| We're just moving into some new office space, and among the things that we
| inherited was some three-phase power. I haven't actually seen what the
| outlets look like, because the person who knows where it is isn't in this
| week, but I was kind of curious if there's any way that we can utilize any
| of that? We're a typical office environment, and all of our equipment
| (workstations, servers, etc.) are single-phase.
Larger UPS systems use three-phase power to keep the batteries charged
and power the DC to AC conversion. Some can be had that then produce
only single-phase, but most of them produce three phase back out. But
that's not a big deal as they can all be had with 120 volts.
| To go from the three-phase power to single-phase, is it simply a matter of
| creating three single-phase outputs from the one three-phase (i.e.,
| A+neutral, B+neutral, and C+neutral), and then we can just plug
| (independent) equipment into each of the 3 single-phase outputs?
It depends on what kind of three phase power you have. If you have what
is known as "208Y/120" then what you describe gets you 120 volts. But
if you have "240 delta center tapped", then A+N and C+N gets you 120 volts
but B+N gets you 208 volts. With 208Y/120, A+B or B+C or C+A gets you
208 volts, while with 240 delta, A+B or B+C or C+A gets you 240 volts.
You could have another three phase system or voltage, in which case you
likely have a transformer or three changing the voltage so you can get
something at 120 volts. The power coming in from the electric company
could be in "480Y/277", "600Y/347" (primarily in Canada), "480 delta",
or some others. You would have multiple fuse boxes or breaker panels
to support the high and low voltage circuits in these cases.
If you have no transformers, you're probably getting one of the first
two I described. In some locations you might be getting "216Y/125" or
| Also, if we do that, from a power standpoint, do we end up with each of the
| single-phase outputs just being 1/3 of the power rating for the original
| three-phase power?
The way you just described, yes. A phase-to-phase connection, which gets
a different voltage, could have a higher capacity, since it is using two
of the phases and thus gets a higehr voltage with no reduction in current
(unless something else uses up some of the capacity).
If you have 208Y/120, you will most likely have circuits distributed over
the three phases in a circuit breaker panel designed to interleave the
three phases at every 3rd row. A 1-pole breaker gets 120 volts at the
phase it is plugged in to. A 2-pole breaker gets 208 volts (this would
have been 240 volts if it were single phase), but can be used with 120
volt loads. A 3-pole breaker gets the full three phases.
Whatever the amperage rating of a breaker is, you get that much power at
each of the poles/wires coming from it, however many that is. So if you
have a 20 amp 1-pole breaker, it can provide 2400 watts to 120 volt loads.
If you have a 20 amp 2-pole breaker, it can provide 4800 watts total to
2 sets of 120 volt loads, or 4156 watts to 208 volt loads. A 20-amp
3-pole breaker would let you have a total of 7200 watts for 3 sets of
120 volt loads. 3 sets of 208 volt loads could use up to 4156 watts
each as long as the total of 2 does not exceed 4800 watts and the total
of all 3 does not exceed 7200 watts. And of course it can supply a real
three phase load up to 7200 watts. Electrical codes generally require
planned loads to not exceed 80% of capacity, so derate the above figures
to determine what you could plan for.
There are PDUs (power distribution units, or power strips) for computer
rack cabinets that plug into a three phase receptacle and provide three
sets of 120 volt outlets. If everything is rated 20 amps, you'd have
that 20 amps three times (16 amps maximum planned usage on each set).
You might want to check what you can, or are, doing with a UPS, before
jumping into how power is fed to your computers.
| As often happens, I'm probably asking what might seem like dumb/naive
| questions, but I'm really glad to have found this NG :)!
You're well ahead of most. The first big step is knowing you needed to
Try to find out what kind of three phase power it is, and the voltages,
so we can focus on your real options. Also, if you can, find out what
type(s) and size(s) of circuit breaker panel(s) you have.
One important thing to remember is that in a situation outside of an
owner-occupied single family house, virtually every jurisdiction does
require using the services of a licensed electrician to do any work.
People here can give you hints, advice, general direction, and explain
your options. But wiring the building is not a do-it-yourself project;
not even changing out a receptacle or light switch.
| Phil Howard KA9WGN | http://linuxhomepage.com/ http://ham.org/ |
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