system (par 7b) or to the end of the branch line off point C.) By allowing,

roughly, a fourth or a fifth of the total voltage drop for this particular section,

you should have enough drop left for the later sections. Using a No. 4 wire, the

actual voltage drop resulting is approximately 1.1 percent. The results of these

computations should be tabulated as shown in figure 17.

(4) Section BC.

The load carried by this section is the sum of the loads

beyond point B or 6.82 KVA.

The distance is 300 feet.

Allow up to 2 percent

voltage drop in this section.

From figure 9 a No. 6 wire may be used, which

actually drops the voltage by 1.8 percent.

The cumulative voltage drop to this

point is now 1.1 + 1.8 = 2.9 percent.

(5) Section CD.

The load is 4.57 KVA; the length is 150 feet.

Since the

cumulative voltage drop to point C is 2.9 percent, you are still 2.1 percent under

the specified maximum.

Actually, this is quite a bit of allowance for this last

section and, consequently, a very small economical wire could be selected.

However, a No. 8 is the smallest allowable size. Hence, using this for section CD

will result in an additional 1.0 percent voltage drop making the cumulative drop to

point D 1.1 +1.8 + 1.0 = 3.9 percent. Figure 17 shows the complete tabulation for

the feeder line.

As shown, the wire sizes selected have created a total voltage

drop of 3.9 percent to point D. Therefore, there is an allowable 1.1 percent drop

for each group of buildings being served by the branch lines off point D. The next

usual step is to determine the size of wire for the branch circuits at the end of

the feeder line.

These last branches are critical because they will have the

largest accumulated voltage drop.

(6) Branch line design.

Figure 18 shows the group of buildings (16 through

22) for which wire size will be determined next. Design of these branches varies

slightly from that for the feeder in that the equivalent distance of the branch

must be computed. The equivalent distance is the ratio of total KVAX ft to total

KVA and is computed as follows:

Since you had a 3.9 percent cumulative voltage drop to point D, there is an

allowable 1.1 percent remaining for this particular branch. Figure 9 is now used

by entering on it a load of 1.65 KVA, a distance of 232.2 feet, and a 1.1 percent

voltage drop. Using a No. 8 wire, the voltage drop induced in the branch line will

be approximately 0.5 percent. Hence the total voltage drop from the generator to

building 19 is 3.9 + 0.5 = 4.4 percent, which is within specifications.

Integrated Publishing, Inc.
6230 Stone Rd, Unit Q
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