Question 10.10: A manufacturing plant has a cooling pond located 4000 ft fro......
A manufacturing plant has a cooling pond located 4000 ft from the plant. Six pumps are used to circulate water between the pond and the plant. In cold weather, however, the pumps can freeze and fail to supply water. The plant owner decides to connect electric-resistance heaters to each pump to prevent this problem. The six heaters are connected to a two-conductor cable from the plant at a junction box (Figure 10 –12). The heaters operate on 480 V and have a total current draw of 50 A. What size copper conductors should be used to supply power to the heaters if the voltage drop at the junction box is to be kept to 3% of the applied voltage? Assume an average ambient temperature of 20° C.
The first step in the solution of this problem is to determine the maximum amount of resistance the conductors can have without producing a voltage drop greater than 3% of the applied voltage. The maximum amount of voltage drop can be computed by multiplying the applied voltage by 3%:
480 \mathrm{~V} \times 0.03=14.4 \mathrm{~V}
The maximum amount of resistance can now be computed using Ohm’s law:
R=\frac{E}{I}
R=\frac{14.4 \mathrm{~V}}{50 \mathrm{~A}}
\mathrm{R}=0.288~ \Omega
The distance to the pond is 4000 ft Since two conductors are used, the resistance of both conductors must be considered. Two conductors 4000 ft long will have the same resistance as one conductor 8000 ft long. For this reason, a length of 8000 ft is used in the formula:
\mathrm{CM}=\frac{\mathrm{K} \times \mathrm{L}}{\mathrm{R}}
\mathrm{CM}=\frac{10.4 ~\Omega-\mathrm{CM} / \mathrm{ft} \times 8000 \mathrm{~ft}}{0.288 ~\Omega}
\mathrm{CM}=\frac{83,200 ~\Omega \text {-CM }}{0.288~ \Omega}
\mathrm{CM}=288,888.9
300-kcmil (thousand circular mils) cable will be used for this installation.