How do I calculate the resistance of a heating element?

Using P = V²/R, rearranged to R = V²/P. For a 3kW immersion heater at 230V: R = 230² / 3000 = 52,900 / 3000 = 17.6Ω. The resistance is fixed by the element's physical construction (wire length, diameter, and material). A lower resistance means higher current and more power. When measuring a cold element with a multimeter, the resistance will be slightly lower than the calculated hot resistance because the element's resistance increases with temperature.

What current does a 3kW immersion heater draw?

A 3kW immersion heater at 230V draws: I = P/V = 3000/230 = 13.0A. This is within the rating of a standard 16A MCB and 1.5mm² cable. For comparison: 2kW = 8.7A, 3kW = 13.0A, 4kW = 17.4A, 6kW = 26.1A. A 3kW immersion heater is the standard size for UK domestic hot water cylinders and is typically connected to a dedicated 16A radial circuit via a double-pole switch.

Can I connect elements in parallel or series?

Yes — parallel connection gives the same voltage across each element but increases total current and power (P_total = P1 + P2). Series connection shares the voltage and reduces total power. For example, two 3kW elements in parallel = 6kW total (26A at 230V — needs 32A MCB). Two 3kW elements in series = 1.5kW total (6.5A). Parallel is used when you want more heat; series is rarely used except for temperature control in some industrial applications.

What MCB and cable do I need for an immersion heater?

For a standard 3kW immersion heater (13A): 16A MCB Type B and 2.5mm² twin and earth cable on a dedicated radial circuit. For a dual-element 6kW cylinder: two separate circuits or a single 32A circuit with 4mm² cable. The circuit must have 30mA RCD protection and a double-pole isolating switch near the cylinder. BS 7671 requires the switch to be within 3m of the appliance for maintenance isolation.

Why does my heating element draw more current when cold?

Most heating elements use nichrome (nickel-chromium) wire, which has a positive temperature coefficient — its resistance increases with temperature. When cold, the resistance is lower, so the initial current (inrush) is higher than the running current. For a 3kW element: cold resistance might be 15Ω (15.3A), hot resistance 17.6Ω (13A). This is why Type B MCBs (3-5× trip) are suitable — the inrush is moderate. Tungsten elements (incandescent lamps) have much higher inrush (10-15×).

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Heating Element Calculator — Power, Resistance & Current

Calculate element resistance from power rating, determine current draw, thermostat compatibility, MCB rating, and cable size for heating element circuits.

Element Power Rating

Power rating per element

Supply Voltage

230V UK single-phase

Element TypeApplication type

Number of Elements

Multiple elements if applicable

How We Calculate This

This calculator uses Ohm's law and the power equation to determine heating element electrical characteristics.

Key Formulas

R = V² ÷ P (resistance from voltage and power)

I = P ÷ V (current from power and voltage)

P = V × I = I² × R = V² ÷ R

Common Element Ratings

Immersion heater: 3kW (13A) standard domestic
Kettle: 2.2-3kW (10-13A)
Oven element: 2-3kW per element
Storage heater: 1.7-3.4kW per brick
Heat output: 1W = 3.412 BTU/h

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.