Cable Size Voltage Drop Calculator
Check your cable is thick enough before the volt drop trips you up.
Result
How it is calculated
The tool assumes a copper conductor with resistivity ρ = 0.0175 Ω·mm²/m. Voltage drop is ΔV = k × L × I × ρ / A, where L is the one-way cable length in metres, I the current in amps, A the cross-section in mm², and k = 2 for single-phase (out and back) or k = √3 for three-phase. The percentage is ΔV / U × 100 with U the supply voltage, and the voltage at the load end is U − ΔV. The minimum size uses the same formula with the allowed drop U × limit / 100 in place of ΔV. A blank limit means 3 %; a limit that is typed but not a number, zero, or above 100 shows no result, and so does any phase other than single or three, or a drop as large as the supply voltage. Cable temperature, power factor and bundling are ignored.
Worked example
A single-phase 230 V circuit carries 32 A over 40 m of 6 mm² copper, with a 3 % limit. ΔV = 2 × 40 × 32 × 0.0175 / 6 = 44.8 / 6 = 7.47 V, which is 3.25% of 230 V, so the load end sees 222.53 V. The allowed drop is 230 × 3 / 100 = 6.9 V, so the smallest size is 44.8 / 6.9 = 6.49 mm². The 6 mm² cable misses the limit, and you would move up to the next larger size you can buy.
Common mistakes
- Entering the total loop length (out and back) as the length. The tool already doubles it for single-phase, so enter the one-way distance from supply to load.
- Using 230 V for a three-phase circuit. For the three-phase option the voltage is the line-to-line value, such as 400 V.
- Reading the minimum area as a standard size. 6.49 mm² is a calculated figure, so round up to a size you can actually buy.
- Checking only the volt drop and forgetting the current rating of the cable. This tool does not test ampacity, so a size that passes here can still be too small for the load.
- Entering the limit as a fraction. The field takes a percentage, so 3 means 3 %, not 0.03.
About this calculator
Voltage drop is the loss of voltage along a cable run caused by the conductor's own resistance, calculated from Ohm's law as current multiplied by the cable's total resistance (which depends on the conductor's cross-sectional area, length, and material — usually copper), doubled for a single-phase circuit to account for both the outgoing and return conductors. Electrical codes such as the UK's BS 7671 and the US NEC recommend keeping voltage drop under roughly 3-5% of the supply voltage, since excessive drop causes dimming lights, underperforming motors, and equipment that fails to operate within its rated tolerance at the far end of a long cable run.
Electricians calculate this before wiring a run — particularly long runs to outbuildings, workshops, or EV chargers — because a cable that's perfectly adequate for the current-carrying (ampacity) rating can still fail the voltage-drop check if the run is long enough, meaning you sometimes need a thicker conductor than the basic ampacity table would suggest, purely to keep the drop within limits.
This calculator takes your cable length, conductor size, load current, supply voltage and allowed drop (3% unless you change it), for single- or three-phase, and returns the voltage drop in volts and percent, the voltage left at the load and the minimum copper conductor size that keeps the drop within your limit — compare it with your cable yourself, and check the ampacity separately.
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