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Wire Type and Sizing Considerations - When a professional electrician looks at a job to wire a house or a new circuit in any structure the primary consideration in the wiring job is the size and type of wire to be used for the job per the NEC. The same for a plumber when looking at a job the plumber will size the pipes based on the number of gallons needed to carry the proper amount of water to its destination. If the pipe is too small then the water needed will not reach its destination. This is almost the same for electrical wire except under-sizing electrical wire is dangerous.

Wire Type and Sizing Considerations - New Circuits and Wiring for Equipment Circuits

Wire Type and Sizing Considerations
An electrical wire carries voltage but it is the current rating of the power loads which is the primary factor in determining the size and type of wire to be used for the job. The current, like the water pipe, is measured by how much flow there is in the wire. If the wire is too small and the flow too fast then wire gets hot. If it gets too hot then the insulation melts and a dangerous condition exists. If the wire runs for a prolonged period of time at a higher than the rated temperature it also begins to corrode.

There are corrosive effects at terminal connections. Eventually, the corrosion and the excessive heat will cause the wire to become brittle. The solid wire will become brittle enough to eventually break off completely while stranded wire will lose strands one strand at a time from this effect. This causes the wire to become smaller and the heat increases. Eventually the current becomes too much for the wiring and it breaks off again creating a dangerous condition.

Wire Type and Sizing Considerations

The equipment or device being supplied electricity from this wire, when it breaks, ceases to function while a live wire is hanging inside a control panel or other panel. This is why it is very important to properly size electrical wires for any type of electrical work and not to overload existing circuits in homes and businesses. Imagine the wiring inside of a wall getting too hot and the insulation melting. This creates a dangerous electrical situation and also is a fire hazard.

National Electrical Code Ampacity Tables

Wire Type and Sizing Considerations

* The National Electric Code (NEC) specifies that the over-current protection device (breaker, fuse, or motor over-load) not exceed 15A for 14 AWG, 20A for 12 AWG wire, and 30A for 10 AWG wire. Always follow the current NEC tables for sizing.

Maximum Ampacity for Copper
and Aluminum
Wire Size
Copper
Aluminum
167°F  (75°C)
194°F  (90°C)
167°F (75°C)
194°F (90°C)
*14
20
25
 
.
*12
25
30
20
25
*10
35
40
30
35
8
50
55
40
45
6
65
75
50
60
4
85
95
65
75
2
115
130
90
100

High Performance HVAC

Wire Type and Sizing Considerations


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