Voltage Drop in Solar Systems: Panel, Battery & Inverter Wiring Guide

Voltage Drop in Solar Panel Arrays

Solar panel arrays present unique voltage drop considerations because they're both the power source and subject to varying output depending on conditions. Understanding how panels behave helps optimize system wiring.

Panel Electrical Characteristics

Solar panels have several important voltage specifications:

Voc (Open Circuit Voltage): Maximum voltage with no load connected, typically 40-50V for a 12V-nominal panel
Vmp (Maximum Power Voltage): Operating voltage at maximum power point, typically 18-20V for a 12V-nominal panel
Imp (Maximum Power Current): Operating current at maximum power, typically 5-10A per panel
Isc (Short Circuit Current): Maximum current with outputs shorted, typically 5-15% higher than Imp

For voltage drop calculations, use Imp (maximum power current) as this represents normal operating conditions. However, wire must be rated for Isc (short circuit current) for safety.

Series vs. Parallel Connections

How panels are connected affects both voltage and current in the wiring:

Series strings: Voltage adds up while current stays the same as a single panel. A string of 4 panels at 40V Vmp each produces 160V but only the current of one panel (say 10A). Higher voltage means lower current for the same power, reducing voltage drop concerns.

Parallel connections: Current adds up while voltage stays the same. Four panels in parallel produce the voltage of one panel but 4× the current. Higher current increases voltage drop, requiring larger wire.

For long runs from panels to charge controller, series strings are generally preferred because the higher voltage allows smaller wire with acceptable voltage drop.

Temperature Effects on Panel Voltage

Panel voltage decreases as temperature increases—approximately 0.3-0.5% per degree Celsius above 25°C (77°F). On a hot summer day, a panel rated at 40V Voc at standard test conditions might produce only 35V. This reduction in source voltage makes the wire's voltage drop a larger percentage of the available voltage. Account for temperature effects when calculating acceptable voltage drop margins.

Panel to Charge Controller Wiring

The connection from the solar panel array to the charge controller is often the longest wire run in the system, making voltage drop an important consideration.

Voltage Drop Impact on MPPT Controllers

Modern MPPT (Maximum Power Point Tracking) charge controllers constantly adjust their input characteristics to extract maximum power from the panels. They can partially compensate for voltage drop in the panel wiring by operating at a slightly different point on the panel's power curve. However, excessive voltage drop still reduces system efficiency and available charging current.

For this connection, a common recommendation is to limit voltage drop to 2-3% of the panel array's Vmp (maximum power voltage). This keeps the MPPT controller operating efficiently while allowing reasonable wire sizes for long runs.

Example Calculation

Consider a system with:

4 panels in series: 4 × 40V = 160V Vmp
Panel current: 10A Imp
Distance from panels to controller: 50 feet
Target: 2% maximum voltage drop (3.2V)

At 160V, the relatively low current and high voltage make wire sizing straightforward. Using 10 AWG copper (1.24 Ω/kft):

VD = (2 × 50 × 10 × 1.24) / 1000 = 1.24V (0.78%)

10 AWG is more than adequate for this application. The high string voltage makes voltage drop much less problematic than in low-voltage portions of the system.

Wire Type Requirements

Panel wiring must be rated for outdoor use and exposure to sunlight. Common options include:

PV Wire: Specifically designed for solar applications, UV resistant, rated to 600V or higher
USE-2: Underground service entrance cable, suitable for exposed outdoor use
Outdoor-rated conduit with THWN-2: Protected wiring in conduit

Charge Controller to Battery Bank

The wiring between the charge controller and battery bank operates at battery voltage (typically 12V, 24V, or 48V) and carries the full charging current. This connection requires careful sizing due to the lower voltage and higher current compared to the panel side.

Critical for Battery Charging

The charge controller measures battery voltage through this wiring. If significant voltage drop exists in these wires, the controller sees a lower voltage than the batteries actually have. This can cause:

Overcharging: Controller thinks batteries need more charge than they do
Premature bulk-to-absorb transition: Controller switches charging stages too early
Inaccurate state-of-charge readings: Controller's battery monitoring is wrong

For this reason, the controller-to-battery connection should be kept as short as possible with voltage drop under 1-2%.

