Enter connected loads by category, apply diversity factors, and calculate
diversified maximum demand and estimated supply current.
Current: 1φ = kW × 1000 / (V × PF) | 3φ = kW × 1000 / (√3 × V × PF). For design guidance only. Always verify with a qualified engineer.
About this maximum demand calculator
This free maximum demand calculator estimates the diversified electrical load and supply current for a building or distribution board. You enter connected loads by category, lighting, socket outlets, HVAC and ventilation, catering, hot water, motors and plant, IT and comms, plus any custom categories. Apply a diversity factor to each, and the tool returns the diversified maximum demand and the corresponding supply current. It is aimed at UK building-services and electrical engineers, contractors and estimators sizing supplies, switchgear and submains at design stage.
How the maximum demand is calculated
The diversified maximum demand is the sum of each category's connected load multiplied by its diversity factor, Σ(kW × DF). Apparent power is found from the power factor (kVA = kW / PF), and supply current follows the standard expressions: for single phase, I = kW × 1000 / (V × PF); for three phase, I = kW × 1000 / (√3 × V × PF). The default diversity factors are typical UK commercial values drawn from the IET On-Site Guide (which accompanies BS 7671), and every factor is user-editable so you can match the building type and your own design assumptions. Defaults assume three-phase at 400 V (230 V single phase) and a power factor of 0.95.
Frequently asked questions
What is a diversity factor?
A diversity factor scales a connected load down to reflect that not every item runs at full power at the same time. The calculator multiplies each category's kW by its factor; the defaults follow typical IET On-Site Guide values but are fully editable.
Can I add my own load categories?
Yes. Eight common categories are provided with sensible defaults, and you can add custom load rows, edit the kW and diversity factor for any row, zero all loads, or reset back to the defaults.
Does it handle single-phase and three-phase supplies?
Both. Choose single or three phase and set the voltage and power factor; the supply current is calculated with the matching formula. Results are indicative for design guidance only. Always verify with a qualified engineer.