Heating Element Calculator — Power, Resistance & Current

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.

A 3kW immersion heater at 230V draws: I = P/V = 3000/230 = 13.0A. This is within the rating of a standard 16A MCB, wired in 2.5mm² twin-and-earth cable (the immersion-heater convention — 2.5mm² is rated 27A at Method C per BS 7671 Table 4D5, leaving ample margin and headroom for long runs). 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.

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.

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. BS 7671 requires a readily accessible means of local isolation (a double-pole switch within easy reach of the cylinder) so the element and thermostat can be safely switched off for maintenance — there is no specified distance in metres. Use our Circuit Breaker Sizing Calculator to verify the correct MCB rating.

Most heating elements use nichrome (nickel-chromium) wire, which has a positive temperature coefficient — its resistance increases with temperature. When cold, the resistance is slightly lower, so the initial current (inrush) is a little higher than the running current. But nichrome has a very low, near-flat coefficient (α ≈ 0.00017/°C), so the change is small: resistance rises only about 5-7% from cold to operating temperature. For a 3kW element that is roughly cold 16.5Ω (~14A) versus hot 17.6Ω (13A) — a modest inrush, not a large surge. This is why Type B MCBs (3-5× trip) cope easily. By contrast, tungsten elements (incandescent lamps) have a much steeper coefficient and 10-15× inrush.