Sizing PDUs for GPU Racks Without Tripping Breakers

Nameplate ratings mislead when sizing power distribution for GPU racks, and the failure mode is a tripped breaker partway through a training run rather than anything gradual.

The problem is transient behaviour. Accelerated workloads draw in synchronised bursts, and the instantaneous peak across a rack of GPU nodes runs well above the steady-state figure that gets used for planning. When every node in a rack begins the same collective operation at the same moment, their peaks coincide rather than average out, which is the opposite of how mixed server workloads behave.

How much headroom

Plan against measured peak rather than nameplate or steady state, and leave meaningful headroom above it. Breakers trip on the transient, not on the average, and a rack sized to its steady-state draw will run for weeks before the right combination of jobs coincides.

Per-outlet metering is what makes this manageable rather than theoretical. Rack-level totals cannot tell you which node moved when the number rises, and at 40kW a rack there is very little margin between what is used and what the feed supplies. Per-outlet data turns capacity planning into arithmetic and allows a rack to be filled to its real limit rather than to a cautious guess well below it.

Practical sequence

Establish the committed per-rack figure from the facility first, in writing. Size the PDU to that rather than to the sum of nameplate ratings. Then measure actual draw under a representative workload during commissioning, before the rack is filled, and use those numbers for the remaining positions.