Wire Sizing Calculator

Code-aware calculator with cost analysis, educational tooltips, and real-world examples for DIY solar installers in Canada.

📊 Quick Reference - Common Solar Wire Sizes

10 AWG
30A Max
Charge Controllers
8 AWG
40A Max
Inverter DC Input
4 AWG
70A Max
Battery Banks
2 AWG
95A Max
Main DC Bus
DC (2-wire) or AC (2-wire line-neutral).
Nominal operating voltage.
**Continuous load current** (Used for 125% OCPD rule).
Source to load. Used for round-trip voltage drop.

💡 Common Off-Grid Application Examples (V2.1)

Solar Panel to Charge Controller (48V, 10A, 50ft)
**8 AWG**
Battery Bank to Inverter (24V, 150A, 5ft)
**2/0 AWG**
Subpanel Feeder (120V AC, 30A, 100ft)
**8 AWG**

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

The basic formula for calculating voltage drop in DC circuits:

Voltage Drop (V) = (2 × K × I × L) / A Where: • K = Resistivity constant (12.9 for copper) • I = Current in amperes • L = One-way distance in feet • 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.

Percentage Voltage Drop

Voltage Drop (%) = (Voltage Drop ÷ System Voltage) × 100

Wire Size (Circular Mils) Calculation

To find the minimum wire size needed:

A = (2 × K × I × L) ÷ (System Voltage × Max Voltage Drop %)

CEC 125% Continuous Load Rule

Design Current = Actual Continuous Current × 1.25
Example: A charge controller drawing 40A continuously requires wire sized for 40 × 1.25 = 50A minimum.

Wire Gauge Conversion

Common AWG sizes and their circular mil areas:

14 AWG = 4,107 CM 12 AWG = 6,530 CM 10 AWG = 10,380 CM 8 AWG = 16,510 CM 6 AWG = 26,240 CM 4 AWG = 41,740 CM 2 AWG = 66,360 CM 1/0 AWG = 105,600 CM 2/0 AWG = 133,100 CM

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.

Provincial Electrical Authorities

  • Electrical Safety Authority (ESA) - Ontario: Provincial oversight for electrical installations, including solar systems.
  • BC Safety Authority - British Columbia: Electrical permits and inspections for BC solar installations.
  • Alberta Electrical System Operator (AESO): Grid connection requirements for Alberta solar systems.
  • SaskPower - Saskatchewan: Utility requirements for solar interconnection.
  • Technical Standards & Safety Authority (TSSA) - Ontario: Additional oversight for electrical systems in Ontario.

Academic & Technical Resources

  • Sandia National Laboratories: "Photovoltaic Array Performance Model" - Comprehensive technical reference for PV system design and performance.
  • NREL Technical Report: "Best Practices for Operation and Maintenance of Photovoltaic and Energy Storage Systems" - Government research on solar system reliability.
  • IEEE Standard 1547: Standard for Interconnecting Distributed Resources with Electric Power Systems - Industry standard for electrical interconnection.
  • Solar Power International (SPI): "Solar Installation Best Practices" - Industry guidelines for safe and effective solar installations.

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.

Safety & Installation Guidelines

  • OSHA Construction Standard 29 CFR 1926.95: Personal protective equipment requirements for electrical work.
  • NFPA 70E: Standard for Electrical Safety in the Workplace - Safety practices for electrical installations.
  • Solar Energy Industries Association (SEIA): "Solar Installation Safety Guidelines" - Industry best practices for safe installation procedures.
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.