What Size Cable Do You Need for an Immersion Heater?
For a standard 3kW immersion heater (13A at 230V), use 2.5mm² cable on a dedicated 16A or 20A circuit with a double-pole switched fused connection unit. For dual-element heaters or long cable runs, 4mm² provides better voltage drop margin.
Quick Reference: Cable Size for Immersion Heater by Standard
| Standard | Cable Size | Conditions | Reference |
|---|---|---|---|
| BS 7671 | 2.5mm² | Dedicated 16A or 20A circuit | Table 4D1A |
| NEC | 12 AWG | 20A circuit, water heater | Table 310.16 |
| AS/NZS 3008 | 2.5mm² | Dedicated circuit | Table 13 |
| IEC 60364 | 2.5mm² | Dedicated circuit | Table B.52.4 |
Values shown are for copper conductors in typical domestic/commercial conditions. Actual size depends on installation method, ambient temperature, and cable length. Use our free cable sizing calculator for your exact scenario.
Typical Applications for a Immersion Heater
- 3kW immersion heater
- Dual element immersion
- Unvented cylinder heater
Calculate Your Exact Cable Size
These are general guidelines. Your specific installation needs exact calculation accounting for cable route length, installation method, ambient temperature, and cable grouping.
Frequently Asked Questions
Is 1.5mm² enough for an immersion heater?▼
It depends. 1.5mm² is technically rated above 13A clipped direct, but 2.5mm² is standard practice for immersion heaters. The continuous load nature of immersion heaters makes 2.5mm² the safer and code-compliant choice.
What fuse size for an immersion heater?▼
A 13A fused spur or 16A MCB is standard for a 3kW immersion heater. The heater is connected via a double-pole switched fused connection unit (FCU) with a 13A fuse.
Does an immersion heater need its own circuit?▼
Best practice is a dedicated radial circuit, though a fused spur from a ring final circuit is acceptable for standard 3kW immersion heaters. Larger or dual-element heaters should have dedicated circuits.
Can I use a timer with an immersion heater?▼
Yes. Most immersion heaters are controlled by a time clock or programmer. The timer is wired between the consumer unit and the immersion heater switch, or a smart switch can be used.
What cable for a dual immersion heater?▼
Dual element immersion heaters (typically 3kW + 3kW) usually only run one element at a time via a diverter switch. 2.5mm² on a 16A circuit remains adequate. If both elements can run simultaneously, use 4mm² on a 32A circuit.
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