Size copper and aluminum busbars for any solar or electrical application. Calculate cross-sectional area, current density, and temperature rise – NEC compliant.
Busbars are the backbone of power distribution in solar combiner boxes, switchgear, and distribution panels. Undersized busbars cause overheating, voltage drop, and potential fire hazards. This calculator ensures your busbar dimensions meet current density limits and temperature rise requirements for safe, code-compliant installations.
Calculates busbar dimensions for both copper and aluminum conductors with correct resistivity, current density limits, and cost comparisons.
Ensures busbar cross-section meets industry-standard current density limits (copper: 1.2–2.0 A/mm², aluminum: 0.8–1.2 A/mm²) for safe operation.
Estimates temperature rise above ambient based on current loading, helping you verify the busbar won't exceed insulation or enclosure ratings.
Busbar sizing is critical in solar combiner boxes, switchgear, and distribution panels. Use this calculator when:
Size the main busbar in DC combiner boxes to handle the combined current from all PV string inputs with proper safety margins per NEC 690.8.
Specify busbar dimensions for commercial solar switchgear and distribution panels where current ratings exceed standard wire gauges.
When fabricating custom electrical panels or battery enclosures, calculate the exact busbar cross-section needed for your current, temperature, and material requirements.
Enter the maximum continuous current the busbar must carry (include NEC 125% factor for continuous loads).
Select busbar material — copper or aluminum.
Choose standard busbar dimensions or enter custom width and thickness.
Set ambient temperature and maximum allowable temperature rise.
Review the recommended busbar size, current density, voltage drop per meter, and power dissipation.
The minimum cross-sectional area (mm² or in²) required for the busbar to carry the specified current within safe current density limits.
Standard busbar width × thickness that meets or exceeds the minimum cross-section, typically in standard incremental sizes.
Actual A/mm² at the specified current — should stay within limits (copper ≤2.0 A/mm², aluminum ≤1.2 A/mm²) for continuous duty.
Estimated temperature increase above ambient in degrees Celsius — critical for enclosed panels and combiner boxes.
Millivolts per meter of busbar length, important for long busbar runs in combiner boxes.
This calculator uses established electrical engineering principles and NEC requirements to determine safe busbar dimensions for any current-carrying application.
Worked example: A 200A main panel with a 200A busbar rating. Solar backfeed breaker needed: 40A (for a 7.6 kW inverter). NEC 705.12(B)(3) check: 200A (main) + 40A (solar) = 240A. Maximum allowed: 200A × 1.20 = 240A. Result: exactly at the 120% limit — PASS. If upgrading to a 10 kW inverter requiring a 50A breaker: 200 + 50 = 250A > 240A limit — FAIL, main breaker derate to 175A required: 175 + 50 = 225A ≤ 240A — PASS.
Calculations sourced from SurgePV’s Busbar Size Calculator — surgepv.com/tools/busbar-size-calculator/
Common copper busbar dimensions with approximate continuous current ratings in enclosed installations (1.2–1.6 A/mm² current density).
| Width × Thickness (mm) | Cross-Section (mm²) | Current Rating (enclosed) | Common Application |
|---|---|---|---|
| 15 × 3 | 45 | 54–72A | Small combiner boxes |
| 20 × 3 | 60 | 72–96A | Residential combiner |
| 25 × 3 | 75 | 90–120A | Residential sub-panel |
| 30 × 5 | 150 | 180–240A | Main panel busbar |
| 40 × 5 | 200 | 240–320A | Commercial combiner |
| 50 × 5 | 250 | 300–400A | Commercial distribution |
| 60 × 10 | 600 | 720–960A | Large commercial/utility |
| 80 × 10 | 800 | 960–1280A | Utility-scale solar |
| 100 × 10 | 1000 | 1200–1600A | Utility-scale switchgear |
* Copper busbars at 35°C ambient, 30°C temperature rise. Aluminum busbars require ~60% larger cross-section for equivalent current rating.
Solar is a continuous load per NEC. Size the busbar for 125% of the maximum expected current, not just the nameplate current. This is a common oversight in combiner box design.
A combiner box on a rooftop in Phoenix can reach 60°C+ ambient inside. This significantly reduces the busbar's current-carrying capacity. Always use the actual worst-case enclosure temperature, not room temperature.
The weakest point is often the connection, not the busbar itself. Ensure bolted connections have adequate contact area and torque. Loose connections create hot spots that can cause fires.
For busbars longer than 1 meter (common in large combiner boxes), calculate the voltage drop along the busbar length. Uneven current distribution can cause the far end to have significantly higher drop.
For enclosed panels and combiner boxes, use 1.2–1.6 A/mm² for copper busbars. For open-air installations with good ventilation, you can go up to 2.0 A/mm². Higher current densities increase temperature rise and power losses.
Copper is standard for most solar applications due to its higher conductivity (60% better than aluminum), smaller size, and better corrosion resistance. Aluminum is used in large commercial installations where weight and cost savings justify the larger cross-section needed.
Sum the maximum current from all input strings (Isc × 1.25 per NEC 690.8), then size the busbar for that total current with appropriate current density limits. Don't forget to account for future expansion if additional strings will be added.
Industry standard limits temperature rise to 30°C above ambient for enclosed installations and 40°C for open installations. In a combiner box that might reach 50°C ambient in summer, a 30°C rise means the busbar could reach 80°C — verify this is within enclosure and connection ratings.
Yes. NEC 408.30 requires busbars to be braced for the available short-circuit current. This determines the mechanical strength needed — a different calculation from the thermal (current-carrying) sizing this calculator performs.
For most combiner boxes (under 1 meter of busbar), voltage drop is negligible. For longer runs in commercial installations (3+ meters), calculate V_drop = I × R × L and verify it stays under 0.5% of system voltage.
Size AC breakers and conductors for solar inverter output.
Find the correct AWG gauge for solar AC and DC circuits.
Calculate voltage drop across any solar wiring run.
Check NEC conduit fill compliance for solar wiring.
Design optimal PV string configurations for any inverter.
Calculate the right solar system size based on energy usage.
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