Wire Sizing Guidelines

Consider a 60A MPPT charge controller output to a 12V battery bank located 6 feet away:

Target: 1% voltage drop (0.12V at 12V nominal)
Using the voltage drop formula: R ≤ (0.12 × 1000) / (2 × 6 × 60) = 0.167 Ω/kft
Required wire: 2/0 AWG (0.0967 Ω/kft) or larger

Even for short runs, high charging currents require substantial wire gauge. This is why locating the charge controller close to the batteries is strongly recommended.

Battery Bank Interconnects

Within the battery bank, all interconnecting cables should be the same length and gauge to ensure equal current sharing between parallel strings. Unequal cables cause unequal charging and discharging, reducing battery life and capacity. Use short, heavy cables for all battery interconnections.

Battery to Inverter Wiring

The battery-to-inverter connection carries the highest current in most solar systems and is the most critical for voltage drop. At 12V or 24V with a large inverter, currents can exceed 200A, demanding very heavy cabling.

Why This Connection Matters Most

An inverter converts DC battery power to AC at a fixed output voltage (120V or 240V). To deliver constant AC power, the inverter must draw varying DC current as battery voltage fluctuates. When battery voltage drops due to discharge or voltage drop in the cables, the inverter draws even more current to maintain output, creating a feedback loop that can cause problems.

Excessive voltage drop in inverter cables causes:

Reduced inverter efficiency (more power lost in cables)
Lower maximum inverter output (hits current limits before power limits)
Premature low-voltage shutdown (inverter sees lower voltage than batteries)
Cable heating and potential fire hazard at high currents

Current Calculations

To find the DC current draw for a given inverter load:

I = P / (V × η)

Where P = AC power, V = battery voltage, η = inverter efficiency (typically 0.85-0.95)

For a 3000W load on a 12V system with 90% inverter efficiency:

I = 3000 / (12 × 0.90) = 278A

This extreme current is why many larger systems use 24V or 48V battery banks. At 48V, the same 3000W load requires only 69A—a quarter of the 12V current.

Solar System Current Table

The following table shows typical currents in solar system components:

Component	12V System	24V System	48V System
1000W Inverter (max)	95A	48A	24A
2000W Inverter (max)	185A	95A	48A
3000W Inverter (max)	280A	140A	70A
100W Solar Panel	6A	3A	1.5A
400W Solar Array	24A	12A	6A
60A Charge Controller	60A max	60A max	60A max

Cable Length Recommendations

Keep battery-to-inverter cables as short as possible—ideally under 3 feet for large inverters. If longer cables are necessary, increase wire size substantially. A common guideline is to allow no more than 2% voltage drop at maximum inverter current.

String Sizing and Voltage Drop

How you configure your solar panel array affects both the charge controller requirements and the voltage drop in panel wiring. Optimizing string configuration can simplify wiring while maximizing system efficiency.

Higher Voltage Strings Reduce Wire Size

For the same power, higher voltage means lower current and less voltage drop. Consider two ways to wire 1000W of panels to an MPPT controller 75 feet away:

Configuration A: 10 panels parallel at 40V (25A total current)

Using 8 AWG: VD = (2 × 75 × 25 × 0.778) / 1000 = 2.92V (7.3% drop)
Needs 4 AWG or larger to stay under 3%

Configuration B: 2 strings of 5 panels in series at 200V (5A total current)

Using 14 AWG: VD = (2 × 75 × 5 × 3.14) / 1000 = 2.35V (1.2% drop)
14 AWG easily meets voltage drop requirements

The higher voltage configuration uses much smaller, less expensive wire while achieving better voltage drop performance.

MPPT Controller Voltage Windows

MPPT controllers have minimum and maximum input voltage specifications. Design string voltage to operate well within this window under all conditions—accounting for both cold temperatures (higher voltage) and hot temperatures (lower voltage). A typical MPPT controller might accept 30-150V input, allowing substantial flexibility in string configuration.

Matching Strings

When running multiple strings in parallel, all strings should be identical (same number and type of panels) and have equal length cable runs. Mismatched strings can cause current imbalance and reduced system performance. If cable lengths must differ, use larger wire on longer runs to equalize resistance.

NEC 690 Requirements

NEC Article 690 provides requirements specifically for solar photovoltaic systems. Understanding these requirements helps ensure code-compliant installations.

