[BBLISA] Looking for a solution to power 120VAC device on 208/220VAC circuit

[Dean Anderson]

220/208 are two legs of a three phase system with return.  Each leg is
120V, out of phase by 120degrees with the other leg.

240 is a single phase ciruit, often constructed of 2 120V lines with
exactly opposite phase.  A 120v line can be obtained by splitting each
of these legs.  You cannot mix 208 and 240!  Sparks and (even fire) will
probably be the result.

If you follow back to bigger and bigger transformers---All grid power
comes from 3 phase generators and is split into single phase circuits.
Sometime a leg is split off from 208/240 for 120V.  Care must be taken
so that the legs remains fairly balanced in load.

Some care also has to be given to what you are going to do with the
legs. There are some bad ideas:  For example, one may be tempted with,
eg, Cisco 6500, 7500 w/ redundant power supplies to split each leg of a
240 or 208 and feed one leg into one cisco power supply, and the other
leg into the other redundant power supply. Even though the power
supplies produce DC to the shared redundant power planes, there is
always ripple (variation in voltage throughout cycle) in AC-DC
conversion.  If the ripple is not in phase with the other power supply's
ripple, DC power will be constantly be transferred between the power
supplies, creating heat in the power supplies, and eventually causing a
failure.  All redundant power supplies in a single machine must share
the same leg and be in-phase.

		--Dean

On Tue, 2 Jun 2009, John P. Rouillard wrote:

> 
> Hi all:
> 
> At one of my companies colo-centers, we have a 220/208 VAC power
> distribution unit in the cabinet. I need to power a 120VAC wireless
> access point which takes in 120VAC @ .3 amps (36 watts or so).  It's
> not worth getting a 120 VAC circuit installed for this.
> 
> While I could probably jury rig a cable that just connected a hot and
> the neutral, that breaks so many rules. So I am looking for a
> commercial transformer that can do the conversion.
> 
> All the stuff I have seen has european style plugs and not US standard
> plugs. Ideally the device would have an IEC C13 (shielded) plug on
> it. To make it more fun, I think there is a short cord that connects
> the IEC C13 to a NEMA 5-20R (yeah I know) since I seem to remember
> plugging my laptop (which supports 120-240VAC) in there.
> 
> I suppose I could also try to find a 240 VAC transformer to DC and
> just replace the 120VAC transformer that came with the access point.
> 
> Does anybody have any pointers/ideas/recommendation on how to power
> the access point?
> 
> --
> 				-- rouilj
> John Rouillard
> ===========================================================================
> My employers don't acknowledge my existence much less my opinions.
> 
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> 

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