Electrical cable size calculator
Work out the cross-section of a copper electrical cable from the current or power, the length and the circuit breaker. The tool checks the current-carrying capacity for the installation method, ambient temperature and grouping, then the voltage drop, and keeps the larger of the two sizes.
Circuit and protection
1 for a heater or hob, 0.8 to 0.9 for a motor or heat pump.
Automatic: smallest standard rating at or above the design current.
Actual cable route from the board to the appliance, without counting the return.
Usual values: 3% for lighting, 5% for other uses.
Cable and installation
Number of loaded circuits run together (same conduit, trunking or bundle).
Selected size
Minimum copper cross-section
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How does this tool work?
Sizing an electrical cable means checking two things at once: that the cable carries the current its circuit breaker lets through without overheating, and that enough voltage still reaches the appliance. This calculator runs both checks for a copper cable, single or three-phase, and keeps the larger of the two sizes. It also shows which criterion sets the result, the derated current-carrying capacity of the cable and the resulting voltage drop. The values come from official cable rating tables and a published voltage drop formula, cited at the bottom of the page.
The design current I_B is either entered directly or derived from the power: I_B = P ÷ (U × cos φ) in single-phase, P ÷ (√3 × U × cos φ) in three-phase. The breaker must satisfy I_B ≤ I_n ≤ I_z: it lets the normal current through but protects the cable. In automatic mode the tool takes the smallest standard rating (6, 10, 13, 16, 20, 25, 32, 40 A…) at least equal to I_B.
For the current-carrying capacity, the tool reads the rating table for the cable type (single-core cables or multicore cable), the insulation (PVC at 70 °C or XLPE at 90 °C), the installation method (A, B, C or E) and the number of loaded conductors (2 in single-phase, 3 in three-phase). These values apply at 30 °C to a single circuit; they are multiplied by a temperature factor and a grouping factor. The size kept is the smallest whose derated capacity reaches the breaker rating.
For the voltage drop, the tool applies ΔU = b × I_B × (ρ/S × cos φ + λ × sin φ) × L, with b = 2 in single-phase (go and return) and √3 in three-phase, a copper resistivity in service of 23.7 Ω·mm²/km and a reactance of 0.08 Ω/km. It keeps the smallest standard size that stays under the chosen percentage. The final size is the larger of the two, between 1.5 and 240 mm².
How to use this tool
Describe the circuit
Choose single or three-phase and the voltage, then the current or power of the appliance and its cos φ. Leave the breaker on automatic or enter the one fitted.
Describe the cable and its route
Enter the one-way length, the maximum drop, the cable type, insulation, installation method, ambient temperature and number of circuits run together.
Read the size and the criterion
The minimum size appears with the criterion that sets it. Compare it with your national wiring rules before choosing the cable.
The two sizing criteria
The size kept is the smallest standard cross-section that meets both the current-carrying capacity condition and the voltage drop limit.
Design current from power
I_B = P ÷ (k × U × cos φ), with k = 1 in single-phase and √3 in three-phase
A 7,360 W hob on 230 V with cos φ = 1 draws 32 A.
Protection condition
I_B ≤ I_n ≤ I_z, with I_z = I_t × C_a × C_g
I_t is the table value for the installation method; C_a and C_g correct for temperature and grouping.
Voltage drop
ΔU = b × I_B × (ρ ÷ S × cos φ + λ × sin φ) × L
b = 2 in single-phase, √3 in three-phase; ΔU % = ΔU ÷ U × 100.
- I_B, I_n, I_z
- design current, breaker rating and derated current-carrying capacity of the cable, in amperes
- C_a, C_g
- ambient temperature and grouping correction factors
- ρ
- resistivity of copper in service: 23.7 Ω·mm²/km
- λ
- linear reactance: 0.08 Ω/km
- S, L
- cross-section in mm² and one-way length in km
Use cases
Dedicated circuit for a heavy appliance
Hob, water heater, EV charger: start from the power on the data sheet and check the matching cable size and breaker.
Supplying a garage or garden building
Over 30 to 50 m, see from what length the voltage drop calls for a larger cable than the current does.
Checking a quote or an existing cable
Make sure a proposed size can carry the breaker rating under the real installation, temperature and grouping conditions.
Examples
32 A over 5 m: the voltage-drop-only trap
On a short run the voltage drop is small, and a calculation limited to that criterion would suggest 1.5 mm², rated about 17.5 A for this installation method: it would overheat. The table requires 4 mm² (32 A). National rules can ask for more: in France, for example, a 32 A cooking circuit needs 6 mm².
