What Size Wire Should I Use?
For residential applications, the most common wire sizes used are 12 gauge and 14 gauge ROMEX. These wires cover most of your wall outlets and lighting circuits. You will have a few larger gauge wires for things like your stove, dryer, water heater, or air conditioner. Wire sizing is very important and something your inspectors should be looking for. Wire size requirements are specified by the National Electrical Code (NEC) and should not be violated as there safety reasons for the specified sizes. If in doubt, talk to a licensed electrician or your inspector.
The table below specifies the rated amperage for common copper and aluminum wire sizes. Wires are rated either 60C, 75C, or 90C. This has to do with how much heat the insulation on the wire is rated to withstand. The table has columns for copper and aluminum wires. You'll typically be looking at copper unless you are feeding something like a subpanel where it becomes more cost efficient to run aluminum. The amperage values in the table are maximums and should not be exceeded. As a rule of thumb, if your wire runs are over 100ft you should use the next larger size wire to avoid excessive voltage drops. You should also think about bumping up a size if the cable is in a conduit or grouped with a bunch of other wires in a place where they can't dissipate heat easily.
Ampacities of Insulated Conductors
| Size | Temperature Rating of Conductor | Size | |||||
| AWG | 60ºC | 75ºC | 90ºC | 60ºC | 75ºC | 90ºC | AWG |
| (140ºF) | (167ºF) | (194ºF) | (140ºF) | (167ºF) | (194ºF) | ||
| Types | Types | Types | Types | Types | Types | ||
|
T TW UF |
THW THWN XHHW USE |
RHH THHN XHHW |
T TW UF |
THW THWN XHHW USE |
RHH THHN XHHW |
||
| 0 | Copper | Aluminum | |||||
| 14 | 20 | 20 | 25 | ---- | ---- | ---- | ---- |
| 12 | 25 | 25 | 30 | 20 | 20 | 25 | 12 |
| 10 | 30 | 35 | 40 | 25 | 30 | 35 | 10 |
| 8 | 40 | 50 | 55 | 30 | 40 | 45 | 8 |
| 6 | 55 | 65 | 75 | 40 | 50 | 60 | 6 |
| 4 | 70 | 85 | 95 | 55 | 65 | 75 | 4 |
| 3 | 85 | 100 | 110 | 65 | 75 | 85 | 3 |
| 2 | 95 | 115 | 130 | 75 | 90 | 100 | 2 |
| 1 | 110 | 130 | 150 | 85 | 100 | 115 | 1 |
| 0 | 125 | 150 | 170 | 100 | 120 | 135 | 0 |
| 00 | 145 | 175 | 195 | 115 | 135 | 150 | 00 |
| 000 | 165 | 200 | 225 | 130 | 155 | 175 | 000 |
| 0000 | 195 | 230 | 260 | 150 | 180 | 205 | 0000 |
| 250 | 215 | 255 | 290 | 170 | 205 | 230 | 250 |
| 300 | 240 | 285 | 320 | 190 | 230 | 255 | 300 |
| 350 | 260 | 310 | 350 | 210 | 250 | 280 | 350 |
| 400 | 280 | 335 | 380 | 225 | 270 | 305 | 400 |
| 500 | 320 | 380 | 430 | 260 | 310 | 350 | 500 |
The values in this table come from NEC 2008, table 310-16. In addition to these requirements, NEC 240.4 specifies overcurrent protection or breakers for some of the common residential sizes as follows:
14 AWG Copper: 15 amperes
12 AWG Copper: 20 amperes
10 AWG Copper: 30 amperes
Where do wire size numbers come from?
Considerations that go into the requirements on wire sizing include voltage drop and heat buildup. The smaller a given wire is in diameter, the higher its resistance per foot. See the copper wire resistance table here. When the resistance is higher, the wire will heat up more, and there will be more voltage drop from one end of wire to the other. This is why on longer runs you need to consider using a larger gauge wire. Long runs of wire can create enough voltage drop that your appliances at the end of the line might be given a voltage that is outside of their operating range (ie. 200V on a 240V appliance). This can damage equipment like motors or compressors that are designed for a specific voltage range.
As a rule of thumb you should target less than 3% voltage drop on a given circuit. There are some voltage drop calculators here that can help you determine how much voltage drop you'll have with a particular length of wire.
One thing to keep is mind is that you can never go wrong by using a larger wire than is required. Things can go catastrophically wrong if you use wire that is too small, particularly if paired with breakers sized for a larger gauge wire.