Sizing Wires for PV Systems
Introduction
Choosing the right wire sizes in your PV system is important for both performance and safety reasons.
- If the wires are undersized, there will be a significant voltage drop in the wires resulting in excess power loss.
- In addition, if the wires are undersized, there is a risk that the wires may heat up to the point in which a fire may result.
An electrical wire carries current much as a water hose carries water. The larger the diameter of the water hose, the less resistance there is to water flow. Moreover, even with a large diameter hose, shorter hoses have better flow than longer hoses. Longer hoses have more resistance than shorter ones of the same diameter. Electrical wires behave in the same manner. If your electrical wires (the copper gauge) are not large enough or if the cable is longer than needed, then the resistance is higher resulting in less watts going to either your battery bank or the grid.
Copper wires are sized using the gauge scale: American Wire Gauge (AWG). The lower the gauge number, the less resistance the wire has and therefore the higher current it can handle safely.
The following chart "Electrical cable size chart amps" shows the ampacity for wires in a conduit per NEC 310.17 Table Rated 90℃ (194℉).
Electrical cable size chart amps
Taking 10-gauge wire as an example, the recommended current carrying capacity can reach 55A for lengths shorter than 18ft. However, when the length reaches 60ft, the recommended current carrying capacity drops to only 18-24A.
Meter ⇔ Feet Converter
When selecting which wire gauge to connect use it is also important to consider:
- How long the cable needs to be?
- How many continuous Amps goes through the wire?
Between Solar Panel and Charge Controller (Solar Adaptor Kit)
Solar Adaptor Kit (Model: RNG-AK, sold in pairs)
Formula to calculate the current capacity required for the wire: Wire Amp Rating ≥ Number of solar panels in parallel × Short Circuit Current (Isc) Amps*1.25*1.25
Round up the result and take the wire length into consideration.
EXAMPLE：
Let's say if we have three 200W panels connected in parallel, each producing 20 volts and the rated short circuit current (Isc) is 10 amps, the total output would be 20 volts and 30 amps.
Parallel Volts = V1 = V2
Parallel Amps = A1 + A2
Formula: Wire Amp Rating ≥ Number of solar panels in parallel × Short Circuit Current (Isc) Amps*1.25*1.25
In this case, Wire Amp Rating ≥ 3 × 10A*1.25*1.25.
It needs to be no smaller than 46.88A.
If the distance between the solar panel array and the charge controller is 13ft, 10 gauge wires would be the right size to use by referring to the "Electrical cable size chart amps" chart.
Between Charge Controller and Battery Bank (Tray Cable)
Tray Cable (Model: RNG-TRAYCB, sold in pairs)
Formula to calculate the current capacity required for the wire: Wire Amp Rating ≥ Charge Controller Rated Amps*1.25
Between Battery Bank and Inverter
Battery/Inverter Cable (Model: RNG-INVTCB)
Formula to calculate the current capacity required for the wire: Wire Amp Rating ≥ Inverter Continuous Power Rating / % Peak Efficiency / System Voltage*1.25
≥ 1000W / 0.90 / 12V*1.25
Round up the result and take the wire length into consideration.
In between Batteries
For the battery, first identify the battery's Max Continuous Charge/Discharge Current.
Battery Interconnect Cable (Model: RNG-BATTERYCB)
Formula to calculate the current capacity required for the wire: Wire Amp Rating ≥ Battery Max Continuous Charge/Discharge Current*1.25
Round up the result and take the wire length into consideration.
