Why Devices Draw More Power Starting Than Running

An appliance rated at a modest power figure can pull many times that for a fraction of a second when switched on. The surge is normal and it shapes how circuits are designed.

A stationary motor looks like a short circuit

A running motor generates a voltage opposing the supply, and that opposition is what limits the current it draws. It only exists while the motor is turning.

At the instant of switch-on, nothing is turning and nothing is opposing. The only limit is the resistance of the windings, which is deliberately low.

Current therefore spikes to several times the running figure and falls as the motor accelerates. A heavily loaded motor takes longer to spin up and holds the surge for longer.

Power supplies charge empty capacitors

Electronic supplies contain large capacitors that smooth the rectified mains. When first connected, those capacitors are empty and behave almost like a direct connection.

The resulting inrush is brief but sharp, and it scales with the size of the supply. A rack of equipment switched on together produces a combined surge that is genuinely large.

Designers limit it with a resistor in the path that is bypassed once charging is complete, or with a component whose resistance falls as it warms.

Filament and heating loads share the pattern

The resistance of metal rises with temperature, so a cold element draws more current than a hot one at the same voltage.

Incandescent lamps were the classic example, drawing a large multiple of their running current for the moment before the filament reached temperature.

This is why such lamps almost always failed at switch-on. The surge stressed the thinnest, most worn section of filament at precisely the moment it was coldest.

Protection devices are built to tolerate it

If breakers responded to every surge, nothing with a motor could be switched on. Their instantaneous trip threshold is therefore set well above the sustained rating.

Different breaker curves exist for exactly this reason, with more tolerant characteristics used on circuits feeding motors and less tolerant ones on general lighting.

A breaker that trips only at switch-on and never during use is usually reporting a mismatch between the load's surge and the curve chosen, not a fault in either.

The surge is visible elsewhere in the house

Drawing a large current briefly causes a small voltage drop across the supply wiring, since no cable has zero resistance.

That drop reaches every other device on the same circuit, which is why lights dip momentarily when a large appliance starts.

Frequent, pronounced dipping points at wiring resistance rather than at the appliance. The surge is the same, and the circuit is showing how much it minds.