Consumer Unit Sizing Calculator — How Many Ways Do I Need?

This calculator determines the consumer unit size you need by counting all required circuits for your property and adding spare ways for future expansion, following BS 7671 18th Edition guidelines.

The formula

Total ways = Number of circuits + Spare ways (min 20% or 2 ways)

The calculator then rounds up to the nearest standard single-row domestic consumer unit size sold in the UK: 6, 8, 10, 14, 16, or 20 ways. Above 20 ways you move to a split-load or dual-row board.

Standard UK circuit MCB ratings

• Lighting circuits: 6A Type B MCB
• Ring main (sockets): 32A Type B MCB
• Cooker circuit: 32A Type B MCB
• Electric shower (up to 9.5kW): 45A Type B MCB with 10mm² cable (9.5kW ÷ 230V = 41.3A; 40A only suits showers up to ~8.5kW)
• Immersion heater: 16A Type B MCB
• EV charger (7.4kW): 32A device, normally a Type A RCBO (BS 7671 Section 722)
• Smoke detectors: 6A Type B MCB