How To Look At A House
McGarry and Madsen's home inspection blog for buyers of
site-built, mobile/manfuactured and modular homes
What is the right electric wire size for a circuit breaker in an electric panel?
Saturday, September 29, 2018
The correct size for residential wiring depends on what the wires are serving and, in the case of major appliances such as an air conditioning condenser, what size circuit breaker the manufacturer specifies for it on the data plate on the side of the unit. Here’s a chart of standard wire sizes and corresponding breaker amperages for copper wiring in home circuits. The ambient temperature of the area where the wiring will be installed, the length of the wire run, along with the category of wiring used, may require an upward adjustment to these sizes. But they are a rule-of-thumb guide that we use in our inspections when checking the size of wiring connected to circuit breakers in an electric panel. Many, but not all, larger wires have the size number embedded in a repeating strip of specification lettering along the side. However, we don’t recommend that a homeowner remove the dead front (cover plate) of an electric panel to check the wiring or any other reason—for their personal safety. Examining a “live” electric panel is best left to a professional.
The size number is followed by the letters “AWG,” which stands for American Wire Gauge. We recognize the smaller #14, #12 and #10 wires by sight, and an experienced inspector can scan a panel rather quickly. Undersize wires—a #10 wire (too small) connected to a 60-amp breaker, for example—means the wire will overheat if these is too much current flow and possibly ignite a fire without the circuit breaker tripping. Conversely, a #8 wire (larger than required) connected to a 30-amp breaker is not a problem. In fact, a one-size larger wire is often necessary to avoid a voltage drop problem when the length of the wire run is long, such as from a main panel in a home to a subpanel in a detached barn.
Multi-strand aluminum wiring is sometimes used for service cables or major appliance circuits, like a range or heat strip in an air handler, and is an acceptable alternative to copper for some circuits. Because it is a little more resistant to the flow of electricity, the wire size required is typically one size larger than the same circuit in copper. Also, an anti-oxidant paste must be applied at wire connections when using aluminum wiring.
Also, see our blog post How dangerous is old electrical wiring?
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