Voltage Drop Calculator — NEC Formula, Single & 3-Phase

Calculate voltage drop and percentage for any wire run using the NEC formula. Single-phase and three-phase, copper or aluminum, full AWG table.

July 9, 2026

Circuit Type

Conductor Material

Wire Gauge (AWG)

Current (Amps)

One-Way Distance (ft)

Source Voltage (V)

Voltage Drop

4.97 V(4.14%)

Voltage at the load end ≈ 115.03 V

Above 3% but within the 5% combined guideline

Single-phase: VD = (2 × K × I × D) ÷ CM. Three-phase: VD = (1.732 × K × I × D) ÷ CM. K = 12.9 for copper, 21.2 for aluminum (circular-mil-ohms/foot at 75°C). CM = circular mil area of the selected wire gauge per NEC Chapter 9, Table 8. The 3%/5% thresholds are NEC informational-note recommendations, not universal hard code requirements — always verify against your local code and a licensed electrician before final wire sizing.

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Reference Values

Last verified: Jul 9, 2026

Category	Range	What It Means	Status
Copper (K constant) ★	12.9 CM·Ω/ft	Resistivity constant for copper conductors at 75°C, used in the standard NEC voltage drop formula. Lower resistance than aluminum means smaller voltage drop for the same wire size and run.	★ Best
Aluminum (K constant)	21.2 CM·Ω/ft	Resistivity constant for aluminum conductors at 75°C. About 64% higher resistance than copper, so aluminum runs need a larger wire size to hold the same voltage drop.	Okay
Branch circuit voltage drop ★	≤3%	NEC 210.19(A) Informational Note recommends keeping branch circuit voltage drop at or under 3% for reasonable efficiency.	★ Best
Feeder voltage drop ★	≤3%	NEC 215.2(A) Informational Note recommends keeping feeder voltage drop at or under 3% for reasonable efficiency.	★ Best
Combined feeder + branch circuit	≤5% total	NEC's combined recommendation for total voltage drop from the service point to the farthest outlet — a guideline for efficient operation, not a mandatory code requirement in most installations.	Good
Above 5% total	>5%	Exceeds the NEC recommendation. Expect dimming lights, reduced motor torque and starting current issues, and extra heat in the conductor. Re-check wire size or shorten the run.	Poor

Source: K-constant and circular mil values from NEC Chapter 9, Table 8 (Conductor Properties), as referenced in IAEI Magazine 'Voltage Drop Formulas', EC&M 'Code Calculations', and EEPower 'NEC Basics: Computing Voltage Drop'. NEC 210.19(A) and 215.2(A) Informational Notes for the 3%/5% guideline.

Worked Examples

12 AWG Copper Branch Circuit, 120V Single-Phase

Conductor: Copper, 12 AWG (6,530 CM)
Phase: Single-phase
Current: 20 A
One-Way Distance: 50 ft
Source Voltage: 120 V

3.95 V drop (3.29%)

(2 × 12.9 × 20 × 50) ÷ 6,530 = 25,800 ÷ 6,530 = 3.95 V. 3.95 ÷ 120 × 100 = 3.29% — just over the 3% branch circuit guideline, so a 10 AWG conductor would be the safer choice for this run.

10 AWG Copper Branch Circuit, 240V Single-Phase

Conductor: Copper, 10 AWG (10,380 CM)
Phase: Single-phase
Current: 30 A
One-Way Distance: 100 ft
Source Voltage: 240 V

7.46 V drop (3.11%)

(2 × 12.9 × 30 × 100) ÷ 10,380 = 77,400 ÷ 10,380 = 7.46 V. 7.46 ÷ 240 × 100 = 3.11% — just above the 3% recommendation, worth stepping up to 8 AWG on a long run like this.

8 AWG Copper Feeder, 208V Three-Phase

Conductor: Copper, 8 AWG (16,510 CM)
Phase: Three-phase
Current: 40 A
One-Way Distance: 150 ft
Source Voltage: 208 V

8.12 V drop (3.90%)

(1.732 × 12.9 × 40 × 150) ÷ 16,510 = 134,056.8 ÷ 16,510 = 8.12 V. 8.12 ÷ 208 × 100 = 3.90% — exceeds the 3% feeder guideline; upsizing to 6 AWG brings this back under the target.

6 AWG Aluminum Branch Circuit, 240V Single-Phase

Conductor: Aluminum, 6 AWG (26,240 CM)
Phase: Single-phase
Current: 50 A
One-Way Distance: 75 ft
Source Voltage: 240 V

6.06 V drop (2.52%)

(2 × 21.2 × 50 × 75) ÷ 26,240 = 159,000 ÷ 26,240 = 6.06 V. 6.06 ÷ 240 × 100 = 2.52% — within the 3% branch circuit guideline despite aluminum's higher resistivity constant.

