Electrical Calculations - Basic Mathematics for Water and Wastewater Operators - page 154

R 1 R 2

15 ohms R 3 120 V 15 ohms 12 ohms

FIGURE 10.25

Illustration for Example 10.21.

E
I

E
I

S
T

R = or R =

T

where R T is the total resistance of all of the parallel branches across the voltage
source E S , and I T is the sum of all the branch currents.

■ Example 10.21
Problem: Given that E S = 120 volts and I T = 26 amperes, what is the total resistance
of the circuit shown in Figure 10.25?

Solution: In Figure 10.25, the line voltage is 120 volts and the total line current is 26
amperes; therefore,

E
I

120
26

S
T

R === 462 . ohms

T

Other methods are used to determine the equivalent resistance of parallel cir-
cuits. The most appropriate method for a particular circuit depends on the number
and value of the resistors; for example, consider the parallel circuit shown in Figure
10.26. For this circuit, the following simple equation is used:

R
N

R

eq =

(10.15)

where
R eq = Equivalent parallel resistance.
R = Ohmic value of one resistor.
N = Number of resistors.

A

R 1 = 10 ohms R 2 = 10 ohms

FIGURE 10.26 Two equal resistors connected in parallel.