C H A P T E R  4

System Power and Cooling Requirements

This chapter provides information about important power issues relating to your servers. Your server documentation provides more detailed power information.

The design of your electrical power system must ensure that adequate, high-quality power is provided to each server and all peripherals at all times. Power system failures can result in system shutdown and possible loss of data. Further, computer equipment that is subject to repeated power interruptions or fluctuations experiences a higher component failure rate than equipment that has a stable power source.

Power Requirements

Each system, when properly configured and installed, must receive sufficient incoming AC power to supply all installed components. The data center should be able to provide a stable, dual-current path to the installed equipment. In addition, the power infrastructure must be designed to maintain system uptime even during disruption of the main power source. It is important to use dedicated AC breaker panels for all power circuits that supply power to your systems. The power system should be designed to provide sufficient redundancy, eliminate all single points of failure, and allow the isolation of a single system for testing or maintenance without affecting the power supplied to other systems.

Power Sources

It is important to secure multiple sources of power when possible. Ideally, multiple utility feeds should be provided from different substations or power grids. This setup provides power redundancy and backup.

The systems provide AC input fault tolerance via redundant power supplies. Therefore, it is prudent to attach to each primary power supply a common power cord from one power grid that can supply power to all systems, and to attach another power cord from a different power grid to the redundant supplies. If a primary power grid goes offline, a backup power grid will provide power to the redundant supplies to keep the systems operating. See Power Supplies for information about power supply redundancy.

For the Sun Fire V890 server, use the 10-amp power cords that are supplied with the server. The V890 server uses 200 to 240 VAC input only.

UPS and Backup Generator

Using an online uninterruptible power supply (UPS) and a backup power generator provides a good strategy for obtaining an uninterruptible source of power. The online UPS filters, conditions, and regulates the power. It protects the systems from fluctuating voltages, surges and spikes, and noise that may be on the power line. The battery backup for the UPS should be capable of maintaining the critical load of the data center for a minimum of 15 minutes during a power failure. This is typically sufficient time to transfer power to an alternate feed or to the power generator.

The backup power generator should be able to carry the load of both the computer equipment and the supporting heat, ventilation, and air conditioning (HVAC) equipment. The generator should include dual power distribution switch gear with automatic transfer switching. To offset the possibility of a generator failure, power system designers often include a temporary generator for secondary backup.


Grounding design must address both the electrical service and the installed equipment. A properly designed grounding system should have as low an impedance as is practically achievable for proper operation of electronic devices as well as for safety. It is important to use a continuous, dedicated ground for the entire power system to avoid a ground differential between various grounds. Grounding design in the United States should comply with Article 250 of the U.S. National Electrical Code unless superseded by local codes. Use an antistatic wrist strap when working inside the chassis.

All properly installed Sun systems are grounded through the power cable. However, there are reasons for installing an additional mechanism to equalize potential. Problematic or deficient conduits can negatively affect another system, especially with respect to the possibility of spreading voltages. Additional grounding points help to avoid leakage current, which prevent system malfunctions. Therefore, additional cables might be used to connect Sun systems and cabinets to the data center's potential equalization rail. Enlist the aid of a qualified electrician to install grounding cables.

Emergency Power Control

A primary power switch that can disconnect all electronic equipment in the data center is specified by NFPA 70 and NFPA 75 (National Fire Protection Association specifications) at each point of entry to the data center. The primary switch should disconnect power to all computer systems and related electronic equipment, HVAC equipment, UPS, and batteries. Multiple disconnects for separate parts of the power systems are also acceptable, but in both cases, the switches must be unobstructed and clearly marked.

Power Constraints

All servers covered by this guide are shipped with a sufficient number of power supplies to provide all power needed by all Sun supported configurations.

Sun does not test many third-party products that are compatible with Sun servers. Therefore, Sun makes no representations about those products or about the power requirements for products not supplied by Sun.

Power constraints can occur in two areas:

To maintain a safe facility, you must ensure that the AC current draw does not exceed the maximum current limit for your power outlet. In the United States and Canada, the maximum is 80% of the outlet's total capacity, which is 12 amps for
15-amp circuits and 16 amps for 20-amp circuits, and so forth. For areas outside of the United States and Canada, contact local agencies for information about local electrical codes.

