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How to Size a Busway: Ampacity Calculation Step by Step

By DHYN Engineering Team  ·  August 10, 2026

Quick answer: Size a busway in five steps. Add up the load current, apply a demand factor for your building type, add 20 to 25 percent spare capacity for growth, correct the rating for the ambient temperature where the duct will run, then check voltage drop and short-circuit rating. Pick the next standard size above your corrected number. Standard ratings run from 400A to 6300A.

Most oversize and undersize mistakes happen in the first ten minutes of a project, when somebody picks a busway rating off a breaker size or a guess. The calculation is not hard, but it has five steps, and skipping any one of them costs money or safety later. Here is the process we use when a client sends us a single-line diagram, with real numbers at the end.

Step 1: Add up the real load current

Start from the loads, not from the main breaker. List every load the busway will feed, in kW or kVA, then convert to current. For a three-phase system the formula is I = P / (1.732 x V x power factor). A 500 kW factory floor at 400V with a 0.9 power factor draws 500,000 / (1.732 x 400 x 0.9), which is about 802A. That number is your starting point, nothing more.

If the consultant has already done a load schedule, use it and check that the power factor assumption is realistic. Motor-heavy plants sit at 0.8 to 0.85 until compensation is added. Offices with LED lighting and UPS systems run closer to 0.95.

Step 2: Apply a demand factor

Not everything runs at once. NEC Article 220 and IEC 60364 both recognize demand factors, and so does every experienced designer. A riser feeding 40 apartments never sees all 40 kitchens and air conditioners at full load simultaneously.

Building or load type	Typical demand factor	Notes
Residential tower riser	0.4 to 0.6	Falls as the number of units grows
Office floors	0.7 to 0.85	Higher if the HVAC is on the same duct
Factory production line	0.7 to 0.9	Depends on shift pattern
Data center white space	0.9 to 1.0	Assume IT load is continuous
Transformer to main switchboard	1.0	No diversity allowed here, ever

The last row matters. A feeder busway between a transformer and its switchboard carries whatever the transformer can deliver, so you size it at 100 percent of the transformer rating.

Step 3: Add growth margin

Loads grow. Tenants add servers, factories add machines, and pulling out an undersized busway five years in is painful. The common practice is 20 to 25 percent spare capacity on top of the demand-corrected load. On projects where the client tells us expansion is already planned, we go to 30 percent and the busway usually pays for the upgrade by avoiding a second installation.

Step 4: Correct for ambient temperature

Busway ratings in the catalog are verified by temperature rise tests under IEC 61439-6 at a reference ambient of 35 degrees C averaged over 24 hours. If your duct runs through a hot electrical room, a roof level, or a vertical shaft that acts like a chimney, the real ambient is higher and the ampacity is lower.

Actual ambient temperature	Approximate derating factor
35 degrees C (catalog basis)	1.00
40 degrees C	0.97
45 degrees C	0.93
50 degrees C	0.88
55 degrees C	0.82

These factors are typical for sandwich bus ducts; the exact curve belongs to each manufacturer, so ask for it. Vertical risers deserve special attention. The air at the top of a 30-storey shaft can be 10 degrees warmer than at the bottom, and the duct up there is the same duct you specified at the bottom.

Step 5: Check voltage drop

Ampacity tells you the duct will not overheat. It says nothing about whether the voltage at the far end is still healthy. Most consultants want the drop on a busway feeder held to 3 percent or less, following the same logic as NEC's recommendation for feeders. For a three-phase run, Vd = 1.732 x I x L x (R cos phi + X sin phi), where R and X come from the manufacturer's data sheet per meter of duct.

Take a 1250A copper sandwich duct, 60 meters long, carrying 1030A. With R = 0.045 milliohms per meter and X = 0.030 milliohms per meter at a 0.9 power factor, the drop works out to about 5.7V, or 1.4 percent of 400V. Comfortable. Run that same load 150 meters and you are at 3.6 percent, which means stepping up a size or splitting the run. This is exactly why aluminum busways, with their higher resistance, need a voltage drop check on anything over 50 meters.

Step 6: Confirm the short-circuit rating

The last check is the one people forget. Your busway must survive the prospective fault current at its position in the network until the upstream breaker clears. IEC 61439-6 expresses this as the rated short-time withstand current Icw, typically for 1 second. Common values are 30, 50, 65 and 80 kA. Close to a large transformer the fault level climbs fast: a 2000 kVA, 400V transformer with 6 percent impedance can push roughly 48 kA into a bolted fault, before motor contribution. A 30 kA duct on that feeder is a liability, no matter how generous its ampacity looks.

A worked example, start to finish

A client asks for a busway feeding a 630 kW production hall at 400V, 0.9 power factor, 45 degrees C ambient near the ceiling, 60 meter run, fault level 50 kA at the board.

Step	Number	Result
Full load current	630,000 / (1.732 x 400 x 0.9)	1010A
Demand factor (production line)	x 0.85	859A
Growth margin	x 1.20	1030A
Standard size selected	Next rating above 1030A	1250A
Temperature check at 45 degrees C	1250 x 0.93	1162A, still above 1030A, OK
Voltage drop over 60 m	Formula above	About 1.4 percent, OK
Short-circuit	Fault level 50 kA	Specify Icw 50 kA / 1s

Final specification: 1250A copper sandwich bus duct, IP54 for an indoor dry hall, Icw 50 kA for 1 second, temperature rise test report to IEC 61439-6. Done properly, that is a 20 minute exercise.

The three sizing mistakes we keep seeing

First, sizing from the breaker instead of the load. A 1600A breaker protecting 900A of real load does not need a 1600A busway, unless growth justifies it. Second, ignoring the shaft temperature on vertical runs, which quietly eats the margin you thought you had. Third, quoting peak current instead of continuous current. Busways are rated for continuous duty, and a load profile with 15 minute peaks every hour belongs in the demand factor discussion, not in a knee-jerk size upgrade.

If you send us the load schedule, this whole calculation comes back with the quotation, free. It takes us less time than it takes you to chase the numbers alone.