What is the Phase to neutral voltage on a 400 volt 3 phase power supply?

The line-to-neutral (not 'phase-to-neutral'!) voltage on a 400V 3-Phase wye-connected system is 230V.

Line voltage is the voltage as measured between any two (2) line conductors (hence its name!). Line voltage is often referred to as the 'line-to-line voltage'.

Phase voltage is the voltage as measured between any single line and neutral. Phase voltage is often referred to as a 'line-to-neutral' voltage.

"400V" on a 400V 3-Phase wye-connected power supply indicates its line voltage. Line voltage in a wye system is always the phase voltage multiplied by the square-root of "3" (1.732) and reflects the vector sum of two individual phase voltages present in a three phase system.

Thusly, if the line voltage is "400V", then the phase voltage is 400V divided by the square-root of "3" (1.732), which is 230V.

Examples of this for North American power systems are 120/208V, 277/480V and 347/600V. Examples for other areas of the world are 220/380V, 230/400V and 240/415V.