Generator cord gauge
Cord gauge (AWG) sets how much current a generator cord can carry safely and how much voltage it loses over distance: 10 AWG for 30 A circuits, 6 AWG for 50 A, and thicker (lower-number) wire the longer the run.
Undersized cords overheat and starve appliances of voltage — a motor that receives low voltage draws more current and runs hotter, which is how refrigerator compressors die during outages. For ordinary extension cords to individual appliances, 12 AWG is a sensible minimum for anything with a motor.
Since generators must sit at least 20 feet from the house, voltage drop is not theoretical: at 20–50 feet, buy the gauge rated for the full circuit amperage — a 30 A L14-30 cord should be 10 AWG, a 50 A cord 6 AWG.
| 30 A generator cord (L14-30) | 10 AWG four-conductor (10/4) |
|---|---|
| 50 A generator cord (14-50 / CS6365) | 6 AWG four-conductor (6/4) |
| Common extension cords, as marked | 16 AWG ≈ 13 A, 14 AWG ≈ 15 A, 12 AWG ≈ 20 A, 10 AWG ≈ 30 A |
| Copper resistance (NEC Chapter 9, Table 8) | 14 AWG 3.14, 12 AWG 1.98, 10 AWG 1.24, 8 AWG 0.778, 6 AWG 0.491 Ω per 1,000 ft |
| Voltage-drop target | 3% on a branch circuit — an NEC informational note, advisory rather than mandatory |
| Jacket markings to look for | SJTW, STW, SOOW — the W means the cord is rated for outdoor and wet use |
The arithmetic, so you can stop guessing
Voltage drop = 2 × (one-way length in feet) × (current in amps) × (resistance per 1,000 ft ÷ 1,000). The factor of two is there because current travels out and back. The resistances come from NEC Chapter 9, Table 8, for uncoated copper: 14 AWG is 3.14 Ω per 1,000 ft, 12 AWG is 1.98, 10 AWG is 1.24, 8 AWG is 0.778, and 6 AWG is 0.491. Every rule of thumb about cord length is just this formula with the numbers already plugged in.
Worked out: 100 feet of 12 AWG carrying 15 A on a 120 V circuit drops 5.9 V, just under 5%. The same run in 14 AWG drops 9.4 V, about 7.8%, which is enough to matter to a motor. Fifty feet of 10 AWG carrying 30 A at 240 V drops 3.7 V, about 1.5%. Fifty feet of 6 AWG at 50 A and 240 V drops 2.5 V, about 1%. The pattern worth internalising is that 240 V circuits tolerate distance far better than 120 V circuits, because the same absolute drop is half the percentage.
The target most people quote — 3% on a branch circuit, 5% including the feeder — comes from an informational note in the NEC, which means it is advice rather than an enforceable requirement. It is good advice. NEMA MG-1 permits motors to run at ±10% of nameplate voltage, so 5% lost in a cord on top of a generator already sagging under load can put a compressor outside the range it was designed for.
Why undervoltage kills motors
A loaded motor behaves roughly like a constant-power device. Drop the supply voltage and it draws more current to deliver the same shaft power. More current through the same windings means more resistive heating, and winding insulation life falls by roughly half for every 10 °C of extra temperature. That is the mechanism behind refrigerator and freezer compressors dying in the weeks after an outage rather than during it — the damage was done by a long undersized cord, and it shows up later.
Starting is worse than running. A motor’s starting torque falls with the square of applied voltage, so 10% low voltage costs about 19% of starting torque. A compressor that cannot break away sits at locked-rotor current — several times its normal draw — until the internal thermal overload opens, then tries again a few minutes later. An undersized cord feeding a marginal generator is precisely the combination that produces this loop.
The cord itself is also absorbing that lost energy as heat. A 100-foot 12 AWG cord carrying 15 A is dissipating close to 90 W along its length, spread over the whole run. Uncoiled, it sheds that easily; coiled on a reel, the inner turns have nowhere to lose heat and the jacket softens. Always unwind a cord completely before putting load on it, even if you only need ten feet of it.
Buying and using cords for generator duty
Read the jacket. The type letters are printed along it: S means service cord rated 600 V, SJ means junior service rated 300 V, T is a thermoplastic jacket, O is oil-resistant (OO meaning both jacket and insulation), and W means rated for weather and water — outdoor use. SJTW and SOOW are the two you will see on generator cords, and a cord without a W has no business lying in wet grass for three days. The gauge and conductor count are printed alongside: 10/4, 12/3, 6/4.
Sizing by rule of thumb: for ordinary appliance extension cords running from the generator, treat 12 AWG as the floor for anything with a motor, and step to 10 AWG beyond about fifty feet. For the generator-to-inlet cord, match the connector — 10 AWG for a 30 A L14-30, 6 AWG for a 50 A 14-50 or CS6365. Never adapt upward: putting a 30 A generator cord behind a 50 A inlet and breaker leaves the cord without overcurrent protection it can rely on.
What not to do: do not daisy-chain extension cords, because every joint is added resistance and a wet-weather failure point and the drop is set by the total length anyway. Do not run cords under rugs or through doorways where they get crushed. Do not use indoor-rated cords outdoors. And do not own a double-male "suicide" cord — plugging a generator into a wall receptacle back-feeds the panel with no isolation, energises the utility service drop through the transformer, and leaves live pins exposed in your hand.
Frequently asked questions
What gauge extension cord do I need for a generator?
For appliance cords running from the generator, 12 AWG is a sensible minimum for anything with a motor, stepping to 10 AWG beyond about fifty feet. For the main cord from the generator to a house inlet, match the connector: 10 AWG four-conductor for a 30 A L14-30, and 6 AWG four-conductor for a 50 A 14-50 or CS6365. Use only outdoor-rated cord, marked with a W in the type code.
How long can a generator cord be?
Long enough is decided by voltage drop, not by a fixed limit. A 10 AWG cord at 30 A on a 240 V circuit loses about 1.5% over 50 feet and about 3% over 100 feet, so 100 feet is workable. A 12 AWG cord at 15 A on 120 V is already near 5% at 100 feet. Compute it: drop = 2 × length × amps × ohms-per-1,000-ft ÷ 1,000, and aim to keep it under 3%.
Can I plug two extension cords together?
You can, and it is a bad habit. Every connection adds resistance and creates another point where water gets in, and the voltage drop is governed by the combined length regardless. If you need a hundred feet, buy a hundred-foot cord in the right gauge rather than joining two fifty-foot cords in the wrong one.
What does SJTW mean on an extension cord?
SJ is a junior service cord rated 300 V rather than the 600 V of an S cord, T is a thermoplastic jacket, and W means it is rated for weather and water — that is, outdoor use. SOOW is the heavier alternative: a 600 V rubber cord with oil-resistant jacket and insulation, more flexible in cold weather and more durable, which is why generator cords are commonly built from it.
Does a longer cord reduce how much power my generator delivers?
The generator still produces its rated watts; you lose some of them as heat in the cord, and the appliance sees a lower voltage. A motor responds to that lower voltage by drawing more current, which makes the heating worse. The generator is not weaker — the delivery is lossier — but the practical effect at the appliance is identical to having a smaller generator.