Calculating Max. Demand Kilovolt-Amperes and Required Cable Current Rating

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 //

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.

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

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)