Service size is the ceiling on your whole house. Everything else in the panel divides up what it lets in, which is why it is the first number an electrician asks for and the number that decides whether the thing you want to install is a morning's work or a month's planning.
What sets it
Three things in series, and the smallest one wins: the cable running in from the street, the meter base it passes through, and the main breaker at the top of your panel. Upgrading the panel alone does not raise the service if the cable and meter base are still sized for the old one.
That is the detail most people miss, and it explains a lot of quotes. Swapping a panel is contained work inside the house. Raising a service means the utility disconnects you, equipment outside gets replaced, and an inspection follows.
Why 100A and 200A
These are not arbitrary. They are the standard residential sizes manufacturers build panels and breakers around, which is why a quote will move you between them rather than to some number in between.
A hundred amps was generous for a house built in the sixties: lights, a stove, maybe an electric dryer. It is tight for a house that has since added central air, a heat pump, and a car that charges overnight. Nothing failed. The house simply started asking for more than it was built to supply.
Capacity is not the same as usage
A house rarely draws anything close to its service size. Service size is about the worst simultaneous moment — a January evening with the heat running, the oven on, and the car charging — not the average.
This is why the code calculation applies demand factors rather than adding up every appliance at full tilt. The question is not what your house uses, but what it could plausibly ask for at once.
How to spot it in your house
- Open the panel door and read the number on the main breaker handle at the top. That is your service size.
- If there is no obvious single large breaker, you may have a split-bus panel, which is older and worth having looked at.
- A label inside the door sometimes states the panel rating, but the panel can be rated higher than the service actually feeding it — the main breaker is the reliable answer.
The next question
Check what your house actually needsRelated terms
- Main breakerThe single large breaker at the top of your panel that cuts power to the whole house. Its rating is your service size.
- Panel upgradeReplacing the breaker panel inside your home, the box the circuits land in, usually with the main breaker at the same time.
- Load calculationThe arithmetic that adds up everything drawing power in a house to work out what service size it actually needs, set out in OESC Rule 8-200.