Derating Main Panel Breaker When Installing PV System

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mfountai

Member
Location
Orland, CA, USA
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:)
 

SolarPro

Senior Member
Location
Austin, TX
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).
 
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Smart $

Esteemed Member
Location
Ohio
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...???
 

Carultch

Senior Member
Location
Massachusetts
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.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
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.
 

SolarPro

Senior Member
Location
Austin, TX
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.)
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
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!

Well if it's a 200A service then it would allow 100A.
And if it's a 125A busbar with a 100A main breaker (quite common), then it would allow 88A, which is awfully close to 100.

If the rule were currently 150%, I'd guess that would have eliminated about 80% of the supply-side taps and service upgrades I've done in the past year.
 

mfountai

Member
Location
Orland, CA, USA
Thank you, Carultch

Thank you, Carultch

I appreciate the explanation of the rules, but I must be a bit dense. Isn't the current running through the bus bar is only going to be as high as the branch breakers allow. So it seems to me, regardless of the supply breakers the branch breakers are the controlling factor for current. If that is correct, what difference does the additional source of supply make?

Thank you for taking the time to try and straighten me out on this.
Mark
 

Smart $

Esteemed Member
Location
Ohio
I appreciate the explanation of the rules, but I must be a bit dense. Isn't the current running through the bus bar is only going to be as high as the branch breakers allow. So it seems to me, regardless of the supply breakers the branch breakers are the controlling factor for current. If that is correct, what difference does the additional source of supply make?

Thank you for taking the time to try and straighten me out on this.
Mark
In many cases, the branch breakers combined will permit more current through the busbar than it is rated for. For example, say you have a 42 space 200A panelboard. All but the PV breaker are 20A single pole. That's 20 x 20A breakers per leg... 400A. In this instance, the load and its diversity would be what keeps the main breaker from tripping.
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
mfountai, your reasoning is fine, which is why the 2014 code allows one method to be that all the branch breaker ratings don't exceed the rating of the busbar. However, as Smart$ described, you'll rarely find this to be the case on existing panels, especially residential panels.
 

Carultch

Senior Member
Location
Massachusetts
I appreciate the explanation of the rules, but I must be a bit dense. Isn't the current running through the bus bar is only going to be as high as the branch breakers allow. So it seems to me, regardless of the supply breakers the branch breakers are the controlling factor for current. If that is correct, what difference does the additional source of supply make?

Thank you for taking the time to try and straighten me out on this.
Mark

There also is another rule that allows the sum of the branch breakers (excluding the main supply) to be limited at the ampacity of the busbar.

It is rare that you will have this situation, unless it is a panelboard that isn't utilized to the capacity of most. One place you might see this, is if you supply ancillary loads from your PV system's AC combiner, such as convenience outlets, vehicle chargers, monitoring systems, or parking canopy lighting.

Panelboards are most often loaded with a lot more breakers than either their busbar or main breaker ampacities. And the reason this happens, is due to load diversity. Not all circuits are operated at full capacity, and the NEC has load calculation methods for "adding them up" and taking advantage of statistical load diversity factors, with a safety factor included as well.
 

Smart $

Esteemed Member
Location
Ohio
There also is another rule that allows the sum of the branch breakers (excluding the main supply) to be limited at the ampacity of the busbar.
Should be neither the sum of the load breakers nor the sum of PV breakers shall exceed the busbar rating.

... One place you might see this, is if you supply ancillary loads from your PV system's AC combiner, such as convenience outlets, vehicle chargers, monitoring systems, or parking canopy lighting.

...
True... but you still have to carry the PV System amount line-ward to the service equipment.
 

mfountai

Member
Location
Orland, CA, USA
Thank you all.

Thank you all.

I appreciate all of the answers/teaching you all have given me. I can almost understand the safety issue here. That being, since the branch breakers will usually allow more current than the main breakers/bus bar can handle, adding the PV circuit could ?possibly? allow more current than the main breaker alone would allow. Therefore, the rating of the bus bar needs to be considered when installing the PV supply.

Do I have that correct?

Thanks again. This is not my area of expertise but I do teach issues of basic electricity and I want to make sure I have a reasonable understanding.
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
Do I have that correct?

I think that you have it exactly.
The next lesson, if you choose to go farther, is why the PV breaker has to be at the opposite end of the bus from the main breaker when applying the 120% rule.
The final question, why the CMP chose 120%, is unanswerable, so do not worry about it. :)
 

Carultch

Senior Member
Location
Massachusetts
The next lesson, if you choose to go farther, is why the PV breaker has to be at the opposite end of the bus from the main breaker when applying the 120% rule.

The answer is simply Kirchhoffs current law. If a panel is fed from opposite ends, at some point the current on the busbars will diminish to zero. This way, the PV system current will at no point in the panel, add up with the main supply current to a value larger than either of the currents.

If a panel is fed from the same side, then immediately after both sources are connected, the current equals the sum of the two.

The final question, why the CMP chose 120%, is unanswerable, so do not worry about it. :)

Indeed. 120% seems like a particularly uncommon number in the NEC, that it makes you believe there is a reason behind it.

By Kirchhoff's current law reasoning alone, you might think that it should be able to be as much as 200%. I know that the reason the CMP is shy to allow this, is the concern for the extra heat generation in the branch breakers, due to the extra source of current. Instead of having 200A of current flow among all the breakers in a 200A panel, you would have 400A of current flow through all the breakers in a 200A panel.

Worst case scenario, all this might do is nuisance trip the breakers. No possibility of overcurrenting the busbars without overload protection.
 
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