Extension Cord Voltage Drop Guide — AWG, Round-Trip & Welder Cords

Why Extension Cords Are Different

Permanent branch circuits are sized with NEC Chapter 3 conductors, ambient correction, and continuous-load rules. An extension cord is a portable assembly: copper conductors in a flexible jacket, often smaller than the breaker feeding the receptacle, and always carrying current out and back through the same cable length.

That last point matters. If you lay out 100 feet of cord between the receptacle and the tool, the conductor length in the voltage-drop formula is 200 feet (out and return). Using one-way distance understates drop by roughly half and is a common field mistake.

The Cord Voltage Drop Formula

For single-phase (typical cord loads):

VD = (2 × L × I × R) / 1000

Where L is one-way length in feet, I is load amps, and R is ohms per 1,000 ft for the AWG (copper). Percent drop is (VD / V) × 100. Shop extension cords feeding 120/240V tools are almost always treated as single-phase round-trip.

Typical copper resistance values used for planning (approximate Ω/kft): 16 AWG ≈ 4.99, 14 AWG ≈ 3.14, 12 AWG ≈ 1.98, 10 AWG ≈ 1.24, 8 AWG ≈ 0.778, 6 AWG ≈ 0.491. Cord manufacturers publish exact values — use them when available.

3% and 5% Limits — What They Mean for Cords

NEC Informational Notes discuss 3% branch and 5% feeder-plus-branch as design targets for reasonable operation — they are not hard code limits for every temporary cord. In practice:

• ~3% or less — preferred for motors, compressors, and anything that must start under load.
• ~5% — common ceiling for lighting and general tools before heat, torque loss, and nuisance trips become likely.
• Above 5% — expect soft starts, overheating at connections, and welders that refuse to hold arc voltage at the machine.

At 120V, 3% is only 3.6V; at 240V, 3% is 7.2V. Low-voltage tools feel cord drop first.

Welder Cords and High Inrush Tools

Stick and MIG machines often draw high primary current, especially on 120V units. A 15A nameplate welder on a 14 AWG 100 ft cord can lose enough voltage that the arc becomes unstable even though the breaker holds. Prefer:

• Shorter runs — move the receptacle or use a closer temporary distribution point.
• Heavier AWG — 10 AWG or 8 AWG for long welder feeds when ampacity allows.
• 240V when available — same watts at half the amps halves I×R loss for the same cable.

For permanent welding cable sizing (electrode/work leads), use the wire gauge calculator for welders — that is secondary cable, not SOOW extension cord, but the voltage-at-the-arc problem is the same idea.

Quick Field Checks

Frequently Asked Questions

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