What a proper assessment needs
Before this topic can be assessed for a real installation, the person responsible for the design or verification needs to establish the relevant facts:
- Real input power and apparent power where relevant
- Voltage, phase allocation and power factor
- Duty cycles and simultaneous operation
- Starting and transient loads
- Measured demand and documented assumptions
Factors to consider
Electrical input power is not always the useful output power printed on equipment. A motor rating may describe shaft output. Adding kilowatts without accounting for power factor and phase allocation does not establish the maximum current in each supply conductor.
Keep the evidence with the decision
Record the equipment, installation conditions and the source of the data used. Where a manufacturer supplies product-specific instructions or coordination data, the exact product matters. A generic example should not silently replace that information.
For UK installation work, use the applicable edition of BS 7671 together with relevant guidance and manufacturer instructions. The IET’s publication hub is the starting point for current editions and supporting material. This brief does not reproduce the standard or establish compliance with it.
IET · BS 7671 and supporting guidance ↗Related calculations
Our arithmetic tools show their scope, inputs and formulas explicitly. They can help you understand a relationship between quantities. They cannot replace the installation assessment described above.