How to Calculate Voltage Drop
Voltage drop is the loss of voltage along a conductor caused by its resistance and reactance. Too much drop means motors run hot, lights dim, and electronics misbehave, which is why every major wiring standard sets recommended limits. This calculator follows the standard engineering method and lets you check the result against the NEC, IEC 60364, or BS 7671.
The voltage drop formula
Start with the one-way resistance of the conductor, then apply the load current, power factor, and a phase factor. The phase factor is 2 for single-phase and the square root of 3 for balanced three-phase.
Single-phase: Vdrop = 2 × I × (R·cosφ + X·sinφ)
Three-phase: Vdrop = 1.732 × I × (R·cosφ + X·sinφ)
% Drop = Vdrop ÷ Vsystem × 100
Here ρ is resistivity (about 0.0175 Ω·mm²/m for copper and 0.0282 for aluminum), L is the one-way length, A is the conductor area, and X is the cable reactance. For small conductors at high power factor the reactance term is small and is often omitted, but it matters for large or long runs.
Voltage Drop Limits by Standard
The headline numbers are similar worldwide, but the framing and reference points differ. The table below summarises the commonly applied limits. Always confirm against the edition and national annex that governs your project.
| Standard | Lighting / Branch | Power / Other (Combined) | Nature |
|---|---|---|---|
| NEC (US) | 3% (branch) | 5% (feeder + branch) | Recommendation |
| IEC 60364 | 3% (lighting) | 5% (other uses) | Design requirement |
| BS 7671 (UK) | 3% (lighting) | 5% (other) | Design requirement |
Note that some IEC national annexes and private-supply installations permit higher figures (for example 6 percent lighting and 8 percent other where the installation is fed from its own transformer), so treat the 3 and 5 percent values as the common public-supply case.
Copper vs Aluminum and AWG vs Metric
US installations specify conductors in AWG and, for larger sizes, thousand circular mils (kcmil), while most of the world uses metric mm². The table below shows common AWG sizes with their approximate metric area, which is how this calculator converts between the two systems.
| AWG / kcmil | ≈ mm² | AWG / kcmil | ≈ mm² |
|---|---|---|---|
| 12 | 3.31 | 1/0 | 53.5 |
| 10 | 5.26 | 2/0 | 67.4 |
| 8 | 8.37 | 3/0 | 85.0 |
| 6 | 13.3 | 4/0 | 107 |
| 4 | 21.2 | 250 kcmil | 127 |
| 2 | 33.6 | 350 kcmil | 177 |
| 1 | 42.4 | 500 kcmil | 253 |
Aluminum has roughly 60 percent more resistance than copper for the same area, so an aluminum run drops more voltage than an equivalent copper run. For long feeders, though, upsizing aluminum is often cheaper than using copper.
Worked Example
Consider a 100 A three-phase load on a 480 V system fed by 150 ft (about 45.7 m) of 1/0 AWG copper (≈ 53.5 mm²) at 0.85 power factor. The one-way resistance is 0.0175 × 45.7 ÷ 53.5 ≈ 0.0149 Ω. The drop is 1.732 × 100 × 0.0149 × 0.85 ≈ 2.2 V, or about 0.46 percent — comfortably within the 3 percent limit. If the same run used 6 AWG (≈ 13.3 mm²), the drop would rise to roughly 8.8 V (1.8 percent), still acceptable but much closer to the limit on longer runs.
Frequently Asked Questions
How do you calculate voltage drop in a cable?
First find the one-way conductor resistance: resistance = resistivity x length / cross-sectional area. Then multiply by the load current, the power factor, and a phase factor of 2 for single-phase or 1.732 (the square root of 3) for three-phase. The result is the voltage drop in volts. Dividing by the system voltage and multiplying by 100 gives the percentage drop.
What is the maximum allowed voltage drop?
It depends on the standard. The US NEC recommends 3 percent for a branch circuit and 5 percent for feeder plus branch combined. IEC 60364 and BS 7671 commonly use 3 percent for lighting circuits and 5 percent for power and other circuits, measured from the origin of the installation. Exact figures vary by edition and national annex, so always confirm against the version that applies to your project.
What is the difference between NEC, IEC and BS 7671 limits?
All three aim to keep voltage at equipment within usable bounds, but they frame it differently. NEC limits are recommendations in informational notes (3 percent branch, 5 percent combined). IEC 60364 and BS 7671 treat the limits as design requirements and split them by circuit type, typically 3 percent for lighting and 5 percent for other uses. This calculator lets you pick the standard and applies the matching limit automatically.
Why does the phase factor use the square root of 3?
In a balanced three-phase system the line-to-line voltage drop is the square root of 3 (about 1.732) times the current times the conductor impedance, because the line-to-line voltage is the square root of 3 times the phase voltage. Single-phase circuits use a factor of 2, because the current flows out on one conductor and returns on the other, doubling the conductor length in the loop.
How do I fix excessive voltage drop?
Increase the conductor size (the most direct fix), shorten the cable run, raise the system voltage, reduce the load current, or split a large load across multiple feeders. Increasing the cross-sectional area lowers resistance proportionally, so going up one or two standard sizes usually brings a marginal run back within the limit.
Should I use copper or aluminum for long runs?
Aluminum has about 60 percent higher resistivity than copper, so for the same size it produces more voltage drop. However, aluminum is lighter and cheaper, so for long feeders it is often more economical to use a larger aluminum conductor than a smaller copper one. This calculator lets you compare both materials directly.
Does cable reactance matter for voltage drop?
For small conductors and high power factor, resistance dominates and reactance can be ignored. For larger conductors, longer runs, or lower power factor, the reactive term (reactance times the sine of the phase angle) becomes significant and ignoring it understates the true drop. NEC Chapter 9 Table 9 and the IEC and BS impedance tables exist for this reason, and this calculator can include reactance when enabled.
This calculator is a planning aid. Final conductor selection must be verified by a qualified electrical engineer against the applicable edition of the relevant standard (NEC, IEC 60364, or BS 7671), including ampacity, temperature correction, grouping, and conduit fill — not voltage drop alone.