This calculator determines minimum cable size using the method described by the British Standard BS7671 – 18th Edition – IET Wiring Regulations which is the national standard in the United Kingdom for electrical installation and the safety of electrical wiring.
This is a simple calculator which ignores cable operating temperature and short-circuit rating. For the most accuracy use Cable Pro Web software which fully complies with BS 7671:2018.
Please enter the parameters below and press Calculate.
Inputs explained
- Voltage (V) – This is the voltage of the supply which is 230 V for single phase and 400 V for 3 phase supply by default. In general, BS 7671 applies to circuits supplied at nominal voltages (Uo) up to and including 1000 volts AC or 1500 volts DC.
- Phase – Select the phase arrangement as either single phase, three phase, two phase or DC.
- Load – Specify a value for the load whether it be in Amps, kilowatts, kVA or horsepower. Note for a three phase load this should be the current of the highest loaded phase.
- Maximum voltage drop (%) – the cable will be automatically sized to meet the maximum voltage drop requirement. Refer to our article Voltage drop limits for low voltage installations.
- Power factor – enter the load power factor (assumed lagging). The voltage drop calculation in this calculator considers power factor which is more accurate.
- Cable length (m) – this is the length of the cable from the point of supply to the load.
- Cores – specify whether the cable type will be single core or multicore (2 core or 3/4 core depending on phase selection).
- Armour – specifies whether the cable is armoured or unarmoured which changes the current rating table of BS7671.
- Conductor – select the conductor material which affects current-carrying capacity and the voltage drop due to different resistance values.
- Insulation – the insulation type which affects the maximum allowable operating temperature and hence the current rating. Cables with higher allowable temperature (which depends on the insulation type) will have a higher current rating.
- Installation – select the installation method of the cable circuit which matches your installation. Note that if the installation method changes along its route then choose the worst case (resulting in the lowest possible current rating) along its entire length.
Results explained
The minimum active cable size is provided which meets the current-carrying capacity and the maximum voltage drop requirements.
Note that a cable size larger than this minimum is also acceptable and may be more economical in terms of electrical losses over its lifetime.
The table number used from British Standard BS 7671:2018 is provided with the result for your reference and checking of the results by you.
Related Resources:
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