# 208V to 208/120 transformer

You get 208 volts coming in via an L6-XXP plug. Output is an isolated 120/208 volt split phase. So how would this be done?
http://www.apcc.com/products/resource/include/techspec_index.cfm?base_sku=SURT003
One possible idea is:
|<------- 208V input ------->| | | * * \/\/\/\/\/\/\/\/\/\/\/\/\/\/ ================================== /\ /\/\/\/\/\/\/\ /\/\/\/\/\/\/\ /\ * * * * * | | | | | A B C D E
A-C = 120V C-E = 120V B-D = 208V A-E = 240V
C might even be bonded to ground.
Another possibility is that it is not really 120/208, but some compromise voltage like 112/224 output (so they don't have so many transformer taps).
How would I be able to determine exactly what they are doing? They do not seem to provide schematics for anything.
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| Phil Howard KA9WGN | http://linuxhomepage.com/ http://ham.org/ |
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On 21/12/09 20:25, in article snipped-for-privacy@news1.newsguy.com,

I downloaded the manual, but you're right, it doesn't tell me great deal. It does say that there's an input selector switch for 208 or 240 V, so there must be an extra primary tap, but that's not really relevant to your question.
It doesn't even explicitly state that it's a split phase output, rather that a single-ended 240 V, though the use of a 14 series NEMA connector does seem to imply that it is a split phase output. Your diagram does seem the most likely configuration, but there are other possibilities; two separate secondary windings for example. If you have one of these things you could try metering between A and B, and between D and E; if you see about 32 V in each case then it would seem to confirm your drawing.
If this really matters for your application then I would suggest asking APC themselves, though at least in this country (UK), they don't seem to be keen on talking to end users, so if it's the same over there you might need to go through a dealer; they have access to more technical documentation than end users do, and they could ask APC themselves. If the exact design of the thing is Important to you then I wouldn't assume anything about it.
I'm glad I'm in Europe, and we don't have all this complication of numerous different Voltages, connectors, grounding points and three phase systems; here it's all just 230/400 V wye, or star as we would call it; much simpler. :)
Merry Christmas, and a happy new year.
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On Fri, 25 Dec 2009 11:50:26 +0000, Stephen Furley

Actually, it may well be relevant. 208V output may only be available with 208V input. IOW, only a center tap on the secondary (no connections B&D).
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I don't think that is the case, though I cannot be certain from the information available. The web page linked to above specifies 208 V input; only when you download the installation manual does it say that this is in fact 208/240 switchable. It also says "If 240V utility is used, program the UPS output for 240V. See the UPS user manual". This seems to imply that it can produce some other Voltage, presumably 208 V, when fed with a 240 V input; this would require the 208 V tappings on the secondary. It does however seem to ignore the possibility that you might want a 208 V output with a 240 V input. Also, it's described as being an 'Isolation/step-down transformer'. There is nothing about some outputs being isolated, while others are not.
It does sound to me like a tapped secondary for all output Voltages, but I cannot be sure from the information available. The best people to ask are APC. I do find that the information generally available to the public on their website is somewhat incomplete. I've had to contact a dealer several times in the past to clarify details of a product specification.