Busbar Size Calculator
Size busbars for DC combiners, AC panels, battery banks, and general distribution. NEC 690.8 and IEC compliant — ampacity, voltage drop, short-circuit withstand, and 120% busbar rule in one free tool.
Busbar Size Calculator
Select your application mode, enter current and system parameters, and get busbar size, voltage drop, short-circuit withstand, and NEC 120% rule compliance instantly.
| Property | Copper | Aluminum |
|---|
| Dimensions | Area (mm²) | 30°C Rise (A) | 50°C Rise (A) | Req. Area @ 1.4 A/mm² | Weight (kg/m) |
|---|
Source: CDA copper.org, IEC 60287. Values for bare copper, edge-mounted, open air at 40°C ambient. Derate for enclosure and orientation. Click a row to verify those dimensions.
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Four Application Modes
One calculator covers every busbar sizing scenario in solar and electrical work - from PV combiner boxes to AC panels, battery banks, and utility distribution.
DC Combiner
PV string combining with NEC 690.8 125% Isc multiplier. Default 80A at 600V. Sized for continuous DC current from multiple solar strings.
AC Panel
Inverter interconnection sizing with NEC 120% busbar rule compliance check. Default 32A at 240V. Includes solar backfeed breaker position analysis per NEC 705.12.
Battery Bank
High-current DC discharge sizing for energy storage systems. Default 200A at 48V. Handles the large conductor cross-sections required for battery discharge at low voltage.
General Distribution
Utility and commercial distribution sizing. Default 100A at 480V. Supports single-phase, three-phase AC, and DC circuits with full derating chain.
Why Electrical Professionals Use This Tool
More than a simple ampacity lookup. Full derating chain, short-circuit verification, and NEC/IEC compliance in one calculation.
NEC & IEC Compliant
Follows NEC 690.8 for PV conductor sizing, NEC 705.12 for the 120% busbar rule, and IEC 60949 for short-circuit withstand verification. References exact code sections in all outputs.
Full Derating Chain
Applies enclosure type, bar orientation, altitude, and parallel bar grouping factors in sequence - not just a single lookup. Shows each derating step in a transparent breakdown table.
Short-Circuit Withstand
Optional IEC 60949 short-circuit verification checks whether the selected busbar survives fault conditions. Enter fault current (kA) and clearing time to get a pass/fail result with required minimum area.
How It Works
From application mode to permit-ready busbar specification in five steps.
Select Application Mode
Choose DC Combiner, AC Panel, Battery Bank, or General. Each mode pre-fills default current, voltage, and circuit type and activates the relevant compliance checks (e.g. NEC 690.8 for DC Combiner, 120% rule for AC Panel).
Enter Load Parameters
Input current directly in amps or via power (kW) and voltage. Set continuous load factor (1.25 for loads running over 3 hours), power factor for AC circuits, and number of parallel bars per phase.
Set Material & Environment
Choose copper or aluminum, system of units (metric or imperial), enclosure type (open, ventilated, sealed), bar orientation, and installation altitude. Each factor applies a derating multiplier to the base current density.
Optional: Short-Circuit & 120% Rule
Enable IEC 60949 short-circuit verification by entering fault current and clearing time. For AC Panel mode, enter panel busbar rating, main breaker, and solar breaker size to check 120% rule compliance per NEC 705.12(B)(3).
Get Your Results
Recommended busbar size (W × T mm), cross-sectional area, rated ampacity, voltage drop, current density, skin effect status (AC), short-circuit pass/fail, and NEC 120% compliance - all with a full derating breakdown table.
Built for Every Solar & Electrical Professional
Solar Installers & Electricians
Size DC combiner busbars to NEC 690.8, verify AC panel busbars meet the 120% rule, and confirm battery bank busbars handle peak discharge current - all before ordering materials or submitting for permit.
System Designers & Engineers
Run short-circuit withstand verification for fault conditions, compare copper vs aluminum cost and weight trade-offs, and check skin effect for high-current AC circuits - all within a single tool.
Commercial & Utility Projects
Handle three-phase AC distribution sizing with altitude derating for high-elevation sites, parallel bar configurations for high-current feeders, and IEC 60949 fault withstand verification for commercial-grade equipment.
Calculation Methodology
Every result is derived from a sequential derating chain - no hidden assumptions.
Design Current
Base × [1.25 NEC 690.8] × Continuous Factor Base current from direct input or calculated from power (P ÷ V for DC; P ÷ V ÷ PF for AC). NEC 690.8 adds 125% for PV conductors. Continuous load factor adds another 25% for loads running over 3 hours.
Derating Factor Chain
Enclosure × Orientation × Altitude × Grouping Sealed enclosures derate to 0.70×; edge-mounted bars outperform flat-mounted; altitude above 2,000 m reduces convective cooling; parallel bars grouped together derate at 0.90× for 2 bars, 0.80× for 3, 0.73× for 4+.
Required Busbar Area
Design Current ÷ Adjusted Current Density (A/mm²) Base current density: 1.4 A/mm² for copper (ETP), 0.9 A/mm² for aluminum (6101-T6). Multiplied by total derating factor to give adjusted density, then divided into design current for minimum cross-sectional area.
Voltage Drop
ρ(T) × L ÷ A × I [DC] | × √3 [3φ] Resistivity is temperature-corrected using α = 0.00393/°C (copper) or 0.00390/°C (aluminum). Three-phase circuits apply the √3 factor. Result in millivolts over the specified busbar length.
