Economical Size of Wire

Author: E.E. Kimberly

If the cross-sectional area of the wire used be doubled, the power loss would be only half as much for the same current. The choice of size of conductor is determined by four major considerations:

1. The mechanical strength must be sufficient to withstand stresses of suspension. For interior wiring, except fixture cord, the minimum size is No. 14. Fixture cord may be as small as No. 18.

2. The voltage drop in the line should not exceed 3% for lamp circuits and 10% for motor circuits.

3. The temperature rise must not exceed the maximum permissible temperature of the insulation used. See Tables A and B of the National Electric Code in the Appendix.

4. The annual cost of the energy lost in the conductor should be approximately equal to the annual investment costs of those parts of the distribution system which are proportional to the weight of the copper. This balancing of fixed charges against the cost of energy lost establishes a relation such that the sum of the annual investment costs and the cost of wasted line energy for a year will be a minimum.

The problem involved in choosing the size of conductor will be further discussed in Chapter 23 under the subject, Most Economical Size of Cable.