Calculation reviewed: 20 October 2026 · Engineering Desk Tools · How we check calculations
How this cable size calculator works
Enter the load in kW or amps, the supply, cable length and the voltage drop you can accept, then choose the conductor, insulation, cable type and how the cable is installed. The calculator finds the smallest cable that passes two checks: it must carry the design current after temperature and grouping derating, and the voltage drop over the run must stay within your limit. It also tells you which check decided the size, so you can see whether a longer run or a hotter location is driving the result.
How to calculate cable size in sq mm (formula)
Three-phase current: Ib = P × 1000 ÷ (√3 × V × PF)
Single-phase current: Ib = P × 1000 ÷ (V × PF)
Current check: Iz × Ct × Cg ≥ Ib
Voltage drop: ΔV = k × Ib × L × (R cos φ + X sin φ) ÷ 1000, with k = √3 (three-phase) or 2 (single-phase)
P is the electrical input power in kW, V the line-to-line voltage (three-phase) or line-to-neutral voltage (single-phase), PF the power factor and L the one-way length in metres. Iz is the tabulated current rating for the installation method, Ct the ambient temperature factor and Cg the grouping factor. R is the AC resistance of the conductor at its maximum operating temperature in Ω/km and X the reactance, about 0.08 Ω/km for multi-core cables. For a motor, use the electrical input power (shaft kW ÷ efficiency) or the nameplate full-load current; the motor current calculator and kW to amps calculator can help.
Worked example: 15 kW load at 400 V, 60 m
A 15 kW three-phase load at 400 V with a power factor of 0.85 is fed by a copper XLPE multi-core cable on a cable tray, 60 m long, in 40 °C air. The allowed voltage drop is 3 %.
- Design current: Ib = 15 000 ÷ (1.732 × 400 × 0.85) = 25.5 A.
- Current check: at 40 °C the temperature factor is 1.0. A 2.5 mm² cable is rated 29 A on a tray, so it carries the current.
- Voltage drop at 2.5 mm²: R = 9.45 Ω/km, so ΔV = 1.732 × 25.5 × 60 × (9.45 × 0.85 + 0.08 × 0.53) ÷ 1000 ≈ 21.4 V = 5.3 %. Too high.
- Try 4 mm²: R = 5.88 Ω/km gives about 13.3 V = 3.3 %. Still above 3 %.
- Try 6 mm²: R = 3.93 Ω/km gives about 9.0 V = 2.2 %. Passes.
The answer is 6 mm², and the size is decided by voltage drop, not current. On a 20 m run the same load would only need 2.5 mm². Long runs are where most undersized cables come from, which is why the calculator always checks both.
Cable current carrying capacity table (mm²)
The calculator uses the manufacturer ratings below. They are for one circuit of 0.6/1 kV multi-core cable at 40 °C air or 35 °C ground (hot-climate base conditions); in cooler places the cable can carry more, and the calculator applies the temperature factor for you.
Source: Bahra Cables low voltage power cable catalogue, ratings calculated to IEC 60287 (in ground, 0.5 m depth, soil 1.2 K·m/W) and BS 7671 (in air). “–” = size not listed for that cable. Single-core and armoured ratings are in the calculator.
Installation methods
- On cable tray or in free air (E): the cable is in open air on a perforated tray or ladder, spaced from the wall. Heat escapes easily, so ratings are the highest.
- In conduit or pipe (B): the cable runs in conduit or trunking on a wall. Trapped air lowers the rating.
- Buried direct in ground (D): laid directly in the soil. The rating depends on soil temperature and thermal resistivity.
- In underground duct: pulled through a buried duct. The air gap around the cable makes it lower than direct burial.
If several circuits run together, enter a grouping factor from your wiring rules, for example about 0.8 for two touching multi-core cables on a tray or about 0.7 for three. Leave it blank for a single circuit.
Temperature derating factors
What the calculator does not check
This tool sizes a cable for continuous current and voltage drop. It does not choose the protective device, check short-circuit withstand or earth-fault loop impedance, account for harmonics, direct sunlight, deep burial or poor soil, or voltage dip during motor starting. Ratings vary slightly between manufacturers. Treat the result as a first selection and confirm it against your local wiring rules (IEC 60364, BS 7671, IS 732, AS/NZS 3008 or NEC) and the cable maker's datasheet before you order.
Cable size questions
How do you calculate cable size in sq mm?
Work out the design current (three-phase I = P × 1000 ÷ (√3 × V × PF)), divide it by the temperature and grouping factors, and pick the smallest size in the rating table whose current rating is at least that value. Then check voltage drop over the cable length; if it exceeds your limit, go up one size until it passes.
What is the cable size and current rating formula?
The cable must satisfy Iz × Ct × Cg ≥ Ib, where Ib is the design current, Iz the tabulated rating for the installation method, Ct the ambient temperature factor and Cg the grouping factor. Voltage drop is ΔV = √3 × I × L × (R cos φ + X sin φ) for three-phase circuits and twice I × L × (R cos φ + X sin φ) for single-phase.
What size aluminium cable replaces a copper cable?
An aluminium conductor carries roughly 78 % of the current of a copper conductor of the same size, so it usually needs one or two sizes larger, about 1.6 times the copper cross-section. Aluminium is normally used from 16 mm² upwards; check the lug and terminal ratings as well.
What voltage drop limit should I use?
IEC 60364-5-52 suggests 3 % for lighting and 5 % for other uses when the installation is supplied from a public low-voltage network, and 6 % and 8 % when it has its own transformer. Your local code or project specification may set a different limit, so enter the value that applies to your job.
How do you calculate earthing cable size?
A common simplified rule from IEC 60364-5-54 (same material as the line conductor): up to 16 mm², the protective conductor is the same size as the line conductor; from 16 to 35 mm² it is 16 mm²; above 35 mm² it is half the line conductor size. For an exact size use the adiabatic equation S = √(I²t) ÷ k with the fault current and disconnection time.
Does this calculator choose the breaker or check short-circuit current?
No. It sizes the cable for continuous current and voltage drop only. You still need to coordinate the protective device (Ib ≤ In ≤ Iz), check the cable can withstand the fault current until the device trips, and confirm earth-fault loop impedance.