Short-Circuit Withstand (IEC 60949)
Min Area = (I² × t) ÷ k I = fault current (A), t = clearing time (s), k = material/temperature k-factor (copper: 159 at 30°C, 141 at 40°C, 115 at 70°C; aluminum: 97/93/76). Actual area must exceed minimum for a PASS result.
NEC 120% Busbar Rule
Main + Solar ≤ Busbar × 1.20 (opposite end) Solar breaker at the opposite end from the main allows 120% of busbar rating as the total OCPD sum (NEC 705.12(B)(3)(2)). Any other position limits the total to 100% of busbar rating. Maximum solar breaker size is calculated and displayed.
Copper vs. Aluminum Busbars
Both materials are used in solar and electrical installations. The right choice depends on current level, space, and budget.
| Property | Copper (ETP) | Aluminum (6101-T6) |
|---|---|---|
| Current Density | 1.4 A/mm² | 0.9 A/mm² |
| Conductivity | Higher - smaller cross-section for same current | ~61% of copper - larger bar needed |
| Skin Depth (60 Hz) | 8.5 mm | 10.9 mm - less skin effect at same thickness |
| Weight | Heavier | ~3× lighter per volume - better for large busbars |
| Relative Cost | Higher material cost | Lower material cost - offset by larger size needed |
| Corrosion | Better in humid/coastal environments | Requires anti-oxidant compound at connections |
| Best For | High-current DC, space-constrained enclosures | Large switchgear, weight-sensitive commercial installs |
Pro Tips for Busbar Sizing
Always apply the continuous load factor for solar
Solar PV systems produce current for more than 3 hours per day - they're continuous loads by definition. Always use the 1.25 continuous load factor on top of NEC 690.8's 125% Isc multiplier. Skipping either factor is a code violation and an inspection failure.
Mount busbars on edge, not flat
Edge-mounted busbars have significantly better natural convection than flat-mounted bars of the same cross-section. For the same current, an edge-mounted bar runs cooler and requires less derating - which can mean a smaller, cheaper bar for the same application.
Check skin effect on AC circuits above 1,000A
For AC circuits above 1,000A, busbar thickness can exceed the skin depth (8.5 mm for copper at 60 Hz). Current concentrates near the surface, making the core effectively useless. Use wider, thinner bars or parallel bars to keep thickness within 1.5× skin depth.
Run short-circuit verification on battery bank busbars
Battery banks can deliver extremely high fault currents - sometimes 10–50 kA - for milliseconds before the fuse clears. A busbar that passes ampacity checks can still fail short-circuit withstand. Always run the IEC 60949 check when fault current data is available for battery applications.
Frequently Asked Questions
How do you size a busbar for solar?
For DC combiner busbars, start with the sum of all string short-circuit currents (Isc), multiply by 1.25 per NEC 690.8, then apply another 1.25 continuous load factor. Divide the result by the adjusted current density (accounting for enclosure, orientation, and altitude derating) to get required cross-sectional area. Select the next standard busbar size at or above that area. This calculator automates all of these steps.
What is the NEC 120% busbar rule for solar?
Per NEC 705.12(B)(3), when a solar backfeed breaker is installed at the opposite end of the busbar from the main breaker, the sum of the main breaker and solar breaker ratings can be up to 120% of the busbar's rated ampacity. For a 200A busbar with a 200A main, this allows a solar breaker up to 40A (200 × 1.20 − 200 = 40A). If the solar breaker is not at the opposite end, the limit drops to 100% of busbar rating.
What is current density and what should it be for busbars?
Current density is amps per square millimeter (A/mm²) of busbar cross-section. Standard base values are 1.4 A/mm² for copper and 0.9 A/mm² for aluminum - these produce a 30°C temperature rise above ambient. After applying derating factors (enclosure, orientation, altitude, grouping), the adjusted density is lower, requiring a larger bar. Exceeding the adjusted density causes the bar to overheat and reduces insulation and connection life.
What is skin effect and does it matter for solar busbars?
Skin effect is an AC phenomenon where current concentrates near the conductor's surface, reducing effective cross-sectional area. At 60 Hz, the skin depth for copper is ~8.5 mm. For most residential solar AC busbars (under 400A), bar thickness stays well within skin depth and skin effect is negligible. It becomes significant for commercial systems above 1,000A where thick bars may be specified. DC circuits have no skin effect.
Should I use copper or aluminum busbars for solar?
Copper is the standard choice for most solar applications - combiner boxes, AC panels, and battery banks - because its higher current density (1.4 vs 0.9 A/mm²) means a smaller, lighter bar for the same current. Aluminum is commonly used in large switchgear and utility-scale equipment where weight and material cost matter more than space. For coastal or humid environments, copper's superior corrosion resistance is a further advantage. The calculator's comparison table shows both options side by side for your specific inputs.
What is short-circuit withstand and when do I need to check it?
Short-circuit withstand (IEC 60949) verifies that a busbar can survive the thermal energy delivered during a fault before the protective device clears the fault. It's most important for battery bank busbars, where short-circuit currents can reach 10–50 kA, and for commercial/utility AC distribution. For residential solar combiner boxes where fault current is modest, ampacity sizing alone is usually sufficient. Enable the short-circuit check in the calculator whenever you have fault current data from the utility or battery manufacturer.
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