Service rating, busbar rating, main breaker: three numbers, not one
They are three different facts about three different things, they are often three different numbers on the same job, and the solar math runs on the gaps between them.
Answers the Service and panel ratings question during setup.
Everything in this article can be read with the enclosure door open and every metal cover left in place. Do not remove a cover, and do not open a meter enclosure. If a number is not visible that way, it is a question for a licensed electrician, not something to go looking for.
“It is a 200A service, so the answer to all three questions is 200.”
The common misreading. Here is what is actually true.
A setup screen that asks for a service rating, a busbar rating, and a main breaker rating is not asking the same question three times. It is asking what the utility delivers, what the metal bar inside the panel can carry, and what the main breaker trips at. On a very common job those are 200, 200, and 150.
Seeing 200 / 200 / 150 and assuming something is wrong is the usual reaction. Nothing is wrong. A 200A busbar with a 150A main is an ordinary, code-compliant panel, and the difference between those two numbers is exactly what decides how much solar the panel can accept.
What each number actually describes
Service rating: the size of the service the utility brings to the building. It is a property of the service, not of any one box.
Busbar rating: how much current the metal bar inside the panel is rated to carry. Every breaker in the panel clamps onto that bar, and its rating is a physical limit of the panel itself.
Main breaker rating: what the main breaker trips at. This is the number that limits how much the building can actually draw, and it can be smaller than the busbar it protects.
Why the gap between them matters for solar
When solar backfeeds a panel through a breaker, NEC 705.12 limits the total: the main breaker rating, plus 125 percent of the inverter's continuous output current, cannot exceed 120 percent of the busbar rating. That is the 120 percent rule. Note the solar term is the CURRENT, not the breaker size. Older write-ups use the breaker, which is close but slightly conservative and can block a design the code allows.
Two conditions ride with that arithmetic. The solar breaker has to sit at the opposite end of the busbar from the main, and a permanent warning label goes at the breaker saying it must not be relocated. A center-fed panel has no opposite end, so it is handled under a separate provision and never run through this calculation as if it were end-fed.
Run it on 200 / 200 / 150. The busbar allows 240 amps of total supply. The main takes 150 of it, leaving 90 amps for the solar term, which covers an inverter rated up to about 72 amps continuous. Now run it on 200 / 200 / 200: the main takes 200, leaving 40, which covers about 32 amps. Same service, same busbar, and less than half the room for solar, purely because the main breaker is bigger.
The bar is rated 200 and the rule allows 240 because the two sources feed it from opposite ends, and the loads tapped along it draw current out in between, so no single point of the bar carries the full 240. That is also why the breaker has to go at the far end.
In kilowatts: at 240 volts a 40 amp ceiling is roughly 7.6 kW of inverter output and a 90 amp ceiling is roughly 17 kW. Divide the headroom by 1.25 to get the allowed continuous output current, then multiply by the service voltage.
That is why the three numbers get asked separately. Collapse them into one and the calculation either blocks a design that is fine or permits one that is not.
How to get the answer
Each number is printed somewhere different, and all three are readable with every cover in place. Read them off the equipment rather than inferring them from each other.
- 1Service rating: read it off the service disconnect, not the meter. Per NEC 230.79 a service is rated at its disconnecting means, so a 150A main breaker on a house is a 150A service. CL200 on the meter is the METER class, the largest current that meter can measure, and utilities fit the same class across services of different sizes. A CL320 socket is the standard marking on a 400A residential service.
- 2Busbar rating: the label on the inside of the panel door. The marking is often "MAIN BUS RATING", "BUS", or a bare number followed by A near the catalogue number. Where the label gives none, the catalogue number decodes to a bus rating on the manufacturer site, and on a panel with a factory main the bus is at least the main breaker rating. Record it as unknown rather than assuming it equals the service.
- 3Main breaker rating: stamped on the main breaker handle itself. If there is no main breaker, the panel is MLO and there is no third number to record.
- 4If two of them disagree with what you expected, trust the labels. The equipment is the record.
What to photograph
- The meter socket sticker.
- The label on the inside of the panel door, plus the catalogue number.
- The main breaker handle, close enough to read the stamp.
This is guidance for reading equipment that is already visible and for describing it on a drawing. It is not instruction for electrical work. Any work on service equipment, panels, or conductors is for a licensed electrician, and local code and utility rules govern.
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