Skip to content

Immersion Heater Circuit Calculator

Calculate cable size, MCB rating, and voltage drop for immersion heater circuits

Reference Info & Formulas
Quick Reference

3 kW: 2.5mm² T&E, 16A MCB

6 kW: 4mm² T&E, 32A MCB

Always use a double-pole isolator at the cylinder

Requirements

Dedicated radial circuit

20A DP switch at cylinder

Thermostat control

30mA RCD recommended

Heater Details
Select the immersion heater rating
Cable Route
Enter cable run details

Data: BS 7671:2018+A2:2022 — Table 4D5

For guidance only. The responsibility for any electrical installation lies with the qualified person carrying out the work. Always verify calculations independently and apply professional judgement.

How This Calculator Works

Immersion heater circuits are designed as dedicated radial circuits carrying a continuous load at the heater's full rated current.

The design current is calculated from the heater's power rating. A standard 3kW immersion heater draws 13A, while a dual-element 6kW unit draws 26A. No diversity is applied because the heater may run continuously for extended periods during heat-up cycles.

Ib = P / V
P
= Heater power rating in watts (e.g. 3000W)
V
= Nominal supply voltage (230V)

BS 7671 Appendix 4

The cable is selected from Table 4D5 so that its current-carrying capacity — after applying correction factors for ambient temperature, grouping, and thermal insulation — exceeds the MCB rating. The MCB is sized to be the next standard rating above the design current: typically 16A for a 3kW heater or 32A for a 6kW dual-element unit.

Quick Reference — Immersion Heater Circuit Sizing

Common immersion heater ratings and circuit requirements

Table 4D5 (Method C)
Heater RatingDesign Current (A)Cable SizeMCB RatingSwitching
3kW (single element)13.02.5mm²16A20A DP switch
3kW (economy 7, off-peak)13.02.5mm²16A20A DP + timer
6kW (dual element)26.14mm²32A20A DP switch
6kW (in insulated space)26.16mm²32A20A DP switch

Based on Method C (clipped direct), 30°C ambient, no thermal insulation contact. For airing cupboards with insulated walls, apply appropriate correction factors.

Practical Notes

Airing Cupboard Temperature Derating

Airing cupboards often reach 35-40°C, especially with a hot water cylinder inside. At 40°C ambient, the correction factor from Table 4B1 is 0.87, reducing a 2.5mm² cable's capacity from 27A to 23.5A. While still sufficient for a 3kW heater on a 16A MCB, always verify the actual ambient temperature and apply the correct factor.

Dedicated Circuit Required

An immersion heater must have its own dedicated radial circuit from the consumer unit. It must not be spurred from a ring main or share a circuit with other loads. The circuit must include a local double-pole isolating switch positioned adjacent to the heater for safe maintenance access.

Economy 7 and Smart Tariff Wiring

For off-peak tariff heating, the immersion heater circuit is typically controlled by a time clock or contactor switched by the meter's tele-switch output. The timer is wired in series before the DP switch. Modern smart immersion controllers (such as solar diverters) wire in place of the timer, diverting surplus solar PV generation to the hot water cylinder.

Bathroom Zone Requirements

If the airing cupboard opens into a bathroom, the immersion heater switch may fall within Zone 2 (within 0.6m of the bath or shower). All equipment in Zone 2 must be rated IPX4 minimum and the circuit must have 30mA RCD protection. Position the DP switch outside the zone boundary where possible.

Frequently Asked Questions

SparkyHub is a free resource built by Taro Schenker — I build websites for trade businesses too.

Related Calculators

Cable Size

Calculate the correct cable size for any circuit

Shower Cable

Cable size, MCB, and voltage drop for electric shower circuits

Electricity Cost

Calculate how much it costs to run any electrical appliance

Breaker Selector

Select the correct MCB type, rating, and RCD requirements for any circuit

Built by Taro Schenker · Need a website for your trade business? Get in touch