DC Power in the Data Center: Myths


As a proponent and engineer of DC power plants and distribution systems for data centers, I’ve been interested to read some rather misinformed diatribes about why DC power shouldn’t be used in data centers. In this article, I’ll address these myths.

“DC power is limited to 48VDC because any higher voltage won’t break an arc.”

This statement is a combination of a quarter true (but out of context) and three quarters incorrect. 48VDC is indeed a common standard, having been used in the telephone industry since the very beginning of automatic switching systems. In later years, this has also become a standard for DC powered network equipment, driven by the movement of data networking equipment and servers into the telephone office.

There are a number of different reasons for the use of 48VDC as a standard (not the least of which is that it’s touch safe), but telephone environments also typically use 120-130VDC up to 190VDC.

So, where does “won’t break an arc” come from? It applies only to circuit breakers. The AC sine wave helps to extinguish the arc inside a circuit breaker when it trips. Does this mean we can’t use circuit breakers with DC? No, it means we simply have to use circuit breakers rated for DC use. Myth #1 busted.

“DC power needs huge expensive copper bus bars that are a nightmare to hang.”

It is true that 48VDC power requires larger conductors than 120VAC or 208VAC, however, the issue is far from as bad as it’s made to sound. First, there is approximately a 20% energy saving involved in eliminating all the AC-DC conversions in an AC powered data center. Compare the reduced cost of the energy saving to the requirement for
larger conductors. Also, the 20% efficiency boost means smaller conductors than might otherwise be expected. Second, consider using a higher DC voltage to provide the same power at lower current (since we busted myth #1), just same way AC power does.

So why not just use higher AC voltages? Since almost no data center equipment uses voltages higher than 240VAC directly, this means transformers to reduce the voltage. Transformers mean more waste and more heat from that waste that needs to be removed from the data center, requiring more energy and equipment expense for cooling systems. DC on the other hand uses efficient solid state DC-DC converters, saving energy and reducing heat in the data center. Myth #2 busted.

“I’ve seen data centers with DC power and nobody wanted to use it so it must be worthless.”

You can’t just build something and hope somebody will just happen along and want it. DC power in the data center has to be promoted as a selling point for the data center and customers have to be encouraged with at least some of the rewards of the savings.

DC power in the data center is good for the environment and good for the wallet and those should be the only reasons required for everyone to adopt it!

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