Voltage drop calculator (IEC 60364)
Work out the voltage drop of a copper cable in volts and %, the voltage at the load and the smallest cable size that stays within your limit. Uses the IEC 60364-5-52 Annex G method, the same as the Cable Calc app.
Copper conductors, ρ₁ = 0.0225 Ω·mm²/m (1.25 × ρ at 20 °C) and λ = 0.08 mΩ/m, as in Annex G. For three-phase enter the line-to-line voltage (e.g. 400 V); the % is referred to the phase-to-neutral voltage. Guidance only: check the standard, the cable maker's data and your national wiring rules. Voltage drop is only one check: the cable must also carry the current after derating.
Recommended limits (IEC 60364-5-52, Table G.52.1)
| Supply | Lighting | Other uses |
|---|---|---|
| A: supplied directly from a public LV network | 3% | 5% |
| B: own LV supply (private transformer or generator) | 6% | 8% |
Worked example
A 230 V single-phase circuit carries 16 A over 30 m of 2.5 mm² copper at cos φ = 0.95: ΔU = 2 × (0.0225 × 30 / 2.5 × 0.95 + 0.00008 × 30 × 0.31) × 16 ≈ 8.2 V, which is 3.6% of 230 V. That is over a 3% lighting limit, so the next size up, 4 mm², brings it down to about 5.2 V (2.2%).
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Questions
How do you calculate voltage drop?
IEC 60364-5-52 Annex G uses ΔU = b × (ρ₁ × L / S × cos φ + λ × L × sin φ) × IB, where b is 1 for three-phase and 2 for single-phase, ρ₁ is the conductor resistivity in service (0.0225 Ω·mm²/m for copper), L the length in metres, S the cross-section in mm², λ the reactance (0.08 mΩ/m) and IB the design current. The percentage is ΔU divided by the phase-to-neutral voltage.
What is the maximum voltage drop allowed?
IEC 60364-5-52 Table G.52.1 recommends 3% for lighting and 5% for other uses when the installation is supplied directly from a public low-voltage network, and 6% / 8% when it has its own LV supply. National rules can be stricter, so check yours.
Why is three-phase voltage drop lower than single-phase?
In a balanced three-phase circuit no current returns in the neutral, so the drop is counted on one conductor (b = 1) instead of out and back (b = 2), and it is compared with the phase-to-neutral voltage.
Is this calculator suitable for AWG cables?
No. It uses metric cross-sections in mm² and IEC values. For North American installations use the NEC method and AWG sizes.