Zerdly

Maximum Demand Calculator

Estimate the diversified maximum demand of an electrical installation from its total connected load. Loads are rarely all on at once, so a diversity (demand) factor scales the connected total down to a realistic peak, and an optional supply voltage converts that to a three-phase demand current.

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How to use this calculator

Enter Values

Enter your values and press Calculate to see results.

Before you rely on this: First-pass guide only. Verify safety-critical or regulated work against the relevant standards, your project requirements and a qualified professional.

How to use this calculator

  1. Enter the total connected load in kVA (the sum of all connected equipment ratings).
  2. Enter a diversity / demand factor in percent, or leave it blank to use the 70 % default.
  3. Optionally enter the three-phase supply voltage in volts to also get the demand current in amps.

How it works

Maximum demand equals the connected load multiplied by the diversity factor divided by 100. When a three-phase supply voltage is given, the demand current is MD x 1000 / (sqrt(3) x voltage). The diversity factor accounts for the fact that not all loads draw their full rating simultaneously.

Worked example

Worked example. A 250 kVA connected load with a 70 % diversity factor gives a maximum demand of 175 kVA. At 415 V three-phase the demand current is 175000 / (1.732 x 415) = 243.46 A.

Common mistakes

  • Guessing the diversity factor instead of using AS/NZS 3000 tables or metered data for the installation type.
  • Applying a line-to-line voltage in a single-phase formula (or vice versa) when converting to current.
  • Sizing the supply to the raw connected load, which oversizes cables and switchgear and wastes cost.

Frequently asked questions

Why is the default diversity factor 70 %?

70 % is a common general-purpose starting point, but the correct value depends on the load type and should be taken from AS/NZS 3000 maximum-demand tables or measured data.

Why does the demand current use sqrt(3)?

For a balanced three-phase load, apparent power S = sqrt(3) x V(line) x I(line), so the line current is S / (sqrt(3) x V).

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