How to Calculate the Amp Rating of a Wire

To calculate amps you need the voltage and wattage.

Ampere, or more commonly known as amps, is the measurement unit of electrical current. Amps coincide with volts and watts in a three-part equation. With two of the parts, the third is realized. To calculate the amps in a wire, one must first identify the volts and watts that the wire harnesses. Knowing the amount of amps in a wire is necessary to make the right circuitry connections in a home. When handling wires, however, never touch a live wire, as the risk for serious injury is ever present.

Things You'll Need

• Screwdriver
• Wattage meter

Instructions

1. Flip off the breaker that controls the wire in question. Remove the wire from the device. For example, if the wire is attached to an outlet, unscrew the outlet from the wall and expose the wires in the back.

2. Locate the black wire and remove it from the device. Typically the wire is attached by a brass-colored screw. Loosen the screw and unwrap the wire tip, freeing it completely.

3. Flip the breaker back on to restore power to the wire. Be extremely careful near the wire and never allow a pet or child to come near the exposed wire.

4. Place the wattage tester probe up against the wire tip, without touching the tip with bare hands, and turn the wattage tester on. In a few seconds the reading will settle on a number. Write that number down.

5. Shut the power off at the breaker, then replace the black wire to the outlet's brass screw. Secure the outlet back into the wall. Turn the power back on.

6. Examine the insulated jacket over the wire in question. This can either be performed at the breaker panel or at the outlet before the outlet is remounted. Identify numbers on the jacket such as, 12/2, meaning the wire is #12 gauge with 2 insulated wires inside. Also find a three-digit number with a small v at the end. This is the amount of volts the wire conducts. Write this number down.

7. Divide the number of watts by the number of volts. This final number is the amount of amps.

References

• EBTX: Amps and Volts
• Don Vandervort's Home Tips: How to Wire an Electrical Receptacle
• Hammer Zone: Installing a New Circuit Breaker