What size cable for an electric shower?
An 8.5 or 9.5 kW shower usually needs a 40 A circuit: 6 mm² twin and earth if clipped direct, 10 mm² or bigger if it runs through insulation. Showers get no diversity, and they need their own circuit with 30 mA RCD protection.
- V – Voltage
- I – Current
- P – Power
Step 1: find the current
Shower power is normally quoted at 240 V, so divide by 240 to get the rated current. On a 230 V supply it draws a little less, but design for the rated figure.
| Shower | Current at 240 V | About at 230 V | Usual breaker |
|---|---|---|---|
| 8.5 kW | 35.4 A | 33.9 A | 40 A |
| 9.5 kW | 39.6 A | 37.9 A | 40 A |
| 10.5 kW | 43.8 A | 41.9 A | 45 A or 50 A (check the maker's instructions) |
A shower runs at full power for as long as it's on, so no diversity is applied: the design current is the full rated current.
Step 2: choose the cable for a 40 A circuit
Run lengths use the 39.6 A rated current of a 9.5 kW shower:
| How the cable is run | Smallest twin and earth for 40 A | Longest run (voltage drop) | Longer run? Next size up |
|---|---|---|---|
| Clipped direct to a wall or joist | 6 mm² (rated 47 A) | up to 39 m | 10 mm²: up to 66 m |
| Above a plasterboard ceiling, insulation up to 100 mm | 10 mm² (rated 45 A) | up to 66 m | 16 mm²: up to 103 m |
| In an insulated stud wall, touching the inner face | 10 mm² (rated 47 A) | up to 66 m | 16 mm²: up to 103 m |
| In an insulated stud wall, not touching the inner face | 16 mm² (rated 42.5 A) | up to 103 m | – |
For a 10.5 kW shower on a 50 A breaker, the cable must be rated for 50 A. That's 10 mm² clipped direct, or 16 mm² above an insulated ceiling.
Other things to check
- Its own circuit from the consumer unit, never a spur off another circuit.
- 30 mA RCD protection: all circuits in a room with a bath or shower need it.
- A double-pole isolating switch, usually a ceiling pull-cord or a switch outside the bathroom.
- Adding or altering a circuit in a bathroom is notifiable under Part P in England.
- Check the maximum Zs for the breaker in the Max Zs table.
Assumes 70 °C flat twin and earth (6242Y), 230 V, an MCB or RCBO, 30 °C ambient, the circuit not bunched with others, and no thermal insulation round a clipped cable. Voltage drop limit 5 % (11.5 V), BS 7671 Table 4D5 values. Grouping, heat or insulation can need a bigger cable: check with the cable size calculator. For study and estimating only; the installing electrician must design and certify the circuit.