Why is three phase 240v not called 360v?

In US the 240V that's delivered to residential is really not 3 phase. It is single phase in Scott T configuration, see http://en.wikipedia.org/wiki/Scott-T_transformer Three phase voltages for a 120V systems is 208V thus the notation 120/208Y where 120 is the single phase voltage and 208 is the Y configuration 3 phase voltage. There is also 277/480Y. The relationship between single phase and 3 phase is multiply by square root of 3. It is not simple voltage addition. See http://en.wikipedia.org/wiki/Three-phase_electric_... In rest of the world, i.e. not North America, the voltages are 240/415Y or 240 in single phase and 415 in 3 phase.

Because 120, 120, 120 three phase is 208V.

120/240 is a single phase system.

Because the Phase/ground voltages are 120 Deg. apart, you multiply

the phase/ground voltage by 3^1/2 to get the phase to phase voltage

on a three phase system.

It's easier to explain with diagrams which we can't do here.

Wiki 'three pahse alternating curent system'

Sorry to say, you have completely misunderstood the meaning of three phase voltages.

It is highly technical but I will try to explain in simple terms.

In electrical generators, three sets of coils generate identical voltages. But they differ in timing. Like a juggler throws 3 balls one by one, the three equal voltages are varying from 0-maximum and then to 0 and then to minimum, one by one, with respect to time. That is why you can not add them arithmetically but you need to use vecto maths to add them.

Hope this is clear to you