Extension Cord Ampacity Calculator

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Table of contents

• How to use
• Common mistakes
• Definitions
• Methodology
• Limitations & assumptions
• Sources

AWG: American Wire Gauge. It is the wire-size scale used for cords. A smaller AWG number means a thicker wire.

Ampacity: The current a cord conductor can carry without getting too hot under the planning conditions used by the calculator.

Voltage drop: Voltage lost as current moves through the cord resistance. Long cords, thin cords, and higher current increase it.

Load current: The current used by the device. If you enter watts, the calculator divides watts by Source voltage to estimate amps.

Continuous load: A load expected to run for 3 hours or more. The calculator uses 80% of the base cord ampacity for this case.

Round-trip resistance: The resistance of both conductors in the cord path, because current travels to the device and back.

Mistake: Entering a watt value in Load amount while Load unit is set to Amps (A).
Fix: Match Load unit to the unit on the device label, or change Load amount to the correct amp value.

Mistake: Doubling Cord length because electricity travels out and back.
Fix: Enter only the one-way Cord length from the outlet to the device; the calculation doubles it internally.

Mistake: Picking a larger AWG number in Cord wire size because it looks stronger.
Fix: Use the actual Cord wire size printed on the cord; with AWG, 12 AWG is thicker than 16 AWG.

Mistake: Leaving Smallest plug, outlet, or printed cord rating higher than the real outlet or cord label.
Fix: Use the lowest rating in the whole chain, such as 15 A if any part is rated 15 A.

Mistake: Choosing No for Will it run for 3 hours or more? when the load will run for a long time.
Fix: Choose Yes for long steady loads so Cord heat limit used is reduced.

Mistake: Treating Answer as a promise that any cord is safe in any condition.
Fix: Also read Safety note, and do not use a coiled, covered, damaged, wet-location-unrated, or permanently installed extension cord.

• This is a planning calculator for common copper extension-cord sizes from 18 AWG through 10 AWG. It does not cover every cord type, insulation rating, conductor material, temperature rating, or special industrial cord.
• The ampacity values are planning values. The printed rating on the actual cord, plug, outlet, breaker, and device instructions can be lower and should control.
• Voltage drop is estimated from conductor resistance. Real voltage at the device can differ because of outlet voltage, connectors, temperature, cord condition, and the device power factor.
• The Load amount should be the normal running load, not a motor startup surge. Motors and compressors can need much more current for a short time.
• A passing result does not make unsafe use safe. Do not run cords coiled, under rugs, through walls, through doors, in wet areas without proper outdoor rating, or as a replacement for permanent wiring.
• If no listed Cord wire size passes, the calculator does not design a different power system. Shorten the cord, reduce the load, use a properly rated circuit, or ask a qualified electrician.

How the calculator uses your entries

The calculator first turns the Load amount into amps. If Load unit is Amps (A), it uses the entered value. If Load unit is Watts (W), it divides watts by Source voltage.

load_amps = load_watts / source_voltage

Next it looks up the selected Cord wire size in the built-in AWG table. The table uses copper resistance in ohms per 1000 ft and base flexible-cord ampacity for 18, 16, 14, 12, and 10 AWG.

round_trip_resistance_ohms = 2 * cord_length_ft * resistance_ohms_per_1000_ft / 1000

The factor of 2 is used because Cord length is one-way length, while the electrical path goes out to the device and back.

Voltage drop and heat

Voltage lost in the cord is current times round-trip resistance.

voltage_drop_volts = load_amps * round_trip_resistance_ohms

voltage_drop_percent = 100 * voltage_drop_volts / source_voltage

voltage_at_device_volts = source_voltage - voltage_drop_volts

The calculator also estimates heat made in the cord.

power_loss_watts = load_amps * load_amps * round_trip_resistance_ohms

Current limits

The heat limit starts with the base ampacity for the chosen AWG. If Will it run for 3 hours or more? is Yes, the calculator multiplies that ampacity by 0.8.

heat_limit_amps = base_ampacity_amps * continuous_factor

The voltage-drop limit is the current that would exactly hit the selected Largest voltage drop to allow.

drop_limit_amps = (source_voltage * max_drop_percent / 100) / round_trip_resistance_ohms

Most current this setup should carry is the smallest of the heat limit, voltage-drop limit, the equipment rating, and the separately selected breaker limit. The breaker limit is multiplied by 0.8 for a continuous load; the equipment nameplate cap is not multiplied again.

max_usable_current_amps = min(heat_limit_amps, drop_limit_amps, rating_limit_amps_if_used, breaker_limit_amps_if_used)

What limits the setup names the smallest limit. In check mode, Answer passes only when the load current is less than or equal to that smallest limit. In find mode, the calculator tests 18, 16, 14, 12, then 10 AWG and returns the first size that passes.

recommended_awg = first_awg_in_order_where(load_amps <= min(heat_limit_amps, drop_limit_amps, rating_limit_amps_if_used, breaker_limit_amps_if_used))

Mini example

For a 12 A load on a 50 ft, 12 AWG cord at 120 V, the round-trip resistance is 2 * 50 * 1.588 / 1000 = 0.1588 ohms. Voltage lost in the cord is 12 * 0.1588 = 1.9056 V, or 1.588% of 120 V. The estimated voltage at the device is 118.0944 V, and heat in the cord is 12 * 12 * 0.1588 = 22.8672 W. With a 15 A plug or outlet limit, the most current this setup should carry is 15 A, so a 12 A load passes the calculated checks.