Recently I’ve been looking at electrical power options for a small but demanding computer cluster (50 computers with GPUs.)

Data center technicians have to consider more complicated power provisioning as rack densities steadily increase and power-hungry hardware like GPUs are rolled out.

The various options in the US from least power efficient to most efficient are:

1. least efficient and simplest to install – one or more single-phase 120 volt AC whips to each rack
2. moderately efficient and moderately complex to install – one or more 3-phase AC whips to each rack of single-phase equipment
3. most efficient and most radical installation – replace all AC power supplies with DC power supplies/none and use DC whips to each rack, since computer motherboards use DC. Telcos have done this for decades.

(Whip is industry slang for a power cable run from a source, like a PDU, to a consumer, like a server or lighting fixture. Typically a whip costs about $1,000 each to install.)

The advantages of 3-phase AC over single-phase AC are many:

• minimum of 2% to 3% more power efficiency of 3-phase over 1-phase, plus additional gains if step-down transformers are eliminated
• 3-phase delivers 1.7 times more amps per whip, reducing number of cables to run
• at very high rack densities (60 amps+/rack), 3-phase may be the only distribution option available
• some large UPS units require 3-phase inputs
• some data centers or grids may favor or require use of 3-phase electricity.
• electric motors are seldom used in data centers, but they will last longer, can be cheaper (no starter required), and will be more powerful if using 3-phase power since maximum power is constantly maintained throughout the AC cycle.

The disadvantages of 3-phase power are:

• slightly more complex to design and wire, but 3-phase is well-known to electricians. Generally very thick electrical cables are used with 3-phase input cables – likely too thick to easily drag several cables into the center of an existing data center floor or to route through existing wall raceway holes.
• more current means more risk – 10 kVA per circuit is common. (A great idea is to use GFCI circuit breakers.)
• each additional whip will likely incur an installation charge
• must maintain balanced power consumption across all wire pairs to maintain efficiency. (mainly applicable to 3-phase motor operation, less of a problem for 1-phase computer loads)

Siemens 60-amp GFCI Circuit Breaker



Hmmm …

Maintaining balanced power consumption across wire pairs when going from 3-phase to 1-phase either requires constant loads, or enough instrument displays/monitoring to enable a human operator to power on the right combination of equipment to balance the loads.

Makes just renting a bunch of cloud servers even more appealing, doesn’t it?

Disclaimer: Although I have managed data center racks, I am not an electrical engineer, and data centers vary.

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