Voltage Drop Calculator

Size conductors per NEC Chapter 9 Table 8. Single & three-phase, copper & aluminum.

System Voltage

Phase

Wire Material

Wire Gauge

One-Way Distance (ft)

Load Current (amps)

Voltage Drop

7.5V

Drop %

3.1%

Voltage at Load

232.5V

3.1% — Acceptable but not ideal

NEC Reference

Per NEC 210.19(A) Informational Note No. 4, conductors should be sized to prevent voltage drop exceeding 3% at the farthest outlet, with a maximum of 5% for combined feeder and branch circuit.

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For exam practice and estimation only — not a substitute for engineered design, manufacturer data, current codes, or a licensed professional's judgment. Verify all values before relying on them.

What Is Voltage Drop and Why Does It Matter?

Voltage drop is the reduction in voltage that occurs as electrical current travels through a conductor. All wires have resistance, and that resistance causes some voltage to be "lost" as heat before the electricity reaches the load. The longer the wire run and the more current flowing through it, the greater the voltage drop.

The NEC doesn't strictly mandate a maximum voltage drop, but Informational Note 1 to NEC 210.19(A) recommends that voltage drop on branch circuits not exceed 3%, and that the combined voltage drop of feeders and branch circuits not exceed 5%. In practice, most inspectors and engineers use the 3%/5% rule as a design standard. Exceeding it causes motors to overheat, lights to flicker, and sensitive electronics to malfunction.

This calculator uses NEC Chapter 9 Table 8 resistance values for copper and aluminum conductors. Values are given for both single-phase (multiply by 2 for the round-trip distance) and three-phase (multiply by 1.732) circuits.

Voltage Drop Chart — Maximum Copper Wire Run for 3% Drop

Maximum one-way run length (in feet) before a single-phase copper circuit at full rated load exceeds the NEC-recommended 3% voltage drop. Calculated with VD = (2 × 12.9 × I × L) / CM using NEC Chapter 9 Table 8 circular mils. Aluminum runs are roughly 61% of these lengths (K = 21.2).

Maximum one-way copper conductor run length for 3% voltage drop at 120 V and 240 V, single-phase
Wire Size	Circular Mils	Load	Max Run @ 120 V	Max Run @ 240 V
14 AWG	4,110	15 A	38 ft	76 ft
12 AWG	6,530	20 A	46 ft	91 ft
10 AWG	10,380	30 A	48 ft	97 ft
8 AWG	16,510	40 A	58 ft	115 ft
6 AWG	26,240	55 A	67 ft	133 ft
4 AWG	41,740	70 A	83 ft	166 ft
3 AWG	52,620	85 A	86 ft	173 ft
2 AWG	66,360	95 A	97 ft	195 ft
1 AWG	83,690	110 A	106 ft	212 ft

Chart values assume the conductor carries its full listed load at 75 °C. Real circuits often draw less, allowing longer runs — use the calculator above for your actual current. Voltage drop is a design recommendation (NEC Informational Notes), not a code requirement; ampacity per NEC 310.16 always governs minimum conductor size.

Frequently Asked Questions

What is the NEC maximum allowable voltage drop?

The NEC recommends (but does not mandate) 3% on branch circuits and 5% combined on feeders and branch circuits per Informational Notes in NEC 210.19(A) and 215.2(A). Some jurisdictions and engineers adopt these as hard limits in their specifications.

Copper vs. aluminum — which has less voltage drop?

Copper has lower resistance per foot than aluminum, meaning copper conductors produce less voltage drop at the same size. Aluminum is lighter and less expensive but requires a larger AWG to carry the same current and maintain the same voltage drop percentage.

How is three-phase voltage drop different from single-phase?

For single-phase circuits, current travels out and back through two conductors, so you use 2 × one-way distance. For three-phase, the formula uses a factor of 1.732 (√3) instead of 2, which reflects the phase relationship between the three conductors — resulting in less voltage drop than a comparable single-phase run.

Does voltage drop appear on the journeyman electrician exam?

Yes — voltage drop calculations are among the most commonly tested math problems on journeyman and master electrician licensing exams. Expect 3–5 calculation questions using the standard formula or resistance values from NEC Chapter 9 Table 8 and Table 9.

What wire size should I use to reduce voltage drop?

Increasing conductor size (going from 12 AWG to 10 AWG, for example) reduces resistance and therefore reduces voltage drop. Use the Wire Size Recommender tool to find the minimum AWG that keeps voltage drop within your specified limit for a given load and distance.

How far can I run 12 AWG wire before voltage drop exceeds 3%?

On a 120 V single-phase circuit carrying a full 20 A load, 12 AWG copper reaches 3% voltage drop at about 46 feet one-way. At 240 V the same conductor and load reaches 3% at about 91 feet, because the allowable drop in volts doubles while the drop itself stays the same. If the circuit draws less than 20 A, the run can be proportionally longer — see the voltage drop chart above for other sizes, or use the calculator for your exact load.

What is the voltage drop formula, and what does K mean?

The NEC voltage drop formula is: VD = (2 × K × I × L) / CM for single-phase, or VD = (1.732 × K × I × L) / CM for three-phase. K is the resistivity constant — 12.9 for copper and 21.2 for aluminum at 75 °C. I is the load current in amps, L is the one-way distance in feet, and CM is the conductor area in circular mils from NEC Chapter 9 Table 8.

Worked example: A 20 A single-phase circuit on 12 AWG copper (CM = 6,530) runs 75 ft. VD = (2 × 12.9 × 20 × 75) / 6,530 = 38,700 / 6,530 ≈ 5.93 V. On a 120 V circuit that is 4.9% — above the NEC-recommended 3%. Upsizing to 10 AWG (CM = 10,380) gives VD ≈ 3.73 V = 3.1%, which passes.