Key Facts: Extension Cord & Generator Cord Gauge
This page is Cord Gauge Decoded's maintained fact reference — the gauge, amperage, and voltage-drop facts our guides are built on, restated in plain prose. General reference only, not an electrical code determination; local code governs and permanent/hardwired work requires a licensed electrician.
Higher gauge number = thinner wire
American Wire Gauge runs backward from what most people guess: a HIGHER number is a THINNER wire. A 16 AWG cord is thinner (and carries less current) than a 12 AWG cord. This comes from the historical wire-drawing process — each gauge step is one more pull through a smaller die. Full explainer: Why Is a Higher Gauge Number Thinner?
Gauge alone doesn't set safe amperage — length does too
A cord's printed gauge reflects a thermal ampacity rating that assumes a fixed reference length. As a cord gets longer, voltage drop — not just heat — becomes the binding constraint, so the same 12 AWG cord that's fine for a 20A tool at 25ft is not automatically fine at 150ft. The full length-by-length breakdown is the Cord Gauge Ampacity Index, and the plain-language explainer is How Long Can an Extension Cord Be?
Voltage drop, explained
Voltage drop is the loss of voltage along a cord's length due to conductor resistance, calculated as
Vdrop = 2 × L × I × R (length doubled for the round trip through hot and neutral). The
commonly-cited safe limit for branch circuits is 3%; above that, motors run hot, tools lose power, and
heating elements underperform. Full explainer: What Does Voltage Drop Mean for Extension Cords?
Generator, welder, and RV cords need heavier gauge than they look
Generator feeds, welder circuits, and RV shore power all draw 20-30A or more, and are frequently run at distance (50-150ft from generator to house, or across a job site) — exactly where voltage drop bites hardest. Our Appliance Amp Draw → Gauge table maps these loads to minimum safe gauge by length; see also gauge for a generator and gauge for a welder.
Indoor vs outdoor cord rating is separate from gauge
Gauge (AWG) and jacket rating are two different specs. SJTW cords are rated for outdoor/wet use; SJT or plain indoor cords are not — regardless of how thick the gauge is. A thick indoor-only cord left out in the rain is a different hazard than an undersized cord. Full breakdown: Indoor vs Outdoor Extension Cord Rating.
How these facts are maintained
These facts are drawn from NEC-referenced flexible-cord ampacity figures, manufacturer nameplate specifications, and standard Ohm's-law voltage-drop calculations, cross-checked against our own dataset pages and updated on the date shown above when a figure changes. We do not publish exact prices, we round toward the safer (lower) amperage wherever sources disagree, and we have not tested any physical cord ourselves. This is general reference information, not an electrical code determination for your specific circuit or jurisdiction — local code governs, and a licensed electrician should be consulted for any permanent or hardwired work. Full methodology at How We Evaluate.