Solar Panel to Battery Wiring Guide: 12V/24V Systems

Wire a 400W solar array the same way you’d wire a 400W inverter, and you’ll buy wire three sizes too thick.

Solar’s different. Your panels output high voltage and low current. Then your charge controller converts that to low voltage and high current for the battery. Three separate circuits, each sized completely differently.

Get the panel circuit right and you’ll use surprisingly small wire (saving money). Get the controller-to-battery circuit wrong and you’ll lose 20% of your solar harvest to voltage drop. Or start a fire.

This guide shows you how to wire all three circuits properly - panel-to-controller, controller-to-battery, and fusing - so you’re not wasting power or money.

The Three Circuits You Need to Wire

Here’s what confuses everyone: same solar array, three different wire sizes.

Circuit 1: Solar Panel Array to Charge Controller

Characteristics:

• Higher voltage (20-100V depending on configuration)
• Lower current (5-30A typically)
• Uses smaller wire than you’d expect

Example: A 400W array of four 100W panels wired in series:

• Voltage: ~80V (4 panels × 20V each)
• Current: ~5A (400W ÷ 80V)
• Wire size: 10 AWG handles this easily for runs up to 25 feet

Circuit 2: Charge Controller to Battery

Characteristics:

• Battery voltage (12V or 24V)
• HIGH current (same watts as panels, but at lower voltage)
• Needs thick wire despite short distance

Example: Same 400W array charging a 12V battery:

• Voltage: 12V
• Current: ~33A (400W ÷ 12V)
• Wire size: 6 AWG minimum even for just 5 feet

See the difference? Circuit 2 carries 6× the current of Circuit 1 because battery voltage (12V) is lower than panel voltage (80V). Same power, vastly different wire requirements.

Circuit 3: Fusing and Protection

Required locations:

• Panel-to-controller: Fuse positive wire at panel side
• Controller-to-battery: Fuse or breaker at battery side (within 18 inches)
• Each string in parallel arrays (prevents backfeed)

The key: fuse each circuit based on ITS specific current, not some generic “solar fuse size.”

Panel to Controller Wiring

This is where series vs parallel configuration makes a huge difference in wire sizing.

Series Configuration

How it works: Connect panels positive-to-negative like batteries in a flashlight.

Effect on voltage: ADDS UP

• 1 panel: 20V
• 2 panels in series: 40V
• 4 panels in series: 80V

Effect on current: STAYS THE SAME

• Each panel produces 5A
• String of 4 panels: still 5A total

Advantage: Lower current means smaller wire gauge

Disadvantage: Shading one panel kills the whole string

Parallel Configuration

How it works: All positives together, all negatives together.

Effect on voltage: STAYS THE SAME

• Each panel: 20V
• 4 panels in parallel: still 20V

Effect on current: ADDS UP

• 1 panel: 5A
• 4 panels in parallel: 20A total

Advantage: Shading one panel doesn’t kill the others

Disadvantage: Higher current means thicker wire

Series-Parallel Hybrid

Most larger systems use a mix: pairs in series, then parallel.

Example: 4 panels configured as 2S2P (two series strings of two panels each):

• Voltage: 40V (2 panels in series)
• Current: 10A (2 strings in parallel)

This balances voltage and current for optimal wire sizing.

Which Configuration Should You Use?

Wire Sizing for Panel Circuit

Because voltage is high and current is low, you can use surprisingly small wire.

Example: 2 × 100W panels in series, 25-foot run to controller

Panel specs:

• Voltage: 40V (2 panels × 20V)
• Current: 5A
• Distance: 25 feet

Let’s check 12 AWG:

• 12 AWG resistance: 0.001588 ohms/ft
• Round trip: 25 ft × 2 = 50 ft
• Total resistance: 50 × 0.001588 = 0.0794 ohms
• Voltage drop: 5A × 0.0794 = 0.397V
• Percentage of 40V: 0.397 / 40 = 0.99% - Excellent!

