Wire ampacity

Wire Size & Amp Ratings

• Ampacity refers to the maximum current a conductor can safely carry continuously without exceeding its temperature rating, as defined in NEC Article 310.16. Current is measured in amperes (amps). Always select the correct wire size for the expected load to prevent overheating or conductor damage.

• The number and type of electrical devices connected to a circuit determine the required ampacity. Typically, general-purpose household circuits are rated for 20 amps, while lighting circuits are often rated for 15 amps, in accordance with NEC 210.3 and 210.19(A)(1).

To determine the correct conductor size:

• Add the total wattage of all devices on the circuit.

• Divide that number by the system voltage (usually 120V or 240V).

• The result gives the current (amps) required. (Formula: Amps = Watts ÷ Volts)

• Always refer to NEC Table 310.16 to match the conductor size and insulation type to the circuit’s required ampacity.