Re: Getting more input power

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Original poster: "Jim Lux" <jimlux-at-earthlink-dot-net> 


----- Original Message -----
From: "Tesla list" <tesla-at-pupman-dot-com>
To: <tesla-at-pupman-dot-com>
Sent: Saturday, January 03, 2004 10:02 AM
Subject: Re: Getting more input power


 > Original poster: "RIAA/MPAA's Worst Nightmare" <mike.marcum-at-zoomtown-dot-com>
 >
 > The 50A 3-phase is 50A each leg. Between a leg and neutral is 12kw with
 > 240v. Using all 3 phases is 1.732 (sqrt 3) times this or close to 21kw.

Three phases, with 240V to neutral, at 50A per phase would be 36kW or so...
The root 3 thing is when you're converting delta voltages to wye voltages.
You need wye (star) voltages when calculating power.  In the US, the typical
distribution is 240V Delta (or 480V delta), giving 138V (or 277V) wye. (Now
you know why you can get fluorescent ballasts for 277V).

In light industrial/office, they'll change the taps such that the 3 phase
power is 208 Delta, which nicely works out to 120V in Wye, so you can run
the usual 115V stuff off it (line vs load voltage specs, which is why you
see 120, 117,115, and 110V, all referring to basically the same circuit)


If you had said you had a 240V 3 phase service at 50 amps, here in the US,
you'd convert the 240V delta voltage to the equivalent wye (138V), multiply
by 50, for 6.9 kW, per leg, multiplied by 3 for a piddly 21kW or so... This
would be a very small service (and probably smaller than they'd ever
install... start at 200A and work up from there...)

The typical residential drop these days is 200A at 240V (single phase) for
48kW (at least my tract house built in 1998 has that service).  Of some note
to the power mavens on the list is that 8 houses with that service are fed
off one (1) 50 kVA transformer, giving you an idea of how they sized the
distribution in our area (no wonder the lights blink in summer when everyone
gets their a/c going). Mind you, the SoCalEdison engineer, when I asked her
about this, replied that she thought it was because the tract was replanned
to double the number of units after they had put the pad mounted
transformers in. She thought that if it got to be a problem, they'd swap out
the 50's for 75's or 100's.  As a practical matter, the a/c is probably the
"big load" and is probably around 20-30A each.

In Australia, or in many parts of Europe, 3phase distribution is much more
common, set up as 220/240V in Wye (or star), giving 380/415V in delta.  It
probably has to do with the "substation to load" distances being much
shorter in Europe distribution practice.  You're less likely to have to run
a service drop 10-20 km across hill and dale, like they do in the US for
rural areas. (This fundamental difference in architecture has led to some
interesting aspects for such things as powerline carrier data transmission
(aka BPL) and switchgear design -> lots of oil or airblast breakers and oil
insulated stuff in the US, in fairly large substations, far from residences,
spaced far apart; Europe-> lots of gas insulated switchgear in underground
vaults within the cities. Having a lot the older oil filled stuff bombed out
of existence in WWII meant that the overall physical plant in Europe tends
to be newer than in the US (true for telecom, too...we've got lots of 70+
year old phone and power lines in the US, even in urban areas.. )