12 AWG works perfectly. Most installers use 10 AWG for added UV resistance and future-proofing, but 12 AWG meets electrical requirements.

Critical rule: Size wire for the CURRENT, not the wattage. A 400W array in series carries 5A. The same 400W array in parallel carries 20A. Huge wire size difference.

Calculate your panel circuit wire size: Use Wire Solved - enter panel current (from configuration), wire length, and select 12V or 24V.

Controller to Battery Wiring

This is where most people undersize wire and lose performance.

Why This Circuit Needs Thick Wire

Your charge controller converts high-voltage, low-current from panels to low-voltage, high-current for the battery.

Same power, different voltage and current:

• 400W at 80V = 5A (panel side)
• 400W at 12V = 33A (battery side)

That 33A needs thick wire even though the controller might be right next to the battery.

The 3% Rule for Solar Charging

Regular accessories tolerate 5% voltage drop (industry standard). Solar charging should target 3% or less.

Why the stricter rule?

Per NEC 690.8, up to 5% voltage drop is acceptable for solar battery circuits. We recommend 3% to maximize solar harvest. You’re already losing 2-5% efficiency in charge controller conversion (MPPT controllers convert with 95-98% electronic efficiency; PWM controllers are voltage-limited and can’t harvest panel voltage above battery voltage, making them less effective when panel voltage significantly exceeds battery voltage). You don’t want to lose another 3-5% in the wire.

Every volt lost to voltage drop is a volt the battery never receives. That’s wasted solar harvest you paid for with expensive panels.

Recommended Wire Sizes

For controller-to-battery runs under 5 feet:

For runs 5-10 feet, go one size thicker.

Pro tip: Mount your charge controller within 5 feet of the battery to minimize wire gauge requirements. Every additional foot costs you money in thicker wire.

Important - Temperature Derating: Wire ampacity ratings assume 30°C (86°F) ambient temperature. If running wire in a hot attic or roof conduit (temperatures can exceed 60°C/140°F), you must apply temperature derating factors per NEC 310.15(B)(3)(a) or use the next larger gauge. Solar-rated wire is rated for 90°C conductor temperature, giving more margin in hot environments.

MC4 Connectors for Panel Wiring

⚠️ CRITICAL SAFETY WARNING: Never connect or disconnect MC4 connectors while the system is generating power (sunny conditions). Always disconnect the battery from controller first, or cover panels completely to stop current flow. Connecting or disconnecting under load creates an arc that damages connectors and creates fire risk.

Most modern panels come with MC4 connectors (locking waterproof plugs). Don’t cut them off.

If you need to extend panel wiring:

• Buy MC4 extension cables (pre-made with connectors), or
• Buy MC4 connectors and crimp them onto wire yourself
• Use 10 AWG minimum for extensions

Don’t use:

• Butt connectors (not weatherproof)
• Wire nuts (will corrode outdoors)
• Twist-and-tape (seriously, don’t)

MC4 connectors are $10 for a pack. They’re waterproof, UV-resistant, and locking. Use them.

Multiple Panel Installations

When you’re adding panels, configuration affects both performance and wire sizing.

Adding a Second Panel to Existing System

Series (recommended for identical panels):

• Connect positive of panel 1 to negative of panel 2
• Free positive and free negative go to controller
• Doubles voltage, keeps current the same

Parallel (for mixing panel types):

• Connect all positives together
• Connect all negatives together
• Doubles current, keeps voltage the same

Critical rule: Only connect identical panels in series. Different wattages or voltages in series will underperform - the lowest-power panel limits the entire string.

Large Arrays (4+ Panels)

Use series-parallel to balance voltage and current.

Example: 6 panels configured as 3S2P

• 3 panels in series = 60V, 5A per string
• 2 strings in parallel = 60V, 10A total
• Wire from array to controller: 10 AWG
• Wire from controller to battery (12V system): 4 AWG (for ~600W / 12V = 50A)

Fusing Each Parallel String

If you have multiple parallel strings, fuse EACH string separately.

