Ohm's Law and the Power Formula

The arithmetic behind every electrical reading you take, and how to use it to predict what a meter should show.

Concept

Apprentice

#fundamentals#electrical#diagnostics#formulas

Safety

Take resistance readings only on de-energized, isolated components and discharge capacitors first.

What it is

Ohm''s law describes the fixed relationship between voltage, current and resistance in a circuit. Knowing any two lets you calculate the third, which turns a meter reading into a prediction you can test.

Volts = Amps x Ohms

Amps = Volts / Ohms

Ohms = Volts / Amps

Watts = Volts x Amps

For resistive loads such as electric heat. Motors add a power factor, so watts are lower than volts times amps.

Why it matters

You can predict what a healthy component should draw and compare it to what it actually draws. A 5 kilowatt electric heat element on 240 volts should pull about 21 amps. If it pulls 10, part of the element is open. If it pulls nothing, the circuit is not being fed.

Compressor windings with a shorted turn read lower resistance and draw high current.
An open winding reads infinite resistance and draws nothing.
A weak capacitor lets a motor draw higher than rated amps while producing less torque.

Worked examples

Situation	Known	Calculation	Expected reading
Electric heat element	240 volts, 11.5 ohms	240 / 11.5	About 21 amps
24 volt contactor coil	24 volts, 12 ohms	24 / 12	About 2 amps
Heat strip bank	240 volts, 21 amps	240 x 21	About 5040 watts

Measure resistance with the power off

An ohm reading taken on an energized circuit is meaningless and can damage the meter. Isolate the component and discharge any capacitors first.

Common mistakes

Measuring resistance of a component still wired in parallel with something else, which gives a false low reading.
Applying the simple watt formula to a motor and assuming the result is real power.
Ignoring voltage drop under load. A circuit that reads correct voltage at rest can sag badly when the compressor starts.