Extension Cord Size Calculator

Extension cord gauge selection is a safety-critical decision. An undersized cord creates resistance, resistance generates heat, and that is a leading cause of residential electrical fires. The two key factors are the current draw of the connected device and the length of the cord run.

The wattage-to-amperage formula:

Current (A) = Wattage (W) ÷ Voltage (V)

For US standard 120V outlets: Amperage = Watts ÷ 120

Two separate limits, and a cord has to clear both

This is the part most cord charts skip, and it is why two charts rarely agree.

The first limit is ampacity: how much current the copper can carry before the insulation cooks. It depends only on the gauge, not the length. For flexible cord the accepted figures are 7 A for 18 AWG, 13 A for 16, 15 A for 14, 20 A for 12 and 30 A for 10.

The second limit is voltage drop, and this is the one that depends on length. Copper has resistance, current through resistance loses voltage, and the motor at the far end sees less than 120 V. Run a saw at 105 V and it draws more current to make the same power, runs hotter, and dies younger.

Voltage drop = 2 × length(ft) × amps × resistance(ohms per 1,000 ft) ÷ 1,000

The doubling is because current goes out along one conductor and back along the other. Resistance per 1,000 ft: 6.385 for 18 AWG, 4.016 for 16, 2.525 for 14, 1.588 for 12, 0.999 for 10.

5% drop, 6 volts on a 120 V supply, is the limit worth holding to. Under 3% is comfortable. Between 3% and 5% is acceptable for lights and heaters but hard on motors. Past 5% you are heating the cord instead of running the tool.

Worked example: A circular saw pulling 1,800 W on a 75-foot cord.

• Current: 1,800 ÷ 120 = 15 A
• AWG 16 is rated for 13 A. Fails on ampacity before length even matters.
• AWG 14 clears ampacity at 15 A, but drop is 2 × 75 × 15 × 2.525 ÷ 1,000 = 5.68 V, which is 4.7%. Legal, and enough to make a saw labour.
• AWG 12: drop is 2 × 75 × 15 × 1.588 ÷ 1,000 = 3.57 V, or 2.98%. Under the comfortable threshold.

AWG 12 is the right cord, which is also what any contractor would tell you and what a 12/3 cord is sold to do.

Safety rules:

• Never use a lower AWG number as a substitute for a higher one: AWG 16 is thinner than AWG 12
• Never daisy-chain extension cords (one plugged into another)
• Fully unroll coiled extension cords before use: a coiled cord acts as a heat trap
• Never run cords under rugs or carpets
• Replace any cord with cracked insulation, exposed wire, or a warm plug immediately

Cord ratings to look for:

• SJ / SJT: light-duty indoor cords
• SO / SOW: heavy-duty outdoor cords
• UL Listed: verified safety standard
• SJTW: all-weather rated