Originally Posted by Subaru297
I'd be interested in the correct formula.
Ok, here you go....
To calculate it out, you need to know a few numbers (resistance). You can not just add up all the lengths of your 3 cables and run it against an online cable calculator because your circuit is a series-parallel circuit. You have 3 legs 1(leg from controller to junction box) 1 (leg from panel #1 to junction box), 1 (leg from panel #2 to junction box). Look up the resistance for that cable size (the numbers are stated in resistance for each foot). Multiply the length of each leg * the resistance per foot number and write it down. Once you have the resistance for each leg, the formula will be a series parallel calculation.
Calculate Resistance in parallel (Leg#2, Leg#3) first:
1 1 (this is 1/R1 + 1/R2.... it will not allow me to put spaces)
__ + ___
Now calculate TOTAL Resistance of the circuit in series (leg #1 + Combined parallel total from above calculation of (Leg#2, Leg#3):
Resistance of Leg#1 + Calculated Leg (Leg#2, Leg#3) = Total cable resistance
You now have the Total Cable Resistance Rt
V (volts)/ rt = Voltage drop
1000' of cable = 0.6282 Ohms
1' of cable = .0006282 Ohms
Or once you cut the cables to their required lengths you can take an Ohm meter and measure the resistance of each of the cables.
Have fun, just use 8AWG for the two panel legs and a 6AWG down, this should allow for future panels
Really had to dust off some of those unused brain cells...
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