Voltage Drop CalculatorSolar wire size calculator
Wire size for solar runs by voltage drop: panels to the charge controller or inverter, and the controller to the battery. Enter the string’s Vmp and Imp and the one-way length.
Smallest wire for 2%
Copper| Wire size | 8 AWG |
| Metric, same or larger | 10 mm² |
| Area | 8.37 mm² |
| Voltage drop | 1.03 V |
| Percent drop | 1.43 % |
| At the load | 70.97 V |
Within 2% target
At other limits
- 8 AWG at 2%
- 10 AWG at 3%
- 16 AWG at 10%
Worked out with
- Copper at 75 °C (K = 12.9)
- 60 ft one way; the return wire is added
- 11 A, the current the load draws
- DC, 72 V
8 AWG. Drop 1.03 volts, 1.43 percent. Within 2% target.
Result from https://voltagedropcalculator.app/solar-wire-size-calculator?phase=dc&voltage=72¤t=11&length=60&material=copper&target=2&mode=find. This checks voltage drop only, not code compliance.
- The fuse or breaker doesn't get bigger. Voltage drop sets the wire size, not the protection. Keep the fuse the circuit needs.
- Check the wire fits. Look up the largest wire the terminals and lugs take. If 8 AWG won't fit, ask an electrician how to connect it where you live.
- Cheaper ways to the same result: the same power at 144 V (half the current) needs 14 AWG.
How this is worked out
V = 2 × K × I × L ÷ CM
2 × 12.9 × 11 A × 60 ft ÷ 16,510 CM = 1.03 V
1.03 V ÷ 72 V × 100 = 1.43 %. At the load: 72 V − 1.03 V = 70.97 V.
- L is the one-way length; the 2 counts the return conductor.
- K is 12.9 for copper and 21.2 for aluminum, in ohm·CM per foot on a 75 °C basis.
- Not checked here: temperature derating, conduit fill, power factor and code compliance.
Which numbers to enter
- Voltage: the string’s Vmp (maximum power voltage). For panels in series, add their Vmp; in parallel, it stays one panel’s Vmp. For the controller-to-battery run, use the battery voltage.
- Current: the string’s Imp (maximum power current). For strings in parallel, add their Imp. Don’t use Isc or Isc × 1.56 here: those are for the ampacity check.
- Length: one way, from the array to the controller. The calculator counts the return wire.
- Target: 2% is the usual DC target. The NEC sets no voltage-drop limit for PV, Morningstar sizes for 2% or less, and Victron for 2.5% or less.
Worked example: two panels, 60 ft away
Two panels, each with Vmp 36 V and Imp 11 A (example values; use your panel’s label), 60 ft one way from the charge controller, at 2%:
| Wiring | Smallest size | Drop |
|---|---|---|
| Parallel: 36 V, 22 A | 3 AWG | 1.80 % |
| Series: 72 V, 11 A | 8 AWG | 1.43 % |
Series wiring doubles the voltage and halves the current for the same power, so the drop falls and 8 AWG does the job of 3 AWG. Before choosing series, check that the string’s open-circuit voltage on the coldest morning stays under the controller’s maximum input voltage. The calculator above starts with the series case.
Controller to battery
This run is low voltage and high current. A 12 V battery charging at 50 A from a controller 6 ft away needs 4 AWG at 2% (1.55 %). Victron advises mounting the controller within about 3 m (10 ft) of the battery and keeping the cables short, because drop on this run leaves the battery undercharged. Many controllers also have a maximum terminal size, so check it before buying cable.
Ampacity: a separate check
Voltage drop sets the size on long runs; heat sets it on short ones. In the US, the NEC takes the maximum PV circuit current as 1.25 × the panels’ Isc and asks the conductor to carry 125% of that before derating (the familiar 1.56 × Isc), or the full maximum current after temperature and conduit-fill derating, whichever needs more. Check the size this page gives against that rating too. The ampacity guide explains which check wins.
Mistakes that cost energy
- Sizing for ampacity only. A wire that is “rated” for the current can still lose several percent on a long ground-mount run.
- Putting Isc × 1.56 into the drop formula. It gives a bigger wire than the drop needs; use Imp for voltage drop.
- Entering the round trip. Enter the one-way length; the return wire is counted.
Exposed wiring in the array uses cable made for it, such as PV wire or USE-2 in the US. Follow the panel and controller manuals and your local code.
Frequently asked questions
Which current and voltage do I use for solar voltage drop?
The operating values at the maximum power point: the string’s Imp and Vmp from the panel label. Voltage drop is about the power the array actually delivers. Isc and Voc are for the safety checks: the conductor’s ampacity and the controller’s maximum input voltage.
What is the 1.56 factor for solar wire?
It is an ampacity check, not a voltage-drop input. The NEC takes the maximum PV circuit current as 1.25 times the panels’ short-circuit current (Isc), and the conductor must carry 125% of that before derating, so 1.25 × 1.25 = 1.56 × Isc. Check the wire’s rating against that; size for voltage drop with Imp.
What voltage drop is acceptable for solar panels?
The NEC sets no limit for PV circuits. 2% on the DC side is the usual rule of thumb; Morningstar sizes for 2% or less and Victron for 2.5% or less, and inverter makers often allow about 5% for the whole system.
Does wiring panels in series reduce wire size?
Yes. Series wiring adds the voltages and keeps the current of one panel, so the same power flows at a lower current and the drop falls. In the example on this page, two panels 60 ft away need 3 AWG in parallel but only 8 AWG in series. Keep the cold-weather open-circuit voltage under the controller’s maximum input.
How long can the cable from the controller to the battery be?
As short as you can make it. That run is low voltage and high current, so a few feet matter. Victron says to mount the controller close to the battery, within about 3 m (10 ft), and that drop there leaves the battery undercharged.
Do I enter the one-way or round-trip length?
One way, from the array to the controller (or the controller to the battery). The calculator counts the return wire. Entering the round trip doubles the drop.
Related: 12V wire size calculator, boat wire size calculator, voltage drop chart, wire gauge chart (AWG to mm²), how the calculator works.