Why? If one string shorts, unfused parallel strings will backfeed current into the short. This can melt wire or cause a fire.

Fusing rule: Each parallel string gets its own fuse rated at 125% of panel short-circuit current (Isc) - fuse sizing explained.

Check your panel datasheet for Isc (short-circuit current). Typical 100W panel has Imp (max power current) around 5A and Isc around 5.5A - but this ratio varies by panel design (some are 1.15-1.2× Imp).

For a panel with 5.5A Isc:

• Fuse calculation: 5.5A × 1.25 = 6.9A
• Round up to standard fuse: 7.5A or 10A fuse

Important: Fuse based on Isc (from datasheet), not Imp. Isc is what flows during a short circuit.

Solar Charging System Components: Panels, Controllers, Batteries & Wiring

Quick overview of what you’re connecting:

Solar Panels generate 18-22V for “12V” rated panels (voltage is higher than nominal to charge batteries). Output varies with sunlight from 0 to rated current.

Charge Controllers regulate panel output to safely charge batteries. MPPT controllers convert high panel voltage to battery voltage with 95-98% electronic efficiency. PWM controllers clip panel voltage to battery voltage (can’t harvest voltage above battery level), making them less effective when panel voltage significantly exceeds battery voltage.

Battery Bank stores energy at system voltage (12V, 24V, or 48V). Can be single battery or multiple in series/parallel.

Wiring connects it all. Three different circuits with three different sizing requirements.

How to Install Solar Panels: Complete Wiring Guide for RV/Marine

Let’s wire a real system from planning to power-on.

Planning Phase

1. Determine your power needs

Calculate watt-hours per day:

• LED lights: 20Wh
• Phone charging: 10Wh
• Laptop: 50Wh
• 12V fridge: 400Wh (running 8 hours)

Example total: 500Wh/day

2. Size your solar array

Rule of thumb for sunny climates: Array should produce 1.5× your daily usage to account for inefficiency, charging losses, and cloudy days.

Adjust multiplier based on location: use 2-2.5× in northern climates or for winter-heavy usage. Check NREL solar maps for your location’s peak sun hours.

Example: 500Wh/day × 1.5 = 750Wh daily production needed

With 4 peak sun hours typical: 750Wh ÷ 4 hours = ~190W minimum array size

Adding margin: 200-400W array recommended

3. Choose system voltage

Most van/RV builds are 12V or 24V.

4. Pick controller type

MPPT vs PWM Charge Controllers: Which to Choose?

Note: Both controller types are electronically efficient. MPPT’s advantage is converting high panel voltage to usable battery voltage, not superior electronic efficiency. PWM clips voltage above battery level, effectively discarding that extra voltage potential.

For arrays over 200W, MPPT pays for itself in extra energy harvest within 1-2 years.

Component Selection

Solar panels:

• Monocrystalline (most efficient, more expensive)
• Polycrystalline (less efficient, cheaper)
• Flexible thin-film (least efficient, lightest weight for RV roofs)

Charge controller sizing: Sized for panel wattage and battery voltage.

Example: 400W array / 12V battery = 33A → Need 40A controller minimum (allows 20% overhead)

Wire:

• Panel-to-controller: 10 AWG for most setups under 30 feet
• Controller-to-battery: Calculate with Wire Solved based on controller maximum output current

Fuses/breakers:

• Panel side: 125% of string Isc (typically 7.5-15A per string)
• Battery side: 125% of controller max output, not exceeding wire ampacity

Installation Steps

BEFORE YOU START: Critical Safety Rules

⚠️ These rules prevent equipment damage and fire:

1. Battery connection sequence: Per NEC 690.71(B), ALWAYS connect battery to controller FIRST, then connect panels. Without battery voltage reference, unregulated panel voltage can damage controller circuitry even with internal protection circuits. Connecting panels first can destroy your controller.

