Building Services Engineering 5th Edition Handbook

Electrical installations 301
For three-phase current:
line current =
VA
V × √ 3 A
= kVA
kV × √ 3 A
The results of the power calculations are given in Table 13.2.
The answers required are as follows.
1. The total input power kilovolt-amperes required for site is 120.8 kVA.
2. The incoming supply cable capacity at 415 volt, three-phase, 50 Hz required is:
120.8 × 1000
current =
415 × √ 3
current = 168 A
This is the input current to the site at the voltage of that cable. This is not the same as a
total of the currents calculated in Table 13.2 as these larger numbers only appear at their
reduced voltages in the relevant sub-circuits.
3. Assume that the crane, pump and tools are running for 25% of an 8-h working day, 5 days
per week for 48 weeks, 20 of the tungsten lamps are for security lighting 16 h every night,
and the remaining 40 tungsten lamps and the flood lamps are used for 3 h per day, 5 days
per week for the winter period of 20 weeks. The crane, pump and tools are working for:
0.25 × 8 h
day × 5 days × 48 weeks = 480 h
week
The security lamps are working for:
16 h
day × 7 days × 52 weeks = 5840 h
week
The other lamps are working for:
3 h
day × 5 days × 20 weeks = 300 h
week
Table 13.2 Building site plant schedule.
Equipment Power (kW) Number kW kVA V A
Tower crane 75 1 75 93.75 415 130.4
Sump pump 5 1 5 6.25 240 26
Lamps 0.1 60 6 6 240 25
Flood lamps 0.4 12 4.8 4.8 240 20
Hand tools 0.4 20 8 10 110 90.9
Total 98.8 120.8 n.a. n.a.