How can we size transfer pump and booster pumps for commercial office building (G+2)?

Selection of pump depends upon two main factors

1. flow rate 

2. head

for transfer pump 

agree the time required to full the overhead tank from underground water tank

divide that time with the total volume of water, you will get flow rate,

next calculate the head (vertical displacement + losses in piping), you will get the head.

booster pump

Calculate the total loading units of the fixtures installed in the building 

(use UPC for calculation of loading unit)

from loading unit you will find the flow rate of the booster pump,

now calculate the head of the booster pump 

Transfer Pump is usually meant for transfer the domestic water from U/G water tank to overhead water tank. Firstly; the consumption of the water is to be known which depends upon the occupants & their usage for a commerical buidling. Refer to IPC for estimating the water flow-rate in USGPM based on the WSFU (Water Supply Fixture Units). It reveals the pipe size too. Transfer pump flow-rate can be estimated based on this pipe size with 25% safety. The ideal configuration transfer pump shall be 1 duty, 1 Standby.

Booster pump is usually to have a minimum pressure at the plumbing fixture. Booster pump are usually meant for the most nearest washroom/toilet area. For example, a booster pump on the roof shall be meant for having min. pressure at 2nd flr. or for the WCs operated by flush valve. The ideal configuration transfer pump shall be 1 duty, 1 Standby.