Power and Electrical Components

Voltage, amps and AC or DC before anything else

Every electrical part has a voltage rating and a current rating, and both must be at least what the circuit delivers. Check whether the circuit is AC or DC, because a switch, relay or fuse rated for AC often has a lower rating on DC, where an arc does not stop on its own. Note the voltage, such as 120, 240 or 12 V, the amps, and whether the system is single or three phase.

Wire size ties it together. A breaker or fuse protects the wire, not the appliance, so its amp rating must not exceed what the conductor can carry, and the plug and outlet must match that rating. Mismatched ratings are a common cause of overheating. Work inside panels, generators and mains wiring is for qualified electricians, and local codes may require a permit or inspection.

Protect, switch, then power: how the aisle is laid out

The aisle follows the path of the current. Protection comes first, with breakers and circuit protection and fuses; control comes next, with switches, relays and timers; outlets and receptacles deliver the power; and supplies, transformers, inverters and generators create or convert it. Begin with the source and the load, then choose the protection, then the control.

Conversion parts need the most care. A transformer changes voltage but not frequency and is rated in volt-amperes, a power supply converts AC to a regulated DC voltage, and an inverter does the reverse from a battery. Choose the output voltage and current from the load with some headroom, and check the input range. For stepping 240 V down to 120 V or vice versa, see voltage converters and transformers.

Questions about Power and Electrical Components

Why must the breaker match the wire gauge?

A breaker's job is to trip before the wire overheats. If the breaker is rated higher than the wire can carry, an overload can heat the wire dangerously without tripping. Use the breaker size that suits the smallest wire in the circuit, as set by your local code.

Can I use an AC-rated switch on a DC circuit?

Only if it is also rated for DC at that voltage and current. DC arcs are harder to extinguish than AC, so DC ratings are usually lower. Look for a separate DC rating on the switch or relay, especially for battery and solar systems.

What do VA and watts mean on a transformer or power supply?

VA is volt-amperes, the apparent power a transformer handles, and watts is the real power consumed. For simple loads they are close, but motors and power supplies draw more VA than watts. Choose a rating above the load's VA, not just its watts.