Sizing a shower circuit
Electric showers are single-phase, near-purely resistive loads, so the design current is simply the shower's power rating divided by 230V. A dedicated radial circuit then needs a protective device rated at or just above that current, and a cable sized to carry it — corrected for the actual installation conditions and checked for voltage drop, exactly as for any other circuit.
Common UK shower ratings and design current
| Shower rating | Design current Ib |
|---|---|
| 7.5 kW | 32.6 A |
| 8.5 kW | 37.0 A |
| 9.5 kW | 41.3 A |
| 10.5 kW | 45.7 A |
What decides the final cable size
- Run length — from consumer unit to shower unit; longer runs need larger cable to stay within the 5% voltage drop limit.
- Installation method — clipped direct (method C) rates higher than a cable buried in insulation (Ci correction) or run through a loft with other circuits (Cg correction).
- Ambient temperature — a cable run through a warm loft space needs the Ca correction applied.
This is why generic "9.5kW shower = X mm²" advice is unreliable — the load is fixed, but the three factors above are specific to your installation. Enter them into the calculator above for a result that's actually checked against your run, not a rule of thumb.