Code-aware calculator with cost analysis, educational tooltips, and real-world examples for DIY solar installers in Canada.
Calculated Results
Minimum OCPD (Fuse/Breaker) Size
Minimum Required Ampacity (After Derating)
Recommended Wire Size (AWG/kcmil)
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(Based on both Ampacity and Voltage Drop)
Required Size (Based on Derated Ampacity)
Required Size (Based on Voltage Drop)
โ ๏ธ Code Warning: The wire size required for Voltage Drop is larger than for Ampacity. Use the LARGER size shown above.
๐ก Common Off-Grid Application Examples
Solar Panel to Charge Controller (48V, 10A, 15m)
8 AWG
Battery Bank to Inverter (24V, 150A, 1.5m)
2/0 AWG
Subpanel Feeder (120V AC, 30A, 30m)
8 AWG
๐ก Pro Tips for Wire Selection
1. Always Size Up: When in doubt between two wire sizes, choose the larger. The cost difference is minimal compared to the risk of undersizing.
2. Check Both Calculations: Your final wire size must satisfy BOTH ampacity (heat) and voltage drop (efficiency) requirements. Always use the larger result.
3. Consider Future Expansion: If you might add panels or batteries later, size wire for future capacity now. Replacing wire is expensive and disruptive.
4. Temperature Matters: Roof-mounted conduit can reach 70ยฐC+ in summer. Always use worst-case ambient temperature for calculations.
5. Use Quality Connectors: Proper crimped connections are critical. Poor connections cause 80% of solar system failures. Invest in quality terminals and crimping tools.
๐ Wire Gauge Quick Reference
| AWG |
Max Amps Cu
(75ยฐC) |
Max Amps Al
(75ยฐC) |
Diameter
(mm) |
Common Use |
| 14 | 20A | 15A | 1.6mm | Lighting circuits |
| 12 | 25A | 20A | 2.0mm | General outlets |
| 10 | 35A | 30A | 2.6mm | Small charge controllers |
| 8 | 50A | 40A | 3.3mm | Medium solar arrays |
| 6 | 65A | 50A | 4.1mm | Large charge controllers |
| 4 | 85A | 65A | 5.2mm | Battery banks (short runs) |
| 2 | 115A | 95A | 6.5mm | Inverter DC input |
| 2/0 | 175A | 145A | 9.3mm | Large battery banks |
How Wire Sizing Works
Proper wire sizing in off-grid solar systems is critical for two main reasons: safety and efficiency. When electricity flows through wire, it encounters resistance, which causes voltage drop and heat generation.
Voltage Drop
As electricity travels through wire, some voltage is "lost" due to the wire's resistance. This lost voltage reduces the power available to your equipment and can cause poor performance. In solar systems, excessive voltage drop means you're not getting the full power from your panels or batteries.
Ampacity (Current Capacity)
Every wire size has a maximum safe current capacity called ampacity. Exceeding this limit causes dangerous overheating, which can melt insulation, start fires, or damage equipment. The National Electrical Code (NEC) provides ampacity ratings for different wire types and installation conditions.
Safety Factors
Off-grid solar systems require additional safety considerations. Both the Canadian Electrical Code (CEC) and US National Electrical Code (NEC) mandate that continuous loads (running for 3+ hours) must not exceed 80% of the circuit's rated capacity. This is why we apply the 125% rule - multiply your continuous current by 1.25 when sizing wire and breakers.
Important: Undersized wire is a leading cause of electrical fires in solar installations. Always err on the side of larger wire when in doubt. Check with your local electrical inspector for specific requirements in your province.
Formulas & Calculations
Voltage Drop Calculation (DC)
The basic formula for calculating voltage drop in DC circuits:
Voltage Drop (V) = (2 ร K ร I ร L) / A
Where:
โข K = Resistivity constant (12.9 for copper, 21.2 for aluminum)
โข I = Current in amperes
โข L = One-way distance in feet (metres ร 3.28084)
โข A = Wire cross-sectional area in circular mils
Why multiply by 2? Because current must travel to the load AND back, creating a round-trip that doubles the resistance.
Voltage Drop Calculation (AC)
Voltage Drop (V) = (2 ร K ร I ร L ร PF) / A
Power Factor (PF) accounts for the phase difference between voltage and
current in AC circuits with inductive or capacitive loads.
Use PF = 1.0 for purely resistive loads (heaters, incandescent lights).
