Sizing of backup generators in residential buildings

The requirement of backup generators for buildings and their sizing is a much debated topic.  It is a compromise between the economics of cost and the need for continued availability of power. The general tendency is to size the generator on the basis of experience of smaller buildings where it is provided as an afterthought. This makes it difficult for the MEP team to justify the size requirement for the generator.

In modern high rise buildings generator backup is required for the following:

1.       Lifts.

2.       Water supply pumps.

3.       Sewage treatment plant.

4.       Fire pump; if the backup pump is also electrical.

5.       Emergency lighting.

6.       Any other loads deemed necessary.

The loads in the list above can be segregated into two types,

Essential loads – These loads do not need immediate power backup, but require power in case of extended power failure.

Life safety systems – These systems need to have immediate power backup, extended power failure can have serious effect.

Of the above items 4 and 5 are life safety services.

The straight forward method of sizing a generator can be done in the following steps

1.       Make a list of all the equipments to be fed by the generator with the electrical loads and demand load for each item.

2.       Segregate the load into two columns for the following two conditions

·         Power failure condition.

·         Fire condition

3.       Find the higher load of the two.

The generator is to be sized to cater to the larger load.

A sample table shown below can help in making the decision regarding the sizing as well as the basis of the decision for record. The loads are listed in column 3 and they are copied to columns 4 or 5 as per the type of load.

Sl.No	Load	kW	Essential	Life Safety
1	Lift -1	15	15
2	Lift -2	15	15
3	Lift - 3 - Fire lift	15	15	15
4	Service lift	20	20
5	Water supply
6	Domestic Water Pump	10	10
7	Flushing water pump	7.5	7.5
8	Sewage treatment plant	45	45
9	Fire pumps	70		70
10	Jockey pump	5	5
11	Emergency lighting	5	5	5
	Total		137.5	90

In the case above, the load on the generator will be 137.5 kW.  

So the next largest size of generator should be chosen. 

 Hope the same is helpful. Do let me know your comments and suggestions on the method above.

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