Extension cord gauge / voltage drop

What gauge extension cord do I need, and how much voltage will it drop?

For a 1,500 W space heater (12.5 A at 120 V) on a 100 ft cord, a 10 AWG cord is the first common size that stays inside the 3% voltage-drop guideline: it drops 3.04 V (2.53%) and wastes 38 W as heat. The same heater on a 100 ft 16 AWG cord drops 12.21 V (10.18%) — double the 5% limit — and on 14 AWG it drops 7.68 V (6.40%). Shorter changes everything: at 50 ft, 12 AWG drops just 2.41 V (2.01%), while 14 AWG is already at 3.20%. Gauge and length decide together, so check both before you buy.

Why a thin cord starves the tool at the end of it

Every conductor resists current a little. In a cord, that resistance turns some of your 120 V into heat along the way, so the appliance at the far end runs on whatever voltage is left. The site's wire data (resistance at 75 °C, the values the calculator uses) shows how steep the difference is: 16 AWG copper resists 4.884 ohms per 1,000 ft, 12 AWG resists 1.931, and 10 AWG just 1.215. Those are the calculator's 75 °C values — the 20 °C textbook figures scaled up by about 22%, because a cord under load is a warm cord.

A cord is a round trip — current goes out on one conductor and back on the other — so the calculator doubles the length before applying the resistance. Voltage drop is then simply current × resistance, and the power wasted as heat is current² × resistance. Double the length and you double the drop. Double the current and you quadruple the heat.

Worked example: a 1,500 W space heater, two cord lengths

A space heater is the load in the calculator's own appliance list: 1,500 W, which is 12.5 A at 120 V. Run the numbers for that heater on four common gauges at 50 ft and 100 ft:

Voltage drop for a 1,500 W (12.5 A) load at 120 V, calculated with the wire resistances in the extension cord calculator.
Cord	Resistance (round trip)	Voltage drop	Drop %	Wasted as heat	Voltage at heater
16 AWG × 50 ft	0.49 Ω	6.11 V	5.09%	76 W	113.9 V
16 AWG × 100 ft	0.98 Ω	12.21 V	10.18%	153 W	107.8 V
14 AWG × 50 ft	0.31 Ω	3.84 V	3.20%	48 W	116.2 V
14 AWG × 100 ft	0.61 Ω	7.68 V	6.40%	96 W	112.3 V
12 AWG × 50 ft	0.19 Ω	2.41 V	2.01%	30 W	117.6 V
12 AWG × 100 ft	0.39 Ω	4.83 V	4.02%	60 W	115.2 V
10 AWG × 50 ft	0.12 Ω	1.52 V	1.27%	19 W	118.5 V
10 AWG × 100 ft	0.24 Ω	3.04 V	2.53%	38 W	117.0 V

At 25 ft almost any of these cords is fine for this heater — length, not the heater, is what pushes a setup over the line.
A heater mostly turns the lost voltage into slightly less heat. A motor load is less forgiving: it draws harder to make up torque, runs hotter, and may fail to start (see below).

Two separate checks: ampacity first, then voltage drop

Passing one check does not mean passing the other. Ampacity is the fire-safety limit — the most current a gauge may carry at all. Voltage drop is the performance limit — how much voltage survives the trip. A 14 AWG cord is rated for 15 A, so our 12.5 A heater is legal on it; at 100 ft it still drops 6.40% of the voltage. Legal, but weak.

Ampacity by gauge, from the calculator's wire data.
Gauge	18 AWG	16 AWG	14 AWG	12 AWG	10 AWG	8 AWG
Max current	10 A	13 A	15 A	20 A	30 A	40 A

The 3% and 5% lines

The calculator flags results against two limits: 3% drop for a branch circuit, and 5% for feeder plus branch combined. Under 3% is its “safe” zone; between 3% and 5% it warns; over 5% it calls the setup a danger — equipment damage and overheated conductors become real risks, not rounding errors.

Motors deserve an extra margin. The calculator's data notes that motors draw 3–7× their running current while starting, typically 5×. A cord that drops 2% for a running motor can dip far deeper for the second it starts, which is exactly when the motor needs full voltage to get turning.

Run your own numbers

Your load, your length, your voltage: run them through the calculator and it returns the drop, the wasted watts, and a safety verdict for the exact cord you own or are about to buy.

Extension Cord Voltage Drop Calculator

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