How to calculate PDU load?
Calculate PDU load in 3 steps. Get the watts, amps and volts formula, the 80% rule and a worked 208V example to avoid tripped breakers.
PDU load is the total power your devices draw, compared with the power the PDU and its circuit can safely supply.
The basic formula
Watts = volts x amps. To find amps from watts, use amps = watts / volts. For three-phase power, multiply by 1.732: watts = volts x amps x 1.732.
Apply the 80% rule
Electrical codes in many regions treat a continuous load as safe only up to 80% of the circuit rating. This derating protects the circuit breaker and wiring from overload. Local rules vary, so confirm with a licensed electrician.
| Circuit | Maximum | Safe continuous load (80%) |
|---|
| 120V, 20A single-phase | 2,400 W | 1,920 W |
| 208V, 30A single-phase | 6,240 W | 4,992 W |
| 208V, 30A three-phase | 10,808 W | 8,646 W |
Worked example
A rack holds 10 servers that draw 350 W each. That is 3,500 W, or about 16.8 amps at 208V. On a 208V, 30A circuit, the safe limit is 4,992 W, so the load is about 70% of the safe limit. That works, with room to add one or two devices.
Find the real draw
- Read the draw from a metered PDU if you have one.
- Otherwise use the manufacturer spec sheet, not the nameplate maximum.
- If a label lists VA, multiply by the power factor to get watts.
- Add everything into a power budget for the rack.
- Divide by the safe limit to get a load percentage.
Redundant power
With A and B feeds, keep each below 40% so one feed can carry the whole rack if the other fails. Also check each outlet bank, and stagger server start-up so inrush current does not trip a breaker.
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