16 Jan
2009
16 Jan
'09
7:27 a.m.
>>>> "Guy" == Guy Sotomayor
<ggs at shiresoft.com> writes:
Guy> On Jan 15, 2009, at 11:57 AM, Sridhar Ayengar wrote:
> I thought that main breakers in the US
split-phase system were
> rated for amps across the full 240V of the main feed?
Guy> In many cases, the power is 208v 3-phase (that's measured
Guy> between any two phases). If you look at each phase to ground
Guy> you see ~110-120v.
Right, that's called 120/208 Y (or "wye"). You have three phase wires
plus a neutral. So if phase to neutral is 120 volts nominal, then
phase to phase is 2 x 120 / cos (30degrees) which is 208 volts.
Guy> Single phase 220v is sometimes done by using
Guy> two of the three phases. You don't get the full voltage but
Guy> usually equipment doesn't care too much.
No, that's not how it is done.
In the USA, homes don't get 3 phase power. Instead, the transformer
feeding the house has a 240 volt secondary which is center-tapped
where the center is grounded (neutral). So you have two "hot" wires
180 degrees out of phase, 240 volts between them and 120 volts between
each and neutral. And it uses only one transformer, and draws on only
one phase from the high voltage line along the road. In fact, away
from densely populated areas you will often see just
one wire on top
of the pole, whic is one of the three high voltage phases.
Guy> All of the US DEC gear that uses 3-phase is the 208v 3-phase
Guy> variety. There's another version of 3-phase that doesn't allow
Guy> this. I'm not an electrician so I don't know the exact
Guy> differences.
There are lots of choices. If you want to be able to power 120 volt
appliances and need 3 phase power, then 120/208 Y connected is the
logical approach. If you need 3 phase but no 1 phase (say, for
motors) then 240 volt delta (no neutral, three phase lines, 240 volts
between phases) works well. Need more power? Use 480 instead of 240.
I've seen references to 277/480 Y, so you have 480 phase to phase for
motors and single phase stuff (like lights) get 277. That doesn't
show up in homes but it sounds like you might find it in factories.
A different scheme for getting 120 in a three phase setting that I've
seen mentioned in books uses a three-phase transformer wired delta
fashion, 240 volts between phases, with one of the three center-tapped
and the center grounded. So from those wires you can get 120 volt
(phase to neutral).
There are probably other schemes, especially in power hungry
factories...
paul