How to size Wiring and Cabling for your System

WIRE VOLTAGE DROP CALCULATIONS EXAMPLE
Solar pv stand alone system operates at nominal voltage 12-volt, 24-volt & 48-volt. The Dc power is equal to voltage multiply by current & voltage is equal to current multiply by resistance. As the size of power plant increased, the current increased. For example in 240 watt power plant at 24-volt the current is 10 amp, but for 480 watt power plant it will be 20 amp. The resistance is measured in ohms per 1000 feet. From it is clear that as the current and wire length increases the voltage drop will be increases and energy will lost in heat. This increase in voltage drop will drop the power.
The solar energy is very costly so voltage drop is kept minimum 3% to 5%, by keeping the wire length minimum..
Allowable % voltage drops are: -
For Dc system: total: 1 to 6 = 4.18%
1. Module wiring : 0.69%
2. Array to Junction box: 0.72%
3. Junction box to Combiner box: 1.82%
4. Combiner box to Charge controller: 0.43%
5. Charge Controller to Disconnects: 0.26%
6. Disconnects to Inverter: 0.26%
For Ac Circuits: total: 1&2 = 0.36%
1. Inverter to disconnect: 0.06%
2. Disconnect to service panel: 0.3%
Over all Dc & Ac =4.18%+0.36% =4.54%
Example: Calculate wire size between the Junction box to combiner box at a distance 40 feet at a site. Consider the 24-volt Dc system, module Imp current =7.0 amp (printed on module).
From above point no 1, voltage drop from junction box to combiner is 1.82%
Voltage drop/system voltage =% loss in voltage
Voltage drop = 1.82x24/100=0. 4368
Voltage drop = 2 x L x Imp x resistance of wire.
Where L is distance between Junction box to Combiner box.
Resistance of wire = Voltage drop/(2x40x7) =0.00078 ohms
The resistance of wire is measured in ohms/1000Feet.
So the resistance of 1000 Feet wire is 0.78 ohms.

From NEC table the resistance of stranded wires per 1000 Feet is as follow.
1. 14AWG = 3.140 ohms 2. 12AWG =1.980 ohms 3. 10AWG = 1.240 ohms
4.8AWG= 0.778ohms 5. 6AWG=0.491ohms
By comparing, 8SWG wire is suitable for our application.