Circuit Sizing Requirements

NEC 690.8 requires that PV circuit conductors be sized for 125% of the maximum circuit current. For PV source circuits (panel wiring), the maximum current is the sum of the parallel module short-circuit currents (Isc). This 125% factor is mandatory, not optional.

Example: An array of 4 parallel strings, each panel with 10A Isc:

Maximum circuit current: 4 × 10A = 40A
Required conductor ampacity: 40A × 1.25 = 50A minimum

Voltage Drop Recommendations

NEC 690 references the general voltage drop informational notes in Articles 210 and 215. While not mandatory requirements, the NEC suggests limiting voltage drop to levels that ensure efficient operation. For PV systems, this typically means:

Panel to combiner/controller: 2-3% maximum
Combiner to inverter (for utility-interactive): 1-2% maximum
Battery circuits: 1-2% maximum

Conductor Types

NEC 690.31 specifies that PV source circuit conductors exposed to sunlight must be listed and labeled as sunlight resistant, or installed in raceways. Single-conductor cables must be type USE-2, PV wire, or similar outdoor-rated cable.

Grounding Requirements

NEC 690 includes detailed grounding requirements for PV systems, including equipment grounding conductor sizing. The EGC must be sized according to NEC 250.122 based on the overcurrent device rating, which may require larger conductors than voltage drop alone would indicate.

Calculating Solar Wire Sizes

Proper wire sizing requires understanding each circuit's characteristics and applying appropriate safety factors.

Solar Wire Sizing Chart

The following table provides recommended wire sizes for common solar system connections, targeting 2% voltage drop:

Current (A)	5 ft	10 ft	15 ft	20 ft	30 ft
12V System (0.24V max drop)
20A	10 AWG	8 AWG	6 AWG	6 AWG	4 AWG
50A	6 AWG	4 AWG	2 AWG	1 AWG	1/0 AWG
100A	2 AWG	1/0 AWG	2/0 AWG	3/0 AWG	4/0 AWG
200A	1/0 AWG	3/0 AWG	250 kcmil	350 kcmil	500 kcmil
48V System (0.96V max drop)
20A	14 AWG	12 AWG	12 AWG	10 AWG	10 AWG
50A	10 AWG	8 AWG	8 AWG	6 AWG	4 AWG
100A	6 AWG	4 AWG	4 AWG	2 AWG	1 AWG

Always verify these values using the voltage drop calculator for your specific installation. Remember to also verify that the selected wire size meets ampacity requirements per NEC 690.8.

Step-by-Step Process

Determine maximum circuit current (for PV source circuits, use Isc × number of parallel strings)
Apply 125% factor per NEC 690.8 to get minimum ampacity requirement
Calculate acceptable voltage drop based on circuit voltage and target percentage
Select wire size that meets both ampacity and voltage drop requirements
Choose appropriate wire type for the installation environment

Frequently Asked Questions

Wire size depends on the panel configuration (series vs. parallel), total current, and distance. For high-voltage series strings (common with MPPT controllers), relatively small wire works because current is low. For parallel configurations at low voltage, larger wire is needed. Target 2-3% voltage drop and verify ampacity is at least 125% of the short-circuit current per NEC 690.8.

The battery-to-inverter connection operates at low voltage (12V, 24V, or 48V) but carries very high current. A 3000W inverter on a 12V system draws nearly 280A at full power. Large cables are needed to carry this current without excessive voltage drop and heat. This is why larger systems use 48V or higher battery banks—the same power at higher voltage requires much less current.

Use larger wire, shorter cable runs, or higher system voltage. For panel wiring, configuring panels in series rather than parallel reduces current and voltage drop. For battery systems, mounting the charge controller and inverter close to the batteries minimizes cable length. Upgrading from 12V to 24V or 48V cuts current substantially for the same power.

Yes, but MPPT charge controllers partially compensate for wire voltage drop by adjusting their operating point. However, excessive voltage drop still reduces the power delivered to the batteries—that power is wasted as heat in the wires instead. Keeping panel wire voltage drop under 2-3% ensures most of the captured solar energy reaches your batteries.

For small systems under 1000W with short cable runs, 12V is simple and economical. For 1000-3000W systems, 24V significantly reduces cable sizes and costs. For larger systems over 3000W, 48V is strongly recommended—it allows reasonable cable sizes and reduces losses throughout the system. The higher voltage also matches common MPPT controller and inverter input ranges.