Warehouse Lighting Wire Size Calculation
Thirty 42-watt fixtures are nominally 1260 watts, which is 10.5 amps at 120V. 

Your longest run has the lights evenly spaced between 150' and 310' and the halfway point between is 230'.  

Hit a voltage drop calculator

Set it for 5% voltage drop, as your lamps can handle that... and ask it for at least 3 points: 


10.5 amps (max) at the 150' (start) distance
8 amps (3/4) at the 190' (1/4 way) distance 
5.25 amps (half) at the 230' (halfway) distance


TLDR: I get 12AWG for each of those calcs.

For each test, the calculator will recommend a wire size and give you the expected voltage drop to be found there. Its recommendation is applicable from the panel to the point you asked about.  

They say to use 3% voltage drop, but that is silly, 5-6% will work fine. But don't push it to 10% (unless you really want to) because if you upgrade to multivoltage lamps, those will answer voltage drop by drawing more current in proportion, throwing calculations off.  

Is voltage drop OK?

It's not unsafe to have high voltage drop.  The 20A breaker definitively protects your 12 AWG wire if your wiring methods are legal. (an example of "illegal" would be more than 9 conductors in a single conduit without derate). Of course the voltage drop is heating the building.  Electric heat is a fairly expensive heating method, especially if you're trying to air-condition the place!  So get out the sharp pencil and figure out the voltage drop, multiply by amps to get watts, figure the cost of the power and additional A/C load, and see if you can cost-justify thicker wires. 

The voltage drop calc will keep you legal -- however it doesn't know about your 20A breaker.  14AWG wire is never legal downstream of a 20A breaker, 12 is your minimum.  

Did you catch the part about "9 conductors per conduit"?

That's kind of an issue.  If you want to cram 12 circuits in a single conduit, that is 24 conductors. That requires a 45% conduit derate, forcing you to 8 AWG wire.  Don't do that, unless you would anyway for other reasons.

It might help to re-label your circuits as 15A circuits and fit 15A breakers. 10-20 wires per conduit would be legal with 12AWG wire on a 15A circuit.**  You can only have 12A of continuous loads on a 15A circuit, but you are under 12A.  

How do big warehouses work around this?


Multi-wire branch circuits.  In an MWBC, the neutral does not count as a conductor since it only carries differential current. With 120/240 split-phase, you can get 4 split-phase MWBCs (8 hots) in a single conduit without derate. 
Three-phase "wye" MWBCs such as 120/208 "wye". This adds a third "hot" conductor.  With 3-phase, you can fit three MWBCs (9 hots) before hitting the derate limit.
277V voltage.  With 277/480 3-phase, you can get more than double the power in the same conduit. 


Combine all these tricks, and a single 1/2" conduit with twelve 12AWG wires can carry 39,888 watts** - enough power to light up an entire CostCo.  (that being 9 circuits x 277V x 20A x 80%, the latter the continuous derate). 



** why is this? The 50% derate comes off the 90 degree C column in the table, so 12AWG is 30A@90C. 10-20 wires calls for a 50% derate, or 15A.  (and just so you know, 7-9 wires does require a 70% derate, but 70% of 30A is 21A... and 12AWG is limited to 20A for other reasons.)