2. MC4 connector safety: NEVER connect or disconnect MC4 connectors while system is generating power (sunny conditions). Always disconnect battery from controller first, or cover panels to stop current flow. Arcing under load damages connectors and creates fire risk.

3. Fuse ALL parallel strings: Each parallel string needs its own fuse. Unfused parallel strings will backfeed into a shorted string, creating fire risk.

4. Use solar-rated wire outdoors: Standard household wire (Romex) has insulation that degrades in UV exposure. Use solar-rated wire with UV-resistant insulation for any outdoor or roof runs.

Step 1: Mount panels

Secure to roof with mounting feet, tilting rack, or ground mount. Face south (northern hemisphere) at latitude angle for optimal year-round performance. Ensure structural mounting - panels see high wind loads.

Step 2: Connect panels in desired configuration

Wire panels in series, parallel, or series-parallel based on your planning. Use MC4 connectors - they’re waterproof and locking.

Step 3: Run wire from panels to controller location

Use 10 AWG solar-rated cable with MC4 adapters. Route through roof gland or sealed entry point with proper waterproofing. Don’t drill a hole and shove wire through - water intrusion will cause problems.

Step 4: Mount charge controller

Install in ventilated area within 5 feet of battery. Controllers generate heat during operation - don’t install in sealed enclosure. Allow airflow around unit.

Step 5: Install fuses on panel input

Install inline fuse holders on positive wire between panels and controller. For single string: 125% of string Isc. For parallel strings: fuse each string separately at panel side.

Step 6: Run wire from controller to battery

This is the high-current circuit. Size properly using Wire Solved based on controller maximum output current. Use properly crimped ring terminals with heat shrink. This isn’t the place to cheap out on connections.

Step 7: Install fuse/breaker on battery positive

Install within 18 inches of battery positive terminal. Rated at 125% of controller max output current, not to exceed wire ampacity.

Example: 40A controller with 6 AWG wire (65A ampacity) → use 50A fuse (40A × 1.25 = 50A, which is under wire ampacity).

If using 8 AWG wire (50A ampacity) with same controller, still use 50A max fuse even though it matches wire limit.

Step 8: Connect controller to battery FIRST

CRITICAL SEQUENCE - per NEC 690.71(B):

1. Connect battery positive to controller (through fuse)
2. Connect battery negative to controller
3. Verify controller powers on and shows battery voltage
4. THEN connect panel positive
5. Finally connect panel negative

Reversing this sequence can damage the controller.

Step 9: Verify system operation

Controller display should show:

• Battery voltage (12V, 24V, or 48V as appropriate)
• Panel input voltage (if sun is out)
• Charging current (if sun is out and battery isn’t full)

Testing the System

Monitor first few days:

• Battery voltage should rise during sunny periods
• Panel input voltage should match your configuration (20V per panel in series)
• Controller shouldn’t be overheating
• Wire connections shouldn’t be warm to touch (warm wire = undersized gauge)

If battery isn’t charging:

1. Measure panel voltage with multimeter (should be 18-22V per “12V” panel in full sun)
2. Verify controller shows panel input voltage on display
3. Check all fuses with multimeter
4. Confirm battery isn’t already full (controllers taper to zero current at 100% charge)

Real-World Solar Wiring Scenarios

Let’s size wire for actual installations.

Scenario 1: Single 100W Panel, 12V Van Build

Setup:

• 1 × 100W panel (18V Vmp, 5.5A Imp)
• 10 feet from panel to charge controller
• PWM charge controller (10A rated)
• Controller mounted 3 feet from battery

Panel-to-Controller Wire:

Panel circuit specs:

• Voltage: 18V
• Current: 5.5A
• Distance: 10 feet (20 feet round trip)

Check 12 AWG:

• 12 AWG resistance: 0.001588 ohms/ft
• Round trip resistance: 20 ft × 0.001588 = 0.03176 ohms
• Voltage drop: 5.5A × 0.03176 = 0.175V
• Percentage: 0.175V / 18V = 0.97% - Excellent!

