconnection. (200A + 40A)/200A = 120%. Consider another example of a 400A panel with a 300A main, and 175A of PV interconnection. (300A + 175A)/400A = 118.75%.

In NEC2014, they changed the language so that rounding quirks on the PV breaker calculation aren't a show stopper. However, it is rare for that to make a difference, and the general sense of the rule still applies.

More than likely, when you get a main distribution panelboard for a building, you install a main breaker that matches the size of your main panelboard bus. For example, a 200A breaker with a 200A bus. It is rare that you would start with a smaller breaker than your busbar amps, unless you are designing with PV interconnections in mind.

If you come to an existing building where this is the case, it does limit your ability to interconnect the size of system you might have in mind. There are several options:
1. Install a system not exceeding the limits of the 120% rule, with the existing bus and main in place.
2. Install the system with a line side interconnection, upstream of the main breaker.
3. Replace the entire main panelboard, with one of an upsized busbar
4. With an NEC approved load calculation, or with historical peak building load data, reconsider what the main breaker has to be. Reduce it a few sizes, so that your system can still be interconnected as a branch breaker on the opposite end.

While the 120% rule might be counterintuitive (you might think it should be a 200% rule per the conceptual laws of current flow), the reason is the heat generated in the breakers throughout the panelboard. More power passing thru the panelboard than the overall panelboard rating can heat up and nuisance trip the branch breakers. I'm not sure where 120% comes from, but it limits the panelboard breaker heating so that it is less of a problem.

5. IMO the 120% rule is ridiculous in many cases. I think there should be complete exception to it (backfed PV can equal service rating) for residential PV with 200 amp services.

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Originally Posted by electrofelon
IMO the 120% rule is ridiculous in many cases. I think there should be complete exception to it (backfed PV can equal service rating) for residential PV with 200 amp services.
We could go off on a protracted discussion on that, but the Code is the Code.

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Originally Posted by electrofelon
IMO the 120% rule is ridiculous in many cases. I think there should be complete exception to it (backfed PV can equal service rating) for residential PV with 200 amp services.
You can do that. You just need to make a supply-side connection. The size of a supply-side connection is only limited by the size of the service.

(FWIW, the Code-Making Panel working on Article 705 in for NEC 2017 is evaluating whether the 120% allowance can be expanded to a 150% allowance.)

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Originally Posted by SolarPro

(FWIW, the Code-Making Panel working on Article 705 in for NEC 2017 is evaluating whether the 120% allowance can be expanded to a 150% allowance.)

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Originally Posted by jaggedben
I concur.

10. FWIW, the Code-Making Panel working on Article 705 in for NEC 2017 is evaluating whether the 120% allowance can be expanded to a 150% allowance.
I'll be less jovial and say "its a start" Actually it doesnt sound like much more, but 100 amps of backfed PV! Wow that would be luxurious!

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