Basic House Wiring -Connections and Code 15-amp vs. 20-amp. Most circuits and devices in a home are rated as 15-amp or 20-amp. What this means varies a bit from one item to another. In a way, it all starts with the size of the wire. 14-gauge wire is the smallest allowed for the permanent wiring of a circuit. Matching Wire Size to Circuit Amperage - The Spruce There is the potential for danger anytime a device or appliance tries to draw more power on a circuit than the wire gauge is rated for. For example, plugging a heater rated for 20 amps into a 15-amp circuit wired with 14-gauge wire poses a distinct danger.