Van Nuys, Los Angeles, CA · December 2021 – February 2022 · LADWP / LADBS

The cheap fix needed two empty slots. There were none.

A large modern house with a low-slope tile roof and an array big enough to need a serious breaker. The usual answer is to derate the main breaker and keep the existing panel. Here the panel was full, and full is a different problem from small.

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

A main panel may be backfed as long as the main breaker and the solar breaker together come to no more than 120 percent of the busbar rating, which is why lowering the main breaker so often avoids a service upgrade. That solution still needs somewhere to land the solar breaker. In this Van Nuys house every position in the main panel was occupied, so the derate was not available at any breaker size, and Cali Energy redesigned the interconnection around new service equipment instead.

At a glance

Size recorded by the utility

11.9 kW AC-CEC

Roof

Low-slope concrete tile, several planes

First design

Derate the main breaker, keep the panel

Why it failed

No spare breaker positions in the panel

Designed instead

New service equipment sized for the backfeed

Interconnection agreement

Approved two weeks after filing

Net meter

Set in early March 2022

Utility / AHJ

LADWP / LADBS

The story

The roof was the easy part. A large single-story house with a modern low-slope concrete tile roof, several planes facing different directions, and plenty of room. The array went on in groups rather than one block, following the planes the sun actually reaches, and the utility recorded the finished system at 11.9 kilowatts AC-CEC.

The hard part was two hundred feet away, on the side wall.

An array this size needs a substantial breaker to push its output into the house. The question every design has to answer is where that breaker goes and whether the panel can take it. The rule is about the busbar — the copper spine every breaker clamps onto. The main breaker and the solar breaker together may come to 120 percent of what that spine is rated to carry, so a 200-amp busbar allows 240 amps of supply. With a 200-amp main that leaves 40 amps, which is rarely enough for a full-size array.

The standard way out is to leave the panel alone and lower the main breaker. Drop a 200-amp main to 175 and the allowance rises to 65 amps, which carries most residential arrays: the panel is untouched, no utility crew is needed, and the cost is a breaker and an afternoon. It is the first thing a good designer tries.

The design team came back with a single sentence that closed that door. The existing panel was full. Every position was taken, and a derate does not create space — it only changes a number. There was nowhere to put the solar breaker at 175 amps, at 150 amps, or at any other size.

The usual ways of making room had already run out. Tandem breakers, which fit two circuits into one position, are only allowed in the positions a panel is listed for, and those were taken by circuits that could not share. A subpanel would have needed a feeder breaker in the main panel, which is the same missing position by another name.

So the interconnection type changed. Instead of backfeeding an existing panel, the project moved to new service equipment: a panel with the capacity and the positions for both the house and the array, and a point of connection the utility would accept. That is a bigger job. It touches the utility's side of the meter, it needs its own inspection, and it changes the schedule.

It also changed who had to do what. Partway through the review, the utility's service representative wrote to the homeowner directly asking for photographs of the existing equipment. The homeowner forwarded the request and asked whether he should take the photographs himself or whether we already had them. We had them, and they went in the same week — but the moment is worth remembering. A utility will contact the customer, and a customer who does not know whether to act can lose a week being polite.

With the equipment settled, the rest moved quickly. The application went in at the end of January, the site assessment cleared in the first week of February, and the interconnection agreement was approved two weeks after filing. The utility completed its interconnection review the same day it approved the agreement, and the net meter went on at the start of March.

What a homeowner should take from this: when an installer quotes a derate, ask to see a photograph of the inside of the panel door. The question is not only what the main breaker says. It is whether there are two empty positions next to it. A quote built on a derate that the panel cannot physically accept is a quote that will change after the contract is signed.

How it ran

Two weeks from application to an approved agreement and about six to the meter, once the interconnection type matched what the house could actually take.

What we did

• Laid out an array in groups across the planes of a low-slope tile roof.
• Tested the derate option against the existing panel and established that it could not be used.
• Redesigned the interconnection around new service equipment sized for the backfeed.
• Filed the interconnection application and supplied the site photographs the utility asked for.
• Carried the project through site assessment and interconnection agreement approval.

System specifications

Is your project like this one?

• Your installer has quoted a main breaker derate and you want to know if it works.
• Your main panel has no empty breaker positions left.
• You are planning an array large enough to need a serious backfeed breaker.
• You have a tile roof with several planes and want the array split sensibly.
• You are on LADWP and want the service equipment settled before the schedule is set.

Cali Energy in Van Nuys

Frequently asked questions

What does it mean to derate a main breaker for solar?

It means replacing the main breaker with a smaller one so the solar breaker can be added without exceeding what the panel's busbar is rated to carry. A 200-amp main dropped to 175 frees enough headroom for most residential arrays. The house loses some theoretical capacity it was almost certainly never using, and the panel, the meter and the utility service all stay exactly as they are.

When does a derate not work?

When the panel has no free positions for the solar breaker. A derate changes the rating of the main breaker, not the physical size of the panel, so a full panel stays full. It also does not help where the busbar rating is already too low for the house's real load, or where the panel is old enough that no current breaker fits it. In those cases the service equipment has to be replaced.

Is replacing the service equipment much slower than a derate?

It is a different kind of job rather than simply a longer one. New service equipment needs its own plan review, its own electrical inspection and a point of connection the utility signs off on. On this project the change was made at the design stage, and the interconnection agreement was still approved two weeks after the application. Discovering it after the crew arrives is what costs time.

Why did the utility email the homeowner instead of the installer?

Because the account is in the homeowner's name, and a service representative chasing site information often writes to the account holder first. It is not a sign that something is wrong. The right response is to forward it to whoever is running the project rather than to act on it alone, because the utility usually wants the photographs in a specific form and the installer already has them.

Does Cali Energy replace service equipment in Van Nuys?

Yes. Where a panel cannot take a backfeed, the work is designing and building new service equipment the utility will connect to, and taking it through plan review, electrical inspection and the interconnection. Cali Energy does that as part of the solar project rather than handing it to the homeowner, and our Van Nuys work runs through LADBS for permits and LADWP for the connection. License CSLB #1032379.

What this would look like on your roof

Related

• Forty-two modules, and the panel stayed where it was Van Nuys, CA · 16.8 kW on tile · 175 A derate, no service upgrade 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 main panel had no room for the solar breaker Studio City, CA · 16 modules · a quad breaker instead of a new panel Solar

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