Want to move to all 208V for server racks

Lamar Owen lowen at pari.edu
Thu Dec 2 12:14:26 CST 2010

On Thursday, December 02, 2010 12:51:42 pm Kevin Day wrote:
> For standard 110V service, you use a single-wide breaker and send one hot phase + neutral and you get 110V. The difference between two phases is 208 volts though, so you use a double wide breaker and can send to device without using a neutral wire. Just 2 hots and a ground. If that's all you're doing (you don't need legacy 110V service anywhere) you skip the ground wire going into the panel entirely.

The photo of the Square-D QO plugin breaker panel was nice; thanks.  Our Liebert 'precision power' PDU's have what appear to be GE panelboards on the 120/208 side, 42 position, two panelboards per PDU. They are wired like the Square-D in you picture.

However, there is one thing in your reply that needs clarification.  A wye connected branch circuit panelboard must always be equipped with a grounded conductor (neutral) AND a grounding conductor (ground) to meet code.  That is unless the panel is listed for delta use in a 'neutral-less' arrangement and no single-pole breakers are present in the panel.  The grounding conductor must never be used as the grounded conductor; the ground is only for fault currents, never for load currents, and the grounded conductor must be bonded to the grounding conductor only at the service disconnect.  

That's in a simple single grounding conductor system; many datacenters are wired with separate safety and signal grounding conductors (ours is) that require special precautions to be taken.  The signal ground is often called the technical ground in the industry, and is, per NEC, always bonded to the safety (or NEC) ground at the service disconnect.  That makes it real fun.

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