Electrical and solar
Cable size calculator
This calculator finds the smallest copper cable that will safely carry a load over a given distance, and the circuit breaker to protect it. Enter the load in kilowatts or amps, whether the supply is single-phase (230 V) or three-phase (400 V), the one-way length of the cable and how it is installed.
What you need
This calculator works out the figures in your browser, which needs JavaScript switched on. The method below shows how to work it out by hand.
Estimates for planning and budgeting. Real quantities depend on the site and the workmanship, so check with your builder before you order.
A cable has to pass two tests. It must carry the current without overheating, which depends on how it is installed: a cable in conduit inside a wall or roof runs hotter than one clipped to a surface. And the voltage lost along the cable must stay small enough for the equipment to work properly, which matters on long runs to a borehole pump, an outbuilding or a gate motor. The calculator checks both and shows every size it considered.
Ratings are for PVC-insulated copper cable at normal ambient temperature, from the tabulated values used under BS 7671 and IEC 60364, which South Africa's SANS 10142-1 and Zimbabwean practice broadly follow. Electrical installations in Zimbabwe must be done and certified by a licensed electrician; use this to plan and to check, not to replace them.
How it is worked out
- Design current: single-phase I = P ÷ (230 × power factor); three-phase I = P ÷ (√3 × 400 × power factor).
- Breaker: the smallest standard rating (6, 10, 16, 20, 25, 32, 40, 50, 63, 80, 100, 125 A) at or above the design current.
- Current check: the cable's tabulated rating for the installation method, times the derating factor, must be at least the breaker rating.
- Voltage drop = mV/A/m × current × length ÷ 1,000, expressed as a percentage of the supply voltage.
- Good practice minimums apply whatever the sums say: 1.5 mm² for lighting, 2.5 mm² for plugs and appliances, 4 mm² to a sub-board.
What it assumes
- Copper conductors, PVC insulation rated 70 °C, ambient about 30 °C.
- Single-phase uses two-core ratings; three-phase, three-core.
- Earth conductor sizing, fault loop impedance and discrimination are for the electrician to verify.
Common questions
What size cable for a 3 kW geyser?
A 3 kW geyser draws about 13 A. On a typical run, 2.5 mm² copper on a 20 A breaker is the usual answer; a very long run may need 4 mm² for voltage drop.
What size cable for a stove?
A domestic stove is usually on its own 32 A circuit in 6 mm² cable, sometimes 4 mm² for a short run with a smaller stove. Check the stove's rating.
Why does cable length matter?
Every metre of cable loses some voltage. On long runs, such as to a borehole pump, the cable often has to be bigger than the current alone requires, or the motor starts poorly and runs hot.
Want exact quantities for your plan?
A calculator works one wall or one slab at a time. A bill of quantities works from your actual drawings and counts everything, which is what builders price against and what stops you buying twice.