Switchgear Busbar Capacity Calculator
Enter bar size, material, number of bars and temperature limits to get continuous ampacity and short-circuit withstand.
Step-by-step
- Enter valid values to begin.
Continuous rating is an empirical fit to DIN 43671 free-air copper bar data (about ±10%). Multi-bar factors (1, 1.8, 2.4, 2.9) assume bar spacing equal to bar thickness. Manufacturer tested ratings and the assembly standard (IEC 61439-1, UL 891) override this estimate.
Switchgear Busbar Capacity: Ampacity Is a Heat Problem, Not a Cross-Section Problem
Core Engineering Principles
Rectangular copper bar looks simple, but its current rating depends on how well it sheds heat. A bar produces I²R heat through its whole volume and loses it through its surface by convection and radiation. A flat, wide bar has more surface per unit of area than a square one, so it carries more current for the same copper. That’s why the bar is wide and thin, why two thin bars with a gap carry more than one thick one, and why a painted bar runs cooler than a bright one, since a dull surface radiates better.
The standard tables come from tests: a certain temperature rise over ambient, vertical orientation, in free air. This page uses a curve fitted to the DIN 43671 copper bar data. Current scales with the square root of cross-section, the 0.39 power of perimeter and the 0.61 power of temperature rise. Aluminum is about 78% of copper for the same size. Real switchgear in an enclosure runs less than the free-air rating, so a derating factor is applied. The bar must also carry the fault. The short-time current rating, usually one second, is set by how much heat the bar can absorb without softening: I = k × A / √t.
Short-time Ith = k × A / √t, k ≈ 143 (Cu) or 94 (Al) A·√s/mm²
NEC & Standard References
IEC 61439-1 sets temperature-rise limits for assemblies: 70 K at terminals for external conductors, and the busbars may run hotter if adjacent insulation permits. ANSI C37.20.1 covers metal-enclosed low-voltage switchgear with a 65 °C rise at 40 °C ambient on bus bars, and silver or tin plating at joints. IEC 60865-1 gives the thermal and mechanical short-circuit calculation, and UL 891 covers switchboards. NEC 408.30 through 408.56 cover switchboard construction, and 408.36 requires the panel’s overcurrent protection. A manufacturer’s tested rating always overrides a calculated number like the one on this page.1. Area = 10 cm²; perimeter = 22 cm; rise θ = 30 K.
2. I = 20.4 × 300.61 × 220.39 × 100.5 = 20.4 × 7.96 × 3.34 × 3.16 = 1,715 A.
3. In an enclosure derated to 0.80: 1,372 A.
4. Short-time (1 s): 143 × 1,000 mm² / 1 = 143 kA thermal withstand.
Rated for a 1,200 A main with margin; for 1,600 A you’d go to two bars per phase.
- Joints are the weak point. Plate, torque and re-torque bolted joints; a poor joint can run 30 K hotter than the bar.
- Vertical vs flat. A horizontal bar flat on its side loses 10 to 15% of its rating.
- Skin and proximity effect matter above about 10 mm thickness at 60 Hz; multiple bars are often more effective than one thick bar.
- Bracing. Short-circuit forces try to bend the bar. Spacing of the supports determines the peak current rating, not just the thermal rating.
- Infrared scan every year. Hot joints are the leading cause of switchgear fires.