To find the right rating, you need to calculate your total connected load and then size the busway with enough "headroom"—usually 125% of the continuous load—to handle future expansion. It’s also critical to factor in ambient temperature and voltage drop, as these environmental variables can significantly impact how a specific busway performs over a long distance.
If you’re staring at a one-line diagram trying to pin down a number, the first thing you need to do is sum up the Full Load Amps (FLA) of all the equipment connected to that specific run. But here’s the catch: you aren’t just building for today. In the world of industrial power, we always talk about “future-proofing.” A common rule of thumb is to size your busway at about 125% of your continuous load. This ensures that if you add a few more racks in your data center or another CNC machine on the factory floor next year, you won’t be ripping out your infrastructure to upgrade.
Temperature is the silent killer of busway efficiency. Most standard ratings are based on an ambient temperature of around 35°C or 40°C. If your busway is running through a high-heat ceiling plenum or a poorly ventilated mechanical room, the conductor’s ability to dissipate heat drops. In these cases, you’ll need to derate the system, which essentially means you’ll choose a higher ampere rating than the load suggests just to handle the ambient heat without stressing the insulation.
Lastly, don’t ignore the voltage drop, especially on longer runs. If your run exceeds 100 feet, the resistance in the busbars can cause the voltage to dip at the far end, which can be hard on your motors and sensitive electronics. Sometimes, you’ll find yourself sizing up to a higher ampacity not because you need the current, but because you need the larger conductor cross-section to keep the voltage stable. When in doubt, always run a quick voltage drop calculation—it’s much cheaper to catch this on paper than during the commissioning phase.
