Amperes (or Amps) are the standard measure of electrical current. Much like water flowing through a pipe, Amps measure how much electricity is moving through a wire at any given time. The Amp draw of a circuit is dependent on the needs of the devices plugged into it, but is always limited by a branch circuit breaker.

Volts are the standard measure of electrical potential and are a fixed value for every circuit. Volts are measured with respect to a reference point (usually between the two respective legs of the circuit). As one increases the electrical potential between two points, the amount of potential energy available increases. For example, consider a rock dropped from 10 feet versus one dropped from 100 feet. The rock dropped from 100 feet will be going much faster when it hits the ground than one dropped from 10 feet and exert more energy on the ground as it hits. Until the rocks are dropped, that energy exists only as potential. Similarly, a 208V circuit has about 1.73 times the potential energy of a 120V circuit, when drawing the same amount of current.

Watts are a measure of the total power being consumed by a system. The simplest calculation is: Watts = Volts x Amps. This is the measurement that power companies use for billing and is also known as Volt-Amps. A true RMS calculation of Watts takes into account the shape and phases of the voltage and current wave forms being delivered to a circuit to provide a reading of the power being used by a circuit. Any difference between the RMS value of Watts and the Volt-Amps value indicates inefficiencies in the way power is being used by the equipment on the circuit.

RMS stands for Root-Mean-Squared. It refers to a complex calculation involving the volts and amps over a period of time determining how power is being consumed by the devices on a circuit.

Apparent Power is the instantaneous calculation of Volts x Amps and is the measurement by which the power companies bill.

Real Power is the RMS value of Watts.
When divided by Apparent Power, it is a measurement of the efficiency of power usage on a circuit.

Power Factor is the ratio of Real Power to Apparent Power. 100% is perfect power. Lower values of Power Factor indicate that the circuit is wasting energy. Power Factors below 60% can cause significant damage to electronic equipment. Circuits with Power Factors below 40% need power conditioning.

NEC stands for National Electric Code and is sponsored by the NFPA (National Fire Protection Association). These guidelines are used by the industry to safeguard life and property from hazards arising from electricity.

The electrical power consumption of electrical appliances is measured in watts (W). Wattage (watts value) is a product of the rated voltage (V) and current (I). W = V x I. The higher the voltage the lower the current required to produce the same watts. The same size wire can carry 2x as much power (watts) @ 250V verses 125V.

Perform the following for a quick estimate of the power needed in a cabinet:

Add the power ratings in watts (W) on the regulatory labels of the equipment you want to put in the cabinet {Sometimes, the labels indicate amps (A) instead of watts (W). In this case, multiply voltage (V) and current (A) values to get an approximate value for power.}

The National Electrical Code (NEC) requires that the current drawn from a branch circuit does not exceed 80% of the circuit’s rating. As a result, the maximum input current rating that can be applied to a Geist PDU is 80% of the plug rating. For example a PDU with a 20 A rated plug will have a de-rated current of 16 A to be in compliance with the NEC. The de-rated current value shown for each Geist unit is the input current rating for the PDU and serves to remind users not to violate the code requirements.

By using the de-rated values of the input and output circuit, you will be able select a PDU with the proper configuration (including output circuit rating, receptacle type, and plug type) for your required application.

Heat is measured in watts. Power is measured in watts. Almost all electrical energy used in computing is converted to heat. A typical computer power supply is about 70-80% efficient. This means that for every 100 watts it draws, between 20 and 30 watts are converted directly into heat without ever being used by the computer. As the computer processes information, the rest of the power is dissipated throughout the system as heat. In addition, computer power supplies are typically most efficient at  80% of their maximum rated load. Since all power can be counted as heat, adding the watt ratings of all equipment in a cabinet will give a relatively 1:1 relationship to heat generated. I.E. 40 servers x 300 watts each = 12,000 watts (12kw) heat. Refer to page 15 for a chart relating power usage to heat dissipation and air conditioning requirements.

Monitoring current at the outlet level allows you to pinpoint potentially system-critical failures and take preventative measures. A gradual rise in system current draw might indicate a power supply or cooling failure. A sharp spike might indicate a short circuit. Outlet level monitoring can also help keep track of where power is being used. Per outlet monitoring is also an easy way to accommodate departmentalized power billing within large companies whose data centers serve several departments. RacSense Ultra (RCU) and some Ethernet (EM) Products

A "Plenum" is an air space often used for HVAC airflow and for cables and piping. In the data center, this is typically the space below a raised floor. A “Plenum Rated” cable refers to structured cabling permitted by building code for use in plenum spaces. Plenum rated cable has a slow-burning, fire-resistant casing that emits little smoke. Article 645 of the National Electric Code (NEC), titled Information Technology Equipment, discusses the use of power cables under a raised floor.

Geist is pleased to offer UL® Listed DP-1 Rated cable, with an FT-4 flame rating, as an option available on most Geist PDU’s. Please contact us at 800-432-3219 for assistance with your specific application or additional information.

A 3  Delta is a 4 wire system consisting of 3phase ( ) conductors (X, Y and Z) and a Ground. Without the neutral present, only phase to phase wiring is possible. The potential between two phase conductors is typically 240V or 208V.

A 3 phase 120/208V WYE is a 5 wire system consisting of 3 phase conductors (X, Y and Z), neutral and ground. Each phase is 120º from each other. The potential between any 2 phase conductors is 208V. The potential between any phase and neutral is 120V. Thus this setup can be wired to provide 120V, 208V or a combination of both voltages within one PDU. In a balanced system the neutral current is zero.

1. Less wire under the floor improves airflow and reduces wiring confusions. A 20A 3  installation contains 5 wires where the equivalent single phase system would require 9 wires (3x3).
2. Fewer whips to pull saves you time and money. A 3  system has one whip for the electrician to bring to the cabinet where the equivalent single phase system would have 3 whips. This saves both material and labor cost.
3. Simplified load balancing reduces technician installation and troubleshooting time. With all 3 phases available in a single cabinet, load balancing can be achieved at the cabinet level where similar type equipment is often found. In a single phase system, a minimum of 3 cabinets may need to be looked at to balance the same load.

A circuit breaker is a device that opens and closes an electrical circuit. It senses the current flow and operates automatically when the current exceeds its rating. Human intervention is required to switch the circuit back on.

A surge protector limits the magnitude of the voltage in an electrical circuit without interrupting the current flow. It is used to prevent voltage spikes from damaging electrical appliances.

Yes. However, the building in which you install the PDU should have an appropriately sized branch circuit breaker in the breaker box. Breakerless PDU’s must distribute power to receptacles sized for the building installation ampacity.

The receptacles on a 30A PDU are usually 15A or 20A outlets. The NEC requires that these components be protected by a 20A branch circuit breaker. Most 30A units break the primary feed circuit into multiple internal circuits.

The receptacles in a 30A PDU are divided into two independent groups. A 30A PDU distributing to 15A or 20A receptacles must be broken down into either 15A or 20A circuits internally. By opting for 20A internal circuits, PDU circuit balance is less critical. One circuit may be loaded to >15 amps. This would not be possible if each breaker were rated at 15 amps.

PDUs with 3 Phase WYE input connectors allow for the option of distributing 120 volts and 208 volts in a single power strip. The 3 Phase WYE connections consists of 3 Hot Phases, 1 Neutral and 1 Ground. 208 Volt output is achieved across 2 phase connectors and 120 Volt output is achieved across 1 Phase Connector and the Neutral Leg.