The current capacity of a wire is called ampacity for the number of amperes it can carry. Manufacturers of wire and wire insulation rate the wires for how much current they can carry safely.

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## Current

The diameter or gauge of a wire is related directly to its cross-sectional area. The maximum number of electrons that can move through a conductor can be determined from the cross-sectional area. Amps or amperes are the unit of measure for how many electrons are flowing through a conductor. The ampacity of wire is not affected by the voltage.

## Voltage

Voltage is the pressure that moves electricity. At zero volts, no work is performed. As voltage increases, so does the ability of electricity to do work. If the voltage is low, it takes more amperes of current to do the work, while at higher voltages fewer amperes will do the same amount of work.

## Wire Insulation

Wire insulation begins to break down when its rated temperature is exceeded. As a wire conducts electricity, the resistance of the wire causes it to generate heat. If the wire gets too hot, the insulation will melt or burn. Wire ampacity is limited to the amount of current it will carry without exceeding the insulation rating.

## 18-Gauge Wire

18-gauge wire coated with 90 degree C insulation can carry a maximum of 18 amperes of current. In general, a wire should not carry its maximum allowed current to allow for a margin of safety. Given a safety margin of 25 per cent, 18-gauge wire with 90 degree C insulation should be limited to 13.5 amperes.