Voltage Drop Calculation: Formula, 3% Rule, Examples

Why voltage drop matters

Every foot of wire has resistance, so a long run always delivers a little less than the panel puts out. Too much of it and motors run hot, lights dim, electronics misbehave — and the voltage you lost didn't vanish, it turned into heat sitting in the wire.

The voltage drop formula

VD = factor × K × I × L ÷ CM
factor = 2 for single-phase, 1.732 (√3) for three-phase
K = 12.9 for copper, 21.2 for aluminum (ohm-circular mils per foot)
I = load current in amps; L = one-way length in feet
CM = conductor area in circular mils from NEC Chapter 9 Table 8, for example 6,530 for 12 AWG and 10,380 for 10 AWG
Percent drop = VD ÷ system voltage × 100

How to calculate voltage drop in PipeMaster

Open Voltage Drop under Electrical (Pro).
Under Circuit, tap Copper or Aluminum, then 1-Phase or 3-Phase, and pick the Wire Size (AWG).
Under Load, set Current, One-Way Length and System Voltage with the steppers.
Read Voltage Drop, Percent Drop and Voltage at Load under Result.
If the drop is above 3%, an orange line reads Exceeds the 3% branch-circuit guideline (NEC 210.19 informational note).

Worked examples: single phase and 3 phase

PipeMaster results
Circuit	Wire	Load and run	Voltage Drop	Percent Drop	Voltage at Load
1-phase, 120 V	12 AWG Cu	20 A, 100 ft	7.90 V	6.58 %	112.1 V
1-phase, 120 V	10 AWG Cu	20 A, 100 ft	4.97 V	4.14 %	115.0 V
1-phase, 120 V	8 AWG Cu	20 A, 100 ft	3.13 V	2.60 %	116.9 V
1-phase, 240 V	10 AWG Cu	30 A, 150 ft	11.18 V	4.66 %	228.8 V
1-phase, 240 V	8 AWG Cu	30 A, 150 ft	7.03 V	2.93 %	233.0 V
3-phase, 480 V	2 AWG Cu	100 A, 200 ft	6.73 V	1.40 %	473.3 V
3-phase, 480 V	2 AWG Al	100 A, 200 ft	11.07 V	2.31 %	468.9 V

Check the first row by hand: 2 × 12.9 × 20 × 100 ÷ 6,530 = 7.90 V, and 7.90 ÷ 120 = 6.58%.

Choosing a wire size for voltage drop

Start with the size the ampacity requires, run the drop, then step up one size at a time until Percent Drop sits under your target. In the table, both the 20 A run at 120 V and the 30 A run at 240 V only clear 3% at 8 AWG — on a long 120 V branch it is worth starting there rather than checking three sizes first. Never go below the size ampacity requires, and remember a bigger wire also changes conduit fill and box fill down the line.

Is voltage drop a code requirement?

In the NEC, the familiar 3% for a branch circuit and 5% for feeder plus branch appear as informational notes, which are recommendations rather than enforceable rules in general. Energy codes, job specifications, equipment instructions and a few specific NEC articles can make a limit mandatory, so check with your inspector.

Limits of this calculator

System Voltage moves in 12 V steps from 12 to 600 V: 120, 240 and 480 V are exact, 208, 230, 277 and 400 V cannot be entered.
Length is in feet only, in 5 ft steps; wire sizes run from 14 AWG to 4/0.
It uses the DC K constants with no power factor or reactance, so for large conductors or motor loads treat it as an estimate.
It is a US NEC and AWG tool, not an IEC or metric-cable calculator.

Questions and answers

How much voltage drop in 100 feet?

It depends on the load, wire size and voltage. A 20 A load on 12 AWG copper over 100 ft one way drops 7.90 V: 6.58% at 120 V, or 3.29% at 240 V. Double the current or the length and the drop doubles.

Does the calculator handle 3 phase?

Yes. Choose 3-Phase and it uses 1.732 instead of 2. A 100 A load on 200 ft of 2 AWG copper at 480 V drops 6.73 V, 1.40%.

Why can't I enter 208 volts?

The System Voltage stepper moves in 12 V steps, so 208 and 277 are skipped. You can apply the formula above by hand, or read the Voltage Drop in volts, which does not depend on the system voltage, and divide it by 208 yourself.

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Voltage drop calculation: the formula, the 3% guideline and worked examples