Percentage Voltage Drop
Voltage Drop (%) = (Voltage Drop รท System Voltage) ร 100
Wire Size (Circular Mils) Calculation
To find the minimum wire size needed:
A = (2 ร K ร I ร L) รท (System Voltage ร Max Voltage Drop %)
CEC 125% Continuous Load Rule
Design Current = Actual Continuous Current ร 1.25
Example: A charge controller drawing 40A continuously requires wire sized for 40 ร 1.25 = 50A minimum.
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Frequently Asked Questions
Q: Can I use smaller wire for short cable runs?
A: Not necessarily. While voltage drop may be acceptable on short runs, you must still meet ampacity requirements. A 100A current requires appropriately sized wire regardless of distance. Always check both voltage drop AND ampacity calculations.
Q: What happens if I use wire that's too small?
A: Undersized wire creates two major problems: dangerous overheating (fire risk) and excessive voltage drop (poor system performance). Your equipment may not function properly, batteries won't charge efficiently, and you risk electrical fires.
Q: How do battery bank cables differ from solar panel wiring?
A: Battery cables typically carry much higher currents (50-200A+) and require larger wire sizes (2 AWG to 4/0 AWG). Solar panel wiring usually carries lower currents (10-40A) but may span longer distances. Battery cables should be kept as short as possible and use welding cable for flexibility.
Q: What's the difference between THWN, USE-2, and welding cable?
A: THWN: Building wire, good for conduit runs and AC circuits. USE-2: Solar-rated cable, UV resistant, perfect for outdoor DC solar wiring. Welding Cable: Extremely flexible, ideal for battery connections but not outdoor rated without protection.
Q: Should I always follow the 3% voltage drop rule?
A: The 3% rule is a good general guideline, but you can be more flexible: 2% for critical loads (medical equipment), up to 5% for non-critical loads (lighting). In low-voltage systems (12V), even 3% can cause noticeable performance issues.
Q: Is it worth upgrading to larger wire than calculated?
A: Often yes! Slightly larger wire costs little extra but provides: lower voltage drop (better efficiency), cooler operation (longer life), future expansion capability, and greater safety margin. It's cheap insurance for your system.
Q: Can I splice or extend solar cables?
A: Yes, but use proper MC4 connectors for solar panel connections. For DC circuits, use appropriate splice boxes or junction blocks. Never use wire nuts in DC solar applications - they can fail over time due to thermal cycling.
Q: What about temperature derating for wire ampacity in Canadian climates?
A: Canadian installations face unique challenges - from extreme cold to hot summers. Wire ampacity decreases in hot conditions but cold weather can make cables brittle. If cables run through hot attics, direct sun, or are bundled together, you must apply CEC derating factors. High-temperature locations may require wire sized 20-30% larger than standard calculations.
References & Standards
Canadian Electrical Code (CEC)
- CEC Section 64: Solar Photovoltaic Systems - Canadian requirements for solar installations including wire sizing, grounding, and safety disconnects.
- CEC Section 14: Protection and Control - Defines overcurrent protection requirements and the 125% continuous load rule.
- CEC Section 4: Conductors - Contains ampacity tables and derating factors for Canadian installation conditions.
- CEC Table 2: Allowable ampacities for copper conductors - Primary reference for wire current capacity in Canada.
US National Electrical Code (NEC) - For Reference
- NEC Article 690: Solar Photovoltaic (PV) Systems - Similar principles to CEC Section 64, widely referenced internationally.
- NEC Article 240: Overcurrent Protection - Harmonized with CEC protection requirements.
- NEC Article 310: Conductors for General Wiring - Ampacity tables similar to CEC Section 4.
Canadian Standards & Certifications
- CSA C22.2 No. 230: Solar Modules - Canadian safety standard for photovoltaic modules and systems.
- CSA C61215: Crystalline Silicon Terrestrial Photovoltaic Modules - Design qualification and type approval.
- CSA C22.2 No. 0.3: Test Methods for Electrical Wires and Cables - Standards for Canadian wire testing.
- Canadian Standards Association (CSA): Primary certification body for electrical equipment used in Canada.
๐ Calculation Standards:
Wire sizing formulas follow Canadian Electrical Code (CEC) Section 4 ampacity tables and Section 64 solar PV requirements. Aluminum ampacity set at 84% of copper per CEC Table 2. Bundling derating applies at 4+ conductors per CEC/NEC. AC voltage drop includes power factor correction. Distance inputs in metres. Last updated: February 2026.
Important Disclaimer: This calculator uses principles common to both Canadian (CEC) and US (NEC) electrical codes. While wire sizing fundamentals are similar, always consult with a licensed electrician and your provincial electrical authority for final approval. Each province may have specific amendments to the CEC, and local requirements can differ. Contact your local electrical inspector before beginning any solar installation.