Finding current and resistance for an immersion heater using V, I, R and power relationships.
Plumbers meet basic electrical calculations when fitting things like immersion heaters and pumps. This example uses Ohm’s law and the power formula together. The numbers are chosen for the worked example.
The relationships. Ohm’s law says voltage V = current I x resistance R. Power P = V x I. Rearranged, current I = P / V, and resistance R = V / I.
The situation. A 3 kW (3000 W) immersion heater runs from a 230 V supply.
Step 1 — find the current. I = P / V = 3000 / 230 = about 13.0 amps (A).
Step 2 — find the resistance. R = V / I = 230 / 13.0 = about 17.7 ohms.
Why it matters. A current of about 13 A is why a 3 kW immersion heater is normally supplied on its own suitably rated circuit with the correct cable size and protective device, rather than sharing a lightly rated circuit. The cable and protection must be chosen for the current the load actually draws.
Takeaway. I = P / V and R = V / I let you size and check a simple resistive load. Electrical installation work in dwellings is controlled under Approved Document P; design and certification must be done by a competent person to the current wiring standards, and circuits must always be safely isolated before work.
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