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volunteer work for a local church

parking lot lighting w/ 4 GE M400 400W fixtures (3.9A each w/ HPS bulbs) on 2 poles (2 fixtures per pole)

fixture operating voltages: 108V - 131V per GE tech. support

120V AC (1 PH) 3-wire, copper conductor with PVC conduit

One 20A circuit feeds the fixtures

All 12 AWG wire (already is place when I arrived)

Relevant Distances:

segment 1: panel to center of parking lot (this point is geometrically between the poles): 175 feet

segment 2: center point to base of pole 1 or 2: 60 feet

segment 3: pole base to top of pole: 31 feet

segment 4: pole out to fixture at arms end: 14 feet

Wiring:

3 wires run from panel to center point of parking lot (175 feet) and feed:

3 wires running north to pole 1 (60 feet) and 3 wires running south to pole 2

These wires run to the base and up to the top of the pole (31 feet) where they feed:

3 wires running to fixture 1 (14 feet) and 3 wires running to fixture 2

My question: In calculating the voltage drop for each fixture via a spreadsheet, I did it in segments as follows:

using Vd=2kIL/Cm (with 12.9 for k and 6530 for Cm) for each segment

* segment 1 *

load amperage: 4 fixtures*3.9A = 15.6A

length: 175 feet

* segments 2 and 3 *

load amperage: 2 fixtures*3.9A = 7.8A

length: 60 + 31 = 91 feet

* segment 4 *

load amperage: 1 fixtures*3.9A = 3.9A

length:14 feet

I estimated the total voltage drop (sum of voltage drop from each segment) at each fixture to be 13.8V or 11.5% of the supplied 120V.

However, I have several "knowledgeable" people within this church arguing that #12 is adequate for the parking lot lighting.

Based on the calculations (and I do realize they are mere estimates if they are indeed correct), the lights will burn dim if the even come on at all.

Since I am probably rusty, I just wanted some advice on my calculations before proceeding.

Thanks in advance