Q & A of the Day – How to calculate office block maximum demand?

Question: I have been asked by the utilities manager to work out the maximum demand of an office block, which also includes three-phase supplies for comfort cooling and heating air conditioning cassettes to cover: 4 x 32A three-phase supplies; 23 x 5A single-phase supplies; and 2 x 10A single-phase supplies. The rest of the power is to include 12 lighting circuits totalling 7.8kW and 10 x 32A ring mains totalling 100 sockets - these are to be used for computer work stations only. A 5 x 32A ring mains is to be used to for kitchens and general heating/water heating a printing room, and also a communications room.

I am working out the maximum demand, and maximum demand using diversity, to ascertain the rating for the main cutout fuses. Have you a quick calculation for the kVA rating, as I think that the electrical distributor will require this?

Answer: As you indicate, the calculation of maximum demand is complex and involved. In existing buildings, it is possible to measure loads with a clamp meter at various times in (say) a 24 hour period which would allow a pattern to emerge and highlight maximum peak times and loads. On new projects, the load connected will have to calculated using maximum figures and some diversity where applicable. The kVA load is the kW load multiplied by the power factor (PF). For most projects, the kVA load is similar to the kW rating

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