The Golden State: Where & How to Live, Secure, Visit, Enjoy and Thrive in California
Practical Electricity And House Wiring
The National Electrical Code limits the amperages that different kinds of wire are permitted to carry, and also differentiates between wires in conduit or cable (where the heat generated by the current is confined to the wires), and wires on insulators as in knob-and-tube work (where the heat generated is dissipated into the surrounding air). The following tabulation shows these maximum amperages as denned in the 1947 Code:
Voltage Drop.—As already explained, any current flowing through a wire creates some heat; the heat represents wasted electricity. Doubling the number of amperes in any given size of wire increases the wasted power by four times. However, forcing too many amperes through a wire also leads to excessive voltage drop. Let us go back to the water pipes of an earlier chapter.
You perhaps have noticed that in a city where all homes are served from one water-tower providing say 40 pounds pressure, the homes near the water tower have practically 40 pounds pressure, while those farther away, particularly those served by small diameter mains, have much lower pressures. Exactly the same is noticeable when sprinkling with two different size hose—a 3/4. inch hose will sprinkle much farther and will handle more water than a 1/2 inch hose connected to the same faucet. More pressure is lost in a small diameter hose or pipe than in a large one.
In similar fashion electrical pressure (voltage) is lost in any wire; for any given amperage, the smaller the wire, the more the loss. For example, a voltmeter at the point where the wires enter your house may show 115 volts, but the same voltmeter directly across the wires where they are connected to the motor on your washing machine may show only 105 volts—the balance has been lost (and wasted) in the wires to that point. The actual number of volts lost depends entirely on the number of amperes carried, the length of wire and the size of the wire.
