# Clarifying how circuit breakers work

I just wanted to clarify my understanding of circuit breakers. I'm planning on putting a new one in my house.

The circuit breaker has a maximum amount of current the main breaker panel can hold.

Each switch has a maximum current, which may vary depending on the switches used.

There is a limit to how much current can pass through the switches in total, which is the sum of the maximums for each of the switches all together.

Each individual switch's maximum current must be less than or equal to the main breaker's maximum capacity.

The total amount of current for all switches must be less than or equal to the main breaker's maximum capacity.

The current for each circuit must be less than or equal to the maximum current for that circuit's switch.

Is my understanding correct?

I just wanted to clarify my understanding of circuit breakers. I'm planning on putting a new one in my house.

Based on the rest of your question, what you are calling a circuit breaker is what is normally called a circuit breaker panel or main panel (if it is the only one) or subpanel.

The circuit breaker has a maximum amount of current the main breaker panel can hold.

The main breaker is a specific circuit breaker which limits the maximum current for the entire panel.

Each switch has a maximum current, which may vary depending on the switches used.

These switches you refer to are each an actual circuit breaker. Each one has a maximum current allowed. They do typically look like a switch, and in fact they are used as a switch both for safety (to turn off a circuit for repairs) and also are usually now switch rated, meaning they can be used as switches for convenience. But they are commonly referred to as circuit breakers, and the term switch by itself normally refers to an item that can turn a circuit on/off but does not actually provide protection.

There is a limit to how much current can pass through the switches in total, which is the sum of the maximums for each of the switches all together.

There is a limit for the panel set by the main breaker.

Each individual switch's maximum current must be less than or equal to the main breaker's maximum capacity.

Correct. Each circuit breaker must be equal to or smaller than the main braeker.

The total amount of current for all switches must be less than or equal to the main breaker's maximum capacity.

Not really. The total actually in use must be less than the main breaker. But the sum of the capacities of all the circuit breakers can be (and usually is) more than the main breaker, they just aren't all maxed out at the same time.

The current for each circuit must be less than or equal to the maximum current for that circuit's switch.

Correct.

• Thanks for going item by item. Super helpful Apr 9, 2023 at 17:12
• Aside: Another factor is the interrupting rating of the device, i.e. the maximum fault current that the device can interrupt without damage. A main breaker will typically have a higher interrupting rating that the branch circuit breakers it feeds.
– HABO
Apr 9, 2023 at 18:27

Main breaker cannot be more than the panels rating, it can be less. It will also depend on what your power company supplies you.

Branch circuit breakers be rated for the size of the wires for that circuit. 15 amp breakers for circuits with 14 gauge wires, 20 amp breakers for circuits only containing 12 gauge wires, and so on.

Most panels will have branch circuit breakers adding up to more than the main breaker rating. Most branch circuits do not use their full rating. A 20 amp circuit might only use a few amps most of the time.

• Agreed Crip: nearly all main panels are over-subscribed, It's not like: "OK, I've got ten 20 amp/120v breakers on one leg and ten on the other leg and I'm now maxed out." If all the breakers were fully drawing 20 amps, you'd have to rob Fort Knox to pay your power bill!!!! OK, that said, unless you are running grow lamps continuously (IE MJ!) or a bit coin mining installation, you should be fine. What will the additional breaker be used for? Apr 9, 2023 at 19:15

## Breakers protect wires and equipment

The reason to put a circuit breaker on something is that the thing will be damaged or create a hazard if too much current flows.

For instance in a home panel, most circuit breakers are there to serve branch circuits, where the primary concern is to avoid overheating the wires in the walls. For instance, a circuit might use 2.5 mm2 or 14 AWG wire, and need 15-16 amp circuit breaker protection.

An electric service to a house has wires of limited size both from the utility pole and also to the circuit breaker panel, and those must be protected by an appropriate sized breaker such as 64A, 100A, 200A, etc. corresponding to the wire size.

A large appliance such as a range/hob/cooktop might need a 32A or 40A circuit breaker so that if a short develops inside a heating element, and current flow increases, power is cut before severe damage occurs or a fire starts.

In an industrial context, with all machines running near their maximum most of the time, the supply feeding the circuit panel may need to be approximately that of the sum of the circuit breakers. For instance a shop with a 20A compressor, 10A dust collector and 20A jointer may need 50A to its breaker panel, because all those machines could be running at the same time. However, consider a home shop with only one craftsman. If you're running the 20A jointer, you are not running the 10A band saw or 20A planer. So those appliances don't require an enlarging of the supply.

This concept is called "load diversity" and it applies in a big way to dwellings. That is because dwellings are well-studied and there is good research to support that the maximum possible loads are almost never used. Your nation's electrical codes (by default: NEC in 100-127V country and European regs everywhere else) have formulas for calculating residential load. So "oversubscription" (when the branch circuit breakers add up to more than the main breaker) is perfectly permitted under these rules.

Note that if you are doing something bizarre, like taking a big American house (for its high capacity air conditioning) and sticking Bitcoin miners on every circuit, then you can no longer rely on the assumptions in those Load Calculations, and must compute based on actual hard load.