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Circuit Capacity

Circuit capacity depends on the voltage and current ratings of wires and circuit breakers. When a circuit is rated for 15 amps, the wires and circuit breakers must be rated for at least 15 amps. The circuit is rated for the lowest current rating of wire or circuit breaker. For example, if the wire is rated for 15 amps, the circuit should not have a 20 amp breaker even if the breaker trips frequently.

It is very important that a 20 amp (or higher) circuit breaker NOT be used on a circuit with a wire rated for 15 amps or less.

Electrical Panel and Circuit Wiring Diagram

Electrical Circuits

From power company

Main breaker

120 V A phase

120 V B phase

120 V B Phase

120 V A Phase

Household Circuits Diagram

These diagrams show a typical circuit path from the power company through the main electrical panel and to the outlets (light sockets, switches, and receptacles).

Single pole circuit breakers


The potential difference between the A phase and B phase is 240 volts.

Ground bar

Neutral bar

240 V

Ground and neutral are connected together

in one location only, at the neutral point either at the power company transformer or in the main electrical panel.

Electrical Panel

Electric meter

Two pole

circuit breaker

for electric oven, dryer and/or air conditioner

240 V A & B



(ground not shown)

120 V


Basic Electricity

Electrical Voltages

Electrical Ground

Low Voltage

Electrical Definitions

Ohm’s Law

Electrical Circuits

Multi-Wire Branch Circuits

Circuit Current Ratings

NM Cable (Romex®)

Wire Connectors