Sizing a circuit: watts, amps, wire and breaker

Every circuit has the same four numbers that must agree: the load it has to carry, the breaker that protects it, the wire that is protected, and the length of the run. Get them in the right order and the answer is straightforward. Get them out of order, for example by sizing the wire for the breaker you happen to have, and the result is a wire that overheats before the breaker trips.

This planner is for understanding and checking a design. Wiring a circuit is regulated work: it usually needs a permit and an inspection, and a licensed electrician should do it or check it.

Step 1: Find the load in watts

Add up what will run on the circuit at the same time, from the labels on the equipment. A space heater, a microwave and a hair dryer are each about 1,500 watts, and a motor or compressor draws more at start-up than while running. For motors, the nameplate gives the full-load amps.

Step 2: Convert to amps

Amps are watts divided by volts (and by the power factor for motors and some electronics). The same load takes half the current at 240 volts as at 120, which is why big loads use 240 volts.

Step 3: Choose the breaker

A load that runs for three hours or more is a continuous load, and the breaker is sized for 125% of it, which is the same as keeping the load to 80% of the breaker rating. Breakers come in standard sizes, so round up to the next one, and never put a bigger breaker on a circuit to stop it tripping.

Step 4: Choose the wire

The wire must carry the breaker's current without overheating, which depends on its size and its temperature rating, and for small wires the breaker is limited too: 14 AWG copper to a 15 amp breaker, 12 AWG to 20 amps and 10 AWG to 30 amps. Aluminum needs a larger size than copper.

Step 5: Check the voltage drop on a long run

On long runs, and on low-voltage systems such as solar, RV and landscape lighting, the wire loses voltage. A drop of 3% or less on a branch circuit is the usual aim, and the fix for too much is a thicker wire.

Step 6: Add up the whole panel and special loads

Charging an electric car, a generator, a battery bank or a new workshop adds to the loads on the service. The cost to run any of them depends on the kilowatt-hours used.

Before you start

• The permit and inspection for new circuits.
• The main breaker panel, with a free slot and enough capacity for the new load.
• Equipment labels, which give the maximum breaker size and the current.
• GFCI and AFCI requirements for the room or location.
• A licensed electrician to do or check the work.

Mistakes that cost people money

• Fitting a bigger breaker because the circuit keeps tripping.
• Sizing the wire for the load but not the breaker, or the other way round.
• Forgetting that a continuous load needs 125%.
• Ignoring voltage drop on long or low-voltage runs.
• Mixing wire sizes in one circuit so the smallest wire sets the limit.

Frequently asked questions