4/0 AWG Copper Feeder, 480V Three-Phase Long Run

Conductor: Copper, 4/0 AWG (211,600 CM)
Phase: Three-phase
Current: 200 A
One-Way Distance: 250 ft
Source Voltage: 480 V

5.28 V drop (1.10%)

(1.732 × 12.9 × 200 × 250) ÷ 211,600 = 1,117,140 ÷ 211,600 = 5.28 V. 5.28 ÷ 480 × 100 = 1.10% — comfortably under the 3% feeder guideline even at 250 feet, because the higher 480V source and large conductor size keep the percentage low.

How to Use This Calculator

1

Choose single-phase or three-phase

Match your circuit type — most residential branch circuits are single-phase; commercial/industrial feeders and motor circuits are often three-phase.
2

Select conductor material and AWG size

Pick copper or aluminum, then the wire gauge you're planning or currently have installed. The circular mil value is built in automatically.
3

Enter current, one-way distance, and source voltage

Current is the expected load in amps. Distance is the one-way run from source to load (not round-trip — the formula already accounts for that). Source voltage is the nominal supply voltage (120V, 208V, 240V, 480V, etc.).
4

Read the voltage drop and NEC guideline flag

The result shows voltage drop in volts and as a percentage of source voltage, plus a color-coded flag against the NEC's 3%/5% recommendation.

What Each Value Means

Voltage Drop (VD) (volts): The amount of voltage lost to conductor resistance between the source and the load, in volts. Calculated as (multiplier × K × I × D) ÷ CM, where the multiplier is 2 for single-phase or 1.732 (√3) for three-phase.
Voltage Drop Percentage (percent): Voltage drop expressed as a percentage of the source voltage — the figure compared against the NEC's 3% branch/feeder and 5% combined guidelines. Calculated as (VD ÷ Source Voltage) × 100.
Circular Mils (CM) (circular mils): A unit of cross-sectional area used for round wire in the US, equal to the square of the wire's diameter in mils (thousandths of an inch). Larger circular mil area means lower resistance and less voltage drop for a given current and distance.

Frequently Asked Questions

What is the NEC voltage drop formula?

For single-phase circuits: VD = (2 × K × I × D) ÷ CM. For three-phase circuits: VD = (1.732 × K × I × D) ÷ CM. K is the resistivity constant (12.9 for copper, 21.2 for aluminum, in circular-mil-ohms per foot at 75°C), I is the load current in amps, D is the one-way distance from source to load in feet, and CM is the circular mil area of the conductor from NEC Chapter 9, Table 8. The 2 or 1.732 (√3) multiplier accounts for the round-trip path in single-phase versus the phase geometry in three-phase.

What do the 3% and 5% voltage drop guidelines mean?

The NEC's Informational Notes at 210.19(A) for branch circuits and 215.2(A) for feeders recommend keeping voltage drop at or under 3% on either segment, with a combined total of 5% from the service point to the farthest outlet. This is a recommendation for reasonable operating efficiency, not a mandatory code violation if exceeded in most residential and commercial work — but some jurisdictions, utility interconnection agreements, or engineered specifications do make it a hard requirement, so always check your local amendments and any project specs before treating 3%/5% as optional.

Why does aluminum wire need a bigger size than copper for the same voltage drop?

Aluminum's resistivity constant (K = 21.2) is about 64% higher than copper's (K = 12.9), meaning an aluminum conductor of the same circular mil area produces significantly more voltage drop carrying the same current over the same distance. To hit the same voltage drop percentage as a copper run, an aluminum conductor typically needs to be two AWG sizes larger. This is on top of aluminum's own ampacity derating — always size aluminum conductors independently rather than just substituting for a copper spec.

Does voltage drop count as a code violation if it exceeds 3%?

Not automatically. The NEC's 3%/5% figures live in Informational Notes, which are explanatory and not enforceable code text on their own — an inspector generally can't cite a violation for voltage drop alone unless a local amendment, a referenced standard, or the engineered plans specifically require it. That said, going meaningfully over 5% causes real problems: dimming lights, motors that run hot and lose torque, and equipment that trips or fails to start under load. Treat the guideline as a design target even where it isn't strictly enforced.

How do I fix a voltage drop that's too high?

Three levers, in order of practicality: increase the conductor size (a bigger circular mil area lowers resistance and voltage drop directly), shorten the run if the layout allows it, or raise the source voltage where the system design permits it (a 240V feeder to a subpanel drops half as many volts-per-amp as a 120V run for the same load). Reducing the connected load also helps but usually isn't the intended fix on a fixed circuit.