5. Since the distance from the generators to the laundry is only 50 feet, the
voltage drop in this distance will be negligible, and the load-carrying capacity of
the wires becomes the deciding factor. Referring to table 1, for a 34W 127/220V
system, a No. 4 wire will carry 57- KVA which is well in excess of the required 41.
A No. 6 wire, however, can carry only 38 KVA which is below the required capacity.
Hence, a No. 4 wire is required.
7. In military practice the percent voltage drop is taken as a percentage of the
voltage at the generator, or
.
If
we
let
X
=
the voltage at the generator, then X -208 is the voltage drop, and the above
relationship yields the following:
8. Refer to figure 8, attached memorandum. Enter 250 feet at the bottom right
horizontal scale; proceed vertically to the No. 8 curve; proceed horizontally from
this point to the vertical line representing a load of 6 KVA (on the bottom left
horizontal scale). At this point the voltage drop is read as 3.2 percent using the
lower voltage drop figures.
11. From table 5, attached memorandum, the sag for 150 foot spans at 60F is 26
inches or 2'2".
From figure 10, attached memorandum, the required clearance for
primary feeder lines is 20 feet if they must cross public streets and driveways to
resident garages. The wires must therefore be attached 20' + 2'2" or 22'2" above
the ground.
12. From figure 10, attached memorandum, the required clearance for primary
feeder lines is 28 feet over railroad tracks.
This, plus the 2'2" sag found in
problem 11, gives a required mounting height of 28' + 22" or 30'2" for the wires.
13. Using figure 12, attached memorandum, enter 450 feet on the right vertical
scale, and proceed horizontally to the curve for 4/0 (0000) wire size.
At this
point proceed vertically downward to the bottom horizontal scale and read the
minimum allowable sag to be 8 feet.
14. Your assistant has allowed an excessive voltage drop for this particular
section of feeder,
.
This leaves only 6.0 - 5.5 = 0.5% allowable drop
for sections
AC, IL, and LN.
In effect, he has used up most of his allowable
voltage drop
on section CI and thus left very little for the subsequent sections of
the feeder.
Remember the 6 percent is the total drop allowed from the generator to
the farthest
load.
15. Demand load on section AC is computed as follows:
Group
I
6.50 KVA
II
2.00 KVA
III
2.50 KVA
IV
4.00 KVA
V
2.00 KVA
Building 5
3.00 KVA
Total demand
20.00 KVA
2-3
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