Input
Single-phase 230 V; 32 A; automatic breaker (32 A); 5 m; PVC single-core cables in conduit on a wall (B); 30 °C; single circuit; 3% limitResult
Minimum size: 4 mm² (current-carrying capacity); voltage drop alone would allow 1.5 mm²; I_z = 32 A; ΔU = 1.9 V, i.e. 0.82%16 A over 30 m: the length sets the size
With 2.5 mm² the drop would reach 9.1 V, i.e. 3.96%, above the 6.9 V allowed. A typical case is a socket in a garage far from the consumer unit.
Input
Single-phase 230 V; 16 A; 16 A breaker; 30 m; PVC single-core cables, method B; cos φ = 1; 3% limitResult
Minimum size: 4 mm² (voltage drop); current alone would allow 1.5 mm²; ΔU = 2 × 16 × (23.7 ÷ 4) × 30 ÷ 1,000 = 5.69 V, i.e. 2.47%11 kW three-phase heat pump
Heat and a neighbouring circuit cut the cable’s capacity by a quarter: a tabulated value of at least 26 A is needed to protect it at 20 A. Without these corrections, 1.5 mm² (22 A in the table) would have seemed enough.
Input
Three-phase 400 V; 11,000 W; cos φ = 0.85; 25 m; XLPE multicore cable clipped direct (C); 35 °C; 2 grouped circuits; 3% limitResult
I_B = 18.7 A; 20 A breaker; factors 0.96 × 0.80 = 0.768; minimum size 2.5 mm² (current); I_z = 30 × 0.768 = 23 A; ΔU = 6.6 V, i.e. 1.64%Tips and best practices
- Start from the breaker, not only the appliance current: the breaker protects the cable, and the cable must carry its rating.
- Measure the real route, bends and risers included, but only once: the factor 2 in single-phase already counts the return.
- A cable surrounded by thermal insulation over more than 0.5 m can lose half its capacity: that case is not calculated here.
- In the US and Canada, conductors are sized in AWG under the NEC or CEC with their own tables; this tool uses metric sizes and IEC-based tables.
- For a motor, the voltage drop at start-up is much higher than in steady operation: have that case checked separately.
Sources and references
- Electrical and Mechanical Services Department (Hong Kong), Code of Practice for the Electricity (Wiring) Regulations — Appendix 5 (rating factors) and Appendix 6 (current-carrying capacities) (opens in a new tab), 2025Page in English
- Schneider Electric, Calculation of voltage drop in steady load conditions (opens in a new tab), Electrical Installation GuidePage in English
- Schneider Electric, Maximum voltage drop limit (opens in a new tab), Electrical Installation GuidePage in English
Frequently asked questions
What cable size for a 16, 20 or 32 A breaker?
It depends on the installation method, temperature, grouping and length. For PVC single-core cables in conduit on a wall, at 30 °C and over a short run, the tables give 1.5 mm² for 16 A, 2.5 mm² for 20 A and 4 mm² for 32 A. National rules may set larger pairings for household circuits, and they take precedence over this calculation.
Why is the selected size larger than the one for current?
Because the voltage drop requires it: over a long run, a cable thick enough not to overheat can still lose too much voltage. The tool shows both sizes and keeps the larger one.
Single-core cables or multicore cable?
Single-core cables are separate insulated wires pulled into conduit or trunking. A multicore cable has its conductors under a common round sheath. Their ratings differ for the same size; flat twin-and-earth cable has its own tables, not used here.
How do I allow for temperature and nearby circuits?
Choose the highest ambient temperature along the route (loft, plant room) and the number of loaded circuits run together. The tool applies the tabulated factors: for example 0.87 at 40 °C for a PVC cable and 0.70 for three grouped circuits.
What maximum voltage drop should I choose?
Usual values for an installation supplied from the public low-voltage network are 3% for lighting and 5% for other uses, from the origin of the installation to the appliance. If the circuit starts at a sub-distribution board, keep a margin for the drop already present upstream.
Why is aluminium not offered?
The rating tables used here are for copper. Sizing an aluminium cable on voltage drop alone could give a section too small for the current; that case needs the aluminium tables.
Is this tool enough for a compliant installation?
No. It checks neither the short-circuit current, nor the disconnection time under fault, nor the minimum sizes required by your national rules. It helps prepare or check a choice; the work itself must be done or inspected by a qualified electrician.
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