To wire multiple solar panels, choose series if your charge controller is an MPPT with a high voltage limit (up to 100V-150V) and you want to minimize voltage drop over long wire runs using 10 AWG PV wire. Choose parallel if you have a PWM controller, severe partial shading issues, or a low-voltage (12V/24V) battery bank. For arrays over four panels, use series-parallel to balance voltage and current. The default recommendation for modern off-grid and hybrid systems is series wiring into an MPPT charge controller using 10 AWG stranded PV wire.

The Direct Answer: Series, Parallel, or Series-Parallel?

Before cutting any wire, you must match your array configuration to your charge controller type and site conditions. Wiring panels in series adds their voltages while keeping the current (amps) the same. Wiring them in parallel adds their current while keeping the voltage the same. Use the decision matrix below to lock in your configuration.

System Condition Wiring Choice Concrete Default Pick
MPPT Controller + Minimal Shading + Long Wire Run (>30 ft) Series String of 3x 100W panels (Voc ~66V) on 10 AWG wire
PWM Controller OR Heavy Partial Shading (trees, chimneys) Parallel 3x 100W panels using MC4 Y-branch connectors with inline 15A fuses
Large Array (>4 panels) or High Power (>800W) Series-Parallel 4x 200W panels wired as 2 series strings of 2, then paralleled via a busbar

The Bottom Line: If you are buying new equipment today, buy an MPPT controller (like the Victron SmartSolar MPPT 100/30) and wire your panels in series. It is more efficient, requires thinner wire, and handles cold-weather voltage spikes better than parallel PWM setups.

Tools, Materials, and Wire Sizing

Solar arrays operate in harsh outdoor environments. Standard indoor wire will degrade under UV exposure and fail. Here is the exact bill of materials for a standard 2-panel series string feeding a 12V/24V battery bank.

Pro-Tip on Wire Sizing: Always use the Short Circuit Current (Isc) rating from the panel sticker, not the operating current (Imp), when sizing wire and fuses. NEC Article 690 requires a 125% safety multiplier on Isc for continuous DC loads.
  • PV Wire: 10 AWG stranded copper PV wire (USE-2 rated, 600V, sunlight resistant). Buy Red and Black to maintain polarity.
  • Connectors: MC4 Male and Female connectors (e.g., Renogy or BougeRV).
  • Charge Controller: Victron SmartSolar MPPT 100/30 (Max 100V PV input, 30A battery output).
  • DC Breaker (Battery Side): 30A DC-rated miniature circuit breaker (MCB) or inline ANL fuse.
  • Battery Wire: 8 AWG THHN stranded copper (Red and Black) for the run between the controller and the battery busbars.
  • Specialty Tools: Dedicated MC4 ratcheting crimping tool (do not use standard pliers), wire strippers, digital multimeter, and a DC clamp meter.

Mains and DC Safety Pre-Wiring Checklist

WARNING: Lethal Voltage and Arc Flash Hazards
Solar panels generate live DC voltage the moment sunlight hits them; you cannot 'turn off' a panel. Treat all exposed MC4 pins as live. Furthermore, if your system includes an AC inverter feeding a home subpanel, you are dealing with lethal AC mains voltage.

AC Mains Procedure: De-energize the main AC breaker feeding the subpanel. Lock/tag it out. Verify dead with a non-contact voltage tester and a multimeter across the busbars (expect 0V AC) before terminating the inverter AC output.
DC Battery Procedure: De-energize the battery bank by disconnecting the negative terminal first. Verify dead with a tested multimeter across the battery busbars. Never connect the solar array to the charge controller before connecting the controller to the battery; the controller needs the battery voltage to initialize its logic board.

Step-by-Step: Wiring the Panels to the Charge Controller

These steps assume a 2-panel series configuration feeding an MPPT controller. Ensure all panel surfaces are covered with a blanket or cardboard while terminating wires to prevent live DC shocks.