See TABLE 5-4 for maximum input current and power consumption for the servers.

Power Supplies

Each server covered by this guide is shipped by Sun with one or more power supplies, which are sufficient to support the maximum configuration of the server.

The systems provide "N+1" power supply redundancy to maintain system uptime. An N+1 redundant power supply configuration does not add to the power capacity of the systems. "N" represents the number of power supplies needed to power a fully configured system. The "1" means that there is one additional power supply in the system to handle the load if a supply fails. When the system is operating normally, all of the power supplies are turned on, even the redundant supplies.

The redundancy configurations of the systems are as follows:

In a 1+1 configuration (that is, two power supplies are installed, each capable of providing enough power for the entire system), both supplies are turned on and are delivering power. Each supply delivers approximately 50% of the power needed by the system. If one supply fails, the supply that is still online will deliver 100% of the power needed to keep the system running.

In a 2+1 configuration (that is, three power supplies are installed, with two power supplies delivering enough power for the entire system), all three power supplies are turned on and are delivering power. Each supply delivers approximately 33% of the power needed by the system. If one supply fails, the supplies that are still online will each provide 50% of the power needed to keep the system running.

Most power supplies cannot support the maximum values on all outputs at the same time because that would exceed the total power supply output capacity. The load must be distributed among the outputs in a way that does not exceed their maximum values or the total output capacity of the power supply.

The servers have built-in protection against exceeding the output capacity of the power supply configuration. Be sure to consult the server documentation to learn how the servers will react during a power overload.

PCI Bus Power

The PCI slots in the Sun servers comply with PCI Local Bus Specification Revision 2.1. The PCI bus in each server is designed to provide 15 watts of power multiplied by the number of PCI slots in the PCI chassis. Thus, a four-slot PCI chassis has a total of 60 watts of power available. These 60 watts can be used in any manner that conforms to the PCI standard. A single PCI slot can support a card that requires up to 25 watts. Here are some examples of how you might populate a four-slot PCI chassis:

Heat Output and Cooling

Servers and related equipment generate a considerable amount of heat in a relatively small area. This is because every watt of power used by a system is dissipated into the air as heat. The amount of heat output per server varies, depending on the system configuration. See TABLE 5-4 for heat output measurements for the servers.

The heat load in a data center is seldom distributed uniformly and the areas generating the most heat can change frequently. Further, data centers are full of equipment that is highly sensitive to temperature and humidity fluctuations. See TABLE 5-5 for the servers' temperature and humidity specifications.

Proper cooling and related ventilation of a server within a cabinet is affected by many variables, including the cabinet and door construction, cabinet size, and thermal dissipation of any other components within the cabinet. Therefore, it is the responsibility of the data center manager to ensure that the cabinet's ventilation system is sufficient for all the equipment mounted in the cabinet.

Do not use the servers' nameplate power ratings when calculating the servers' heat release. The purpose of the nameplate power ratings is solely to indicate the servers' hardware limits for maximum power draw.

Chassis Airflow

The flow of air through the servers is essential to the proper cooling of the servers. Even though the data center air may be at a safe and steady temperature at one location, the temperature of the air entering each server is critical. Problems sometimes arise for these reasons:

All of the servers described in this guide draw in ambient air for cooling from the front and discharge heated exhaust air to the rear. The servers require that the front and back cabinet doors to be at least 63% open for adequate airflow. This can be accomplished by removing the doors, or by ensuring that the doors have a perforated pattern that provides at least 63% open area. In addition, maintain a minimum of 1.5-inch (3.8-cm) clearance between the systems and front and back doors of a cabinet.

The servers are equipped with fans that route cool air throughout the chassis. As long as the necessary air conditioning is provided in the data center to dissipate the heat load, and sufficient space and door openings are provided at the front and back of the servers, the fans will enable the rackmounted servers to work within the temperature specifications for systems in operation. See TABLE 5-5 for temperature specifications. See Cabinet Location for information about recommended placement of cabinets and racks to optimize proper aisle airflow.

