Building Design and Construction Handbook - Merritt - Ventech!

ELECTRICAL SYSTEMS 15.3
Resistance for a given material varies inversely as the area of the cross section
and directly as the length of wire.
Ohm’s law states that the voltage E (volts) required to cause a flow of current
I (amperes) through a wire with resistance R (ohms) is given by
or
E � IR (15.1)
Power P is measured in watts and is the product of volts and amperes:
2
P � EI � (IR)I � IR (15.2a)
��
2 E E
P � EI � E � (15.2b)
R R
Large amounts of power are measured in kilowatts (kW), a unit of 1000 W, or
megawatts (MW), a unit of 1,000,000 W.
Electric Energy. The energy expended in a circuit equals the product of watts
and time, expressed as watt-seconds or watt-hours (Wh). For large amounts of
energy, a unit of 1000 watt-hours, or kilowatt-hours, kWh, is used.
Charges for electric use are usually based on two separate items. The first is
total energy used per month, kWh, and the second is the peak demand, or maximum
kW required over any short period during the month, usually 15 to 30 min.
Power Transmission. Power is usually transmitted at very high voltages to minimize
the power loss over long distances. This power loss results from the energy
consumed in heating the transmission cables and is equal to the square of the current
flowing I, times a constant representing the resistance r of the wires, �/ft, times
the length L, ft, of the wires. Measured in watts,
2
Heat loss � IrL (15.3)
Series Circuits. A series circuit is, by definition, one in which the same current
I flows through all parts of the circuit (Fig. 15.1a). In such a circuit, the resistance
R of each part is the resistance per foot times the length, ft. Also, by Ohm’s law,
for each part of the circuit, the voltage drop is
FIGURE 15.1 Types of electric circuits: (a) series; (b) parallel.