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208/240 power question...

 
 
essenz
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      06-05-2007
I almost always work with pure 120V AC equipment, but now with the new
6500 equipment I am getting, I am little confused about something.

In a house, you have two 120V supply lines of the same phase. In your
breaker panel, you can install a breaker which grabs voltage from each
bus, and you wire a standard 240V device - like a dryer or AC unit,
etc.,.

Here is where I get confused. Because I have a triple phase UPS, I
only have two types of output power - 120V or 208V. I actually have
three 120V phase, and none of them are of the same phase. So its
impossible for me to wire the 240V equipment like I would at home.
However, I have the 208V power.

Is it safe to assume that any 240V appliance can run off 208V? Cisco's
power spec for the 6500 just says anything between 100V and 250V. Can
someone clarify this for me. I just want to be sure I am correct in
assuming that 208V and 240V are the same. Obviously the wiring is
different. 240V is 4 wires (120V supply, 120V supply, return, and
ground), and 208V has 3 wires (208V supply, return, and ground)....

thanks
john

 
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Doug McIntyre
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      06-05-2007
essenz <(E-Mail Removed)> writes:
>I almost always work with pure 120V AC equipment, but now with the new
>6500 equipment I am getting, I am little confused about something.


>In a house, you have two 120V supply lines of the same phase. In your
>breaker panel, you can install a breaker which grabs voltage from each
>bus, and you wire a standard 240V device - like a dryer or AC unit,
>etc.,.


Right. In residential neighborhoods, you end up with two legs in phase,
but opposite voltages coming into your house, center tapped. So the
difference between your two 120V legs is 240V.


>Here is where I get confused. Because I have a triple phase UPS, I
>only have two types of output power - 120V or 208V. I actually have
>three 120V phase, and none of them are of the same phase. So its
>impossible for me to wire the 240V equipment like I would at home.
>However, I have the 208V power.


Right Three phase vs. single phase.
Just dealing with 120V off your 3-phase UPS you get 120V. But, when you
take two legs of the three, you get things 120degrees out of phase
between the two, so by the time one leg is leading, the other is
falling. RMS difference between two legs ends up being 208V
between the two legs because of the phase difference once you work out
the math.


>Is it safe to assume that any 240V appliance can run off 208V? Cisco's
>power spec for the 6500 just says anything between 100V and 250V. Can
>someone clarify this for me. I just want to be sure I am correct in
>assuming that 208V and 240V are the same. Obviously the wiring is
>different. 240V is 4 wires (120V supply, 120V supply, return, and
>ground), and 208V has 3 wires (208V supply, return, and ground)....


You have to be careful of what you define as a 240V appliance.
A 240V dryer or heater might not operate quite right on 208V...

But, something like a 6500 is designed to operate just fine on 208V,
because that is the expected environment it will be going into. Note:
Most computer gear runs just fine off 120V or 208V power or 220/230/240V
power, as the power supplies are all auto-sensing for the voltage
level for all areas around the world, and adjusts appropriately. That
way, manufacturers can just ship the correct power plug for the region
and not have seperate power supplies trying to figure out what power
is what in what area of the world it ships to.

You also have to be mindful of *what* power supplies you put into your
6500. Some are 120V/208V. Some higher-wattage ones are 208V only, to
stay within your current ratings (ie. they'll be 208Vx16A feeds, which
is going to be more current than most wire run for 120V will take, but
works just fine for 208V).

You aren't quite right on the wiring though.

240V residential is normally run with 12-3 NM cable in the house.
Two phases of power, neutral and ground. It doesn't run in EMT or BX
conduit like some areas used to have many years ago.

BUT, 208V commercial is normally run with 3 THHN inside EMT conduit.
You still have two phases, and one neutral, but the ground run is the EMT
conduit itself. The ground will be tied to the neutral at one point, probably
the main PDU for your UPS, which is then grounded to the building ground.

My house is old enough that all oldwork is run in EMT or BX, which was
the designed in ground for the 2 THHN conductors they pulled in each,
so my home wiring is more like commercial environments. But
residential is all done with NM cable now which is cheaper and quicker
to run.

So, ultimately, your 6500 power supplies are designed to work on the
208V feed your 3-phase UPS will generate for it, and you don't have to
worry about it.




 
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