  1. Link the Panels in Series: Take the Male MC4 connector (usually the positive/red wire) extending from Panel 1 and plug it directly into the Female MC4 connector (usually the negative/black wire) extending from Panel 2. You should hear a definitive click.
  2. Prepare the Extension Cables: You now have one unused Female MC4 on Panel 1 (Negative) and one unused Male MC4 on Panel 2 (Positive). Strip 1/4 inch of insulation off your 10 AWG Red and Black PV extension wires.
  3. Crimp the MC4 Pins: Insert the stripped 10 AWG Red wire into a Male MC4 pin and crimp using the dedicated ratcheting tool. Insert the 10 AWG Black wire into a Female MC4 pin and crimp. Snap these into their respective MC4 housings until they click.
  4. Terminate at the Charge Controller (PV Side):
    • Take the Red PV wire (Positive from Panel 2) and terminate it under the PV+ screw terminal on the charge controller. Torque to 0.5 Nm.
    • Take the Black PV wire (Negative from Panel 1) and terminate it under the PV- screw terminal on the charge controller. Torque to 0.5 Nm.
  5. Wire the Controller to the Battery:
    • Connect an 8 AWG Black THHN wire from the battery's negative busbar directly to the controller's BAT- screw terminal.
    • Connect an 8 AWG Red THHN wire from the battery's positive busbar, route it through the 30A DC breaker (breaker OFF), and terminate the other end at the controller's BAT+ screw terminal.
  6. Power Up: Flip the 30A DC breaker to the ON position. The charge controller screen or Bluetooth LED should illuminate, indicating it has detected the battery bank.

Verify and Test: Expected Meter Readings

Do not skip the verification step. Assuming the wiring is correct without testing is how expensive MPPT controllers get fried on a sunny afternoon.

  1. Test Panel Open Circuit Voltage (Voc): Set your multimeter to DC Volts. Before plugging the PV extension cables into the controller, touch the Red probe to the Red MC4 pin and the Black probe to the Black MC4 pin. Expected Reading: For two standard 100W panels in series, expect between 40V and 44V DC (depending on cloud cover and temperature). If you read ~21V, your panels are in parallel or one is disconnected. If you read 0V, check your MC4 crimps.
  2. Verify PV Polarity: Ensure the multimeter reads a positive number when the Red probe is on the Red wire. If it reads a negative number (e.g., -42V), your MC4 gender pins are swapped. Reverse them before connecting to the controller.
  3. Test Charge Current: Once the controller is live and the sun is hitting the panels, clamp your DC clamp meter around the Red PV+ wire. Expected Reading: For two 100W panels, expect roughly 9A to 11A under peak noon sun.
  4. Check Battery Charging Voltage: Measure across the battery busbars with your multimeter. Expected Reading: 13.8V to 14.4V DC for a 12V lead-acid/AGM bank in the absorption phase, or roughly 13.5V for LiFePO4.

The Most Common Botch: The MC4 Crimp and Polarity Reversal

In my years of troubleshooting solar installs, 90% of field failures trace back to two specific mistakes made during the initial wiring phase.

Botch 1: Using Standard Pliers for MC4 Crimps

The Symptom: The system works fine for a week, then you notice a severe voltage drop under load, or worse, the MC4 connector physically melts and fuses together.
The Cause: Standard wire strippers and pliers do not compress the MC4 metal pin evenly around the 10 AWG stranded wire. This creates a high-resistance joint. Under a 10A continuous load, that resistance generates heat (I²R losses), melting the plastic housing and causing an arc flash.
The Fix: Only use a dedicated MC4 ratcheting crimping tool (like the IWISS SN-28B or Renogy crimp kit). The ratchet ensures the exact factory-spec compression depth every time. Give the wire a firm tug after crimping; it should not pull out.

Botch 2: Reversing PV Polarity into the Controller

The Symptom: You plug the PV wires into the controller, hear a pop, and the controller is permanently dead.
The Cause: You wired PV+ into PV- and PV- into PV+. While some modern controllers have reverse-polarity protection, many budget MPPTs will instantly blow their internal blocking diodes or fry the DC-DC converter MOSFETs if hit with 40V+ in reverse.
The Fix: Never trust the color of the wire coming directly out of the solar panel junction box—manufacturers sometimes swap internal wiring. Always test the raw panel leads with a multimeter to identify the true positive and negative before attaching your extension MC4 connectors.

For further reading on NEC compliance and temperature coefficient calculations for cold climates, refer to the Solar-Electric wiring guides and the Renogy technical blog on array configurations.