How to wire an immersion heater circuit

Helpful video reference. Jamie at Immersion Heaters UK Ltd shows the wiring of a domestic immersion heater from the consumer unit through to the heater head, covering MCB sizing, 2.5 mm² T&E cable, double-pole switch connections, and a clear look inside the heater head itself. Watch on YouTube.

1. Understand the circuit layout

The circuit runs from a 16 A or 20 A MCB in the consumer unit, along 2.5 mm² twin and earth cable, to a double-pole switch or timeswitch, and then on to the three terminals inside the heater head: L (line), N (neutral) and E (earth). That is the full path, and understanding it makes every subsequent step much easier.

Inside the heater head there is a built-in thermostat that cuts power once the water reaches the set temperature, and a thermal cut-out that trips if the thermostat fails and the water overheats. Both are part of the heater itself, not something you wire separately.

A fused connection unit (FCU) is sometimes used to supply a small immersion element from a spur off a ring final circuit, but this approach is not ideal for anything above 1 kW. A dedicated radial circuit from the consumer unit is better practice for a standard domestic immersion heater and keeps the load clearly separate.

2. Size the MCB and cable

A standard 3 kW element draws around 13 A at 230 V, so a 16 A MCB and 2.5 mm² T&E cable are correct. Larger elements, up to about 6 kW in some cylinders, draw around 26 A and need a 20 A MCB (the MCB protects the cable, not the element, so the rating matches the cable's capacity after any derating). At short cable lengths 2.5 mm² may still be acceptable for a 20 A circuit, but you must work through the correction factors in the IET On Site Guide for your specific installation conditions.

The double-pole switch must be rated at least 20 A for a standard domestic installation. Many off-the-shelf immersion heater switches are rated at 20 A or 25 A, which covers both MCB sizes neatly.

3. Isolate the supply at the consumer unit

Switch off the relevant MCB, or pull and pocket the rewireable fuse if you have an older board. If your consumer unit has a padlock hasp, use it. The point is that nobody should be able to restore the supply whilst you are working on the circuit.

Go to the switch location and dead-test before you drill a single hole or cut any cable. Both the live and neutral conductors must read zero volts. Only then proceed.

4. Run the 2.5 mm² T&E cable

Route the cable from the consumer unit to the switch position and on to the cylinder. On chased walls, follow the safe cable zones rule: horizontal runs should be within 150 mm of the top of the wall or ceiling junction, and vertical runs should be directly above or below accessories. This protects the cable from future nail and screw damage.

For any surface run, use PVC oval conduit or flat trunking. It looks neater and gives mechanical protection. Before plastering over a chased run or covering any surface run, photograph the entire cable route. That photograph is invaluable if anyone wants to put a screw in the wall later.

5. Fit the double-pole switch or timeswitch

The switch must be accessible but must not be inside the bathroom zone where the hot water cylinder lives. In most homes the airing cupboard is outside a bathroom zone, but check the zone boundaries if yours opens directly into a bathroom.

A timeswitch is worth considering. It lets you restrict heating to off-peak hours, which can meaningfully reduce running costs, particularly on an Economy 7 tariff.

Wire according to the switch manufacturer's drawing. The incoming live and neutral (from the consumer unit) go to the IN terminals. The outgoing live and neutral (to the heater head) go to the OUT or LOAD terminals. Sleeve every earth conductor in green and yellow throughout, including any short tail to the earth terminal on a metal back box.

6. Make connections at the heater head

The heater head is usually protected by a plastic cover held by two screws. Remove it carefully. The terminals are underneath and the thermostat adjustment is often accessible from the same cover.

The three terminals are labelled clearly: L for line (live), N for neutral, and E or the earth symbol for earth. Connect the brown conductor to L, the blue conductor to N, and the green/yellow sleeved earth to E. Tighten each terminal firmly. Over-tightening can crack the terminal block on cheaper heater heads, so stop when the conductor is secure, not when the screwdriver will not turn any further. Replace the plastic cover.

7. Restore power and test

Switch on at the consumer unit. Check that the thermostat inside the heater head is set to around 60°C (the minimum recommended temperature to control Legionella bacteria). You should hear a faint click as the element energises, and after a short while the pipe immediately above the cylinder may feel warm.

After 30 to 45 minutes, draw some hot water from a tap and confirm it is genuinely hot. If the MCB trips the moment you switch on, go back to the consumer unit, switch off, and recheck every connection. The most common cause is a loose live conductor making contact with the earth terminal or the metal cylinder body.

When to call us

Re-wiring an existing faulty circuit or replacing an element is manageable for a competent person. Installing a brand-new circuit from the consumer unit (especially in an airing cupboard with limited space) is where most people sensibly hand over to an electrician.