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

Santa Clarita: the correction needed a structural engineer.

Forty-three modules on a tile roof. At that size the question stops being electrical.

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

Most plan-check corrections are answered by redrawing something. Some are not: when an array is large enough, or the roof is heavy enough, the department asks whether the structure carries the load — and that question can only be answered by a licensed structural engineer, whose calculations become part of the set. On this 17.2 kW Santa Clarita project on a tile roof the correction was exactly that, and Cali Energy sent the plans for engineering rather than trying to redraw around it.

At a glance

System

17.2 kW

Array

43 Q CELLS 400 W modules

Inverters

One Enphase IQ7+ per module

Roof

Tile

Service

Main panel and sub panel, both 200 A

Correction

Required structural engineering

Utility / AHJ

Southern California Edison / Santa Clarita

The story

This one started with a question that turned out to be about the house rather than the panels: the owner wanted the array on a back building, and there was no back building at the address we had been given. That got sorted before anything was drawn, which is the cheapest moment to sort it.

Then the sizing, done from bills rather than from a rule of thumb. Annual usage came out around seventeen and a half thousand kilowatt-hours; the design was taken to roughly twenty thousand because the household asked for headroom, and the electric vehicle was counted separately rather than folded into an average.

That produced forty-three modules at four hundred watts — 17.2 kW, which is a large residential array — on a tile roof.

The electrical side had its own wrinkle. The service had been upgraded years earlier and the house now had two panels, a main and a sub, both fitted with 200 A breakers. Which of them the array connects to decides where the 120 percent arithmetic is done, and the answer here was a derate of the main breaker in the sub panel. Worth noting that this is the opposite conclusion from a project a few miles away where the busbar had headroom and no derate was needed at all: the rule is the same, the panels are not.

And then plan check came back with a correction that could not be answered by redrawing anything.

The department wanted structural engineering.

This is the point where a lot of projects stall, so it is worth being clear about what is being asked. A roof was built to carry its own covering, the weather and a person walking on it. An array adds dead load spread across specific points — the mounts — rather than evenly. On a tile roof the covering is already heavy, and forty-three modules is not a token addition. The question is whether the rafters, at their span and spacing, carry the combination.

Nobody at a solar company can answer that with a drawing. It takes a licensed structural engineer running the calculation for this roof, and the stamped result becomes part of the permit set.

What it costs is time and a fee. What it buys is the difference between an array that is known to be carried and one that is assumed to be. On a roof this size, with tile already on it, that is not a formality worth resenting.

The engineering was commissioned the day the correction landed, and the revised set went back with it. The department approved it a week later with one further revision — the location of the combiner box — and the permit was issued in the middle of November.

One question was worth asking the city directly before it became an assumption: does the breaker derate need a permit of its own? The answer was no, as long as it is shown on the approved plans. That is a useful thing to have in writing, because a derate treated as a separate electrical job adds a second application and a second wait for nothing.

The array went up at the beginning of December, a little over three months after the first bill was read.

Forty-three modules have been producing on that tile ever since, on rafters somebody checked rather than assumed. The only thing this address has needed in the years since is a monitoring question — not a call about the work.

Cali Energy designed, permitted and installed this 17.2 kW array in Santa Clarita.

How it ran

Three months from a bill to an installed array, and the correction that mattered was about the rafters rather than the wiring.

What we did

• Settled which building the array was going on before anything was drawn.
• Sized the system from actual bills and counted the vehicle separately.
• Established where the 120 percent arithmetic belonged across two panels.
• Commissioned structural engineering the day the correction arrived rather than arguing it.
• Asked the city in writing whether the derate needed its own permit, instead of assuming either way.

System specifications

Is your project like this one?

• Your array is large and your roof is tile.
• Your plan check has come back asking for structural calculations.
• Your house has two panels and nobody has said which one the array joins.
• You are sizing a system and own an electric vehicle.

Cali Energy in Santa Clarita

Frequently asked questions

Why would a solar plan check require a structural engineer?

Because the question is whether the roof carries the load, and that is outside what a solar drawing can answer. An array adds dead load at the mount points rather than evenly, and on a heavy covering like tile with a large module count the department wants the rafters checked at their actual span and spacing.

Does a tile roof make structural review more probable?

It does. Tile is already a heavy covering, so the margin left for anything added on top is smaller than on composition shingle. Combine that with a large array and the calculation stops being obvious, which is when it gets asked for.

Which panel does the 120 percent rule apply to when a house has two?

The one the array backfeeds. Where the connection is made to a sub panel, that is where the arithmetic is done and where any derate is taken — which is why the same rule produces a derate on one house and none on the next.

Should a solar system be sized above current usage?

Sometimes, and it should be a deliberate decision rather than a default. On this project the household asked for headroom above roughly 17,500 kWh a year, and the electric vehicle was counted as its own load instead of being absorbed into an average.

Does Cali Energy install large residential arrays in Santa Clarita?

Yes. Santa Clarita permits through its own Building & Safety Division and interconnects through Southern California Edison. Larger lots here carry larger arrays, and on tile that regularly brings structural review into the permit set.

What this would look like on your roof

Related

• The main breaker did not need derating Santa Clarita · the 120 percent rule is about the busbar Solar
• The wood under the array was rotten Granada Hills, CA · 6.00 kW DC · 30 squares off, and what was underneath Solar
• A vent pipe moved one panel off the plan Santa Clarita · shade you can draw, and a failure you cannot Solar
• The array did not go on the roof at all Santa Clarita · ground mount · concrete, posts and a trench Solar

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