Immersion Heater Voltage Drop Calculator
Calculate voltage drop for immersion heater dedicated circuits
Immersion Heater Quick Facts
- Standard 3kW immersion heater draws 13A at 230V
- Dedicated circuit required - typically 16A MCB with 2.5mm² cable
- Immersion heaters run for extended periods - voltage drop affects heating time
- Usually short cable runs from CU to airing cupboard or cylinder location
- Dual element immersions may have two circuits or switching arrangement
Values from BS 7671 Table 4D1B (PVC copper at 70°C)
🔥 Voltage Drop Effect on Heating
At 230V (no drop)
Power = 230² ÷ 17.6Ω
= 3,006W (full rated)
At 218.5V (5% drop)
Power = 218.5² ÷ 17.6Ω
= 2,711W (10% reduction)
Keep voltage drop low for efficient heating and shorter heat-up times.
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Common Immersion Heater Scenarios
Standard immersion (3kW) - short run (10m)
13A with 2.5mm² (18 mV/A/m). At 10m: (18 × 13 × 10) ÷ 1000 = 2.34V (1.02%) ✓
Immersion in loft (15m run)
13A with 2.5mm² at 15m: (18 × 13 × 15) ÷ 1000 = 3.51V (1.53%) ✓
Long run to utility room (25m)
13A with 2.5mm² at 25m: (18 × 13 × 25) ÷ 1000 = 5.85V (2.54%) ✓
Economy 7 with two elements
If switching between elements (not both on), same calculation. If both on, 26A total - use 4mm² cable.
Immersion in outbuilding (35m)
13A with 4mm² (11 mV/A/m). At 35m: (11 × 13 × 35) ÷ 1000 = 5.01V (2.18%) ✓
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For electric showers: shower circuit. Heat pumps: heat pump. Electric underfloor heating: check our main voltage drop calculator. Use our cable sizing calculator for current capacity.
Immersion Heater Electrical Requirements
Standard UK immersion heaters are rated 3kW at 230V, drawing 13A. They require a dedicated radial circuit with 16A MCB protection (15A or 20A also acceptable). The circuit feeds a double-pole switch (usually with neon indicator) mounted near the cylinder, with flex from switch to immersion element. 2.5mm² cable is adequate for most domestic runs.
Immersion Heater Electrical Costs (2024)
Typical UK costs for immersion heater electrical work.
| Work Type | Materials | Labour | Total |
|---|---|---|---|
| New immersion circuit (<10m) | £60-120 | £120-220 | £180-340 |
| Immersion circuit (10-20m) | £80-160 | £150-280 | £230-440 |
| Immersion + timer install | £100-180 | £180-320 | £280-500 |
| Dual immersion wiring | £120-220 | £200-380 | £320-600 |
| Immersion element replacement | £30-80 | £80-150 | £110-230 |
Prices as of 2024. Cylinder and element costs are additional to circuit work.
Voltage Drop Effect on Immersion Heaters
Immersion heaters are resistive loads - power output is proportional to voltage squared. A 5% voltage drop reduces heating power by about 10%. While the water still heats eventually, it takes longer and wastes energy. Keeping voltage drop under 3% ensures efficient operation and minimises running costs.
Economy 7 and Dual Immersions
Economy 7 systems use cheap overnight electricity to heat water. Some cylinders have two elements - bottom (large, for overnight) and top (boost, for daytime). These may share one circuit with a changeover switch, or have separate circuits. If both can operate simultaneously, the combined current (26A) needs larger cable.
Immersion Heater Switching
The double-pole switch near the cylinder must be rated for the immersion load (usually 20A minimum). Timer controls can be added - either at the switch or in the consumer unit. Smart switches allow app control and integration with time-of-use tariffs. All switching must isolate both live and neutral.
Frequently Asked Questions
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