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Voltage drop calculator
Calculate the voltage drop and power loss of a cable for three-phase, single-phase or DC circuits, with IEC 60228 conductor resistances and temperature correction. Free, no signup.
- Copper and aluminium, 1.5–630 mm²
- Results update as you type
- Share any calculation as a link
Voltage drop
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Drop: —
Check the inputs: every value must be a positive number.
- Voltage at the load
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- Load current
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- Conductor resistance
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- Power loss in the cable
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- Loss as share of the load
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Approximate formula from IEC 60364-5-52 (Annex G); the NEC uses the same effective-impedance approach. Accurate for drops up to about 10 %.
How the calculation works
The calculator uses the standard approximate voltage drop formula, with the conductor resistance taken from IEC 60228 and corrected to the operating temperature.
- Three-phase AC (line-to-line drop)ΔU = √3 · I · L · (R cos φ + X sin φ) / n
- Single-phase AC (out and back)ΔU = 2 · I · L · (R cos φ + X sin φ) / n
- DCΔU = 2 · I · L · R / n
- Resistance at operating temperatureRT = R20 · (1 + α (T − 20))With α = 0.00393 /°C for copper and 0.00403 /°C for aluminium.
- Percentage and lossesΔU% = 100 · ΔU / U, Ploss = 3 I² R L / nFor single-phase and DC circuits the losses are 2 I² R L / n.
L is the one-way length in km, R and X are per kilometre, and n is the number of cables in parallel per phase. When you enter power instead of current, the current is I = P / (√3 U cos φ) for three-phase, P / (U cos φ) for single-phase and P / U for DC.
Worked example
A 400 V three-phase circuit, 100 A at cos φ = 0.9, 100 m of copper 35 mm² at 70 °C, X = 0.08 Ω/km:
| Resistance at 70 °C | 0.627 Ω/km |
|---|---|
| Voltage drop | 10.38 V (2.59 %) |
| Voltage at the load | 389.6 V |
| Power loss in the cable | 1.881 kW |
When a single-cable check is not enough
Voltage drop adds up along a feeder and depends on what else is connected. For a whole installation, run a load flow: it gives the voltage at every bus and the loading of every cable at once. See the load flow guide and, for motors, the motor starting guide.
Frequently asked questions
What voltage drop is acceptable?
IEC 60364-5-52 (informative Annex G) suggests 3 % for lighting and 5 % for other uses in installations supplied from a public low-voltage network, and 6 % and 8 % when the installation has its own supply. The NEC informational notes suggest 3 % for a branch circuit and 5 % for feeder plus branch. Local rules and project specifications take precedence.
Where do the R and X values come from?
The calculator uses the IEC 60228 maximum DC resistance of class 2 stranded conductors at 20 °C and corrects it to the conductor temperature. Reactance depends on the cable construction and laying; about 0.08 Ω/km is typical for low-voltage multicore cables. For exact values, switch to “Datasheet value” and enter the manufacturer’s figures.
Which conductor temperature should I use?
The maximum operating temperature is 70 °C for PVC and 90 °C for XLPE or EPR insulation. Using it is conservative: a lightly loaded cable runs cooler and has a lower resistance.
Why √3 for three-phase and 2 for single-phase?
For three-phase circuits the formula gives the line-to-line drop, which is √3 times the drop in one phase conductor. In single-phase and DC circuits the current flows out and back through two conductors.
Is voltage drop enough to size a cable?
No. A cable must also carry the load current without overheating, withstand the short-circuit current, and allow the protective device to disconnect in time. Voltage drop is one of these checks.