How to Wire Solar Panels in Series: Safe Steps, Diagrams & Calculations

When you wire solar panels in series, you connect the positive (+) of one panel to the negative (−) of the next. In a series string, voltages add while current stays (roughly) the same as a single panel. This is the most common method when you need higher voltage to reduce wire losses or to meet the input requirements of an MPPT charge controller or grid-tied inverter.

Series wiring is typically the right choice when you want to:

• Increase array voltage so the system can use smaller gauge wire over longer distances with less loss.
• Meet a controller/inverter requirement such as minimum MPPT voltage (especially in cold weather).
• Build strings for an inverter that expects a higher DC input (often hundreds of volts DC in grid-tied designs).

The main tradeoff: in a series string, shading or a weak panel can reduce the output of the entire string. Bypass diodes help, but good layout and avoiding partial shading remain important.

Series wiring rules: the voltage math you must check first

Before connecting anything, confirm the string voltage will stay within the safe limits of your equipment across temperature extremes. The two key panel specifications are: Voc (open-circuit voltage) and Vmp (voltage at maximum power). You must ensure the string’s cold-weather Voc does not exceed the controller/inverter’s maximum input voltage.

Quick calculation for series strings

For series wiring: String Voc ≈ sum of each panel’s Voc and String Vmp ≈ sum of each panel’s Vmp. Current stays close to one panel’s current (Imp/Isc), so series affects voltage far more than amps.

Example with real numbers

Suppose each panel label shows Voc = 38.0V and Vmp = 31.5V. With 3 panels in series: String Voc ≈ 114.0V and String Vmp ≈ 94.5V.

If your MPPT controller max input is 150V, 114V looks safe at room temperature, but cold can raise Voc. Many panel datasheets include a Voc temperature coefficient (often around −0.28% to −0.35%/°C). If you do not have the coefficient handy, treat cold-weather Voc as potentially 10–20% higher than the label value and confirm with the datasheet before finalizing your string length.

Key takeaway: design series strings from the controller/inverter backward. Max input voltage is a hard limit; exceeding it can permanently damage equipment.

Tools and components you need for a clean, reliable series connection

A correct series build is mostly about using the right connectors and protection. Avoid improvising with household wiring parts; PV strings can carry dangerous DC voltage and require components rated for DC use.

Core items

• PV-rated connectors (commonly MC4-type) that match your panel leads and are from compatible families.
• PV wire (commonly 10 AWG or 12 AWG, sunlight/UV rated) sized for your current and distance.
• A DC-rated multimeter with probes in good condition.
• A DC disconnect or breaker (often near the controller/inverter) for safe servicing.

Protection and combining (as needed)

• String fuses or breakers if you have two or more strings in parallel (series-only single string often does not require string fuses, but follow local code and equipment instructions).
• Combiner box if multiple strings are being merged.
• Surge protection (SPD) where required or recommended, especially for exposed arrays.

Compatibility note: many installers avoid mixing connector brands because “MC4” is often used as a generic term. For best reliability and safety, use matched connector types or manufacturer-approved equivalents.

Step-by-step: how to wire solar panels in series (practical procedure)

This procedure assumes identical panels and typical PV connectors. Work methodically, and treat the array as live whenever exposed to light. If you are not comfortable working with DC power, hire a qualified installer.

1) Identify each panel’s positive and negative leads

Confirm polarity on the panel label and lead markings. If anything is unclear, verify with a multimeter in daylight. Correct identification prevents reverse polarity that can trip protection or damage equipment.

2) Connect Panel 1 (+) to Panel 2 (−)

Plug the connectors firmly until they click/lock. This creates your first series link. Continue the same pattern down the line.

3) Repeat until the string is complete

For three panels: Panel 1 (+) to Panel 2 (−), then Panel 2 (+) to Panel 3 (−). The remaining free ends become your string outputs: String negative is Panel 1 (−) and String positive is Panel 3 (+).

4) Measure the string voltage before connecting to electronics

With the string disconnected from the controller/inverter, measure voltage across the final positive and negative ends. Expect a reading close to calculated String Voc in bright sun. If the measured value is drastically different, stop and re-check every connection and polarity.

