Other than a brief foray in the 1970s when aluminum wiring was popular, copper is king when it comes to house wiring. Copper rates high on the conductivity scale. That is, it’s an efficient pathway for an electrical current.
Conductivity Scale
Silver 100%
Copper 98%
Aluminum 61%
Iron 16%
Nickel 7%
In addition to a wire’s conductivity, its size and the type of insulation around the wire affect its ampacity, or the amount of current (in amps) it can carry before it exceeds its temperature rating. The greater its size, as measured in mils, the more current it can conduct.
Every wire size has a maximum current that it can conduct. The following table shows the most common residential wire sizes and their ratings.
Wire Gauge Rating
Gauge Value Ampacity
14 15 amps
12 20 amps
10 30 amps
8 40 amps
6 55 amps
Appliance and lamp cords use No.16 or No.18 wire, which is quite thin. This might lead you to ask, “Well, if thinner wire has a lot of resistance and can heat up easily, but thicker wire can hold more juice without overheating, why don’t we use thicker wire throughout our homes? Wouldn’t that be safer?” This is a reasonable question, and it has two answers: flexibility and cost.
If you try to bend and fit No.8 wire so you can connect it to a light fixture, you’ll come to appreciate the flexibility of smaller wire. Like just about anything else in life, the larger the size, the greater the cost. There’s a reason home-improvement stores periodically have loss-leader sales on No.12 wire but not any of the thicker stuff. Other factors that affect your choice of wire will be discussed in later chapters.
The society of wire and conductors is a very closed one. No amount of politically correct persuasion will convince one gauge of wire to mingle with another. You should not mix No.12 wire with No.14 on the same circuit, for example. Wire must match up with its circuit breakers or fuses; No.14 wire doesn’t go with a 20-amp breaker, so don’t confuse either party by mixing them together. You can install larger wire on a smaller circuit breaker, but you cannot install a smaller wire on a larger circuit breaker.
Conductivity Scale
Silver 100%
Copper 98%
Aluminum 61%
Iron 16%
Nickel 7%
In addition to a wire’s conductivity, its size and the type of insulation around the wire affect its ampacity, or the amount of current (in amps) it can carry before it exceeds its temperature rating. The greater its size, as measured in mils, the more current it can conduct.
Every wire size has a maximum current that it can conduct. The following table shows the most common residential wire sizes and their ratings.
Wire Gauge Rating
Gauge Value Ampacity
14 15 amps
12 20 amps
10 30 amps
8 40 amps
6 55 amps
Appliance and lamp cords use No.16 or No.18 wire, which is quite thin. This might lead you to ask, “Well, if thinner wire has a lot of resistance and can heat up easily, but thicker wire can hold more juice without overheating, why don’t we use thicker wire throughout our homes? Wouldn’t that be safer?” This is a reasonable question, and it has two answers: flexibility and cost.
If you try to bend and fit No.8 wire so you can connect it to a light fixture, you’ll come to appreciate the flexibility of smaller wire. Like just about anything else in life, the larger the size, the greater the cost. There’s a reason home-improvement stores periodically have loss-leader sales on No.12 wire but not any of the thicker stuff. Other factors that affect your choice of wire will be discussed in later chapters.
The society of wire and conductors is a very closed one. No amount of politically correct persuasion will convince one gauge of wire to mingle with another. You should not mix No.12 wire with No.14 on the same circuit, for example. Wire must match up with its circuit breakers or fuses; No.14 wire doesn’t go with a 20-amp breaker, so don’t confuse either party by mixing them together. You can install larger wire on a smaller circuit breaker, but you cannot install a smaller wire on a larger circuit breaker.
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