0195136047.pdf

212 THREE-PHASE CIRCUITS AND RESIDENTIAL WIRING

The total three-phase reactive power delivered to the load is

√

3 (WC−WA)=

√

3 ( 1183 − 317 )=1500 VAR

which checks with

√

3 VLILsinφ=

√

3 ( 100

√

3 )( 5

√

2 )sin 45°=1500 VAR

For positive-sequenceA–B–C,

tan∅=

√

3

(

WC−WA

WC+WA

)

=

√

3

(

1183 − 317

1183 + 317

)

= 1. 0

or∅=45° with a positive sign, implying thereby that it corresponds to an inductive or lagging

load, which checks with the given load specification. The load power factor is given by

cos∅= 0. 707

and is lagging in this case.

4.4 Residential Wiring and Safety Considerations

Residential electric power service commonly consists of a three-wire ac system supplied by the

local power company. A distribution transformer with a primary and two secondaries formed with

a center tap on the secondary side, as illustrated in Figure 4.4.1, is located on a utility pole, from

which three wires originate. Two of them are known as “hot” wires, while the third is called a

neutral wire, which is connected to earth ground. In accordance with standard insulation color

codes, “hot” wires are denoted by either B (black) or R (red), neutral by W (white), and ground

(or uninsulated wire) by G (green). The functional difference between neutral and ground wires

will be brought out later when considering circuit wiring.

Domestic loads consisting of 120-V appliances and lighting, divided nearly equally between

the two 120-V (rms) secondaries, are connected from hot wires to neutral. Appliances such as

electric ranges and water heaters are supplied with 240-V (rms) power from the series-connected

secondaries, as shown in Figure 4.4.1.

Minimizing the power loss in the lines (known asI^2 R loss) is important from the viewpoint

of efficiency and reducing the amount of heat generated in the wiring forsafetyconsiderations.

Since the power loss in the lines is directly related to the current required by the load, a lower

line loss will be incurred with the 240-V wiring in delivering the necessary power to a load. For

the lower voltage case, however, the size of the wires is increased, thereby reducing the wire

resistance, in an effort to minimize line losses. Problem 4.4.2 deals with these considerations.

120 V

Earth 120 V

ground

2400 V

Primary Secondaries

Neutral 240 V

(white) W

R (Red)

B (Black)

Hot

Hot

240-V

appliances

120-V

appliances

120-V

appliances

Figure 4.4.1Three-winding distribution transformer providing dual-voltage ac supply.