5) Connect the string to a DC disconnect, then to the controller/inverter

Use a DC-rated disconnect/breaker so you can isolate the array. Follow the manufacturer’s wiring order; many charge controllers specify connecting the battery first so the controller can power up and configure safely before PV input is applied. The key outcome is the same: do not hot-plug unpredictably—use a disconnect and follow the manual.

• Keep series string wiring neat, secured, and off sharp edges.
• Maintain correct polarity from the string all the way to PV+ and PV− terminals.
• Label string voltage and disconnect location for future service.

Series vs. parallel: what changes electrically and why it matters

Many systems use series strings because higher voltage reduces current for the same power. This often lowers wire losses and can reduce conductor size and cost (within code constraints).

A practical power-loss illustration

Assume you want to transmit 600W from an array to a controller. At 60V, current is about 10A (600W ÷ 60V). At 120V, current is about 5A. Because resistive wire loss scales as I²R, cutting current roughly in half can reduce voltage drop and heating significantly over long runs.

In practice, many arrays use series strings to hit a target voltage, then connect multiple strings in parallel to increase total current and power. The design objective is to land within the equipment’s voltage window while balancing shading risk, wiring cost, and safety.

Safety and code-adjacent essentials for series strings

Series wiring can produce hazardous DC voltage quickly. Even relatively small arrays can exceed 100V DC, which increases shock risk and arc potential. Use hardware rated for PV DC service and follow local electrical codes and manufacturer instructions.

Minimum safety practices

• De-energize where possible: cover modules or work at dawn/dusk, and use a DC disconnect.
• Never unplug connectors under load. DC arcs are persistent; open the disconnect first.
• Verify polarity with a meter before landing conductors on PV+ / PV− terminals.
• Use PV-rated cable management, strain relief, and weatherproofing to prevent insulation damage and water ingress.

Overcurrent protection: when fuses matter

In a single series string feeding one controller input, each module’s current is limited by the string current, so string fusing may not be required. However, when you have multiple strings in parallel, one string can backfeed fault current into another, so string fuses/breakers are commonly used. Always follow the controller/inverter manual and the applicable electrical code for your jurisdiction.

Troubleshooting: common series wiring mistakes and fast checks

If the system underperforms after wiring solar panels in series, the cause is often a simple connection, polarity, or expectation mismatch. Use these checks to isolate issues quickly.

Symptom-based checklist

• Measured string voltage is near zero: likely a reversed connection creating a canceling loop, a connector not fully seated, or a broken lead.
• Voltage is correct but power is low: check partial shading, soiling, or a panel with a failed bypass diode; confirm all panels are the same model/spec.
• Controller shows PV overvoltage: string has too many panels in series for the temperature conditions; reduce series count or use equipment with a higher input rating.
• Intermittent faults: suspect a loose connector, water ingress, or cable strain at module junction boxes.

A simple validation routine

1. Measure each panel’s Voc individually in similar sunlight; note any panel that is significantly off.
2. Connect the series string and measure total Voc; confirm it matches the expected sum within a reasonable margin.
3. If available, use the controller display/app to confirm the operating voltage sits near calculated String Vmp during good sun.

If your readings do not align with the basic series math, stop and correct the wiring before connecting to expensive electronics. Most failures are preventable with pre-connection voltage checks.

Practical design tips for better performance with series strings

Good series-string performance is as much about layout as it is about wiring. These practical steps reduce losses and avoid avoidable derates.

Keep modules in the same sun conditions

In a series string, one shaded panel can drag down the string. Place panels in the same plane and avoid mixing modules exposed to different shade patterns (chimneys, trees, dormers) within the same series chain.

Minimize voltage drop on the home run

Higher series voltage helps, but you should still aim for a low voltage drop. As a rule of thumb, many designers target ≤ 3% voltage drop for array wiring. Achieving that may require shorter runs, larger wire, or a higher-voltage string within equipment limits.

Use matched panels in each string

For predictable results, build each series string from the same model and similar age. Mixing panels with different current ratings can force the entire string to operate closer to the weakest panel’s capability.

Bottom line: if you want fewer surprises, prioritize correct voltage design, consistent irradiance per string, and verification measurements before energizing the controller or inverter.