A building site and electrical equipment
A building site is to have the following electrical equipment available for use:
- Tower crane, electric motors totalling 75 kW at 415 V;
- Sump pump, 5 kW at 240 V;
- 60 tungsten lamps of 100 W each at 240 V;
- 12 flood lamps of 400 W each at 240 V;
- 20 hand tools of 400 W at 110 V.
To calculate //
- Find the total kilovolt-amperes to be supplied to the site if the power factor of all rotary equipment is 0.8.
- Find the electrical current rating for the incoming supply cable to the site.
- Estimate the cost of electricity consumed on the site during a 12-month contract.
Ok, let’s do it…
For rotating machinery, formulae for power in Volt Amperes [VA] goes like this:
For single-phase current:
For three-phase current:
The results of the power calculations are given in Table 1 below. 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:
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:
The security lamps are working for:
The other lamps are working for:
Table 1 – Building site plant schedule
Table 2 – Building site energy use
|Equipment||Power (kW)||Number||Time (h)||Energy (kWh)|
|Tungsten lamps, security Lamps||0.1||20||5824||11648|
|Total Power Used //||56528|
The total energy used by the systems is found from:
kWh = number of appliances × kW per appliance × operation hours as shown in Table 2.
If electricity costs 8 p per unit (kWh) then the estimated cost for the 1-year contract will be:
Reference: Building Services Engineering // Electrical Installations – David V. Chadderton (Purchase paperback from Amazon)