Cable Size Calculator BS7671

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Cable Size Calculator BS7671

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.
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Your Calculated Cable Size Is:

Why This Result?
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.

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How to use

Calculator 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 to match the load. The options available are single-phase AC, three-phase AC, two-phase AC, or DC.
  • This phase arrangement affects the voltage drop calculation because different voltage drop equations are used.
  • Cable Pro Software performs more accurate voltage drop calculations than this calculator does, resulting in smaller cable sizes.
  • Three-phase AC is used for larger loads and consumer mains. Balanced three-phase conditions are assumed, meaning the current in each of the three phases is the same.
  • To calculate the voltage drop for an unbalanced three-phase load, a conservative approach is to assume a balanced three-phase load condition and perform calculations using the current flowing in the heaviest-loaded phase.

Load

  • The load determines the current-carrying capacity of the cable. The load is specified in Amps, kilowatts, kVA, or horsepower.
  • For a three-phase load, this should be the current of the highest-loaded phase or the total power on all the phases.

Maximum voltage drop (%)

  • The cable size will also be determined by the maximum allowable voltage drop limit.
  • Useful rules of thumb for voltage drop are to allow 0.5% for consumers mains, 1.5-2% for sub-mains, and 2.5 % for final subcircuits.
  • Refer to our article Voltage drop limits for low voltage installations.

Power factor

  • Enter the load power factor (assumed lagging). The power factor is used for accurate voltage drop calculations.
  • A poor power factor will result in a higher voltage drop, especially for large cable sizes (because their impedance is highly reactive).

Cable length (m)

  • This is the distance between the supply point and the load location.
  • The cable return path length is automatically included with the voltage drop calculations for single-phase loads.

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

  • You can select the conductor material of the cable. The type of conductor, due to different impedance values, affects the current-carrying capacity and the voltage drop of the cable.

Insulation

  • The cable’s insulation material affects the maximum allowable operating temperature and, hence, the current rating.
  • Cables with a higher allowable temperature (which depends on the insulation type) will have a higher current rating.

Installation

  • Select the cable circuit’s installation method that matches your installation.
  • If the installation method changes along its route, choose the worst-case scenario (resulting in the lowest possible current rating) along its entire length. Typically, buried cable sections will have a lower current rating than those installed in the free air.

Frequently asked questions

What voltage drop (%) should be used?

  • You should enter an allowable voltage drop limit considering your installation's maximum allowable voltage drop limit.
  • The minimum cable size will be dictated by the voltage drop for circuits that are long or carry a small load current.
  • The maximum allowable voltage drop limits for an electrical installation are covered by the standard AS/NZS 3000.
  • Please take a look at the article Voltage drop limits for electrical installations.

How do I calculate multiple parallel circuits?

  • The minimum cable size may consist of multiple parallel cables. You can increase the number of parallel conductors to reduce the active cable size.
  • For example, a compliant cable size solution may be 1 x 630 mm2 or 2 x 185 mm2. Each solution can carry 1014 A and 918 A, respectively.
  • This free calculator can only automatically size one cable per phase.
  • You can use Cable Pro Web Software for multiple parallel cables per phase.

How can I determine a derating factor?

  • The standards provide derating factor tables for the cable current ratings when your project's installation conditions differ from the standard conditions. You can determine derating factors from the standards tables.
  • This article provides derating factor tables from standards.
  • Cable Pro Web Software has a simple-to-use Wizard for derating factors.

How do I calculate voltage rise instead of voltage drop?

  • Voltage rise is not different to voltage drop calculations.
  • Voltage rise occurs in solar PV systems on the AC side between the power inverters and the network connection when power flows from the inverter back into the network.
  • Australian standards and state regulations cover the requirements and limits for voltage rise. Refer to the article Voltage rise calculations.
  • Cable Pro Software has a selection for voltage rise (%).

How do I enter a 3-phase current?

  • The load current you enter for the cable sizing calculations is the maximum load current per phase.
  • So for a 3 phase supply the load current would be the load on the phase which carries the highest load current (highest loaded phase).

Can I calculate the size for flexible aluminium cables?

  • Cables with flexible aluminium conductors are popular nowadays. However, the standards do not provide current ratings or voltage drop values.
  • Cable Pro Software supports flexible aluminium cables.

What should I enter for power factor?

  • Poor power factor affects cable size significantly because it increases voltage drop, especially for large cables (they have high reactance).
  • The Service and Installation Rules of New South Wales require that an electrical installation must have a power factor of 0.9 or greater lagging.

How do I calculate the cable size for an unbalanced 3-phase system?

If the out-of-balance conditions are inconsistent or intermittent:
  • An approach to calculating the voltage drop for an unbalanced three-phase load is to assume a balanced three-phase load condition and perform calculations using the current flowing in the heaviest-loaded phase.

Where the currents in each phase are different in magnitudes for consistent periods:

  • In this case, voltage drop calculations can be performed by selecting phases as single-phase.