# Bus Bar Size Calculator

Bus bar size calculator for copper and aluminum bars. Enter current, material, and installation type to get cross-section (mm^2), recommended width x thickness, and I^2R loss per meter.

## What this calculates

Bus bars carry current inside switchgear, panelboards, distribution panels, and battery banks. Sizing them wrong causes overheating, melted insulators, and tripped feeders. This bus bar size calculator uses the IEC 61439 approach of a material-specific current density (A/mm^2) derated for installation type and ambient temperature, then converts that to a rectangular width x thickness you can actually order.

## Inputs

- **Continuous Current** (A) — min 0 — Continuous RMS current the bus bar must carry.
- **Bus Bar Material** — options: Copper (1.6 A/mm^2 indoor, 1.2 A/mm^2 enclosed), Aluminum (1.0 A/mm^2 indoor, 0.8 A/mm^2 enclosed) — Copper carries roughly 60 percent more current for the same cross-section than aluminum.
- **Installation Type** — options: Indoor, well ventilated (higher density allowed), Enclosed switchgear / cubicle (lower density), Outdoor, solar exposure (derated) — Enclosed installations restrict convection, so the safe current density drops.
- **Ambient Temperature** (C) — min -20, max 80 — Ambient air temperature around the bus bar. IEC baseline is 35 C.
- **Preferred Bar Thickness** — options: 3 mm, 5 mm, 6 mm, 8 mm, 10 mm, 12 mm — Standard rectangular bus bar thickness. The calculator sizes width to match your chosen thickness.

## Outputs

- **Minimum Cross-Sectional Area** (mm^2) — Minimum rectangular cross-section to carry the current safely.
- **Recommended Bar Width** (mm) — Suggested width for your chosen thickness, rounded up to the nearest 5 mm step.
- **Design Current Density** (A/mm^2) — Derated current density used for sizing.
- **I^2R Loss per Meter** (W/m) — Resistive power loss per meter of bar length at operating current.
- **Bar Ampacity** (A) — Continuous current rating of the recommended size at the chosen conditions.

## Details

How the bus bar sizing calculation works

The standard bus bar sizing calculation starts from a target current density J in amperes per square millimeter. The required cross-sectional area A (mm^2) is simply A = I / J, where I is the continuous RMS current. Split that area into a rectangle of width times thickness, then check the I^2R heat dissipation against the surface area available for cooling.

Typical design current densities come from IEC 61439-1 and long-standing industry practice. Copper in free, indoor air runs at around 1.6 A/mm^2. Copper tucked inside an enclosed cubicle drops to about 1.2 A/mm^2 because convection is restricted. Aluminum carries roughly 60 percent of copper's current for the same area, so the densities are 1.0 A/mm^2 open and 0.8 A/mm^2 enclosed.

Standard bus bar sizes (copper)

  
    Size (mm)Area (mm^2)Indoor Ampacity (A)Enclosed Ampacity (A)
  
  
    20 x 5100160120
    25 x 5125200150
    30 x 5150240180
    40 x 5200320240
    50 x 6300480360
    60 x 10600960720
    80 x 108001280960
    100 x 10100016001200
  

For a bus bar sizing calculation pdf reference you can print and tape to the panel, the standard industrial PDF tables (Copper Development Association Publication 22, Eaton Bus Bar Selection Guide) publish the same numbers with slightly different derating curves. This calculator follows IEC 61439 for ampacity and the CDA handbook for temperature derating, so results land within 5 percent of the published tables.

Picking thickness and width

Bus bars come in a narrow set of standard thicknesses (3, 5, 6, 8, 10, 12 mm) and widths in 5 mm increments. Thicker bars are stiffer and handle short-circuit forces better but cost more per ampere. For amperages under 500 A, 5 mm thickness is usually enough. Above 800 A, 10 mm thickness gives better short-circuit withstand and lets you keep the width under 100 mm.

Always check two other limits after the sizing calculation. First, the short-circuit withstand: a 60 x 10 mm copper bar typically handles around 40 kA for 1 second. Second, the mechanical support spacing, which depends on Faraday force during a fault: I^2 forces scale quickly, so supports every 500 mm are common for bars rated above 2000 A.

Losses and temperature rise

The I^2R loss per meter is what drives temperature rise. This calculator reports power loss per meter in watts, using copper resistivity 0.01724 ohm-mm^2/m at 20 C or aluminum 0.0289 ohm-mm^2/m. A well-designed bus bar runs 25-40 C above ambient at full rating. If the calculator shows loss above 80 W/m you probably need to go up one size or move to copper if you were on aluminum.

## Frequently Asked Questions

**Q: How do I size a bus bar?**

A: Divide the continuous current by the design current density for your material and installation. Copper in open air uses roughly 1.6 A/mm^2, copper in enclosed switchgear uses 1.2 A/mm^2. A 400 A copper bus bar in an enclosed cubicle needs at least 400 / 1.2 = 333 mm^2, so 40 x 10 mm (400 mm^2) is a clean fit.

**Q: What is the formula for bus bar current capacity?**

A: I = J x A, where J is the allowable current density (A/mm^2) and A is the cross-section (mm^2). For copper, IEC 61439 gives J around 1.2 to 1.6 A/mm^2 depending on installation. Multiply by the cross-section to get ampacity: a 80 x 10 mm copper bar (800 mm^2) in an enclosed panel carries about 800 x 1.2 = 960 A continuously.

**Q: Is copper or aluminum better for bus bars?**

A: Copper carries about 60 percent more current per mm^2 than aluminum and has lower voltage drop, so it is preferred for compact switchgear and high-amperage designs. Aluminum is about 30 percent cheaper per ampere and much lighter, so it is common in utility switchgear, generator risers, and data-center PDUs where weight matters.

**Q: Where can I find a bus bar sizing calculation pdf?**

A: The Copper Development Association Publication 22 (Copper for Busbars) and Eaton's Busway Selection Guide are the two most-referenced PDF references, both free to download. This calculator follows the same IEC 61439 derating curves, so the ampacity it returns matches the standard PDF tables for copper and aluminum rectangular bars within a few percent.

**Q: What current density should I design for?**

A: For copper bus bars, use 1.6 A/mm^2 in open indoor air and 1.2 A/mm^2 inside enclosed cubicles. For aluminum, use 1.0 A/mm^2 open and 0.8 A/mm^2 enclosed. Drop another 0.8 percent per degree Celsius above 35 C ambient. For ratings above 3000 A, plan a more detailed thermal analysis because radiation and proximity effects start to matter.

**Q: How do I account for short circuit in bus bar sizing?**

A: After sizing for continuous current, check the short-circuit current rating. Rectangular copper bus bars follow I^2 t = k^2 S^2, where k is about 143 for copper at 200 C final temperature and S is cross-section in mm^2. A 40 x 10 mm (400 mm^2) copper bar can carry about 57 kA for 0.1 s or 18 kA for 1 s. If the panel fault level exceeds this, step up one size.

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Source: https://vastcalc.com/calculators/physics/bus-bar
Category: Physics
Last updated: 2026-04-08