Electrical Plant Sizing for Building Services

The electrical building services engineer is responsible for the proper design and sizing of all electrical plant that provides power to buildings. This simple introductory exercise is aimed at giving students an opportunity to familiarise themselves with the theory and calculation procedures and to go on to size electrical plant required for any application.

1) To understand fully the terms essential, non-essential (normal) and critical loads  and differentiate between them.

2) To appreciate the terms ‘connected’ load and ‘diversified’ or ‘maximum demand’ loads, differentiate between them, apply diversity factors and allocate spare capacity for future expansion.

3) To work out the electrical plant sizes for the transformer, standby diesel generator and UPS.

4) To work out the ratings for the protective devices required to protect all plant. Further, the student must have an understanding of the appropriate types of protective devices on the primary and secondary sides of the transformer as necessary.

5) To explain why it is important to balance the load on the different phases of the electrical plant and to go on to discuss ways in which this is normally done in practice.

6) To draw a schematic diagram showing how a transformer and standby generator and a UPS system are connected together to supply power to buildings under varying conditions. A full explanation of the operation of such an installation must be given.

Observe the laboratory safety regulations, a copy of which is displayed in the laboratory. You are responsible for your own safety and the safety of other people working in the same area.

A new residential and commercial complex is being proposed comprising:

i) Three identical blocks of luxury flats, each having eight flats per floor on four floors.

ii) Two identical office blocks, each having six office units per floor on three floors.

iii) A small shopping centre.

iv) Roads and car park facilities. See the layout drawing of the building complex provided on page 6. Using this information complete the following:

a) Work out the size and type of the transformer required to supply the total complex load, assuming total connected loads of:

· 10kVA for each individual flat,

· 20 kVA for each individual office unit,

· 40 kW at p.f.=0.8 lag for the shopping centre,

· 18 kW at p.f.=0.9 lag for road and car park lighting.

a)Make allowances for 40% and 60% diversity for the flats and offices respectively and allow overall 10% spare capacity for future expansion. It is advisable, though not essential, to use a spreadsheet for these calculations as the resulting tables afford easy scrutiny and comparison. Assume all loads are evenly balanced across the three phases.

b) Work out the transformer primary and secondary currents needed for sizing the cables and protection devices assuming an 11kV/400-230V, 3-phase, 50 Hz supply transformer and suggest suitable types and ratings for the primary and secondary protective devices.

c) Explain why it is important to balance the load on the different phases of the distribution transformer and explain how this is normally achieved in practice in:

1) A high rise building, and

2) Houses in a residential road.

d) A 3-phase, 400 V, 50 Hz standby diesel generator is required to supply items (ii) and (iii) above in the event of a mains power failure. Repeat the calculations for (a) above and size the main generator protective device at the main emergency distribution board. Mention other issues which would need to be considered in practice.

e) Consider the need for UPS systems for parts of the complex in question, list the services likely to be connected to them and propose an appropriate type of UPS system. Work out the UPS rating in kVA and propose an appropriate fuse size for its protection