Start with the cord and tool labels
An extension-cord calculation begins with the actual equipment current and the cord's marked gauge, length, voltage and usage rating. Do not assume every cord with the same AWG has the same allowable current or environmental rating. Use a listed, factory-assembled cord set suitable for the location and equipment.
OSHA warns that flexible cords are more exposed to damage than fixed wiring. Construction cords must be the required three-wire type for grounding where applicable, marked for hard or extra-hard usage, protected from damage and inspected before use.
How do you calculate voltage drop?
Every metallic conductor has internal electrical resistance. As current flows through a run of copper or aluminum wire, part of the source voltage is lost as heat along the cable path. Using Ohm's law (V = I × R) and circular-mil resistivity at 75°C, the voltage lost across a two-wire circuit is:
where:
- VD – Total voltage drop between the supply panel and the load terminals, measured in volts (need a target gauge instead? Use the wire size calculator);
- K – Conductor resistance constant at 75°C (K = 12.9 for copper and K = 21.2 for aluminum in Ω·CM/ft);
- I – Load current flowing through the circuit in amps (estimate from wattage with our watts to amps calculator);
- L – One-way wire distance in feet (the multiplier of 2 already accounts for the return conductor); and
- CM – Conductor cross-sectional area in circular mils from NEC Chapter 9, Table 8.
Three-phase AC voltage drop equation
In a balanced three-phase system, the return currents cancel in phase opposition, so the round-trip factor of 2 is replaced by √3:
Subtract VD from the source voltage (V_load = V_source − VD) to find the actual voltage delivered to the equipment terminals. Explore all related tools in our electrical calculators hub.
Estimated drop at 15 A and 120 V
The table shows the direction of change: doubling length doubles the simplified voltage loss, while a larger conductor reduces it. Actual cord construction, connection resistance, conductor temperature and the operating load can produce a different result.
| One-way cord length | 16 AWG | 14 AWG | 12 AWG | 10 AWG |
|---|---|---|---|---|
| 25 ft | 3.12% | 1.96% | 1.23% | 0.78% |
| 50 ft | 6.25% | 3.92% | 2.47% | 1.55% |
| 100 ft | 12.50% | 7.85% | 4.94% | 3.11% |
Five checks before plugging in
- 01
Read the equipment nameplate
Confirm voltage, current and manufacturer limits. Starting current may exceed running current.
- 02
Read the cord markings
Confirm AWG, length, conductor count and indoor, outdoor, hard-service or temperature ratings.
- 03
Inspect the entire cord
Look for missing pins, cuts, crushed areas, loose fittings, exposed conductors and unapproved repairs.
- 04
Protect the route
Keep cords away from water, sharp edges, vehicle traffic, pinch points and trip hazards.
- 05
Stop if performance changes
Heat, odor, tool slowdown, nuisance tripping or damaged connections require the equipment to be de-energized and evaluated.
Why a larger gauge is not the only answer
- The cord must be rated for the equipment current and environment.
- Grounding continuity and required GFCI protection remain essential.
- A cord does not convert equipment to a different voltage or plug configuration.
- Coiled cords, damaged connectors and poor receptacles can create additional heating.
- Multiple connected cords add connections and total length; use the complete path in the estimate.
- Extension cords are temporary equipment, not a replacement for required permanent wiring.
Use the calculator without turning it into a rating
For listed sizes, enter the marked conductor gauge, copper material, actual current and one-way cord length in the voltage-drop calculator. The 16 AWG chart uses 2,580 circular mils, which sits below the interactive calculator's 14 AWG to 4/0 list; do not substitute 14 AWG for a 16 AWG cord. Treat the result as one diagnostic input. It can help explain why a long cord performs differently, but it cannot determine whether the complete cord set is listed or safe for the task.
When a manufacturer supplies a cord-size table, that product guidance should be used with the equipment instructions. For jobsite decisions, follow the applicable safety program and qualified supervision.