Units of Measurement

A standard unit for measuring the heat generated within, or removed from, a data center is the British Thermal Unit (Btu). The heat produced by electronic devices such as servers is usually expressed as the number of Btu generated in an hour (Btu/hr).

Watts (W) is also a term used to express heat output and cooling. One watt is equal to 3.412 Btu/hr. For example, if you use 100 watts of power, you generate
341.2 Btu/hr.

Air conditioning capacity is also measured in Btu/hr or watts. Large air conditioning systems are rated in tons. One ton of air conditioning is a unit of cooling equal to 12,000 Btu/hr or 3517 watts.

Determining Heat Output and Cooling

TABLE 5-4 lists the minimum, typical, and maximum heat output and cooling requirements for base configurations of the servers. These specification are the measured power ratings, which are calculated for the base server configurations as defined by Sun and listed in TABLE 5-3. Use the nameplate ratings only as a references to the servers' hardware limits that could accommodate future components and not to calculate the servers' current power and cooling requirements.

In addition to the heat load generated by the servers, some cabinets include fans, power sequencers, and other devices that generate heat. Be sure to obtain the heat output values of these devices from your cabinet supplier. Also, when calculating data center cooling requirements, be sure to include heat dissipation for all equipment in the room.

To determine the heat output and cooling requirements of the rackmounted servers, add the Btu or watts for each server in the rack. For example, if one server is putting out 1000 Btu/hr (293 watts) and another one is putting out 2000 Btu/hr (586 watts), the total heat generated is 3000 Btu/hr (879 watts). The air conditioning equipment then should be properly sized to cool at least 3000 Btu/hr (879 watts) to accommodate these two systems.

If you only have wattage measurements and want to obtain the equivalent Btu rating, multiply the total wattage by 3.41 to obtain the Btu/hr. To calculate tons of air conditioning, multiply the total wattage by 0.000285.

See Calculating Cooling Requirements for an example of how to estimate cooling requirements based on the square footage used by the cabinets and racks in the data center.

Using Rack Location Units to Determine Heat Output and Cooling

In the book Enterprise Data Center Design and Methodology by Rob Snevely (available at http://www.sun.com/books/blueprints.series.html) the concept of using rack location units (RLUs) to determine heat output and cooling requirements in the data center is discussed. A rack location is the specific location on the data center floor where services that can accommodate power, cooling, physical space, network connectivity, functional capacity, and rack weight requirements are delivered. Services delivered to the rack location are specified in units of measure, such as watts or Btus, thus forming the term rack location unit.

Since today's data centers house hundreds or thousands of systems with widely varying power and cooling requirements, RLUs can help you determine where greater or less power and cooling services are needed. RLUs can also help you determince how to locate the racks to maximize services. Using square footage calculations for power and cooling assumes that power and cooling loads are the same across the entire room. Using RLUs lets you divide the data center into areas that need unique power and cooling services.

To determine RLUs for heat output and cooling, you must add together the heat output and cooling requirements for all systems installed in the rack. Then assess the RLUs for adjacent racks. For example, suppose you had 24,000 square feet of space in the data center. You might have a 12,000-square foot area where 600 PCs are outputing 552,000 Btu/hour and needing 46 Btu/hour of cooling per square foot. Another 6000-square foot area might contain 48 severs outputting 1,320,000 Btu/hour and needing 220 Btu/hour of cooling per square foot. A third 6000-square foot area might contain 12 high-end servers outputting 972,000 Btu/hour and needing 162 Btu/hour of cooling per square foot.

Using a square footage calculation for this example yields a cooling requirement for all three sections of 2,844,000 Btu/hour, or 118.5 Btu/hour of cooling per square foot. This would exceed the 46 Btu/hour cooling needed by the PCs, but it is much too little cooling capacity required for both server areas. Knowing the RLUs for power and cooling enable the data center manager to adjust the physical design, the power and cooling equipment, and rack configurations within the facility to meet the systems' requirements.