How it works
A cable has to pass two tests. First, its current rating (after allowing for a hot ambient) must be at least the load current. Second, the voltage lost along its length must stay within the limit you set, often 3 % for a feeder.
The calculator works down the standard sizes (1.5, 2.5, 4, 6, 10, 16, 25, 35, 50, 70, 95 and 120 mm²) and shows the smallest size that passes the current test, the smallest that passes the voltage-drop test, and the larger of the two as the recommended size. On long runs the voltage drop decides, not the current.
The current ratings are indicative values for PVC-insulated copper cable in conduit, at 30 °C and scaled for the ambient temperature you choose. Aluminium is taken as 78 % of copper and is offered from 16 mm². Grouping several circuits together, burial, sunlight, harmonics and the starting current of motors are not included.
Formulas used
- Current: I = kW × 1000 ÷ (V × pf) single phase, I = kW × 1000 ÷ (√3 × V × pf) three phase
- Required rating = I ÷ ambient factor
- Voltage drop = 2 × I × L × (R × pf + X × sin φ) single phase, √3 × I × L × (R × pf + X × sin φ) three phase
- R = ρ ÷ cable area (ρ = 0.0206 Ω·mm²/m copper, 0.0339 aluminium, at 70 °C), X = 0.08 Ω/km
Worked example
A 15 kW three phase load at power factor 0.9 on 415 V draws about 23.2 A. At 40 °C the cable rating must be at least 23.2 ÷ 0.87 = 26.7 A, so 4 mm² (28 A in the table) passes the current test. Over 100 m with a 3 % limit the voltage drop decides, and 10 mm² is recommended (drop about 1.8 %).
The calculator opens with these example values, so you can compare its answer with the text above.
Frequently asked questions
How do I choose cable size from load?
Work out the load current, divide it by the ambient derating factor, pick a size whose rating is at least that, then check the voltage drop over your length and increase the size if it is too high.
What voltage drop is allowed?
Many designers use about 3 % for a feeder and 5 % in total from the supply to the load. Your supply authority or the applicable code may differ, so check the rule that applies to your installation.
Why is aluminium only offered from 16 mm²?
Smaller aluminium conductors are rarely used for fixed power wiring, so the calculator does not suggest them.
Can I rely on this for a real installation?
No. It is a learning tool. A real design must follow the cable maker’s datasheet and the applicable standard (for example IS 732 and IS 1554 in India), including grouping, installation method and short-circuit checks.