I am estimating a job and the distance from the panel to the first light is 400' there are then 8 lights spaced out 35' apart totaling about 240' from the first JB, the engineer has #4 specced out for the lights and i would like to downsize after hitting that first JB but I am not sure how to calculate the voltage drop using the different gauge wires from the first JB. I have more information on fixture specs and voltage if needed thanks.
You'd need those specs and voltage, and how the lights are wired. Everything else being equal, daisy-chained from one to another will have more VD than if they were all wired 'star pattern' from one j-box, 2 runs of 4, etc.
The #4 may be to meet 2 or 3% VD for the circuit, future expansion, boilerplate specs, lazy/over-engineering, or any of a number of other reasons. Obviously the last light will only have its own draw and wouldnt need #4. 8 600W HPS @ 208V would need #4 cu to get 2.5% VD @ 400'. The last light would only need #14 cu (2.88A @ 250', 1.94% VD).
There are online calculators that you can use to figure the VD. You can estimate it two ways; #4 all the way, per the specs, and your way. For a bid tho, you have to go by the plans and not what you think you maybe able to install if you get the job. Considering the cost of trenching, conduit, parts and labor, there isnt going to be an enormous difference materials wise between #4 and #6 or smaller the last 250'.
