TL;DR
- Busbar is an enclosed run of copper or aluminium conductors that delivers power along the length of a row. Tap-off boxes plug into the busbar at any point to feed each rack.
- Replaces per-rack cabling at scale: instead of pulling thick cables from the PDU to every rack, you run busbar once and tap off where racks land.
- Typical ratings are 400 A, 800 A, and 1250 A per busbar run, in 3-phase 400 V or 415 V configurations — enough for several 100 kW racks per run.
- The dominant rack-power topology in hyperscale, modern colocation, and any AI build above ~30 kW per rack.
Overview#
A busbar is a rigid, enclosed conductor system mounted overhead along a row of racks. Rather than running individual armoured cables from the floor-level PDU to each rack, the row's entire power need is supplied through a single busbar run, with tap-off boxes plugged in at the position of each rack.
Busbar has been standard in industrial electrical distribution for decades; what changed in the data centre world is density. At 15 kW per rack, conventional cabling was manageable. At 60 kW or 100 kW per rack, the cable bulk and the per-move re-wiring effort makes per-rack cabling impractical, and busbar's plug-and-tap modularity wins.
How It Works#
- Busbar trunking runs along the row, typically suspended from the ceiling above the racks.
- Each rack's tap-off box clips into an access port in the busbar housing, drawing power directly from the conductors.
- Tap-off boxes contain protective devices (MCB or RCBO), a rack PDU output, and optional metering.
- Adding, moving, or upgrading a rack is a physical re-plug operation — no re-wiring of the row.
The 'plug-and-tap' modularity is busbar's headline operational benefit. A new rack lands, an electrician plugs in a tap-off box, and the rack is powered — no shutdown, no re-pulling cable, no PDU re-trenching.
Typical Specifications#
| Parameter | Typical range |
|---|---|
| Conductor | Copper or aluminium |
| Voltage | 230/400 V or 240/415 V, 3-phase |
| Ampacity | 400 A / 630 A / 800 A / 1250 A / 1600 A |
| IP rating | IP54-IP55 typical |
| Tap-off interval | Every 0.5-1.0 m |
| Per-tap rating | 32 A to 250 A |
| Earth bonding | Continuous internal busbar |
| Voltage drop per 100 m (1250 A) | ~1-2 % |
Trade-offs vs Cabled Distribution#
- Capex: busbar is typically 10-20 % more expensive per metre installed than equivalent cable; the saving comes from reduced labour on moves/adds/changes over the life of the room.
- Flexibility: cabling is fixed to specific rack positions; busbar treats the row as a continuous power resource.
- Density: at 100 kW per rack, cable bundles become physically unwieldy; busbar is rigid and compact.
- Fault current: busbar handles much higher fault levels than equivalent cable, which matters at scale.
- Visual appearance: overhead busbar is visible; cabling can be hidden under floor — a minor consideration but real for customer-facing tour rooms.
- Vendor lock-in: tap-off boxes are vendor-specific; a Schneider busbar takes Schneider tap-offs.
When to Use Busbar#
- Any row with racks above ~30 kW average density.
- Any AI cluster build, hyperscale or colocation.
- Rooms where rack churn is frequent — the moves/adds/changes economics dominate.
- Tier III and Tier IV builds with dual A/B power paths: a busbar per path is cleaner than dual cable plant.
Operational Pitfalls#
- Tap-off plugging while live: most modern busbar supports plug-on-live, but procedures and PPE must be in place. Untrained staff plugging in tap-offs is the most common safety incident.
- Phase balance: at high tap counts, mis-distributing single-phase loads across the three phases creates imbalance. PDU-level monitoring is essential.
- Selective coordination: tap-off MCB ratings must coordinate with upstream feeder protection. Misjudged settings cause feeder trips that take a whole row offline.
- Mechanical support: busbar is heavy; ceiling suspension and seismic bracing need engineering review.
- Thermal expansion: long runs expand under high current; expansion joints must be installed and inspected.