A New Look at 3-Phase Power Distribution

Executive Summary

The principles of 3-Phase power are not always well understood by the installer,
whose only task is to power up the equipment being installed in the computer rack.
Load balancing (matching current draw on each phase) is critical in these applications
for multiple reasons:
1) If the three phases are not balanced, heat is generated resulting in higher
cooling costs.
2) Unbalanced loads lead to inefficiency and higher power bills.
3) High loads on a single phase means greater chance of tripping either a PDU or
upstream breaker, and losing power at the rack.
Good practice in the data center is to install rack mounted equipment so that the
current draw is similar on each branch. This is relatively easy if the rack is filled with
only one type of device. Unfortunately this is often not the case. Mixed devices such as
switches, storage devices, blade servers and different brands and types of 1U/2U/3U
servers can create a crazy mesh of power cables in the back of the rack. This can
potentially inhibit airflow and add to the heat problems mentioned above.
The issues above are compounded by the move to higher density racks that demand
more kilowatts of power. Higher power distribution at 60A and 100A force even more
complex PDUs with six, nine, and even twelve branches. The power draw still must be
evenly distributed across these branches.
A solution to these issues is to use an alternating phase PDU. These specially designed
PDUs alternate the phased power on a per-outlet basis instead of a per-branch basis.
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