Electrical Reference Card
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BuyExtension Cord Voltage Drop Calculator
Find cord drop, voltage at the load, and the minimum AWG that stays under 3% or 5%
Enter system voltage, load amps, and cord length to estimate voltage drop on portable extension cords (AWG 16–6). Use Recommend Gauge mode when you need the lightest cord that still passes your drop limit — built for tools, welders, and jobsite runs, not permanent branch-circuit wiring.
Also try our Voltage Drop Calculator and Wire Gauge Calculator for Welders.
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Why Extension Cord Voltage Drop Matters
Extension cords are temporary conductors, but they still lose voltage over distance. Undersized or oversized-length cords starve tools of voltage, cause motors to overheat, and make welders weld poorly. This calculator focuses on portable cord runs — not permanent branch-circuit feeders. For in-wall or conduit wiring, use the full Voltage Drop Calculator.
How the Cord Drop Formula Works
Voltage drop for a single-phase cord uses two conductors (hot and neutral, or two hots on 240 V):
VD = 2 × I × R × (L / 1000)
Where I is load amps, R is ohms per 1,000 ft from a copper (or aluminum) resistance table, and L is the cord length in feet. Percent drop is VD divided by system voltage. A 3% limit is conservative for sensitive electronics; 5% is a common field target for tools.
Common Extension Cord Gauge Mistakes
- Using 16 AWG or 14 AWG for welders, compressors, or high-draw saws
- Treating cord length as round-trip distance instead of one-way length (this calculator already applies the 2-wire factor)
- Ignoring connector and cord ampacity — a thick cord with a 15 A plug is still a 15 A cord
- Daisy-chaining multiple cords, which multiplies drop and heat
FAQ
Is cord length one-way or round-trip?
Enter the physical cord length (for example, 50 ft). The calculator multiplies by two conductors automatically for the round trip.
What drop % should I use?
Use 3% when you want maximum tool performance, or 5% as a practical jobsite limit. Never ignore manufacturer cord recommendations on welders and generators.
How is this different from the branch-circuit voltage drop calculator?
This tool is scoped to portable extension cords (AWG 16–6). The branch-circuit calculator covers building wire sizes through large kcmil feeders and aluminum service conductors.