Recommendation: 10 AWG (12 AWG meets electrical requirements, but 10 AWG provides better UV resistance and durability for roof installation)

Controller-to-Battery Wire:

Battery circuit specs:

• Voltage: 12V
• Current: 8A (100W ÷ 12V, accounting for controller efficiency)
• Distance: 3 feet

Wire Solved calculation:

• Amp Draw: 8A
• Wire Length: 3 feet
• System Voltage: 12V
• Voltage Drop Tolerance: 3%
• Load Type: Continuous

Result:

• Wire Gauge: 12 AWG
• Fuse Size: 10A (8A × 1.25 = 10A)
• Actual Voltage Drop: ~0.5%

Shopping List:

• 25 feet of 10 AWG solar-rated wire (panel circuit with slack for routing)
• 10 feet of 12 AWG red wire (controller to battery positive)
• 10 feet of 12 AWG black wire (controller to battery negative)
• 10A inline fuse holder (panel input)
• 10A fuse or breaker (battery connection)
• MC4 to bare wire adapters
• Ring terminals sized for 12 AWG wire

Calculate wire sizing for your single-panel system: Use Wire Solved - enter your controller output current and distance.

Scenario 2: 400W Array (4 × 100W Panels, 2S2P), 12V RV

Setup:

• 4 × 100W panels configured as 2S2P (two strings of two panels in series, then parallel)
• String 1: 2 panels in series (40V, 5A)
• String 2: 2 panels in series (40V, 5A)
• Strings connected in parallel: 40V, 10A total to controller
• 20 feet from array to charge controller (MPPT 40A)
• Controller 5 feet from battery

Panel-to-Controller Wire:

Panel circuit specs:

• Voltage: 40V
• Current: 10A (both strings in parallel)
• Distance: 20 feet (40 feet round trip)

Check 10 AWG:

• 10 AWG resistance: 0.000999 ohms/ft
• Round trip resistance: 40 ft × 0.000999 = 0.03996 ohms
• Voltage drop: 10A × 0.03996 = 0.4V
• Percentage: 0.4V / 40V = 1.0% - Excellent!

Recommendation: 10 AWG

Controller-to-Battery Wire:

Battery circuit specs:

• Voltage: 12V
• Current: 33A (400W ÷ 12V)
• Distance: 5 feet

Wire Solved calculation:

• Amp Draw: 33A
• Wire Length: 5 feet
• System Voltage: 12V
• Voltage Drop Tolerance: 3%
• Load Type: Continuous

Result:

• Wire Gauge: 6 AWG
• Fuse Size: 40A (33A × 1.25 = 41.25A, round to 40A standard)
• Actual Voltage Drop: ~1.4%

Shopping List:

• 50 feet of 10 AWG solar-rated cable (panel wiring with slack)
• MC4 Y-branch connectors for parallel connection (2× - one for positive, one for negative)
• 10A inline fuse holders (2× - one per string)
• 10A fuses (2× - 125% of 5.5A Isc per string)
• 15 feet of 6 AWG red wire (controller to battery positive)
• 15 feet of 6 AWG black wire (controller to battery negative)
• 40A ANL fuse and holder (battery connection)
• 6 AWG ring terminals

Calculate wire sizing for your multi-panel system: Use Wire Solved - enter total controller output current and wire run distance.

Scenario 3: 800W Array, 24V RV System

Setup:

• 8 × 100W panels configured as 4S2P (4 in series, 2 strings in parallel)
• String voltage: 80V (4 panels × 20V)
• String current: 5A per string
• Total: 80V, 10A (two strings in parallel)
• 25 feet from array to MPPT controller (60A, 24V rated)
• Controller 5 feet from battery bank

Panel-to-Controller Wire:

Panel circuit specs:

• Voltage: 80V
• Current: 10A
• Distance: 25 feet (50 feet round trip)

Check 10 AWG:

• Round trip resistance: 50 ft × 0.000999 = 0.04995 ohms
• Voltage drop: 10A × 0.04995 = 0.5V
• Percentage: 0.5V / 80V = 0.6% - Excellent!

