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Ask the Engineer - How do I get 120V from a 208V circuit?

Posted by Robert Faulkner on Jul 13, 2018 10:00:00 AM

Originally posted February 22, 2013, Updated July 13, 2018

One of the questions we often get is whether Servertech has any PDU's that can convert down from 208V to 120V for those handful of legacy devices that "require" 120V.

The short answer is this:  like Mick Jagger said, you can’t always get what you want.

The real short answer is that no power distribution unit on the market includes step-down transformation.   If you find yourself in a situation in which you are trying to get 120V from 208V, there are a number of practical, soul-searching questions you should ask yourself.

What are we doing here?   This is a good one.  And what I mean is, are you sure that the IT device in question really needs 120V power? The reality is that most server, storage, and network equipment sold within the last 10 years on our planet have full-range (100V-240V) power supplies.  Just because the cable has a straight-blade NEMA 5-15P plug, it does not mean the appliance requires 120V.   Our advice:  double check the specs on the device. If the input connector is a C14, there is a strong probability that it is a full-range supply.

Why this device?  Next, consider whether it is really the best choice to continue using this particular piece of legacy equipment. A cost/benefit analysis may reveal that it is all cost and no benefit.  In other words, it might well be less expensive to buy a new machine with full-range power supplies than to run separate 120V circuits or buy a step-down transformer.  Our advice:  do a little homework to see if you can get yourself out of this situation.

Why me?  If you are still reading, then maybe you are stuck.  Congratulations, you may be one of the few IT managers who has a rust bucket of a server that absolutely, positively must be saved.  Our condolences.  So, if it turns out that 120V is truly required, there are step-down transformers of varying wattage capacity that are available for purchase.  Our advice?   Stick to the less-expensive models with 2:1 conversion, which will result in 104V. 

It isn’t exactly 120V, but you can’t please everybody all the time.

Topics: PDU, rack PDU, How To, single phase PDU

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