From maximum demand to a circuit calculation record

A demand result, cable size and measured test value answer different questions. Use a sequence of checks, keep the assumptions with each answer, and record what remains outside the calculation.

1. Establish the design current

Use the maximum demand calculator where its Appendix C method applies. Record the load groups, phase allocation and heaviest loaded phase. For equipment circuits, use applicable equipment data and the design assessment. The supported demand tool does not cover every installation or replace circuit-protection selection.

2. Choose the cable model and conditions

The cable sizing calculator supports copper V-75 multicore screening. Select the actual installation, thermal insulation, grouping and temperature. Aluminium, XLPE, single-core and parallel arrangements need data and methods outside that tool's current scope.

3. Budget voltage drop along the complete route

The simplified Table C8 voltage-drop calculator evaluates the entered run. Account for upstream consumer mains and submains when selecting this run's allowance. Do not give each segment the complete installation limit.

A worked single-phase example

Take an assessed 32 A design current, a 30 m one-way run, copper V-75 multicore cable in PVC conduit in air, 40 °C, one circuit and a 5% run budget. The screening model gives a 6 mm² active conductor governed by current capacity. Its Table C8 voltage drop is 3.14%; the Table 5.1 copper earth lookup gives 2.5 mm². These values do not establish protective-device suitability or fault disconnection.

Open the circuit worksheet to reproduce these inputs, inspect the working and print one record. The worksheet uses the same existing engines as the calculators; it does not introduce a different sizing formula.

4. Complete the protection and installation checks

Use the fault-loop calculator within its stated device and conductor limits. Review fault current, device breaking capacity, disconnection requirements, short-circuit withstand and manufacturer data separately. Check conduit fill using the actual cable dimensions. A timing margin alone does not establish complete selectivity.

5. Keep calculations distinct from inspection evidence

Calculation PDFs preserve entered assumptions and calculated results. The inspection report builder records actual observations, measured values, instruments and follow-up entered by the inspector. It does not infer compliance from calculator outputs or issue an official certificate.

What should the record contain?

Keep the circuit reference, input values and units, method and edition, installation conditions, governing result, working, and checks not evaluated. Review the licensed standards and equipment data for the actual installation. For the model details, read the cable guide and maximum-demand guide.