Key takeaways
- A load-side solar connection is limited by the busbar rating, not the main breaker alone. The code's 120 percent rule sets the ceiling.
- The arithmetic is simple: busbar rating times 1.2, minus the main breaker, is the largest solar breaker the panel can take.
- Derating the main breaker is a legitimate way to make room, within limits, and it is cheaper than replacing the panel.
- CPS Energy interconnection is a separate process from the city permit, with its own documentation, and it happens before the system is allowed to run.
Homeowners looking at solar in San Antonio usually start with the roof: which direction it faces, how much shade the oaks throw, how many panels will fit. Those matter. But the constraint that most often decides how big a system a house can actually take sits inside the electrical panel, and it is a piece of copper most people have never heard of.
The busbar is the thing that limits you
Inside your panel, the main breaker feeds a pair of metal bars that run the length of the enclosure. Every branch breaker clips onto them. Those bars are the busbar, and they carry a rating, usually stamped on the panel label alongside the service rating.
Under normal operation, power enters the busbar at one end, through the main breaker, and leaves through the branch breakers. Nothing on the bar can see more than the main breaker will pass. Add a solar system connected through a breaker in that same panel and the picture changes: now power enters from two directions, the utility through the main and the array through the solar breaker, and the part of the bar between them can carry both at once.
That is the whole reason the rule exists.
The arithmetic
NEC Article 705 sets the limit for this configuration, and it is known in the trade as the 120 percent rule. The main breaker and the solar breaker together cannot exceed 120 percent of the busbar rating.
Written as arithmetic: take the busbar rating, multiply by 1.2, subtract the main breaker. What is left is the largest solar breaker the panel can accept.
For the most common residential case, a 200 amp busbar with a 200 amp main: 200 times 1.2 is 240, minus 200 leaves 40. A 40 amp solar breaker. That is a real ceiling on system size, and it is why an installer can look at your panel photo and tell you something meaningful before anyone climbs on the roof.
Run the same math on a 125 amp busbar with a 125 amp main and the answer is 25 amps. Same rule, much less room.
What to do when the number is too small
There is more than one answer, and the cheapest one is the one that gets skipped.
Derating the main breaker is the first thing to price. Reducing the main from 200 amps to a smaller size increases the headroom the rule leaves for solar, and swapping a main breaker is a fraction of the cost of replacing a panel. The catch is that the main still has to carry the house, so this only works if a load calculation confirms the smaller breaker is adequate for your actual demand. In a home with gas heat, gas water heating, and a modest cooling load, there is often room. In an all-electric home with two air conditioners, there is not.
A supply-side connection is the second path. Instead of landing solar on the busbar, the system connects ahead of the main breaker entirely, which takes the busbar out of the equation. It is a different install with different requirements, and it is a legitimate answer when the panel simply has no room.
Replacing the panel is the third. Sometimes it is the right call anyway, particularly if the existing panel is full, obsolete, or one of the brands with a documented failure history. If you are replacing it for solar, size the busbar for what comes next rather than for the array alone.
If the panel is coming out regardless, it is worth reading Federal Pacific and Zinsco panels in San Antonio homes first, because those two brands change the calculus from optional to overdue. The work itself is a panel upgrade.
Interconnection is a separate process
This is the step that surprises people, and it is specific to being in CPS Energy territory.
The city permit governs whether the installation is safe and to code. Interconnection governs whether the utility will allow your system to operate in parallel with its grid. They are separate approvals with separate paperwork, and the second one is not a formality.
CPS Energy expects an interconnection agreement along with technical documentation: a one-line diagram showing how the system connects from the array through the inverter to your service, a site plan, and manufacturer specifications for the equipment. Larger systems carry additional engineering requirements. Timelines vary, which is worth building into your expectations rather than discovering after the array is installed and waiting.
None of this is a reason not to go solar. It is a reason to have the panel question answered early, because a panel change discovered late sits in the middle of two approval processes at once.
What to check before you call anyone
- The busbar rating on the panel label, which is often listed separately from the service size.
- The size printed on your main breaker handle.
- Whether the panel has physical space left for a two-pole breaker.
- The panel brand, since some are obsolete and change the plan entirely.
- Whether an EV charger, a pool, or an addition is also in your plans.
With those five items, the 120 percent arithmetic takes about ten seconds and tells you most of what you need to know.
If an EV is part of the picture, run both numbers together rather than in sequence. We covered the charger side in what panel capacity an EV charger actually needs, and the load calculation behind both is the same piece of work. We also handle the electrical inspection that documents what you have before a solar contractor quotes you.
We do panel work for solar projects across the metro, from Cibolo and Schertz to Helotes. Send us a photo of your panel label and we will tell you what your busbar will take.