How to size a busway with this calculator
Busway (busduct) carries the bulk of a building or plant feeder load, and once it is installed and supported it is expensive to enlarge. Enter the connected load, voltage and power factor and the calculator returns the actual load current, a design current with safety margins applied, and the standard ampere rating that fits.
Load current is I = kW × 1,000 ÷ (√3 × V × PF) for three-phase systems and I = kW × 1,000 ÷ (V × PF) for single-phase. NEC 215.2(A)(1) requires feeders to be sized for 125 percent of the continuous portion of the load, so that multiplier is applied along with your spare capacity allowance. The result is matched to standard busway ratings such as 225, 400, 600, 800, 1,000, 1,200, 1,600 and 2,000 A.
If utilization of the chosen rating comes out above 95 percent, consider stepping up one size because temperature rise and voltage drop grow quickly near the rating. NEC Article 368 also covers support spacing, through-floor requirements and the reduced-size feeder rules, so check those before ordering.
This is a preliminary selection based on standard ambient and mounting conditions. High ambient mechanical rooms, long vertical risers and harmonic-rich loads need additional derating, so have a licensed electrical engineer review the final design.
Frequently asked questions
A load that runs three hours or more keeps the conductors fully heated. The NEC therefore requires the feeder and its overcurrent device to be rated at least 125 percent of that continuous load.
Commercial buildings with no defined expansion plan commonly use 10 to 20 percent, while plants expecting new production lines often carry 30 percent or more. Allow for support and riser space too.