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Initial versus loaded state: single-phase resistive loads, unity power factor.
Never use resistance/continuity mode on an energised circuit. Live measurements require competent assessment, suitable equipment and safe access. This calculator is not a safety or compliance test.
Explanations and assumptions
- Current = measured real power ÷ measured voltage, assuming unity power factor. Use actual measured watts, not nameplate ratings.
- Loaded equivalent resistance = loaded voltage ÷ loaded current. This represents ALL loads included in that power reading; it represents the appliance alone only when that appliance is the only measured load.
- Estimated supply-path resistance = voltage reduction ÷ increase in current. This includes the shared upstream line and neutral path, connections and source effects. It does not locate a faulty contact.
- Voltage and power must refer to the same electrical boundary. Appliance-only watts cannot represent total feeder current if other loads are present.
- Assumes stable source voltage, no unrelated load switching and a predominantly resistive supply path. Small differences are sensitive to meter error and supply variation. This is not a loop-impedance test.
- For non-unity power factor: RMS current = watts ÷ (RMS volts × power factor). Motors, electronic loads, significant reactance and three-phase systems need a different model; subtracting RMS current magnitudes is not generally equivalent to subtracting current phasors.
- Zero current indicates no completed operating load path, but does not locate the interruption. Zero voltage sag does not prove sound connections.
- To investigate a particular contact, voltage drop directly across that contact multiplied by its current gives its resistive power loss. Do not attribute the entire supply-voltage reduction to one contact.