19 December 2018
Marc Garner is VP of Schneider Electric UK’s IT division.
A good methodology for selecting the optimal PDU for an IT rack is to follow this five-step approach that prioritises and simplifies the decision-making process.
1. Determine output plug type and quantity
The most common plug types in data centres are C-13 and C-19 connectors defined by IEC 60320. The best option is to choose a rack PDU with a combination of C-13 and C-19 outlets. This ensures there are more of each outlet type than is needed for the initial load, and allows for more equipment to be added later if required.
2. Estimate power capacity
Several methods can be used to estimate rack equipment power load. The nameplate rating, based on power draw at full load, is typically higher than actual usage. When choosing a PDU, first assume a rack power level based on an estimate of total data centre utilisation, e.g., if the overall power delivery is 1MW and the IT load consists of 100 racks, then a maximum power level could be estimated at 10kW per rack.
3. Determine input plug type
The site voltage (208V or 240V) and phase (single or three phase) will dictate the input power needed for the rack. To mitigate overload risk, it is advisable to limit PDUs to 40A or less for single phase, and 32A or less for three phase applications. An appropriate whip length, typically two metres, should be selected to allow the cord enough slack to reach the power outlet.
4. Select visibility and control options
Various levels of visibility into rack power are available. The most advanced is metering, where built-in network management capabilities alert users as loads approach pre-determined thresholds. Many metered PDUs also measure temperature and humidity. In addition, switching capability will allow power outlets to be turned on or off remotely. A rack PDU should be rated for continuous use at up to 50ºC, and metering should have an accuracy tolerance to within five per cent.
5. Select form factor and mounting
Rack PDUs are deployed into the back of a server cabinet in one of two mounting orientations: a horizontal 19-in rack-mounted version used mainly with open-frame racks; or a vertical 0U PDU that positions outlets closer to the equipment they power. The latter is the preferred option for data centres because it consumes no rack space, enabling shorter power cords and therefore less cable management.
