How can I wire a 240 V single-phase outlet from a commercal building with 3-phase service 4- wire?

Answer 1

A three phase four wire commercial distribution service will not give you 240 volts. The nearest voltage you can get is 208 volts. It is a wye connection which gives you 208 volts between the three legs and 120 volt between any of the three legs and ground. This ground point is the systems neutral point.

Answer 2

It depends on where on earth where you live and what voltage your service distribution is. This voltage can easily be obtained if the three phase four wire service is in Europe or the UK. The three phase service would have to be 240/415 volts. Any line leg to ground will give you a voltage of 240 volts. 415/1.73 = 240 volts.

If you are in the US this voltage will have to be obtained by using a transformer. The US nearest voltage, three phase four wire is 277/480. 480/1.73 = 277 volts. The transformer's primary voltage will be what ever your service voltage is, The secondary voltage will be 240 volts. The sizing of the transformer will be governed by the current of the connected load.

Answer 3

If you have 440 Volts 3 phase, neutral system, you can get 240 Volts single phase, by using one of the phase and the neutral to connect your single phase load