Busway, Voltage Drop & Fault Current Sizing Calculator

How the Calculations Work

Rack and row current

Three-phase line current from rack power:

A 120 kW rack at 480V and 0.98 PF draws about 147 A. Under NEC 210.19(A) and 210.20(A), a continuous load — anything expected to run at maximum for three hours or more, which describes essentially all data center load — requires the overcurrent device and conductor to be rated at 125% of the continuous current. That pushes a 147 A rack to a 184 A design basis, and up to the next standard tap-off frame at 200 A.

Feeds per rack and tap-off sizing

A high-density rack is not fed by a single tap-off. GB200/GB300 NVL72-class racks typically take 4, 6, or 8 bus plugs and drop cords, distributed evenly across the bus runs so no single bus failure drops the rack. The tap-off is therefore sized on per-feed current, not rack total:

A 120 kW rack at 480V draws 147 A total. Split across four feeds — one per bus in a 4-to-make-3 row — each feed carries about 37 A normally and 49 A when one bus is lost. The electrical minimum is a 60 A frame, yet 100 A tap-offs are what actually gets installed, because frames get standardized for whip and connector compatibility and to leave headroom for the next hardware generation. The tool reports the calculated minimum and lets you override the frame to check utilization of whatever you are actually specifying.

Busway sizing: two cases, whichever governs

Frame selection is the greater of two independent checks, not a single stacked calculation:

Stacking the 125% continuous multiplier on top of the N-1 contingency double-counts margin and oversizes the bus badly — it is the difference between selecting a 1200 A and a 2000 A frame on the same row. Most distributed-redundant designs treat the N-1 condition as a temporary state and evaluate it at 100%, while the continuous multiplier applies to the normal operating case. For 2N, where either side is expected to run indefinitely, many engineers apply 125% to the contingency case as well — the Continuous Duty Basis selector lets you choose, and the result always names which case governed.

How much of the row each bus carries comes from the topology, covered in depth in our prefabricated HAC guide:

• 2N — two buses, each ~50% loaded normally, each carrying 100% of row when the other is lost.
• 4-to-make-3 — four buses, each ~25% normally and ~33% at N-1. A 24-rack row of 120 kW GB200 racks lands on 4 × 1200 A at 74% normal / 98% at N-1 — the configuration widely deployed today.
• 1N — one bus carrying 100% at all times, with no redundancy.

Voltage drop

Voltage drop across the busway run, using the distribution factor K:

K = 0.5 for load distributed evenly along the run (the normal case for a rack row, where the average load travels half the length) and K = 1.0 for a point load at the far end. The calculator reports drop at the worst-case bus current for the selected topology, since that's the condition the design must satisfy. NEC 210.19 informational notes suggest 3% maximum on branch circuits, but data center practice typically targets ≤1.5% on the busway so the rack PDU sees stable voltage.

Available fault current

Infinite-bus approximation at the transformer secondary:

This is deliberately conservative: it ignores utility source impedance and the impedance of everything between the transformer and the point of application, both of which reduce actual fault current. It also excludes motor contribution, which adds to it. Use it to bracket equipment ratings early — not as a substitute for a study. Busway bracing and tap-off device interrupting ratings must both equal or exceed the available fault current at their point of application.

Notes on the Data

Busway impedance values are representative figures for copper track busway at 75 °C and are used to produce realistic estimates across standard frame sizes. Actual R and X vary by manufacturer, conductor material (copper vs. aluminum), housing design, and configuration — Starline publishes specific values in its product selection guides, and those should replace these defaults once the product is selected. Standard tap-off frames follow common industry ratings from 30 A through 400 A; above that, plug-in units are typically paired or replaced with a dedicated feeder tap.