Data Center Power Series 1 – Selecting the Right Voltage

Data Center Power Series – Part 1

Energy efficiency and optimization in the data center is a growing concern. IT managers struggle to balance complex trade-offs to find the best data center voltage to use. Considerations include utility costs, green initiatives, space and cooling constraints, virtualization and cloud computing options, and ever-increasing computing performance requirements.

As IT professionals begin the daunting task of unraveling the many variables and defining data center specifications to meet business requirements, a similar set of data center power related questions usually arises:

• What voltage should I standardize on and why?
• How do I calculate wattage, voltage, and BTU requirements?
• Which electrical connector configuration should I deploy?
• What does watts per square foot really mean?

These few questions can be significant stumbling blocks to the initial stages of defining data center power requirements and specifications. A few web searches will return many sources of information on these topics, and just as many opinions making the right answers difficult to discern. This whitepaper, the first in a series of four, tackles the voltage question – what is the best data center voltage should I use, and why?

What Voltage Should I Use? 120V, 208V, 240V, AC or DC?

Choosing the right voltage standard for data center design will have a significant impact on power utilization efficiency, cost of implementation, total cost of ownership, cooling efficiency, and space utilization. Specifically in North America, 120V single phase AC power is often used by tradition over sensibility and is dramatically less efficient and more costly than higher voltage options.

Most modern information technology equipment is designed to operate at 100V to 250V to accommodate North American 120/208V, Japanese 100/200V, and 230V used in the rest of the world to allow for worldwide power compatibility. Without going into confusing power calculations and details (readily available on the web if interested) using the highest voltage available is widely regarded as the most efficient choice when striving to reach the 85-90% efficiency range. The emerging standards recommendation for data centers in North America to move to 240V power significantly improves efficiency over 120V distribution, requires less wiring, uses smaller step-down transformers, and is less costly to implement.

North American UPS standard output power of 277/480V can be stepped down to 240V with a single-winding autotransformer. For 120/208V power distribution, an isolation transformer is required with two windings, primary and secondary. The single-winding autotransformer for 240V power is typically 90% smaller and less costly than the PDU isolation transformers required for step down to 120/208V.

While some advocate using DC over AC power citing power efficiency benefits, the availability of DC power delivery systems outside of telecom equipment racks is not widely supported and the proposed efficiency gains are similar to the gains achieved by operating at 240V AC over 120/208V AC. Thus, standardizing on 240V AC power can achieve similar benefits at a much lower cost, with less complexity, and provide greater overall compatibility.

Top 10 Reasons to Use 240V Power over 120V/208V

1. 277/480V to 240/415V step down autotransformer is 90% smaller and less expensive than 120/208V PDU isolation transformer
2. Elimination of large PDU transformers reduces cooling costs and requires less space
3. 240V distribution yields the same power capacity with nearly half the current as 120V
4. Fewer circuit breakers required reduces points of failure
5. Fewer branch circuits to racks reduces wiring cost, weight, and bulk
6. Less wiring bulk increases airflow and cooling efficiency, decreases cable weight
7. Branch circuit conductor wires for 120V distribution are 2-3 times larger than 240V yielding the same power capacity
8. Using higher voltage at lower amps for the same power yield allows for use of smaller conductor wires thus reducing capital costs and reducing line lost due to resistance
9. Nearly every other country outside of North America is already operating at similar 230V distribution – worldwide consistency has many practical benefits
10. Moving from 120V to 240V increases efficiency by 2-3.5% per server – accumulated over hundreds of servers this adds up to significant energy savings and greater capacity

A Powerful Conclusion

Thus the conclusion is to standardize on 240V distribution as the best data center voltage where possible. Then, use the maximum available power supply rated voltages on each device to maximize efficiency. If you have an existing implementation using 120V distribution and the move to a 240V infrastructure is not practical, consider the relatively low cost modifications to provide 208V to appropriately rated devices to maximize power efficiency and data center capacity. At 2-3.5% efficiency gains achieved by moving from 120V to 208V or 240V the costs of electrical modifications can be easily justified by calculating the annual energy savings. For example: at $0.10/kW-Hr with a 1000kW data center load, a 3% efficiency gain results in $26,280 annual energy cost savings.