Whether you are just adding a new fixture or outlet to an old circuit or running a completely new circuit when doing any electrical work around the home that requires adding new wire to an old or new circuit it is extremely important that you select and use the proper size of wire for your application.
For the purposes of this article we are only going to discuss the three most common sizes you may need to deal with in a typical project around the house. Farm wiring or other really long runs like wiring a shed or detached garage may be subject to some different rules and require larger wire sizes because of voltage drop, but we will not be discussing those here.
The load the circuit will carry will determine the size of the breaker and the size of the breaker will determine the size (gauge) of the wire required.
In all the examples we will discuss, with ground means one more conductor than is called by the wire name. This ground wire is usually bare, no insulation but it may also be green.
A 30 amp circuit powers most domestic water heaters, some older or larger air-conditioners, table saws and a few other power hungry devices. These are almost always a double-pole breaker. A 30 amp circuit requires a number 10 (10 gauge) wire. Most 30 amp circuits are 220 volt so you will need 10/3 with ground cable to power these. That means there will be 4 wires total in the cable. Some older circuits may only have 3 conductors.
In older construction you might run across single pole 30 amp circuits. In that case 10/2 with ground would be the correct wire. In an ungrounded system it might even be 10/2 and no ground. If you find any single pole 30 amp circuits I think you should take a good hard look at what it is powering. A small water heater or a dedicated outlet for a piece of equipment is about the only two situations where I might be comfortable with leaving in place.
A 20 amp breaker is the most common size in all new construction. It may be providing power to a dedicated outlet for a kitchen appliance, several rooms of outlets, or even lighting for one or more rooms. A 20 amp circuit requires a 12/2 with ground wire.
20 amp double pole breakers should use 12/3 with ground but you will find a lot of older wiring out there that only uses a 3 conductor cable for a 220 volt 20 amp application. There were millions of homes built in the 1970-80ís with 220 volt baseboard heat using 12/2 with ground.
The 15 amp 110 volt circuit uses 14/2 with ground wire. These lightweight circuits are only suitable for convenience outlets and lighting; though in older homes you may find them anywhere. As long as it is working with no problems or breaker tripping it may be fine to leave alone.
So to summarize this in a couple of easy to recall tables:
30 amps 10 gauge wire
20 amps 12 gauge wire
15 amps 14 gauge wire
220 volt circuits 4 wires
110 volt circuits 3 wires
One final point, if you are in doubt about the wire size always choose the larger size. A larger wire can always be used safely and a few extra cents per foot for the wire is some of the cheapest insurance and piece of mind that you can purchase.
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