How Voltage Drop Steals Power From Long Extension Cords

An extension cord is a resistor with a marketing budget. Every foot of copper adds resistance, and the longer and thinner the cord, the more of the supply voltage is converted to heat before the current ever reaches the tool. On a 50-foot 16 AWG cord drawing 12 amps, the drop is around 4.8 volts — roughly 4 percent of a 120-volt supply. That sounds small until you notice the cord is warm and the saw bogs down.

The reason longer cords are more dangerous is not the voltage drop itself but what the drop represents: power dissipated in the cord. The same current flows through the cord and the load, so watts lost in the conductor equal the drop multiplied by the current. Those watts leave the cord as heat, and at high current in a coiled, undersized cord the heat has nowhere to go.

The Arithmetic Behind the Drop

Voltage drop in a single-phase circuit is computed from the round-trip conductor length, the current, and the resistance per unit length of the wire. Copper resistance for common extension cord gauges at 75°C works out to roughly 4.02 ohms per 1,000 feet for 12 AWG, 6.39 for 14 AWG, and 10.15 for 16 AWG. Right there is the short version: halving the gauge roughly doubles the loss for the same length.

Because current must travel out and back, the effective length is twice the cord length. A 100-foot cord is 200 feet of conductor in the calculation. That doubling is the single most common error when people estimate drop by hand and conclude a run is fine when it is not.

Where the Limits Come From

The National Electrical Code recommends that branch-circuit voltage drop not exceed 3 percent at the outlet and 5 percent overall from the service. Extension cords are not branch circuits, but the same thresholds are a sensible engineering guide, and equipment manufacturers frequently specify tighter limits. Motor-driven tools are the most sensitive: an induction motor running 10 percent below nameplate voltage can draw noticeably higher current to deliver the same torque, which heats the motor and can shorten its life.

The second limit is ampacity. A 16 AWG cord is typically rated 13 amps and a 14 AWG cord 15 amps for short runs, with lower ratings as length increases. Putting a 15-amp table saw on a 16-gauge, 100-foot cord is over the ampacity rating for that length in most manufacturer tables regardless of what the voltage-drop math says.

A Worked Case

A contractor runs a 15-amp circular saw at the end of a 100-foot 16 AWG cord plugged into a 120-volt outdoor receptacle. Round-trip conductor length is 200 feet. Resistance for 16 AWG at roughly 4 ohms per 1,000 feet round-trip basis gives about 2.03 ohms of loop resistance, so the drop at 15 amps is about 30 volts — the saw sees roughly 90 volts. In practice the saw’s current rises as voltage sags, so the real number is somewhat better than that worst-case estimate because the load is not a pure constant-current sink, but the direction is unambiguous. Replacing the cord with 12 AWG at the same length cuts the drop by roughly 60 percent and keeps the saw inside its rating.

The fix is almost always thicker copper or a shorter run, not a bigger breaker. If the circuit is already at 15 or 20 amps, the breaker protects the wiring in the wall, not the cord on the ground.

What to Check on Site

• Cord gauge printed on the jacket — look for 12/3 or 14/3; the first number is AWG, the second is conductor count.
• Coiled cords — a coiled cord cannot dissipate heat and can overheat well before its rating is exceeded in free air.
• Warm plugs — a plug that is noticeably warm indicates resistance across the connection, often from wear rather than length.
• Motor sound — a motor that labors under load it used to handle is often voltage-starved, not worn out.

Comparing Cords and Circuits

Two cords of the same gauge and length behave identically; a single longer cord is usually better than two shorter cords joined, because each connection adds resistance and a failure point. Where a run must exceed 100 feet at high current, running a dedicated 12 AWG cord or relocating the receptacle is the standard remedy. For the underlying circuit math — resistance, current, and power relationships — the electrical calculator handles the Ohm’s law side quickly, and you can layer the cord resistance on top for a specific run.

FAQ

Does voltage drop damage the tool? It can. Motors draw more current to compensate, which raises winding temperature; electronics can brown out and reset. Light bulbs simply dim.

Is a 16-gauge cord ever fine at 100 feet? For a phone charger or a lamp drawing under 2 amps, yes. For anything with a motor or a heater, treat 50 feet as the practical ceiling for 16 AWG.

Can I just upsize the breaker? No. Breakers protect fixed wiring; they do not make a cord safe for more current, and doing so removes the only protection the cord has.

Voltage drop is a linear, predictable loss, which makes it one of the few electrical problems you can fully diagnose with a tape measure, a gauge marking, and a nameplate. Measure the run, read the jacket, note the amps, and compute before plugging in. The calculator homepage links the electrical, conversion, and unit tools needed to turn those three numbers into a safe answer.