By Ed Ritchie

When it comes to higher-voltage power distribution systems in high-density data centers, the Europeans take an approach that offers significant advantages over the tradition in North America. Data centers that move to this new method can save up to 56% in the lifetime cost of the distribution system. And if that’s not enough, additional savings in floor space and weight loading offer even more benefits. However, taking advantage of these benefits does require engineers to adapt to a new voltage configuration.

The basic concept involves using the international 230- to 400-V distribution system instead of the standard 120- to 208-V system typically found in North America. But that doesn’t mean the current system has to be thrown out, according to Rob Bunger, product line manager of infrastructure systems at American Power Conversion (APC), a power and systems manufacturer for sensitive electronic, network, communication, and industrial equipment. This alternative design will operate alongside conventional power distribution operations in existing data centers, and it’s part of an ongoing drive to improve data-center performance and efficiency.

“The industry is looking at a lot of different things to improve electrical efficiency,” says Bunger. “It tested DC in data centers to see if that’s effective, plus whole DC distribution and high-voltage distribution right to the rack. But the first and easiest move that you see people take is running everything they can off 208 volts.”

Power Boost
In North America, servers typically draw power from the 208-V high-voltage (line-to-line) connection, but in the rest of the world, power comes from the 230-V low-voltage (line-to-neutral) connection. (Note: Comparing the line-to-neutral voltages of both distribution methods requires that the comparison be made between 120 V and 230 V. It must not be compared to the line-to-line difference between 208 V and 230 V.)

The benefits between the line-to-line and line-to-neutral approach can be staggering: Given the same circuit current rating, the 230-V distribution provides 92% more power than the 120-V distribution. “The typical way to distribute power is 120 volts, and the amount of power you get out of a 20-amp 120-volt circuit is about 2 kilowatts,” explains Bunger. “Nowadays that’s sort of on the low end of what people can easily put into an IT [information technology] rack enclosure. However, that requires more and more circuits. But if you do 208 volts at the same current draw, such as 20 amps, you almost double the amount of power you get out of the circuit.”

It may sound too good to be true, but the increase becomes readily apparent when calculating the power capacity for a three-phase branch circuit. For example, assume 20-amp circuits are provided to the load in either case. Power capacity for the 120-V line-to-neutral distribution method is calculated as 20 amps x 120 V x 3 = 7.2 kW, while the capacity for the 230-V line-to-neutral distribution method is calculated as 20 amps x 230 V x 3 = 13.8 kW.

Along with the 92% boost, this configuration also increases the power-density capability per rack without adding extra circuit breakers (a precaution necessary with the 120- to 208-V distribution). Additional breakers present additional points of failure resulting in lower reliability.

The availability of the new systems, ultimately, will allow North American data centers to catch up with their European counterparts.

Current Reduced
Looking at it from the perspective of current reduction, it helps to take the same comparison but assume a fixed branch-circuit power capacity. For example, assume 10 kW is available to the load in either case. The power capacity for the 120-V line-to-neutral distribution is calculated as 27.7 amps x 120 V x 3 = 10 kW, while the capacity for the 230-V line-to-neutral distribution is calculated as 14.5 amps x 230 V x 3 = 10 kW. Given the same power, the 230- to 400-V distribution method provides the same power capacity with nearly half the current required by the 120 V.

Eliminating the PDU Transformer
So far, this approach has shown significant improvements in power production and current reduction, but the benefits don’t stop there. This design allows for the elimination of power distribution unit (PDU) transformers to convert the voltage to the 208-V and 120-V single-phase branch circuits utilized by IT equipment.

Bunger notes that generally, and especially in larger systems such as all uninterrupted power supply (UPS) systems above 100 kW, 480 V is the norm, so typical North American systems require a transformer. Although the efficiency of the transformer is in the high 90s, it produces heat and loses power. “Yes, the transformer gets you down to 120 volts for the servers,” Bunger says, “and again, from there you can give it 120 volts or 208 volts, but if you do 208 volts the current is going to be less, and when the current is less you decrease losses in the line.”

As the power losses go down, so do the system’s space requirements. For example, in a North American data center with 30-kW racks, roughly 20% of the space for racks is consumed by PDUs. In addition, the PDUs would make up more than 25% of the weight on the raised floor.

Bunger spent a number of years in Europe, and he notes that it has standardized on 400 V rather than 480 V phase to phase, and that includes what usually comes to the buildings. “If you take the phase to the neutral voltage in that system it’s 230 volts, and that’s what you see in all the regular receptacles even in households. That’s what the equipment and hardware run on. So you don’t need a transformer to take it down because it’s already a usable voltage.”

Making the Connection
The connectors for 230-V circuits are IEC C13 and C19 models. This is the power-cord connector provided by most original equipment manufacturers of rack-mount servers and storage devices. “Just like the kind you see on the back of your computer,” Bunger adds. “That’s all they use in Europe to distribute 230 volts.” Most high-density servers are already provided with the appropriate connector for use with a 230-V system.

Circuit Breakers Go Universal
The branch-circuit breakers used in European systems operate at a higher voltage than those normally found in North American data centers, which aren’t rated for this voltage and cannot be used. Not surprisingly, most European circuit-breaker panels aren’t certified by Underwriters Laboratories for use in North America. But the good news is that manufacturers are introducing compact worldwide circuit breakers and circuit breaker panel boards that are rated and certified for high-voltage use in North America. Such companies as APC offer complete data-center PDUs, remote power panels, and rack power panels that are appropriately rated and certified for high-voltage distribution in North America.

Ultimately, the availability of the new systems allows North American data centers to catch up with their European counterparts. “You’ll have a lot of benefits,” says Bunger. “Fewer losses, fewer transformers, less equipment, smaller wire sizes, and a lot of savings. When you add it all together, it’s quite significant.” The benefits are most apparent for high-density installations, and the new approach can operate alongside conventional distribution designs. “There’s nothing dramatic in the design,” Bunger adds. “You see a lot of people hesitating and defaulting to 120 volts, but this is not strange and it’s not as exotic as it might seem.”

Ed Ritchie writes on energy, transportation, and communication.

DE - July/August 2007