Power Definitions and Terms

An Amp (or Ampere) is the standard measure of electrical current.  Much like water flowing through a pipe, the Amp is a measure of how much electricity is moving through a wire at a given time.  The Amp draw of a circuit is dependent on the needs of the devices plugged into it, and is limited by the branch circuit protection.

A Volt is the standard measure of electrical potential and a fixed value for every circuit. Voltage is measured with respect to a reference point (usually between the two respective conductors of the circuit). Voltage is analogous to pressure in a water pipe. Higher pressures, or higher voltages, allow more energy to flow within a given amount of time for a given wire size. Standard voltages present in most data centers are 120V and 208V in the U.S., and 230V in continental Europe. Some newer U.S. data centers are being designed to utilize 230V.

A Watt is the measure of total work performed by the energy consumed in a system. The calculation is: Watts = Volts x Amps x Power Factor.

RMS stands for Root-Mean-Squared. It is used in conjunction with AC Volts and AC Amps to express an average value. A true RMS calculation takes into account the shape and phases of the wave forms being delivered to a circuit. AC voltage and current are ever-changing values. Using RMS measurements provides useful values.

Apparent Power is the instantaneous calculation of Volts x Amps.

PUE stands for Power Use Effectiveness. PUE is a measure of how efficiently power is being used in a data center, and is becoming the standard benchmarking metric in most data centers. PUE is determined by dividing the total facility power use (Building Watts) by the IT equipment load (IT Watts).

DCIE stands for Data Center Infrastructure Efficiency. DCIE is IT Power divided by Total Facility Power, expressed as a percent (%). DCIE is the inverse of PUE.

Power Factor is used to define the ratio of Real Power to Apparent Power, or how much of the power is being used to do work. Power Factor is therefore a number zero to one but may also be displayed as a percentage. Lower power factors have the additional cost of energy loss in the distribution system and require a larger infrastructure.

The power distribution within the building has several points where losses occur (UPS, transformers, wire runs), so the ideal place to measure the IT power load is at the cabinet level within the power strip. These readings can be collected and aggregated to determine the IT power load. Once an initial assessment of PUE has been made, efforts can be made to improve PUE by applying various methods to improve operational efficiencies in the data center.

Real Power is sometimes referred to as ‘True’ power. Real Power is the actual power being used by the load and is measured in Watts (W). Real Power takes into account the phase angle of the current and this is typically the nameplate rating on IT equipment.

An absolute must in mission-critical applications, the general concept behind power redundancy is to connect critical equipment to two independent power sources. If one line of power is interrupted, the second is able to power the critical equipment. This is accomplished in the following. First, equip the cabinet with two rPDUs, each of which is capable of handling the power requirements of the entire cabinet. Plug one rPDU into the first power source and the other into the alternate. Plug each piece of equipment into both (most data center equipment today has multiple power supplies as a fail-safe).

Apparent power is the calculation of volts times amps. Real power is RMS power (real-time) plus the power factor calculation.

The L22 plug has been approved for use at 200-240/346-415V by UL. The nameplate rating of the rPDU identifies the 230/400V rating in addition to labeling by the plug.

An L22 is a NEMA 4P/5W 277/480V 3~ WYE Twist-lock plug. Vertiv Geist offers this on North American 200-240/346-415V 3~ rPDUs as an alternative to the IEC 309 4P/5W Pin and Sleeve connector. Note that the intended operation of this unit is for 200-240/346-415V only.

The IEC 309 Pin and Sleeve connector is more commonly used in European applications, however it is common in North America as well on devices rated at 60A and higher.

An L22 plug may be desired over the equivalent IEC 309 plug as it is often more commonly found in North America in addition to being a smaller connector.

On a 3 phase rPDU outputting 120V the calculation would be Volts x Amps (80%) x 3 (# of independent conductors). For example, a 30A 3 phase unit outputting 120V would be 120 x 24 x 3=8.6kW. If the rPDU is outputting 208V, the calculation would be Volts x Amps (80%) x SQRT(3). The SQRT(3) is used as line voltage for 208V output is derived from using two hot conductors. For example, a 30A 3 phase unit outputting 208V would be 208 x 24 x SQRT(3)=8.6kW.