What it is

Ohm's law is the relationship between voltage, current, and resistance, and the power formula ties those to watts. You don't need to love math to use them — you need three or four shortcuts that let you predict what a meter should read, so when the meter disagrees you know something's wrong. That's the whole point: the numbers tell you whether a part is healthy before you ever pull it.

How it works

Three quantities, one relationship. Voltage (E, in volts) is the push. Current (I, in amps) is the flow. Resistance (R, in ohms) is the opposition. The law says voltage equals current times resistance: E = I × R. Rearrange it any way you need — current is voltage divided by resistance, resistance is voltage divided by current.

Power is the work being done, measured in watts. Watts equal volts times amps: P = E × I. Combine that with Ohm's law and you can also get watts from amps and resistance, or from volts and resistance. In our trade the version you reach for most is amps = watts ÷ volts, because nameplates love to give you watts (or BTU, which converts to watts) and the breaker cares about amps.

The intuition that matters: at a fixed voltage, lower resistance means more current. A dead short is near-zero resistance, which is why it pulls enormous current and trips a breaker. An open is infinite resistance, which is why it pulls zero current and the load just sits dead.

In the field

You mostly use this to predict and to verify.

To predict a heat strip's draw: take the rated watts, divide by the voltage. A 10,000-watt (10 kW) strip on 240V pulls about 41.7 amps. Now you know what your clamp should read and what size wire and breaker the circuit needs.

To verify a heat strip is whole: with power off and the element isolated, ohm it out. A 240V, 5 kW element should read about 11.5 ohms (240 squared, divided by 5,000 watts). If it reads open, an element is burned out; if your clamp shows it pulling half the expected amps in operation, one of two parallel elements has failed.

To sanity-check a 24V coil: a contactor coil that draws a small fraction of an amp at 24V will ohm out in the low tens of ohms. If it reads near zero, the coil is shorted; if it reads OL, it's open.

Normal values & targets

  • Amps from watts: divide watts by the supply voltage. 240V is the usual residential number; use the actual measured voltage for precision.
  • A 5 kW / 240V heat element ≈ 20.8 amps and ≈ 11.5 ohms cold.
  • A 10 kW / 240V heat element ≈ 41.7 amps and ≈ 5.8 ohms cold.
  • Heat element resistance rises a little once hot, so a cold ohm reading runs slightly lower than the in-operation math — that's normal, not a fault.
  • To convert heat output: 1 kW ≈ 3,412 BTU/h. A 10 kW strip is about 34,120 BTU/h of electric heat.

Common faults & what they mean

  • Clamp reads roughly half the calculated amps on a multi-element heater → one element open, the other still working.
  • Clamp reads zero on a strip that should be energized → open element, open sequencer/relay, or no voltage upstream.
  • Measured resistance much lower than the math predicts → shorted turns or a partial ground in the load.
  • Breaker trips instantly on energizing → near-zero resistance fault (dead short), find it before resetting.
  • Element ohms out fine but pulls low amps in operation → low supply voltage; check the actual volts and recompute.

Tech tips & gotchas

Watts scale with the square of voltage on a fixed resistance, so voltage matters more than people expect. Run that same 240V strip on 208V and it makes only about 75% of its rated heat — that's why a unit sized on a 240V assumption can feel weak on a 208V commercial supply. Always measure the real voltage before you blame the equipment.

Don't mix up the cold ohm reading and the operating number. Heating elements have a positive temperature coefficient — resistance climbs as they heat — so your bench/cold reading will be a touch lower than the watts-and-volts math implies. Close is good; an open or a dead short is the obvious fault.

When a number doesn't make sense, recheck the voltage first. Half the "this part is bad" calls are really a supply voltage problem feeding the math wrong.

Safety / code notes

Branch-circuit and conductor sizing for fixed electric heat follow NEC Article 424; continuous loads are sized at 125%, which is why a 41.7-amp strip lands on a larger breaker than the bare number suggests. De-energize and lock out per NEC Article 110 before taking any resistance reading.