Recommendation: 10 AWG

Controller-to-Battery Wire:

Battery circuit specs:

• Voltage: 24V
• Current: 33A (800W ÷ 24V)
• Distance: 5 feet

Wire Solved calculation:

• Amp Draw: 33A
• Wire Length: 5 feet
• System Voltage: 24V (select 24V in dropdown)
• Voltage Drop Tolerance: 3%
• Load Type: Continuous

Result:

• Wire Gauge: 8 AWG (vs 6 AWG required for 12V!)
• Fuse Size: 40A
• Actual Voltage Drop: ~1.4%

Why 24V is better for larger solar arrays:

Notice the controller-to-battery wire is 8 AWG instead of 6 AWG. Same 800W on a 12V system would need 4 AWG wire because current would be 66A instead of 33A.

24V systems draw half the current for same wattage. Lower current means:

• Smaller wire gauge (lower cost, easier installation)
• Less voltage drop for same wire size
• More efficient overall system

Panel wiring can also run at higher voltage (80V in this example), which further reduces current and wire size requirements.

Calculate wire sizing for your 24V solar system: Use Wire Solved - select 24V system voltage, enter controller output current.

Troubleshooting Solar Systems

⚠️ SAFETY ISSUE - Controller-to-Battery Wire Gets Warm

Symptom: Wire between controller and battery is warm or hot to touch during charging.

Diagnosis: Wire gauge is too small for the current being carried.

Fix: Shut down the system immediately. Warm wire indicates resistance heating - this is a fire risk.

Upgrade to next larger wire gauge. If you’re using 8 AWG, upgrade to 6 AWG. If you’re using 10 AWG, upgrade to 8 AWG. Recalculate with Wire Solved to verify new gauge is adequate.

Test again once upgraded. Wire should remain at ambient temperature during charging.

⚠️ SAFETY ISSUE - No Fuses on Parallel Strings

Symptom: You have multiple panel strings in parallel but no fuse on each string.

Risk: If one string shorts, unfused parallel strings will backfeed current into the short, potentially melting wire or causing fire.

Fix: Install inline fuse holder on positive wire of each parallel string, rated at 125% of that string’s Isc (short-circuit current from panel datasheet). This is required for safety.

⚡ Performance Issue - No Charging / Controller Shows No Input

Symptom: Controller display shows 0V from panels.

Diagnosis:

1. Measure voltage at panel output with multimeter in full sun (should be 18-22V per “12V” panel)
2. If panels show voltage but controller doesn’t see it: wiring issue or blown fuse
3. Check panel-side fuse with multimeter
4. Check for reversed polarity (positive connected to negative by mistake)
5. Verify MC4 connections are fully seated and locked

Fix: Trace wiring from panels to controller, verify all connections are correct and secure.

⚡ Performance Issue - Low Charging Current

Symptom: Controller shows input voltage but very low current (1-2A when you expect 10A).

Diagnosis:

1. Check for partial shading on panels (even shade on one panel in a series string kills output of entire string)
2. Measure voltage drop from panels to controller while system is charging
3. If voltage drop exceeds 2V: wire gauge is too small
4. Verify panel configuration matches controller expectations

Fix:

• Eliminate shading if possible (trim trees, relocate panels)
• Upgrade panel-to-controller wire gauge if voltage drop is excessive
• Verify panels are wired in correct series/parallel configuration

⚡ Performance Issue - Battery Overheating While Charging

Symptom: Battery becomes hot to touch during charging.

Diagnosis: Controller is overcharging or charge settings don’t match battery type.

Fix:

1. Check controller settings - verify battery type selection (flooded lead-acid, AGM, gel, lithium, etc.)
2. Verify charging voltage with multimeter (should be 14.2-14.4V for 12V flooded lead-acid; 14.4-14.6V for AGM; 14.2-14.6V per manufacturer spec for lithium)
3. If lithium battery, confirm controller is set to lithium charging profile
4. Consult battery manufacturer’s recommended charging voltage

This is a controller configuration issue, not a wiring problem.

