Voltage Drop Calculator
Drop in volts and percent, load voltage and cable losses — with the formula shown.
How it was calculated
Other conductor sizes on this run
| Size | Drop | % | Limit | Loss |
|---|
Same current, length, temperature and reactance. Voltage drop says nothing about whether a cable is big enough for the current — check its current-carrying capacity in your wiring rules or the cable datasheet, or use the cable size calculator.
Results are estimates from the formulas shown, not a professional design or certification. Have a qualified engineer verify anything safety-critical (structures, electrical installations, gas or pressure systems).
About the Voltage Drop Calculator
Long cable runs lose voltage: the conductors have resistance (and, for AC, reactance), so less than the supply voltage reaches the load. This calculator works out the voltage drop in volts and percent, the voltage left at the load and the power lost in the cable, for DC, single-phase and three-phase circuits, with conductor sizes in mm² (IEC 60228) or AWG and kcmil, in copper or aluminium.
It uses the formula in IEC 60364-5-52 Annex G, with resistance from the IEC 60028 and IEC 60889 resistivities at the conductor temperature you choose — or the Annex G default, or your cable datasheet — and compares the result with the limits of IEC 60364, BS 7671 or the NEC recommendation. It also tells you the longest run that stays within the limit and the smallest size that would.
How to use it
- Choose the supply (DC, single-phase or three-phase) and enter the voltage — line to neutral for single-phase (230 V), line to line for three-phase (400 V).
- Enter the load as a current in amps, or as power in kW (with the power factor for AC), and the one-way route length.
- Choose copper or aluminium, mm² or AWG sizes, the conductor size and how many conductors run in parallel per line.
- Pick where the resistance comes from: resistivity at the conductor temperature (70 °C for PVC, 90 °C for XLPE at full load), the Annex G default, or a datasheet value. Leave reactance on for AC unless you know better.
- Choose the limit to compare with. Read the drop, the verdict, the longest run and the smallest size within the limit, and the table of other sizes.
Examples
Single-phase, 20 A, PF 1, 25 m, 2.5 mm² Cu at 70 °C
R = 8.252 Ω/km · u = 2 × 8.252 × 0.025 × 20 = 8.25 V = 3.59% · within 5%, over 3% · 4 mm² gives 2.24%
400 V, 30 kW, PF 0.85, 80 m of 16 mm² Cu at 70 °C, λ = 0.08 mΩ/m
I = 50.94 A · u = (1.289 × 0.85 + 0.08 × 0.527) × 0.08 × 50.94 = 4.64 V per phase = 2.01% (8.03 V line to line)
Single-phase, 120 V, 16 A, 50 ft, 12 AWG Cu at 75 °C
R = 6.337 Ω/km · u = 3.09 V = 2.58% · within the NEC 3% branch-circuit recommendation
DC, 12 V, 10 A, 5 m, 4 mm² Cu at 20 °C
u = 2 × 4.310 Ω/km × 0.005 km × 10 A = 0.431 V = 3.59%
The formula (IEC 60364-5-52 Annex G)
u = b × (ρ₁ × L ÷ S × cos φ + λ × L × sin φ) × I_B, and the percentage Δu = 100 × u ÷ U₀.
- b is 2 for single-phase and two-wire DC circuits (the current goes out and back) and 1 for three-phase circuits, where u is the drop in each phase
- ρ₁ ÷ S is the conductor resistance per metre; λ is its reactance per metre (Annex G default 0.08 mΩ/m)
- L is the one-way route length, I_B the design current and cos φ the load power factor
- U₀ is the line-to-neutral voltage: 230 V for single-phase; for a 400 V three-phase circuit, 400 ÷ √3 = 230.9 V
For three-phase circuits the drop between lines is √3 × u, so the percentage is the same either way. Annex G treats a three-phase circuit with only one phase loaded as a single-phase circuit.
Where the resistance comes from
- Resistivity at conductor temperature (default): copper 1/58 Ω·mm²/m at 20 °C with α = 0.00393 /K (IEC 60028), aluminium 0.028264 Ω·mm²/m with α = 0.00403 /K (IEC 60889), corrected with R_T = R₂₀ × (1 + α(T − 20)). A cable at full load runs at its maximum operating temperature — 70 °C for PVC, 90 °C for XLPE and EPR.
- Annex G default ρ₁: 0.0225 Ω·mm²/m for copper and 0.036 for aluminium, the values the standard uses when nothing better is known.
