Conductor Sizing
The simple formula that can help you quickly select the correct conductor size from a single phase inverter to the point of common coupling (POCC), is straightforward when you break it down.
With just a few details from the project and a copy of Table 8 from your NEC code book you can rapidly determine the conductor size that will achieve your maximum percentage of voltage drop allowable in the design of the circuit.
Let’s dive in!
The Formula:
R = (5 x %Vdrop x Vnominal)
(I x D)
Definition of Variables:
- ‘R’ is the conductor’s resistance in Ω/1000 Ft.
- ‘%Vdrop’ is the targeted maximum voltage drop in percent
- ‘Vnominal’ is the service voltage
- ‘I’ is the maximum current of the inverter
- ‘D’ is the one-way distance of the circuit, measured in feet.
See it in action!
Currently I am working on a residential project for a customer of ours in California. They have a ground mounted array located on the property that is 125 feet away from their POCC. The chosen inverter is a SMA Sunny Boy 7000 transformerless inverter mounted at the array. My goal in this design is to keep the voltage drop below 1%.
The service there is 240 volt and the maximum current output of the SB-7000TL-22 is 29.2 amps.
R = (5 x 1 x 240) R = (1,200) R = 0.329 Ω/1000 Ft
(29.2 x 125) 3,650
Now that we have found the resistance of the conductor that is needed, we can look at Table 8 in the NEC code book for a conductor that closely matches that result. I’m using copper conductors so be sure you are looking under the correct column if using other than coated copper conductors.
From the table, the nearest conductor to the result calculated, which must be equal or less resistance than our result, is a #4AWG conductor. This has a resistance of 0.321 Ω/1000 Ft so that should suit nicely to achieve my goal of maintaining the voltage drop of the system to be at 1%.
Also note that you can use this same formula with DC conductors as well.