Voltage Drop Calculation Explained: NEC 3% Rule and Worked Examples

A long wire is not a perfect conductor. It has resistance, and when current flows through it, some voltage is lost along the way. That lost voltage is called voltage drop. If the drop is too large, lights dim, motors overheat, and equipment at the far end does not get the voltage it was designed for.

What Is Voltage Drop?

Voltage drop is the difference between the voltage at the start of a circuit and the voltage at the load. A 120-volt circuit that measures only 112 volts at the far outlet has lost 8 volts, about 6.7 percent. The cause is the resistance of the wire itself.

Three things decide how much voltage is lost: the length of the run, the current, and the wire size. A longer wire has more resistance, a heavier load pushes more current through it, and a thinner wire has more resistance per foot than a thicker wire.

The NEC 3% Rule

The NEC does not set a hard maximum for voltage drop. Instead, NEC 210.19(A)(1) Informational Note No. 4 recommends no more than 3 percent drop on a branch circuit, and no more than 5 percent combined on feeder and branch circuit.

The Voltage Drop Formula

For a single-phase circuit, voltage drop is calculated as VD = (2 x K x I x L) / CM. Here VD is the voltage drop in volts, K is the resistivity of the conductor, I is the load current in amperes, L is the one-way length in feet, and CM is the circular-mil area of the wire. The number 2 appears because current travels out on the hot wire and back on the neutral.

For copper, K is 12.9 ohms per circular mil-foot, and for aluminium it is 21.2. The circular-mil area comes from the wire size: 14 AWG is 4,110 CM, 12 AWG is 6,530 CM, 10 AWG is 10,380 CM, and 8 AWG is 16,510 CM.

Diagram: Voltage drops along the wire. The source supplies 120 V, but resistance in the hot and neutral wires uses up part of it, so the load receives less than 120 V.

Worked Example 1: A Long Lighting Circuit

A 120-volt lighting circuit carries 12 amperes over 100 feet using 12 AWG copper wire. VD = (2 x 12.9 x 12 x 100) / 6,530 = 30,960 / 6,530 = 4.74 volts. That is 4.74 / 120 = 3.95 percent, above the 3 percent recommendation.

Now try the same circuit with 10 AWG copper wire, which has an area of 10,380 CM. VD = (2 x 12.9 x 12 x 100) / 10,380 = 2.98 volts, or 2.49 percent. That is inside the limit, so the fix is to upsize the wire from 12 AWG to 10 AWG.

Worked Example 2: A 240-Volt Water Heater

A 240-volt water heater draws 18.75 amperes through 80 feet of 10 AWG copper wire. VD = (2 x 12.9 x 18.75 x 80) / 10,380 = 3.73 volts, or 3.73 / 240 = 1.55 percent, well inside the limit, so 10 AWG is fine here.

Notice how the higher voltage helps. The same power at 240 volts draws half the current it would at 120 volts, and the 3 percent allowance is twice as many volts, so a 240-volt circuit can run much further on the same wire size.

Diagram: Voltage drop grows with distance, and thinner wire drops faster. At 12 A on a 120 V circuit, 12 AWG crosses 3 percent near 75 feet, and 10 AWG stays below 3 percent past 100 feet.

Key Takeaways

Voltage drop is the voltage lost to wire resistance, and it grows with length and current and shrinks with larger wire. The NEC recommends no more than 3 percent drop on a branch circuit and 5 percent combined on feeder and branch circuit. The formula is VD = (2 x K x I x L) / CM, with K = 12.9 for copper. A 12-ampere, 100-foot, 12 AWG run dropped 4.74 volts (3.95 percent) and needed 10 AWG to fall to 2.98 volts (2.49 percent).

Objective Questions

1. What is voltage drop?

a) The voltage rating printed on a breaker
b) The voltage lost along a wire because of its resistance
c) The voltage of the utility transformer
d) The voltage between two hot legs

Answer: (b) The voltage lost along a wire because of its resistance

Explanation: Voltage drop is the difference between the voltage at the start of a circuit and the voltage at the load, caused by wire resistance.

2. According to NEC 210.19(A)(1) Informational Note No. 4, what is the recommended maximum voltage drop on a branch circuit?

a) 1 percent
b) 2 percent
c) 3 percent
d) 10 percent

Answer: (c) 3 percent

Explanation: The NEC recommends no more than 3 percent voltage drop on a branch circuit, and no more than 5 percent combined on feeder and branch circuit.

3. In the formula VD = (2 x K x I x L) / CM, what is the value of K for copper?

a) 10.4
b) 12.9
c) 21.2
d) 6,530

Answer: (b) 12.9

Explanation: For copper, K is 12.9 ohms per circular mil-foot; for aluminium it is 21.2.

4. A 120-volt circuit carries 12 A over 100 feet on 12 AWG copper (6,530 CM). What is the voltage drop?

a) 2.98 V
b) 3.73 V
c) 4.74 V
d) 6.32 V

Answer: (c) 4.74 V

Explanation: VD = (2 x 12.9 x 12 x 100) / 6,530 = 30,960 / 6,530 = 4.74 V, which is 3.95 percent.

5. Why does a 240-volt circuit suffer less from voltage drop than a 120-volt circuit of the same power?

a) Because 240-volt wire has no resistance
b) Because the same power at 240 volts draws half the current, and 3 percent is twice as many volts
c) Because 240-volt circuits are always shorter
d) Because the neutral carries no current at 240 volts

Answer: (b) Because the same power at 240 volts draws half the current, and 3 percent is twice as many volts

Explanation: At double the voltage the current halves for the same power, and the 3 percent allowance is 7.2 V instead of 3.6 V, so the circuit can run much further.