Bus Bar Size Calculator
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.
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 5 | 100 | 160 | 120 |
| 25 x 5 | 125 | 200 | 150 |
| 30 x 5 | 150 | 240 | 180 |
| 40 x 5 | 200 | 320 | 240 |
| 50 x 6 | 300 | 480 | 360 |
| 60 x 10 | 600 | 960 | 720 |
| 80 x 10 | 800 | 1280 | 960 |
| 100 x 10 | 1000 | 1600 | 1200 |
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.