Are the individual breakers and branch circuits on a service main truly isolated from one another?

Question

United States here. I have a 150-amp main service panel in my house and I'm trying to understand how (if at all) the individual branch circuits (and their breakers) interact with each other and the panel at large.

I have several "slots" open and unused in the panel. Meaning, several locations where more breakers could be added, and hence, more branch circuits.

But, to me, just because the panel has more slots doesn't necessarily mean it can handle more breakers + circuits. If every time I added a new breaker that previously wasn't there, will that some how reduce the amount of voltage or amperage available to the other breakers/circuits? And somehow put more strain on them or hurt their "performance"? Or is each branch circuit completely isolated from the others, making it so that I am free + clear to fill all the unused slots up with breakers and more circuits?

Answer 1

Correct - number of breaker spaces has no bearing on system ampacity. They are two unrelated parameters, like money in your account vs checks in your checkbook.

Every appliance draws the current it needs, and no more. Varying as its needs change. The sum total of those actual draws (for each phase) constitutes the actual load on your panel. Having more breakers in your panel doesn't add any load, the loads do.

Around here, we advocate a gross excess of breaker spaces, since they're dirt cheap. But your whole 150A ampacity could be consumed in 4 spaces. I had a friend with 400A service (two 200A panels). 2 breakers (4 spaces) accounted for 140A of emergency heat on a heat pump. But only when the emergency heat was on, and that means the heat pump proper is not on, so the 16A heat pump can overlap into that same 140A. So it isn't 140A+16A, it's 0A with nothing running, 16A with heat pump running, OR 140A with emergency heat running.

If you overload your panel you will get a main breaker trip. That breaker is there to protect the panel and service wiring from damage from overloading.

No professional electrician will build a panel that will overload. This is determined by a formal process (2 actually) called a Load Calculation. You input square footage of house (as a catch-all for general lighting and receptacles), an allocation for 20A kitchen and bathroom circuits, the nameplate data of most 240V appliances except some wild math on the range, and you add them up. It must be less than the service ampacity or they won't give you an occupancy permit.

That is vanishingly unlikely to trip the main breaker. Back in the dark days, they permitted house with up to six "main breakers" which added up to well over the service ampacity, hanging their hat entirely on the load calc.

The Achilles' heel of this system is upgrades. People add loads and don't redo the load calc. That was particularly a problem with Rule of Six panels, which is why they are outlawed.

Answer 2

Each breaker/circuit is independent of the others, with some specific exceptions:

• MWBC - You can have two breakers next to each other, on different legs of the service, that share a neutral. That is a "Multi-Wire Branch Circuit" (MWBC). An MWBC in some ways is "one circuit" and in some ways it is "two circuits". Not really the issue here, but it is a "two breakers affect each other" situation, and as a result they (a) have to be next to each other and (b) have to be physically connected (double breaker or handle tie) so that turning off one turns off the other. The sharing actually helps on the neutral because the neutral carries the difference between the two instead of adding together.
• AFCI - Due to the way arc fault detection is done, there can be some effect from one circuit on another. But that should be relatively rare, and would likely be an indication of a big potential problem. It isn't so much that the circuits affect each other but rather that the arc could be detected in more places than just the circuit where it happened.

In any case, more breakers is fine. There is no effect on voltage unless you draw so much power that you effectively cause a brown-out for your house. (Think "laser printer starts up and lights dim" - normally happens only on the same circuit but enough usage throughout the house could have impact on everything else in the house.)

There is a concern about current, but more breakers/circuits is an improvement, at least compared to simply putting more stuff on each existing circuit, because it decreases the likelihood that any one circuit will be overloaded because you are splitting the current across more circuits.

All that being said, there is a concern if you dramatically increase the total usage - e.g., by adding EV charging or on-demand water heating - that you could overload the main service. So when a major change is made, you really should have a load calculation to determine if your total service is up to the task. But adding new circuits that either have short term usage (e.g., circuits for tools that get used for a few minutes at a time) or low continuous usage (e.g., LED lighting) generally will have minimal effect on the total building electrical load.