Every conductor has resistance. Push current through it and some of the supply voltage is spent along the cable instead of at the load. That lost voltage is the voltage drop. It is small on short runs and large on long ones, and it is the reason a pump at the far end of a field or a workshop in an outbuilding often needs a thicker cable than its current rating alone would suggest.
The physics in one line
Ohm's law applies to the cable itself: the voltage across a conductor equals the current through it times its resistance, V = I × R. A cable's resistance grows with length and shrinks with cross-sectional area:
where ρ (rho) is the resistivity of the metal, L the length and A the area. Copper's resistivity is about 0.0172 Ω·mm²/m at 20 °C; aluminium's about 0.0282. Double the length and the drop doubles; double the area and it halves. Warm the conductor and its resistance rises, about 0.39 % per degree for copper, which is why calculations use the operating temperature rather than room temperature.
Out and back: why the length is doubled
Current leaves the source on one conductor and returns on another. In a DC or single-phase AC circuit the current flows through the full length of both, so the effective conductor length is twice the route length:
with Rc the resistance per metre of one conductor. In a balanced three-phase circuit the return currents cancel in the neutral, and the drop between lines works out as √3 (about 1.73) times the single-conductor drop rather than 2:
For loads that are not purely resistive, multiply by the power factor (cos φ) to get the resistive part of the drop. The voltage drop calculator applies exactly these formulas and shows each intermediate value.
A worked example
A 230 V single-phase supply feeds a 20 A load 30 m away through 2.5 mm² copper running at 70 °C. IEC 60228 gives 7.41 Ω/km for that conductor at 20 °C; at 70 °C it is 7.41 × (1 + 0.00393 × 50) ≈ 8.87 Ω/km, or 0.00887 Ω/m.
Vd = 2 × 20 × 30 × 0.00887 ≈ 10.6 V, which is 4.6 % of 230 V. The load sees roughly 219 V. Using 4 mm² instead brings the drop to about 6.6 V (2.9 %).
What the standards allow
Limits are set for two reasons: equipment must receive a voltage within its tolerance, and energy dissipated in a cable is wasted as heat. The common references:
- BS 7671 (UK wiring regulations), Appendix 12: for an installation supplied from a public low-voltage network, 3 % for lighting and 5 % for other uses, measured from the origin of the installation to the load.
- NEC (NFPA 70, USA): informational notes at 210.19(A) and 215.2(A) recommend that a branch circuit or feeder drop not exceed 3 %, and that feeder plus branch together not exceed 5 %, for reasonable efficiency of operation. These are recommendations, not enforceable requirements, in most cases.
- IEC 60364-5-52, Annex G (informative): similar values, with allowances for longer runs.
Local codes may adopt or tighten these. Equipment can be more demanding than the code: check motor and electronics datasheets for their supply tolerance.
What excessive drop does
- Motors draw more current at lower voltage to deliver the same torque, run hotter, and may fail to start under load. Starting current through a long cable can pull the voltage down far enough to trip protection or stall the motor.
- Lighting: incandescent output falls sharply with voltage; LED drivers usually cope but may flicker or shut down below their minimum input.
- Electronics and chargers may reset, throttle or fail to charge. Battery chargers and inverters at the end of long DC runs are a frequent case, because low-voltage DC systems carry high current: a 1 V drop matters far more at 12 V than at 230 V.
- Energy: the drop times the current is power lost in the cable as heat, continuously, for the life of the installation.
How to reduce it
- Increase conductor size: the drop is inversely proportional to area.
- Shorten the route, or move the distribution point closer to the load.
- Raise the voltage: the same power at 400 V three-phase or 48 V DC instead of 230 V or 12 V needs far less current, and the percentage drop falls with it.
- Reduce current: split loads across circuits, improve power factor, or use more efficient equipment.
Remember that voltage drop is only one of the sizing checks. The conductor must also carry the current safely for its installation method and ambient temperature, and the circuit's protective device must still operate quickly enough at the far end of the run.
Low-voltage DC deserves special care
Solar arrays, battery banks and 12/24/48 V equipment move a lot of current at low voltage. A 600 W load at 12 V is 50 A; over just 5 m of 16 mm² copper at 50 °C the drop is about 0.64 V, already more than 5 % of a 12 V supply, before the inverter or charger has done anything. This is why battery-to-inverter cables are short and thick, and why DC distribution over any distance moves to 48 V or higher. The battery runtime calculator reports the discharge current, which is the number to feed into the voltage drop calculator for the DC side.
References
- IEC 60228:2004, Conductors of insulated cables
- BS 7671:2018+A2:2022, Requirements for Electrical Installations, Appendix 12 (Voltage drop in consumers' installations)
- NFPA 70, National Electrical Code 2023, 210.19(A) Informational Note No. 4; 215.2(A) Informational Note No. 2
- IEC 60364-5-52:2009, Low-voltage electrical installations — Selection and erection of electrical equipment — Wiring systems, Annex G