How many servers per PDU?
Quick answer: use the PDU calculator.
Two limits decide it: how much power the PDU can deliver, and how many outlets of the right type it has. The lower of the two wins.
1. Usable PDU power
Single-phase: volts × amps. In North America, keep continuous load at 80% of the breaker, so multiply by 0.8. Three-phase: volts × amps × 1.732 (× 0.8 in North America).
| PDU | Input | Usable power |
|---|---|---|
| APC AP8941 | 208 V, 30 A single-phase | 208 × 30 × 0.8 ≈ 5.0 kW |
| APC AP8853 | 230 V, 32 A single-phase | 7.4 kW rated |
2. Real server draw, not PSU rating
A server with two 1100 W PSUs does not draw 2200 W. Use the vendor's power calculator (Dell EIPT, HPE Power Advisor, Lenovo LCP) or a measured peak from the BMC. Add about 20% margin for spikes and future upgrades.
3. A/B redundancy
With dual PSUs on two PDUs (A feed and B feed), each PDU must be able to carry the whole rack if the other feed fails. So the total rack load must fit on one PDU, not two.
4. Outlets
Each server with two PSUs uses one outlet on PDU A and one on PDU B. A C14-inlet PSU needs a C13 outlet. A C20-inlet PSU needs a C19 outlet, and PDUs usually have only 3–6 of those.
Worked example
A/B pair of APC AP8941 (5.0 kW usable each, 21 C13 + 3 C19). Servers measured at 450 W peak, plus 20% margin = 540 W. Power limit: 5000 / 540 ≈ 9 servers. Outlet limit: 21. Answer: 9 servers per A/B pair. Power runs out long before outlets do.
GPU servers flip this: one 8-GPU system can need 6 × C19 outlets and more than 10 kW, so a single PDU may not be enough. See DGX H100 PDU requirements.
Related: PDU power cord compatibility, server to PDU power cord.
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