Understanding 3 phase 480V lighting circuit
I understand that you would use 480V light fixtures as opposed to 277V because the 480V have a lower current draw, resulting in a more efficient system. 


The lamps aren't necessarily more efficient. It's just that there are less losses in power transmission.


  In a simple single-phase lighting circuit, you would have two wires: say, 120V hot wire and a neutral wire. Current flows from the hot to the light fixture and back through the neutral, assuming conventional direction of current flow. 


Correct.


  This completes the circuit and you can calculate the wattage by measuring the current pulled and the voltage supplied. 


You have to remember that they can be somewhat out of phase giving a power-factor &lt; 1 which needs to be taken into consideration for power (watts) calculations but you are on the right track.


  However, I don't see a 3 phase lighting circuit with 480 V light fixtures work like that. Consider a 3 ph system A, B and C. If the A and B phase are connected to the fixture in a "480V single phase" manner, which phase supplies and which one returns current? 




Figure 1. The voltage difference between two phases is a sinewave.

Since there is no neutral connection you can look at it either way. Current will alternate between A and B.


  If A supplies and B returns, then what about a load that's connected across B and C? 


Current will also alternate between B and C.


  That would mean C supplies and B is the one carrying the return current at all times, or else you will have a conductor with two currents flowing in opposite directions. 




Figure 2. Two currents flowing in opposite directions cancel out as indicated at point (1) when the B phase is at zero volts (assuming resistive load). At other times B has to carry the sum of the return currents with a peak value when B is at maximum.


  OR is the fixture drawing current from both phases? If so, then that's not single phase. 


A load connected between two phases has only two wires and sees a sinusoidal voltage applied to it. As far as the load is concerned it has a single-phase supply. (It knows nothing about neutral.)


  Also, wouldn't you use 3 single-pole breakers as opposed to 1 3-pole breaker to supply the 3 phases.


Single-phase breakers would leave the other phase connected and the potentially faulty circuit still live. Good practice would be to use a 2-pole breaker on each load or a 3-phase breaker to isolate everything at once.