Data Center PDU Sizing

Q: How do I size a PDU for a server rack?

Start with measured or nameplate rack kW, convert to kVA using power factor, add 15–25% planning margin, then verify branch ampacity at your PDU voltage (208V or 415V three-phase).

Q: What is the difference between PDU kW and branch breaker amps?

kW is real power; branch breakers limit current (amps). For three-phase: I = kW ÷ (√3 × V × PF). A 10 kW rack at 208V and PF 0.95 draws roughly 26 A per phase before margin.

Q: Should I use metered or switched PDUs for sizing?

Metered PDUs help validate actual draw after install; sizing still uses peak kW plus margin. Switched outlets do not change ampacity requirements—they add control only.

Match rack PDUs to IT kW, branch ampacity, and growth headroom before final UPS and whips are ordered. Use this scenario after rack power density screening.

Who this scenario is for

Best for: Colocation engineers, data center ops, and electrical designers selecting rack PDUs (208V/415V, single or three-phase) for new rows.

Not ideal for: Final NEC ampacity studies, arc-flash labels, or utility service entrance design.

Quick answer

PDU planning kVA ≈ rack kW ÷ PF × (1 + margin). Typical margin **15–25%**. Verify three-phase current: **I ≈ kW ÷ (√3 × V × PF)**. Select PDU nameplate and branch breaker above that current with manufacturer coordination rules.

Use UPS Load Calculator

Key variables

Rack kW: From meter data or rack power density rollup.
PDU voltage: 208V vs 415V changes per-phase amps at the same kW.
Power factor: IT load often 0.9–0.95; use measured PF when available.
Redundancy: Dual-cord racks may need A+B feed sizing, not half on one PDU.
Outlet count: C13/C19 mix must cover nameplate without oversubscribing breaker.

Example: 10 kW rack, 208V 3-phase, PF 0.95

Planning kVA ≈ 10 ÷ 0.95 × 1.2 ≈ **12.6 kVA**. Current ≈ 10,000 ÷ (1.732 × 208 × 0.95) ≈ **29 A** per phase. Select a PDU and branch rated above 29 A with standard breaker sizes (e.g. 32 A or 40 A class per local code).

Next steps

Rack Power Density — estimate kW per cabinet first.
UPS Load Calculator — device-level kW rollup.
UPS Capacity Calculator — upstream kVA with PF.
UPS Runtime for Server Rack — backup at this load.
Step-down transformer sizing — row and hall kVA.

Assumptions and disclaimer

Planning estimates only. Confirm ampacity, breaker curves, and PDU manufacturer derating with licensed design teams.

FAQ

How do I size a PDU for a server rack?

Use rack kW → kVA with PF and margin, then check three-phase amps against branch rating.

What is the difference between PDU kW and branch breaker amps?

Breakers limit amps; convert kW with voltage and PF: I ≈ kW ÷ (√3 × V × PF) for three-phase.

Should I use metered or switched PDUs for sizing?

Metered PDUs validate draw after install; sizing uses peak kW plus margin regardless of outlet switching.