Building Services Engineering 5th Edition Handbook

300 Electrical installations
For1mm 2 cable:
l =
9.6 × 103
12.5 × 44 m = 17.5 m
For 1.5 mm 2 cable:
l =
9.6 × 103
12.5 × 29 m = 26.5 m
For 2.5 mm 2 cable:
l =
9.6 × 103
12.8 × 18 m = 42.7 m
Construction site distribution
A list is made of all electrical equipment to be used on site in order to assess the maximum
demand kilovolt-amperes and cable current rating required. An estimate of the cost of electricity
for running the site may also be made.
EXAMPLE 13.7
A building site is to have the following electrical equipment available for use:
(a) tower crane, electric motors totalling 75 kW at 415 V;
(b) sump pump, 5 kW at 240 V;
(c) 60 tungsten lamps of 100 W each at 240 V;
(d) 12 flood lamps of 400 W each at 240 V;
(e) 20 hand tools of 400 W at 110 V.
1. Find the total kilovolt-amperes to be supplied to the site if the power factor of all
rotary equipment is 0.8.
2. Find the electrical current rating for the incoming supply cable to the site.
3. Estimate the cost of electricity consumed on the site during a 12-month contract.
For rotating machinery:
power, VA =
For single-phase current:
useful power W
PF
= W 0.8
and, kVA =
kW
PF
line current = VA V A
= kVA
kV A