In a 480 Volt 3-phase power supply what is the voltage between each lead?

Assuming this is a WYE connected system: (Most commercial/industrial services are) Your question answered it: 480 volts. The phase to phase voltage on this system is 480 volts and the phase to ground voltage is 277 volts.

Answer

The 'leads' you refer to are more-properly called 'lines', and the voltage between any pair is called a 'line voltage' (not a 'phase voltage', as stated in the original answer). The rated voltages of three-phase systems, regardless of whether they are three-wire (delta) systems or four-wire (star/wye) systems are always quoted in terms of their 'line voltages'. So, to answer your question, the line voltage ('voltage between leads' ) of your three-phase system is 480 V. If your system is a four-wire star/wye system, then the phase voltage voltage, or 'line-to-neutral' (not 'phase to ground') voltage is the line voltage divided by 1.732, i.e. 277 V.