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How Do You Calculate Voltage Drop on a Bus Duct?

By DHYN Engineering Team  ·  August 31, 2026

Quick answer: To calculate voltage drop on a bus duct, use Vd = √3 × I × L × (R cosφ + X sinφ) for a three phase run, where I is the load current, L is the one-way length, and R and X are the per-meter resistance and reactance from the manufacturer's test data. Keep the drop at or below 3 percent on the busway itself and 5 percent total from the transformer to the furthest load. (Answer time: August 31, 2026)

Voltage drop is the calculation most buyers skip, because the ampacity table feels like the whole answer. It is not. We have taken calls from contractors whose 1600 A duct was perfectly sized on paper and still tripped motors on undervoltage, because nobody ran the numbers on a 90 meter run. The math takes ten minutes. Here is how to do it.

The formula, in plain terms

For a three phase busway, the voltage drop along the run is:

Vd = √3 × I × L × (R cosφ + X sinφ)

I is the actual load current in amps, not the duct's nameplate rating. L is the one-way route length in meters. R is the conductor resistance per meter, X is the reactance per meter, and both come from the manufacturer. cosφ is the load power factor, with sinφ from the same angle (at a power factor of 0.9, sinφ is 0.436). For a single phase run, drop the √3 and use a factor of 2 instead, because the current has to go out and come back.

To get the percentage, divide Vd by the system voltage. On a 400 V three phase system, a 12 V drop is 3 percent.

Where R and X come from

Do not compute R from a copper resistivity table unless you have to. The resistance of a bus duct depends on bar cross section, joint design and plating, and the verified values are in the test reports behind IEC 61439-6 compliance. Ask for them. As a rough sense check, these are representative figures for copper sandwich busway at operating temperature:

Rating	R (mΩ per meter, hot)	X (mΩ per meter)
630 A	0.096	0.024
1250 A	0.048	0.016
2500 A	0.024	0.010
4000 A	0.015	0.007

Two things to notice. Resistance roughly halves each time you double the rating, because the copper section doubles. Reactance stays small and shrinks slowly, which is exactly why sandwich busway behaves well on voltage drop. A cable in a tray carries noticeably higher reactance, and at large currents reactance is what eats your voltage.

A worked example: 1250 A duct, 60 meter run

Say the duct feeds a production floor. Actual measured load is 1100 A, power factor 0.9, route length 60 meters, system 400 V three phase. From the table, R is 0.048 mΩ/m and X is 0.016 mΩ/m.

First the impedance term: 0.048 × 0.9 + 0.016 × 0.436 = 0.0432 + 0.0070 = 0.0502 mΩ per meter.

Then the drop: Vd = √3 × 1100 × 60 × 0.0000502 = 5.74 V. As a percentage, 5.74 ÷ 400 = 1.4 percent. Comfortably inside the 3 percent target, so a 1250 A duct is fine here. If the same run were 130 meters, the drop would reach about 3.1 percent and you would step up to 1600 A, not for the current capacity but for the voltage.

The limits people actually design to

IEC 60364-5-52 suggests a maximum of 4 percent for circuits fed from the public LV supply, measured from the origin of the installation. North American practice under the NEC uses a widely followed recommendation of 3 percent on any one feeder and 5 percent total to the furthest outlet. In our own designs we hold the busway run itself to 3 percent, which leaves headroom for the branch circuits after the tap-offs. Motor loads deserve extra care, because a motor starting at reduced voltage draws longer and sags the whole run for its duration.

Mistakes that quietly inflate the drop

The first is using cold resistance. Copper resistance rises about 0.4 percent per degree Celsius, so a duct running at 60 degrees has roughly 16 percent more resistance than the 20 degree datasheet value. Use the hot figure. The second is assuming a power factor of 1. Real loads sit between 0.8 and 0.95, and the reactive term grows fast as power factor falls. The third is treating the run as end-loaded when it is actually distributed. A plug-in busway feeding loads along its whole length drops roughly half what the same current would drop if it all arrived at the far end, and knowing that can save you a size. The fourth is forgetting the last segment. After the final tap-off, the small whip cable to the load adds its own drop, and on long whips it is not trivial.

If you are sizing a duct from scratch, run this calculation alongside the ampacity method in our busway sizing guide. Ampacity tells you what the duct can carry. Voltage drop tells you what your load actually receives.