How to Calculate Busway Voltage Drop

Formula, Example & Engineering Guide

Voltage drop is an important engineering consideration when selecting a busway system. Even if a busway has sufficient rated-current capacity, a long route or high load can still cause excessive voltage drop, which may affect downstream equipment and overall system performance. This is especially important for long feeder routes, rising mains, industrial plants, data centers and large commercial buildings.

For a balanced three-phase system, a commonly used approximation is:

ΔV = √3 × I × L × (R cosφ + X sinφ)

Where I is load current, L is route length, R is resistance per unit length, X is reactance per unit length and cosφ is the power factor. The units of R, X and L must be consistent before calculation.

A practical calculation process is:

Confirm Load Current → Confirm Route Length → Obtain R & X Data → Confirm Power Factor → Calculate Voltage Drop → Calculate Voltage Drop Percentage

For example, a 400 V, 1600 A busway with a 100 m route, power factor 0.90, R = 0.040 mΩ/m and X = 0.020 mΩ/m gives an illustrative voltage drop of approximately 12.4 V, or about 3.1%. Final calculations should always use the actual technical data for the selected busway.

For long routes, engineers may need to consider a larger busway rating, different conductor material or an alternative distribution layout.