- Datasheet: the maximum conductor resistance at 20 °C from the cable maker, corrected to your temperature. Use this for final design: IEC 60228 allows a real stranded conductor more resistance than its nominal area suggests — up to 7.41 Ω/km for 2.5 mm² copper, against 6.90 Ω/km from resistivity — so the datasheet value gives a slightly higher, safer drop.
How much voltage drop is allowed?
IEC 60364-5-52 Table G.52.1 and BS 7671 Appendix 4 (Table 4Ab) give the same figures, measured from the origin of the installation to the load:
- Supplied from a public low-voltage network: 3% for lighting, 5% for other uses
- Supplied from a private low-voltage supply (your own transformer or generator): 6% for lighting, 8% for other uses — although the drop within final circuits should, as far as possible, stay within the public-supply figures
- Where main wiring runs are longer than 100 m, the limits may be raised by 0.005% per metre beyond 100 m, by no more than 0.5% in total
Because the figures run from the origin, the drops of a sub-main and the final circuit it feeds add up: work out each cable here and keep the sum within the limit (or choose Your own limit for what is left after the sub-main).
In the US, informational notes to NEC 210.19 and 215.2 recommend sizing conductors so that a branch circuit drops no more than 3% and feeder plus branch circuit no more than 5%. Those notes are advice, not code requirements.
How to reduce voltage drop
- A larger conductor: the drop falls in proportion to the area (2.5 → 4 mm² cuts it by 37.5%).
- Conductors in parallel: two equal conductors per line halve the drop.
- A shorter route, or a sub-distribution board nearer the load.
- Copper instead of aluminium: aluminium has about 1.64 times the resistance for the same area.
- A higher voltage or three-phase: the same power at a higher voltage needs less current, and three-phase has no return-conductor drop (b = 1).
Sources
- IEC 60364-5-52:2009, Low-voltage electrical installations — Selection and erection of electrical equipment — Wiring systems, Annex G (informative) and Table G.52.1
- IEC 60028, International standard of resistance for copper, and IEC 60889, Hard-drawn aluminium wire for overhead line conductors
- IEC 60228, Conductors of insulated cables — nominal cross-sections and maximum resistances
- BS 7671:2018, Requirements for Electrical Installations (IET Wiring Regulations, 18th edition, as amended), Appendix 4 §6.4 and Table 4Ab
- NFPA 70, National Electrical Code, 210.19 and 215.2 informational notes
Limitations
- The formula is the standard approximation, accurate for the small drops installations aim for; above about 10% it becomes rough, and the equipment would not work properly anyway.
- It uses DC resistance. Skin and proximity effects make the AC resistance of large conductors higher — by a few percent around 240 mm² and more above that — so use the cable maker’s AC resistance for big feeders.
- It checks voltage drop only. A cable must also carry the current without overheating (current-carrying capacity), survive a short circuit and let the protective device disconnect in time — see the cable size calculator.
- Motor starting current and other transient dips are not included; three-phase results assume a balanced load.
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Frequently asked questions
What is an acceptable voltage drop?
Under IEC 60364 and BS 7671, for an installation fed from the public network: 3% for lighting and 5% for other uses, from the origin to the load. With a private supply (own transformer), 6% and 8%. In the US, the NEC recommends 3% for a branch circuit and 5% for feeder plus branch.
How do I calculate voltage drop?
For a single-phase circuit, multiply 2 × the conductor resistance per metre × the one-way length × the current. For 20 A over 25 m of 2.5 mm² copper at 70 °C: 2 × 0.008252 Ω/m × 25 m × 20 A = 8.25 V, which is 3.59% of 230 V. For AC loads with a power factor below 1, the reactance adds a little more.
Should I enter the one-way or the round-trip length?
One-way: the distance from the supply to the load along the cable route. The calculator adds the return conductor itself for single-phase and DC circuits. For three-phase circuits there is no return-path drop in a balanced load.
Why is there a √3 for three-phase voltage drop?
The Annex G formula gives the drop in each phase (b = 1), compared with the line-to-neutral voltage. The drop between two lines is √3 times larger, compared with the line-to-line voltage — so the percentage comes out the same.
What conductor temperature should I use?
For a cable carrying its full rated current, its maximum operating temperature: 70 °C for PVC (thermoplastic) insulation, 90 °C for XLPE or EPR. A lightly loaded cable runs cooler and drops less; 20 °C gives the lowest, cold-cable figure.
Does a bigger cable reduce energy loss too?
Yes. The cable loss is I² × R, so doubling the conductor area halves the loss as well as the drop. The calculator shows the loss in watts and as a share of the load power.