Q: Does an immersion heater need its own circuit?

Yes. An immersion heater must be on its own dedicated radial circuit — not a ring final or socket spur — with a 20 A double-pole switch within reach of the cylinder. In a bathroom, use a 20 A DP pull-cord. This lets the heater be isolated safely for servicing.

Question 1

What size cable and MCB does an immersion heater need?

Accepted Answer

A standard 3 kW immersion heater draws about 13 A (3000 ÷ 230 V), which needs a 16 A MCB. The T&E cable is then sized so its installed (derated) rating is at least the MCB rating — typically 1.5 mm² or 2.5 mm² depending on installation method, grouping and run length. The calculator shows the minimum compliant size for your inputs.

Question 2

Does an immersion heater need its own circuit?

Accepted Answer

Yes. An immersion heater must be on its own dedicated radial circuit — not a ring final or socket spur — with a 20 A double-pole switch within reach of the cylinder. In a bathroom, use a 20 A DP pull-cord. This lets the heater be isolated safely for servicing.

Question 3

What is the maximum cable run for an immersion heater?

Accepted Answer

The limit is voltage drop (5% of 230 V = 11.5 V). For a 3 kW heater on 2.5 mm² T&E that is roughly 40 m, and longer on larger cable. The calculator gives the exact maximum length for your cable size and load.

Question 4

Can an immersion heater run from a 13 A plug?

Accepted Answer

No. A 3 kW immersion is right at the 13 A plug limit and must be on fixed wiring with a dedicated circuit and double-pole isolator, not a plug and socket. Continuous near-13 A loads on a plug are not good practice.

Immersion Heater Circuit Calculator