What is a PDU?
A PDU (power distribution unit) is a device that takes power from one or more upstream circuits and distributes it to multiple pieces of IT equipment. In data centers the term usually means the rack PDU, a strip of outlets mounted vertically or horizontally in a cabinet that feeds servers, storage and switches. It also covers larger floor-standing units that distribute power from the UPS to rows of racks. Rack PDUs range from simple outlet strips to networked devices that meter every outlet and report over SNMP, a web interface or a vendor API.
Types of rack PDU
Vendors use slightly different names, but rack PDUs generally fall into these types:
- Basic: distributes power with no metering or network access.
- Metered: shows total current on a local display, sometimes also over the network.
- Monitored: reports load remotely so it can be tracked centrally.
- Switched: lets operators turn individual outlets on and off remotely.
- Switched with outlet metering: measures each outlet and allows remote switching, often with ports for temperature and humidity sensors.
A worked example: rack power budget
Capacity planning starts with the circuit. In North America, continuous loads are normally limited to 80 percent of the breaker rating. A single-phase 208 V, 30 A circuit can therefore carry about 208 x 24 = 4,992 W, or roughly 5 kW. A three-phase 208 V, 30 A circuit carries about 1.732 x 208 x 24, or roughly 8.6 kW. If each server draws 450 W under typical load, the single-phase PDU supports about 11 servers and the three-phase PDU about 19. Measured outlet data matters here, because nameplate ratings are often far above real draw, and planning from nameplates leaves capacity unused.
Plan from measured peak draw per outlet, and keep enough margin that one feed can carry the whole rack if the other fails.
Redundancy and A/B feeds
Most servers have two power supplies. Each connects to a different PDU, and each PDU connects to a separate upstream path, usually called the A and B feeds. If one feed fails, the other carries the full load. This only works if each feed stays below half of its usable capacity in normal operation. A rack drawing 3 kW on each of two 5 kW feeds looks healthy until feed A trips and feed B must carry 6 kW, which exceeds its limit and can trip its breaker too. Alerting on the combined load of both PDUs catches this risk in advance.
PDU vs UPS and other power equipment
A PDU distributes power. The UPS (uninterruptible power supply) sits upstream and bridges outages with batteries or flywheels until generators start. Between the UPS and the racks there may be floor PDUs, remote power panels or overhead busway. A rack-level automatic transfer switch (ATS) can move single-corded equipment between two sources. Recording which PDU, panel and UPS feed each rack lets operators see which equipment is affected when a power alarm fires.
What to monitor on an intelligent PDU
Networked PDUs report more than total load. Energy readings from rack PDUs are also a common way to measure IT equipment energy, the denominator in The Green Grid's PUE metric. Useful data points include:
- Current per phase, per circuit breaker and per outlet.
- Real power in watts and energy in kWh, for chargeback and PUE reporting.
- Voltage and power factor.
- Breaker trips and outlet state changes.
- Temperature and humidity from attached sensors.
How CloudSino approaches PDUs
CloudSino DCOS lists UPS units and PDUs among the server room infrastructure it supports, alongside servers, storage and network devices. Its energy and capacity functions cover device-level power monitoring, cabinet energy consumption statistics, cabinet capacity analysis, U-position space management and intelligent pre-racking, so teams can check whether a cabinet can take another device. For high-density racks, CloudSino describes PDU power monitoring for 40 kW to 100 kW racks, real-time monitoring of busbar, circuit breaker and ATS status, and capacity warnings based on power margin.
Frequently asked questions
What is the difference between a metered and a switched PDU?
A metered PDU measures current, for the whole unit or per outlet, but cannot turn outlets off. A switched PDU adds remote control of individual outlets, so a hung device can be power-cycled without a site visit. Many current models combine metering and switching in one unit.
Should I choose a single-phase or three-phase PDU?
Three-phase PDUs deliver much more power through one cord and spread load across the upstream electrical system, which suits dense racks and GPU servers. Single-phase is simpler and often enough for low-density racks. With three-phase units, balance outlets across phases so no single phase overloads first.
How do PDUs connect to monitoring tools?
Networked PDUs typically offer a web interface and SNMP with a vendor MIB, and some add a REST API or Modbus. Monitoring systems poll per-outlet and per-phase values and receive SNMP traps for threshold breaches. Use SNMPv3 and a management VLAN, because a switched PDU can turn off production servers.