Introduction
The standard power distribution system for large data centers in North America is a 277/480V three-phase power system supplying distributed Power Distribution Units (PDUs) which convert the voltage to the 208V and 120V single-phase branch circuits utilized by IT equipment. This arrangement is represented by the one-line diagram of Figure 1.
High density server installations where racks can draw from 10 kW to 40 kW per rack place significant strains on this standard power distribution system. This increased rack power density is causing:
- Multiple branch circuits per rack, with corresponding breakers and cabling
- PDUs to become up to 30% of the floor space
- PDUs to become up to 30% of the floor weight loading
- More PDUs installed per IT rack, driving up capital costs
However, these problems are unique to North America. In Europe and the rest of the world the power distribution system is simpler and more efficient. This paper shows that it is practical to take the power distribution system used in the rest of the world and apply it to North American data centers to reduce cost, reduce weight, save floor space, and increase electrical efficiency.
In the past it was not practical to consider alternative higher voltage power distribution systems because a significant fraction of IT equipment operated from 120V. However in the modern high density data center virtually all rack-based equipment operates on 208V (or 230V outside North America). Even devices that can run on 120V are also capable of operating on higher voltage.
Virtually all IT equipment manufactured today is designed for worldwide compatibility. This means it can operate on the North American 120/208 volt system, the Japanese 100/200 volt system, and the 230 V system used in the rest of the world.
Alternative Power Distribution for North America
The alternate power distribution system proposed and described in this paper is an adaptation of the power distribution system used in the rest of the world. This arrangement is represented by the one-line diagram of Figure 2.
The differences between this diagram and the diagram of Figure 1 are subtle, and include:
- Three-phase branch circuit power distribution that is 230 V (line to neutral) instead of 208 V (line to line)
- Elimination of the PDU transformers
- A transformer that is used to change 277/480 V to 230/400 V at the output of the UPS.
These items are explained in more detail in the following sections.
Three-phase branch power distribution – 230V (line to neutral) instead of 208V (line to line)
While the proposed power distribution method sounds like a minor change, it actually creates a very large impact on the power distribution system. First, the typical North American three-phase branch power distribution is at 120/208 Volts, while the rest of the world uses 230/400 V. This is shown in Figure 3.
Servers draw their power from the 208 V high voltage (line to line) connection in North America, while they draw their power from the 230 V low voltage (line to neutral) connection in the rest of the world. It is important to recognize that when comparing the "line to neutral" voltages of both distribution methods, the comparison should be between 120 V and 230 V and NOT between 208 V and 230 V. This "line to line" and "line to neutral" difference becomes readily apparent when the power capacity for a for a three-phase branch circuit is calculated.
For example, assume 20 amp circuits are provided to the load in either case. The 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). Given the same circuit current rating, the 230 V distribution provides 92% more power than the 120 V distribution. This increases the power density capability per rack without adding extra circuit breakers as would be the case with the 120/208 V distribution. These additional breakers present additional points of failure resulting in decreased data center reliability.
