Voltage Drop Calculator and Cable Size Calculator
Work out the voltage drop in volts and percent for 1-phase, 3-phase and DC circuits, and get the smallest copper or aluminium cable size that meets both the drop limit and the current rating. Modelling the installation too? Browse our MEP Revit families.
1-phase, 3-phase and DCCopper and aluminium, mm²IEC 60228 resistance at 70/90 °CAmpacity to IEC 60364-5-52Formula with your numbers
Supply
Line-to-line voltage for 3-phase.
Load
Design current Ib of the circuit.
Active power in kW.
0.9 is typical for mixed loads; use 0.8–0.85 for motors.
Cable
Route length from the board to the load, not the loop length.
Identical cables sharing the load.
Optional: test a size you already have in mind.
Voltage-drop limit
Limits from origin of installation to the load, per IEC 60364-5-52 Annex G (Table G.52.1) and BS 7671 Appendix 4 (section 6.4). Public supply means fed directly from the public low-voltage network; private supply means fed from your own transformer.
Recommended cable size
Enter your circuit to see the result.
Preliminary sizing only. This calculator does not apply derating for grouping, ambient temperature other than 30 °C, thermal insulation or other installation methods, and it does not check protective-device coordination (Ib ≤ In ≤ Iz), fault-loop impedance or short-circuit withstand. Verify the final design against your local code (IEC 60364, BS 7671 or NEC).
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Request a projectSize a cable in three steps
Describe the supply
Choose 1-phase, 3-phase or DC and pick a voltage such as 230 V or 400 V, or type your own.
Enter the load and the route
Give the current in amps or the power in kW with its power factor, the one-way length, conductor, insulation and the drop limit.
Read the recommended size
The calculator returns the larger of the size needed for voltage drop and the size needed for current rating, with a table of nearby sizes and the formula with your numbers.
Voltage drop formula for 1-phase, 3-phase and DC
I is the design current in A, L the one-way length in m, and R and X the resistance and reactance of one conductor in mΩ/m. Divide by 1000 to get volts, then ΔU % = ΔU ÷ U × 100. For 3-phase, U is the line-to-line voltage.
R is the IEC 60228 class 2 maximum DC resistance at 20 °C, corrected to the conductor operating temperature: Rt = R20 × [1 + α × (t − 20)], with α = 0.00393 /K for copper and 0.00403 /K for aluminium, and t = 70 °C for PVC or 90 °C for XLPE. X is taken as 0.08 mΩ/m, typical for multicore LV cable.
Worked example: 3-phase 400 V, 32 A, 50 m, 6 mm² copper PVC, cos φ 0.9. R = 3.08 × 1.1965 = 3.685 Ω/km. ΔU = 1.732 × 32 × 50 × (3.685 × 0.9 + 0.08 × 0.436) ÷ 1000 = 9.29 V, which is 2.32 % of 400 V.
| Installation supplied from | Lighting | Other uses |
|---|---|---|
| Public low-voltage network | 3 % | 5 % |
| Private supply (own transformer) | 6 % | 8 % |
Maximum voltage drop from the origin of the installation to the load, per IEC 60364-5-52 Annex G (Table G.52.1) and BS 7671 Appendix 4. For wiring over 100 m, Table G.52.1 allows 0.005 % per metre beyond 100 m, up to 0.5 % extra.
Copper cable resistance and current rating
| Size (mm²) | R at 20 °C (Ω/km) | R at 70 °C (Ω/km) | Iz PVC 1-phase (A) | Iz PVC 3-phase (A) | Iz XLPE 3-phase (A) |
|---|---|---|---|---|---|
| 1.5 | 12.1 | 14.48 | 19.5 | 17.5 | 22 |
| 2.5 | 7.41 | 8.87 | 27 | 24 | 30 |
| 4 | 4.61 | 5.52 | 36 | 32 | 40 |
| 6 | 3.08 | 3.69 | 46 | 41 | 52 |
| 10 | 1.83 | 2.19 | 63 | 57 | 71 |
| 16 | 1.15 | 1.38 | 85 | 76 | 96 |
| 25 | 0.727 | 0.870 | 112 | 96 | 119 |
| 35 | 0.524 | 0.627 | 138 | 119 | 147 |
| 50 | 0.387 | 0.463 | 168 | 144 | 179 |
| 70 | 0.268 | 0.321 | 213 | 184 | 229 |
| 95 | 0.193 | 0.231 | 258 | 223 | 278 |
Resistance: IEC 60228 class 2, plain copper. Current rating Iz: IEC 60364-5-52 Tables B.52.2, B.52.4 and B.52.5, reference method C (multicore cable clipped direct), 30 °C ambient, no grouping. The copper values match BS 7671 Tables 4D2A and 4E2A. Apply derating factors for your installation.
Voltage drop and cable size questions
Use ΔU = √3 × I × L × (R·cos φ + X·sin φ), where I is the line current, L the one-way length and R and X the resistance and reactance of one conductor per metre. The result is the line-to-line drop, so divide it by the line-to-line voltage (for example 400 V) to get the percentage. For a 32 A load on 50 m of 6 mm² copper PVC cable at cos φ 0.9, the drop is about 9.3 V, or 2.3 %.
For multicore PVC copper cable clipped direct (reference method C) at 30 °C, 4 mm² is the smallest size rated for 32 A on a 3-phase circuit (32 A, IEC 60364-5-52 Table B.52.4). On a single-phase circuit 2.5 mm² is rated only 27 A, so 4 mm² (36 A) is needed again. Long runs often need a larger size for voltage drop, and grouping or high ambient temperatures reduce the rating, so check your length in the calculator.
IEC 60364-5-52 Annex G and BS 7671 Appendix 4 recommend 3 % for lighting and 5 % for other uses when the installation is fed from the public low-voltage network. For an installation fed from its own transformer the values are 6 % and 8 %. These limits apply from the origin of the installation to the load, so sub-main and final-circuit drops add up.
Single-phase: ΔU = 2 × I × L × (R·cos φ + X·sin φ). DC: ΔU = 2 × I × L × R. The factor 2 covers the outgoing and the return conductor, so L stays the one-way length. For example, 10 A over 20 m of 4 mm² copper at 24 V DC drops about 2.2 V, which is over 9 %, so low-voltage DC runs usually need a much larger cable than the current alone suggests.
The current rating (ampacity, Iz) stops the insulation from overheating, while the voltage-drop limit keeps enough voltage at the equipment. On short runs the current rating usually decides the size; on long runs voltage drop does. The recommended size is the larger of the two, before you apply derating for grouping, ambient temperature or the installation method.
Yes. Choose aluminium and the calculator uses IEC 60228 aluminium resistances (α = 0.00403 /K) and the aluminium columns of IEC 60364-5-52 method C. Aluminium sizes start at 16 mm². As a rule of thumb an aluminium conductor needs about one to two sizes larger than copper for the same current and voltage drop.
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