Domakihub How to Wire an Immersion Heater: A Clear Guide
Understand the circuit, the components, and exactly when to call a licensed electrician.
Reviewed by Domakihub home & decor pros · 2026-06-23 · 5 min read
An immersion heater is one of the most common ways to heat domestic hot water in Greek homes, holiday rentals, and hotel rooms — but connecting one to the mains involves a dedicated radial circuit that must be handled by a licensed electrician. The video above shows exactly how the wiring goes together; this article walks through each step, explains the components, and tells you what to prepare before the electrician arrives so the job runs smoothly and safely.
What an Immersion Heater Circuit Actually Does
An immersion heater is essentially a large resistive element — typically 3 kW — screwed into the side or top of a hot-water cylinder. When live current passes through it, it heats the surrounding water to a thermostat-set temperature, usually 60–65 °C. Because it draws around 13 A continuously, it must run on its own dedicated circuit rather than sharing a ring-main socket.
In Greece, the standard supply is 230 V / 50 Hz single-phase. The wiring principles are the same as in the rest of Europe: a 16 A circuit breaker at the distribution board, a minimum 2.5 mm² three-core (live, neutral, earth) cable running to a double-pole switch or fused connection unit beside the cylinder, and from there to the heater element terminals.
Components You Need to Know Before the Electrician Arrives
Understanding the parts speeds up every conversation with your electrician and helps you shop ahead of time. The key components are: the immersion heater element itself (choose the right length for your cylinder — typically 11" or 14"), a bi-metallic thermostat (usually pre-fitted on the element, adjustable via a screwdriver slot), a double-pole isolator switch rated at 20 A and mounted within arm's reach of the cylinder, 2.5 mm² twin-and-earth or three-core cable, and a 16 A miniature circuit breaker (MCB) at the consumer unit. Optional additions include a timer or a smart controller that lets you schedule heating for off-peak hours — particularly valuable for properties on a time-of-use electricity tariff.
The 5 Steps a Licensed Electrician Will Follow
- 1
Isolate and Verify Power is Off
Before touching any wiring, the electrician switches off and locks the MCB feeding the immersion circuit at the consumer unit, then confirms dead using a two-pole voltage indicator at every relevant point — the breaker terminals, the isolator, and the heater terminals. This is not a step to skip: immersion circuits carry enough current to cause cardiac arrest.
- 2
Run the Dedicated Cable from the Consumer Unit
A 2.5 mm² three-core cable (or twin-and-earth with the earth sleeved green/yellow) is routed from a spare 16 A MCB position in the consumer unit to the location of the cylinder. The run is kept as short as possible and is clipped or conduit-run to protect it from mechanical damage. In Greek properties, cable often runs inside PVC trunking or through wall ducts.
- 3
Fit and Wire the Double-Pole Isolator
A 20 A double-pole switch — sometimes called a flex outlet plate — is fixed to the wall near the cylinder. The electrician terminates Line (brown or red) to the L-in terminal, Neutral (blue or black) to the N-in terminal, and Earth (green/yellow) to the earth bar. The switched output terminals connect to the cable that runs down to the heater. A double-pole switch is essential: it breaks both live and neutral simultaneously so the heater is genuinely de-energised when servicing.
- 4
Connect the Heater Element Terminals
The immersion heater has a plastic terminal head, accessed by removing a cap. Inside are typically three terminals marked L (live), N (neutral), and E (earth). The electrician connects the incoming cable cores to their matching terminals, ensures the earth is securely fastened to the heater body stud, and replaces the terminal cover. The thermostat is then set to 60 °C using a flat-head screwdriver — hot enough to prevent Legionella growth in the cylinder.
- 5
Test the Circuit and Commission the Thermostat
With everything connected, the electrician restores power and carries out an insulation resistance test and a continuity check on the protective conductor. If readings are within spec, the heater is switched on and the thermostat behaviour confirmed: the element should draw current until the water reaches the set temperature, then cut out. A clamp meter around the live conductor will show the expected 13–13.5 A load for a 3 kW element on a 230 V supply. The electrician issues an electrical installation certificate for the work.
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FAQ
- Can I wire an immersion heater myself in Greece?
- No. In Greece, wiring a new circuit from the consumer unit is electrical installation work that legally requires a licensed electrician (ηλεκτρολόγος με άδεια). Unlicensed work can void your property insurance, create a fire risk, and lead to fines. You can prepare — sourcing the heater, planning the cable route, confirming the cylinder boss size — but the actual wiring must be done by a qualified professional.
- What cable size do I need for a 3 kW immersion heater?
- A 3 kW element at 230 V draws approximately 13 A. The standard choice is 2.5 mm² copper cable protected by a 16 A Type B MCB. If the cable run is unusually long (over 20 metres) or buried in thermal insulation, a qualified electrician may upsize to 4 mm² to compensate for voltage drop and derating factors.
- How do I know if my immersion heater thermostat has failed?
- Two common failure modes: the water never gets hot (thermostat tripped or element burnt out — reset the thermal cut-out by pressing the small red button under the element cap) or the water overheats and the pressure-relief valve lifts (thermostat stuck closed). If resetting does not restore normal operation, the thermostat or element needs replacing — again, work for a licensed electrician.