Wire Types and Sizes
There are three types of approved interior wiring in general use for modern residential and farm applications.
- Flexible armored cable, usually called BX.
- Nonmetallic cable or Romex and plastic-covered cable.
- Thin-wall conduit, also called EMT.
Although it would seem that the subject of wires would be a simple one, it actually involves many factors including math. For instance, the three major factors that affect the current-carrying capacity of a conductor are the amperage, the wire size, and the wire temperature. The higher the amperage, the smaller the wire, and the higher the wire temperature, the lower will be the wire’s safe current carrying capacity. In other words, if the wiring is to be maintained at a safe temperature, it must be large enough to carry more than the rated amperage of the connected load. Also, if the wiring is exposed to relatively high temperatures, its size should be increased accordingly.
Wires for electric circuits are identified by number. The larger the number, the smaller the diameter of the wire. The wires used for wiring used for wiring homes and average size business establishments range in numbers from No. 000 to No. 14. Wires larger than No. 000 are especially for industrial applications, where exceptionally high voltages and current are required. Wires smaller than No. 14 are used for many applications such as low-voltage control circuits, door-bells, solenoid coils, etc.
The following table lists the wires from size No. 000 to size No. 14 and their maximum current-carrying capacity in amperes. It should be noted that some of the ratings depend on the type of wire insulation. Local codes determine which type is acceptable for a particular locality. In most instances, types RH and RHW are acceptable for indoor wiring. The wiring sizes mentioned in the following discussion pertain to RH and RHW types.
Wiring-size data (enclosed wires)
Maximum ampere rating
Wire Size | Types R,RW, RU,T,and TW | Types RH and RHW |
14 | 15 | 15 |
12 | 20 | 20 |
10 | 30 | 30 |
8 | 40 | 45 |
6 | 55 | 65 |
4 | 70 | 85 |
3 | 80 | 100 |
2 | 95 | 110 |
1 | 110 | 130 |
0 | 125 | 150 |
00 | 145 | 175 |
000 | 165 | 200 |
R = code rubber
RW = moisture resistant rubber
RU = latex rubber
T = thermoplastic
R = code rubber RW = moisture resistant rubber RU = latex rubber T = thermoplastic TW = moisture resistant thermoplastic RH = heat resistant rubber RHW = moisture and heat resistant rubber R = code rubber RW = moisture resistant rubber |
The diameters of electric wires are usually in mils. A mil is 1/1000 inch. It takes 1000 wires each 1 mil in diameter to equal 1 inch. A circular mil is the area enclosed by a circle that is 1 mil in diameter. The area of a circle in circular (CM) is obtained by squaring the diameter (in mils) of the circle. For example, if a wire is 100 mils in diameter, what is its area in circular mils? A = D2 = 100 x 100 = 10,000 CM
Specific resistance is the resistance of a piece of material 1 foot long and with a cross sectional area of 1 circular mil. A piece this size is known as a mil foot or circular mil foot. A piece of copper 1 foot long and with a cross sectional area of 1 circular mil has a resistance of 10.4. The specific resistances for several common metals are listed as follows:
Silver 9.8 ohms
Iron 63.4 ohms
German silver 128.3 ohms
Copper 10.4 ohms
Aluminum 17.2 ohms
Resistance varies directly with the kind of material. A formula for finding the resistance of a conductor should include temperature, length, area, and material. However, ordinary temperature changes have such a small effect that they can be ignored. The following formula is used:
R = KL/A
R = resistance of the conductor in ohms
K = specific resistance of the material
L = length of the conductor
A = cross sectional area (diameter2) of the conductor
Example: what is the resistance of copper cable having a diameter of 0.229 inch and a length of 2500 feet?
R = KL = 10.4 X 2500
= 0.495 ohms
A (229) 2
If aluminum wire was substituted for the copper in the preceding illustration, the resistance would be:
R = KL = 17.2 X 2500
= 0.819 ohms
A (229) 2
Another Example: What is the length of 40.3 mils-diameter iron wire that would have a resistance of 26ohms?
L = R X A = 26 X (40.3)2
K 63.4 = 666.03