Santa Clarita, CA · December 2021 – January 2022 · Southern California Edison / City of Santa Clarita

Santa Clarita: the main breaker did not need derating.

The rule is about the busbar, not the breaker. Read the wrong number and you replace hardware for nothing.

Written by the Cali Energy project team · California contractor license 1032379 · Updated September 2026

Adding solar to an existing panel is governed by what the panel's busbar can carry: the main breaker plus the solar backfeed together are limited to 120 percent of the busbar rating. Derating the main breaker is the standard way to make that sum fit — but only when it does not already fit. On this Santa Clarita project the revised plan proposed dropping the main breaker to 175 A; Cali Energy checked the panel and the busbar was rated 225 A, so no derate was required and the stamped set went to a final in January.

At a glance

System

9.52 kW, 28 Q CELLS 340 W modules

Inverters

One Enphase IQ7+ per module

Proposed

Derate the main breaker to 175 A

Actual busbar rating

225 A

Derate required

None

Also settled

A roofing claim, using a survey photo taken before the work

Utility / AHJ

Southern California Edison / Santa Clarita

The story

Two things happened on this project and they are both about reading the evidence that already exists.

The first was a roofing claim. In late December, with rain forecast, word came that tiles on the roof were out of place and we needed to go back and fix them.

We sent back a photograph — one taken during the site survey in October, weeks before our crew had ever been on that roof. The displaced tiles are in it. That ended the question without an argument, and it is the reason site survey photographs are worth keeping: they establish the condition of a roof on a date, and the date is the whole point.

The second thing is the reason the first final did not pass. The plan set specified IQ7 microinverters; the modules on the roof carried IQ7+ units factory-fitted to them. Those are different devices with different outputs, an inspector compares what is on the roof with what is on the stamped drawing, and this one did. The inspection failed, the plans went back for revision and the set had to clear plan check a second time.

Which produced the third thing, and it is the most useful of the three.

A revised plan set came through in January proposing to derate the main breaker from its existing rating down to 175 A. That is a completely standard move and on most houses it is the right one. Here it was unnecessary, and the reason is a number people routinely read off the wrong component.

The rule works like this. When solar backfeeds an existing panel, the panel's busbar — the metal spine the breakers clamp onto — has a rating, and the main breaker plus the solar breaker together are allowed to reach 120 percent of it. Exceed that and the busbar could in theory be fed more current than it was built for, from two directions at once.

So there are two ways to make the sum work: shrink the main breaker, or have a busbar big enough that you do not have to.

The trap is that people read the number stamped on the main breaker and assume the panel matches it. It often does not. A panel fitted with a 200 A main breaker can have a busbar rated at 225 A, and that extra headroom is exactly what decides whether a derate is needed.

On this panel the busbar was 225 A. Once that is on the table, the arithmetic comes out with room to spare and the derate has nothing left to do.

Why this is worth an owner's attention: derating the main breaker is not free. It lowers the ceiling on what the house can draw at once, which on a property that later adds a car charger, a heat pump or an induction range can become a real constraint. Doing it when it is genuinely required is fine. Doing it because nobody opened the panel door and read the busbar label is a permanent downgrade bought for nothing.

The second inspection was held at the end of January against the corrected set, and the project closed on the permit it started on — with the main breaker still at its original rating.

Cali Energy reviewed, corrected and finaled this project in Santa Clarita.

How it ran

Six weeks between an installed array and a final, and the decisive fact was a label inside a panel door.

What we did

• Answered a roofing claim with a survey photograph taken before our crew had been on the roof at all.
• Read the busbar rating instead of assuming it matched the main breaker.
• Took the IQ7/IQ7+ mismatch straight to a revision instead of arguing it at the counter.
• Declined an unnecessary derate that would have capped the house permanently.
• Took the revised set through to a stamp and a booked final.

System specifications

Is your project like this one?

• Your plan set says the main breaker will be derated.
• You are adding solar to a panel somebody else installed.
• You expect to add a car charger or a heat pump later.
• You have been asked to pay for roof damage you do not think is yours.

Cali Energy in Santa Clarita

Frequently asked questions

What is the 120 percent rule for solar?

The main breaker and the solar backfeed breaker together may reach 120 percent of the busbar rating of the panel they share. It exists because the busbar can be fed from two directions at once — the utility through the main, the array through its own breaker.

Is the busbar rating the same as the main breaker rating?

Frequently not, and that is where the mistake happens. A panel fitted with a 200 A main breaker can carry a busbar rated 225 A. The rule is calculated on the busbar, so reading the breaker instead can produce a derate that was never necessary.

What does derating the main breaker cost me?

It lowers the maximum the house can draw at once. That is harmless until the property adds a car charger, a heat pump or an induction range, at which point the lowered ceiling is a real constraint — bought, in this case, for nothing.

How do I check my own busbar rating?

It is on the label inside the panel door, usually stated as a bus rating in amps and separate from the main breaker size. If a quote proposes a derate, that label is the first thing to look at.

Does Cali Energy install solar in Santa Clarita?

Yes. Projects here are permitted through the City of Santa Clarita Building & Safety Division and interconnected through Southern California Edison. The housing spans several eras, so the service equipment differs street by street and the panel label is worth reading before the design is fixed.

What this would look like on your roof

Related

• The correction needed a structural engineer Santa Clarita · 17.2 kW · 43 modules on a tile roof Solar
• The array did not fit the panel, so the main breaker got smaller Granada Hills, CA · 31 modules · de-rated to 175 A instead of a new panel Solar
• The panel came out. What went back has a bigger busbar Granada Hills, CA · 6.00 kW DC · 225 A busbar, 70 A of headroom left Solar
• The module in the design was no longer made Santa Clarita · 26 modules · found at the order, not at the drawing Solar

This timeline reflects this property and this permitting path. Updated September 2026.