Hooking up solar panels directly to a battery will overcharge, overheat, and permanently destroy the cells. To safely charge a battery bank, you must route the DC current through a charge controller that regulates voltage and current. This guide provides a complete wiring diagram walkthrough for a standard 12V off-grid system: a 200W solar array feeding a Victron SmartSolar MPPT 75/15, which charges a 100Ah LiFePO4 battery, ultimately powering a 1000W inverter.
The direct answer for wire sizing in this 15A controller setup is 10 AWG copper for the charge controller connections, and 2/0 AWG for the battery-to-inverter run. Below, we will trace the exact physical connections, decode the schematic symbols, and verify the system with a multimeter before throwing the final switch.
Decoding Diagram Symbols and Physical Terminals
Before cutting any wire, you must map the abstract symbols on your schematic to the physical screw terminals on the hardware. In a standard solar wiring diagram, you will see a square with plus/minus signs (PV array), parallel horizontal lines of varying lengths (battery), a lightbulb or resistor zigzag (load), and three descending horizontal lines (earth ground).
The table below maps these diagram symbols directly to the physical terminals on the Victron SmartSolar MPPT 75/15, including the exact wire gauges, torque specifications, and overcurrent protection required by Victron's Wiring Unlimited guidelines and standard NEC-style DC practices.
| Diagram Symbol | Physical Terminal Label | Wire Gauge (Copper) | Torque Spec | Overcurrent Protection |
|---|---|---|---|---|
| PV Array (Square with +/-) | PV + and PV - | 10 AWG THHN | 1.5 Nm (13 in-lbs) | 15A DC Breaker (Positive leg) |
| Battery Bank (Parallel lines) | BAT + and BAT - | 10 AWG THHN | 1.5 Nm (13 in-lbs) | 15A DC Breaker (Positive leg) |
| DC Load (Lightbulb icon) | LOAD + and LOAD - | 12 AWG THHN | 1.0 Nm (9 in-lbs) | Inline fuse per load spec |
| Earth Ground (3 descending lines) | Chassis Ground Lug | 8 AWG Bare Copper | 2.0 Nm (18 in-lbs) | None (Grounding electrode) |
Node-by-Node Trace: Source to Load
With the terminals identified, we will trace the physical path of the electrons and the grounding path. Always build the system backward from the battery to the panels, but trace the diagram forward from the source to the load.
Node 1: PV Array to Charge Controller
- Source: The 200W solar panel outputs raw, unregulated DC. For a single 200W panel, the open-circuit voltage (Voc) is roughly 22.4V, and the short-circuit current (Isc) is about 11.5A.
- Conductors: Run 10 AWG UV-rated PV wire from the panel's MC4 connectors down to the combiner box or charge controller location.
- Polarity & Protection: Route the positive (red) PV wire through a 15A DC-rated solar breaker. The breaker must be placed as close to the charge controller as practical. The negative (black) PV wire routes directly to the controller's PV- terminal, bypassing the breaker.
- Termination: Strip 1/2 inch of insulation, crimp on ferrules to prevent stranded wire splaying, and torque the PV+ and PV- screws to 1.5 Nm.
Node 2: Charge Controller to Battery Bank
- Battery Connection: Connect the 10 AWG red wire from the controller's BAT+ terminal to the positive terminal of the 100Ah LiFePO4 battery. Route this through a second 15A DC breaker.
- Negative Return: Connect the 10 AWG black wire from the controller's BAT- terminal directly to the battery's negative terminal or a negative busbar.
- Equipotential Bonding: The DC negative in this specific off-grid setup is typically left floating at the battery, or bonded to chassis ground at exactly one point (usually the battery negative busbar) to prevent ground loops. Do not bond neutral and ground at multiple points.
Node 3: Battery to Inverter and Ground Path
- Inverter Feed: The 1000W inverter will pull up to 85A continuously (1000W / 12V = 83.3A, plus inverter inefficiency). Run 2/0 AWG pure copper battery cables from the battery terminals to the inverter.
- Main Overcurrent Protection: Install a 150A Class T fuse on the positive 2/0 AWG cable within 7 inches of the battery positive terminal. This protects the massive cable run from a dead short.
- The Ground Path: Run an 8 AWG bare copper Equipment Grounding Conductor (EGC) from the solar panel aluminum frame, down to a grounding rod driven into the earth (for ground mounts) or to the vehicle/RV chassis ground (for mobile setups). This path is strictly for lightning dissipation and fault clearing; it carries zero current during normal operation.
Verifying Connections with a Multimeter
Before closing any breakers, you must verify the wiring with a digital multimeter (DMM). Set your meter to DC Voltage (V⎓) and follow this diagnostic sequence.
Step 1: Verify PV Open-Circuit Voltage (Voc)
Leave the PV breaker OFF. Place your red probe on the incoming PV+ wire and your black probe on the PV- wire. In full sunlight, a single 200W panel should read between 21V and 23V. If you read 0V, check your MC4 crimps. If you read a negative voltage, your polarity is reversed. Do not proceed if polarity is reversed.
Step 2: Verify Battery Resting Voltage
Leave the battery breaker OFF. Place your probes directly on the battery terminals. A healthy, fully charged 12V LiFePO4 battery will read between 13.4V and 13.6V. If it reads below 12.0V, the battery is deeply discharged and the MPPT controller may refuse to wake up; you will need to top-charge it with an AC-to-DC smart charger first.
Step 3: Energize and Check Charging Voltage
- Close the Battery breaker first. The Victron MPPT LED should illuminate, indicating it has detected the 12V system profile.
- Close the PV breaker. The controller will begin MPPT sweeping.
- Place your meter probes on the battery terminals. You should now see the voltage climb to the Absorption setpoint (typically 14.2V to 14.4V for LiFePO4). If the voltage stays pinned at 13.4V and the solar input shows 0W, check your PV breaker and ensure the panel isn't shaded.
Edge Cases: Cold Weather Voc and Inverter Inrush
Wiring diagrams on paper assume standard test conditions (STC) of 25°C (77°F). Real-world installations require adjusting for temperature extremes and inductive loads.
The Cold Weather Voc Spike
Solar panel voltage increases as temperature drops. The temperature coefficient of Voc for most monocrystalline panels is roughly -0.29% per degree Celsius. If you wire two 200W panels in series for a 24V system, the STC Voc is about 44.8V. However, if you install this system in Minnesota where winter mornings hit -20°C, the Voc will spike to over 51V. If you are using a charge controller with a strict 50V maximum input limit, this cold-weather spike will permanently destroy the controller's internal MOSFETs. Always calculate your worst-case historical low temperature and ensure the cold-adjusted Voc remains at least 10% below the controller's absolute maximum input rating.
Inverter Inrush and Breaker Nuisance Tripping
When you connect a 1000W inverter to the battery, the inverter's internal DC bus capacitors are completely empty. They will draw a massive, instantaneous inrush current—sometimes exceeding 400A for a few milliseconds—to charge up. If your battery management system (BMS) is rated for exactly 100A continuous with a strict 150A trip threshold, this inrush can trip the BMS, shutting down the battery before the inverter even turns on. To prevent this, use an inverter with a built-in pre-charge circuit, or install a pre-charge resistor inline to slowly charge the capacitors before closing the main 150A Class T fuse.
For comprehensive code compliance, always cross-reference your specific installation with NFPA 70 (National Electrical Code) Article 690, which governs solar photovoltaic systems, as local authorities having jurisdiction (AHJ) will require specific rapid-shutdown devices and disconnect placements that vary by municipality.