⚡ Performance Issue - Battery Drains Overnight

Symptom: Battery charges during day but voltage drops overnight even with no intentional loads running.

Diagnosis:

1. Check for parasitic loads (LED indicators, electronics in standby mode, phantom loads)
2. Disconnect all loads and measure battery voltage over several hours
3. If battery voltage drops while completely disconnected: dead cell in battery (internal short)
4. Check for backfeed through charge controller (rare but possible with some PWM controllers)

Fix:

• Eliminate phantom loads with switches or disconnects
• Replace battery if internal short is confirmed
• Install load disconnect switch to isolate loads when not in use

Common Mistakes to Avoid

Mistake #1: Undersizing Controller-to-Battery Wire

The problem: “The controller is only 5 feet from the battery, 10 AWG should be fine for my 40A controller.”

Reality: 40A through 10 AWG at 5 feet (12V system) = 2.8% voltage drop. You’re losing 2.8% of your solar harvest. Over a year with average 500Wh/day production, that’s ~5 kWh wasted - equivalent to $50 in lost solar harvest if you were buying that power.

Fix: Use Wire Solved to size wire based on actual controller output current and distance. Don’t guess based on “it’s a short run.”

Mistake #2: Connecting Panels Before Battery

The problem: Installation sequence - wire panels to controller first, battery second.

Result: Per NEC 690.71(B), controller requires battery voltage reference. Without it, unregulated panel voltage can damage controller circuitry even with internal protection. This can destroy your controller.

Fix: ALWAYS connect battery to controller FIRST. Verify controller powers on and displays battery voltage. THEN connect panels.

Mistake #3: Mixing Panel Types in Series

The problem: “I have a 100W panel and a 150W panel. I’ll connect them in series to get 250W.”

Result: You get 100W + 100W = 200W, not 250W. In series, current is limited by the lowest-current panel. The 150W panel operates below its capability, wasting potential.

Fix: Only series-connect identical panels (same wattage, voltage, and preferably same manufacturer/model). Mix different panels using separate parallel strings.

Mistake #4: Using Household Wire for Outdoor Solar Runs

The problem: “I have leftover 12 AWG Romex from a house wiring project. I’ll use that for my panel-to-controller run on the roof.”

Result: Romex has THHN insulation rated for indoor use. It’s not UV-resistant. After 6-12 months of sun exposure, insulation becomes brittle, cracks, and exposes bare copper. Water intrusion and short circuit risk.

Fix: Use solar-rated wire with UV-resistant insulation (marked “sunlight resistant” or “solar cable”) for any outdoor wiring. It costs about $0.50/foot more than Romex. Worth every penny.

Mistake #5: Ignoring Temperature Derating for Roof Runs

The problem: “Wire ampacity chart says 10 AWG is good for 30A. My controller outputs 25A, so 10 AWG is perfect.”

Result: Wire ampacity assumes 30°C (86°F) ambient temperature. Roof surfaces and attic spaces can exceed 60°C (140°F) in summer. At elevated temperatures, wire must be derated - 10 AWG might only be rated for 18A in hot conditions per NEC 310.15(B)(3)(a).

Fix: For roof runs or attic installations, either:

• Use next larger wire gauge to provide temperature margin, or
• Use solar-rated wire rated for 90°C and apply proper derating factors, or
• Consult NEC temperature correction tables for your installation environment

What’s Next?

• Wire Sizing 101 - Understanding AWG, ampacity, and the 3-5% voltage drop rule for all DC circuits
• Voltage Drop Explained - Deep dive into why high-voltage panel circuits need smaller wire than low-voltage battery circuits
• How to Use Wire Solved - Step-by-step tutorial: calculate accurate wire recommendations for your solar charging system
• Fuse Sizing Guide - Learn the 125% rule and how to protect each circuit in your solar installation