ElectricalToolbox

Extension Cord Voltage Drop Calculator

Estimate voltage drop on a portable extension cord feeding a tool or equipment — jobsite power, not installed wiring. Enter the cord gauge, length, and load (by current or wattage) to see the voltage reaching the tool and whether the run stays under a practical 3% drop. It uses the same single-phase K-factor method as the main Voltage Drop Calculator, with copper conductor data from National Electrical Code (NEC) 2020 Chapter 9, Table 8.

Cord and load
120 V standard receptacle, 240 V for larger tools.

Estimates single-phase drop on copper portable cord using the K-factor (K = 12.9, circular mils from NEC Chapter 9 Table 8). Practical reference, not a code check.

How to use this calculator

  1. Enter the source voltage — 120V for standard receptacles, 240V for larger tools and site power.
  2. Enter the load as current in amps, or switch to power (W) if that is what the nameplate lists.
  3. Select the cord gauge (16, 14, 12, or 10 AWG) and the one-way cord length, or tap a length preset.
  4. Read the voltage drop, percentage, and voltage at the tool. A run over 3% is flagged with the next gauge that clears it.

NEC reference

Conductor resistance comes from the K-factor (K = 12.9 for copper at 75°C), derived from circular-mil data in NEC 2020 Chapter 9, Table 8. Flexible and portable cord is not sized for voltage drop by any NEC table — the 3% threshold here mirrors the recommendation in the informational notes to NEC 210.19 for installed branch circuits and is used as a practical field guideline, not a code requirement. Cords are copper, so the aluminum K-factor does not apply.

Results are a practical reference for matching portable cords to tools and equipment. Flexible cord is not covered by the NEC's fixed-wiring voltage-drop guidance, so this is not a code-compliance check — follow the tool manufacturer's minimum gauge and length ratings.

How extension cord voltage drop is calculated

Single-phase voltage drop on a cord follows the same relationship as any single-phase circuit:

VD = 2 × K × I × L / CM

K is 12.9 for copper at 75°C, I is the load current, L is the one-way cord length, and CM is the conductor's circular mils. The leading 2 accounts for the round trip — down the hot and back on the neutral. Circular mils for the common cord gauges (NEC Chapter 9, Table 8): 16 AWG = 2,580; 14 AWG = 4,110; 12 AWG = 6,530; 10 AWG = 10,380. Cords are copper, so aluminum's K-factor doesn't enter into it. At these small sizes AC and DC resistance are effectively equal — skin effect doesn't matter until conductors get much larger. This calculator assumes a unity power factor (purely resistive load), the conservative assumption for a general-purpose tool — a lagging power factor lowers the true drop, so results here read as an upper bound rather than an exact figure.

Worked example

A 12A tool on a 50 ft, 14 AWG cord at 120V:

VD = 2 × 12.9 × 12 × 50 / 4,110 = 3.77 V

VD% = 3.77 / 120 = 3.14%

Voltage at the tool = 116.2 V

At 3.14% this is just past the 3% guideline. Swapping to a 12 AWG cord on the same run drops it to 2.37 V (1.98%), and the voltage at the tool climbs to about 117.6 V. This is the typical jobsite call: a 14 AWG cord is fine for short pulls but goes marginal on a 50 ft run with a real load.

Common mistakes

  • Sizing by the cord's amp rating alone. A cord stamped 13A carries that current, but the rating says nothing about how much voltage it loses over 50 or 100 feet.
  • Reading gauge backward. 16 AWG is thinner than 12 AWG — a higher number means more drop, not less.
  • Daisy-chaining without adding lengths. Two 50 ft cords drop voltage like one 100 ft run, plus extra resistance at every connection.
  • Ignoring motor startup. Motor-driven tools pull well above running current on inrush; a cord that's fine at running load can sag enough to stall or overheat the motor on startup.
  • Mishandling distance. Enter the one-way length — the calculator already doubles it for the return conductor.

Frequently asked questions

What gauge extension cord do I need for power tools?

It depends on the tool's current draw and the cord length together, not the tool alone. A 12A tool on a 50 ft cord needs 12 AWG to stay near 3% drop; 14 AWG on that run hits about 3.1%, and 16 AWG is well past it. Field rule of thumb: 16 AWG for light loads under ~25 ft, 14 AWG for medium loads to ~50 ft, 12 AWG for heavy loads or runs past 50 ft, and 10 AWG for high-current tools on long runs. Run your exact load and length above to confirm.

How long can an extension cord be before voltage drop matters?

Voltage drop scales linearly with length, so doubling the run doubles the drop. For a 12A load, 14 AWG crosses 3% around 48 ft, 12 AWG holds to about 76 ft, and 10 AWG to roughly 120 ft. Total length is what counts — two 50 ft cords behave like one 100 ft run. Size for the full distance from the receptacle to the tool.

Will a long extension cord damage power tools?

Low voltage at the tool can. Motors compensate for reduced voltage by drawing more current, which raises winding temperature and shortens motor life — showing up as overheating, lost torque, and nuisance trips. Drops past roughly 5% are where motor tools suffer noticeably. Resistive loads like heaters and lights mostly just run weaker. If a saw bogs down or a compressor struggles to start at the end of a long cord, voltage drop is the usual cause.

Is a 16 AWG extension cord enough for a circular saw?

Usually not, except on a short run. A typical 15A circular saw on a 25 ft 16 AWG cord drops over 6% — enough to cut power and stress the motor on startup. 16 AWG suits drills, lights, and small loads under ~25 ft. For saws, grinders, and other high-draw tools, use 12 AWG, or 10 AWG on longer runs. Check the saw's nameplate amps against the length.

Does extension cord voltage drop fall under the NEC?

Not directly. The NEC's 3% voltage-drop figure lives in the informational notes to NEC 210.19 and 215.2 and applies to permanently installed branch circuits and feeders — not portable cords. Flexible cord is addressed elsewhere in the code for ampacity and use, but no NEC table sizes a cord for voltage drop. The 3% threshold here is borrowed as a sensible guideline because the physics are identical; it just isn't a code requirement for cords.

Can I daisy-chain extension cords?

Electrically, the drops add up — two 50 ft cords equal one 100 ft run, and each connection adds resistance and a failure point. If you must chain them, put the heaviest gauge nearest the tool and size for the total length. A single correctly sized cord always beats several light ones strung together.

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