Pick cord gauge from ampacity first, then check length so voltage drop stays under control. These tables give recommended maximum one-way lengths for common 120V and 240V tool and welder loads.
How to Read This Chart
Each row is a copper cord AWG with a typical continuous ampacity planning value (confirm against the cord marking — SJTW, SOOW, and welding cable differ). Maximum lengths assume round-trip voltage drop near 3% for motor/tool loads at the listed amps. Longer runs need heavier cable or higher voltage.
These are field planning aids, not a substitute for manufacturer ratings or the extension cord voltage drop calculator. For the theory behind round-trip math, see the extension cord voltage drop guide.
Use cords listed for the environment and load. Ampacity and temperature ratings on the jacket control — do not exceed marked ampacity even if voltage drop looks acceptable.
120V — Recommended Max One-Way Length (~3% VD)
| AWG | Typical Ampacity* | 10A load | 12A load | 15A load | 20A load |
|---|---|---|---|---|---|
| 16 AWG | 10–13A | 50 ft | NR | NR | NR |
| 14 AWG | 15A | 75 ft | 60 ft | 50 ft | NR |
| 12 AWG | 20A | 100 ft | 85 ft | 75 ft | 50 ft |
| 10 AWG | 30A | 150 ft | 125 ft | 100 ft | 75 ft |
| 8 AWG | 40A | 200 ft | 175 ft | 150 ft | 100 ft |
| 6 AWG | 55A | 250 ft+ | 225 ft | 200 ft | 150 ft |
*Ampacity column is typical flexible-cord planning range — verify the specific cord listing. NR = not recommended (ampacity or excessive drop).
240V — Recommended Max One-Way Length (~3% VD)
Same cable at 240V carries the same watts at roughly half the amps of a 120V circuit, so allowable length roughly doubles for the same power. Values below are for the listed load amps at 240V.
| AWG | Typical Ampacity* | 10A | 15A | 20A | 30A |
|---|---|---|---|---|---|
| 14 AWG | 15A | 150 ft | 100 ft | NR | NR |
| 12 AWG | 20A | 200 ft | 150 ft | 100 ft | NR |
| 10 AWG | 30A | 300 ft | 200 ft | 150 ft | 100 ft |
| 8 AWG | 40A | 400 ft | 300 ft | 200 ft | 150 ft |
| 6 AWG | 55A | 500 ft+ | 400 ft | 300 ft | 200 ft |
Common Tool & Welder Loads
| Equipment | Typical Draw | Prefer | Notes |
|---|---|---|---|
| Circular saw / grinder | 10–15A @ 120V | 12 AWG ≤ 50–75 ft | Inrush higher than running |
| Air compressor (portable) | 15A @ 120V | 12 AWG short / 10 AWG long | Watch start voltage |
| 120V MIG/stick welder | 15–20A | 10 AWG; keep short | Arc quality fails before breaker |
| 240V welder / plasma | 20–50A | 10–6 AWG per FLA | Match inlet and cord rating |
| Jobsite spider box feed | Per breaker | Per load calc | Do not daisy undersized cords |
How to Use These Tables
- Read the tool nameplate amps (or measure under load).
- Confirm cord ampacity ≥ load (continuous loads: stay under cord rating with margin).
- Find AWG + amp column; if your one-way length exceeds the cell, upsize AWG or shorten the run.
- Verify with the calculator for exact percent drop and non-standard lengths.
5% vs 3% Length Rough Scaling
The length tables target roughly 3% drop for motor-friendly performance. If you only need ~5% for non-critical resistive tools, allowable length is roughly 5/3 ≈ 1.67× the 3% value — still limited by ampacity. Example: a 12 AWG / 15A / 120V entry at 75 ft (~3%) scales to about 125 ft at ~5%, but heat and connector quality often fail before the math does. When in doubt, stay on the 3% side or move the power source closer.
Cord Types You Will See
- SJTW / SJOW — common outdoor-rated jobsite cords; check temperature and ampacity marks.
- SOOW — heavier oil-resistant portable cord; often used for welders and generators.
- Welding cable (secondary) — electrode/work leads are not the same as SOOW extension cords; size those with the welder wire-gauge tools.
Never cover cords with rugs or coil excess under load — heat rise kills ampacity margin even when voltage drop looks fine on paper.
Field Example
A 120V 15A MIG on a 100 ft 14 AWG cord: ampacity may be borderline depending on cord mark, and voltage drop at 15A for 100 ft one-way (200 ft round-trip) is typically well over 5%. Upsize to 10 AWG or shorten to ≤50 ft of 12 AWG. Confirm with the calculator before blaming the machine for a soft arc.
Exact voltage drop for your cord Enter AWG, length, volts, and amps for round-trip percent drop.
VoltKit — Electrical Calculators Offline
Voltage drop, wire gauge, and load tools on Android.
Related Calculators & Guides
Electrical
Electrical Reference Card
One page. Voltage drop lengths, breaker and wire pairing, motor FLC.
$3.99
Buy
Complete Electrical Reference Guide
15 pages. Ampacity, corrections, voltage drop, motor circuits, conduit fill.
$12.99
Buy
Electrical Job Estimator
Quote electrical work and flag panel headroom before you price it.
Excel or Google Sheets. Best on a computer.
$29.00
Buy
Electrical Complete Bundle
Card, 15-page guide and the estimator workbook. All six files.
Excel or Google Sheets. Best on a computer.
$34.99
$45.98 if bought separately — save $10.99
BuyCheckout opens in a new tab.