[geeks] D'OH!

[Greg A. Woods]

[ On Friday, February 1, 2002 at 12:21:50 (-0800), Fogg, James wrote: ]
> Subject: RE: [geeks] D'OH!
>
> UK wiring is SO different than in the states. What you call a cooker is what
> we call a stove (I think). Our stoves include an oven and 4 cooktop burners.
> Stoves are wired for 220V @ 30A, as are our clothes dryers. It is very rare
> to find a 110V branch circuit at more than 20A.

I'm not entirely sure about the USA N.E.C., but it would seem from the
new Canadian Standards Association Canadian Electrical Code Part I book
I just received the other day describes a stove as an "electric range",
and alternatley as a "cooking unit".  One interesting quirk in the CEC
is that a branch circuit for a cooking unit in a dwelling need only have
8kw capacity for units rated at under 12kw (and 8kw + 40% of the
remaining kw over 12kw), whereas the same unit in a commercial setting
would require a branch circuit equal to the unit rating.  I guess the
CSA folks have not met many of our mothers who like to do lots of
cooking!  ;-)

FYI my clothes dryer (a Maytag unit) has its own 20A 220VAC feeder
circuit.

I installed two 20A 220VAC feeder circuits for my two new BestPower UPS
units in my "computer room".

It would appear that 110VAC circuits are allowed for up to 50A with
non-locking receptacles, and up to 60A with locking receptacles.  I
can't easily find the rule for fixed-wired stationary 110VAC equipment
or feeder circuits, though anyone going at more than 20A is wasting
copper -- you get twice the watts at twice the voltage, and a 60A
circuit requires #4-AWG conductors (and that's for a max of 30 feet if I
read the table right)!  Even a 20A cicruit requires #12-AWG conductors
for a short run.  I don't know exactly how far my 20A circuits run, but
I used #10 just to be sure because the UPS will draw up to 16A when
running full out and charging, and they are at least 30' runs.

> It is assumed that anything
> that needs that much power is both stationary and built for 220V. By using
> 220V (or 2 X 110V also known as 2-phase) you save on copper wiring for a
> given amount of wattage (since wattage is a function of voltage X amps).

Ah, no, that's all completely wrong, at least w.r.t. "phases".  (the
wattage stuff is right, of course....)

A circuit with two or more phases has an equivalent number of feeder
lines direct from the generator.  Each phase has the sine wave starting
its cycle at some number of degrees offset from the other(s).  If you
connect multiple phases to the same winding in the same transformer you
will evict the smoke from the wire as there is always potential between
the phases (277VAC between the phases of a 480VAC three-phase system,
for example).

The 110VAC windings on a centre-tapped transformer are always exactly in
phase with each other, and they are fed by a single-phase high-tension
distribution line.

> The
> US uses delta-wound distribution transformers that divide 3 phases across
> three windings.

Yes, but that's in the distribution system, and it's not uniform for all
areas -- some two-phase systems exist.

> A house receives two of these phases from two windings of
> the delta and a neutral line

There's no phase delta in a 220VAC three-wire single-phase dwelling
feeder circuit.

> (neutral is not to be confused with ground,
> even though they are bonded together in the service panel - don't get me
> started on the neutral/ground issue, its a stupid thing). The neutral is the
> "center tap" of the other two windings.

Indeed!  Don't ever confuse a centre-tap with the ground of a
multi-phase system either!

> If you take neutral and one of the
> phases (called a "hot") you get 110V. Take the two hots and you get 220V
> between them.

Yup.

> My understanding of UK wiring (which is probably wrong) is
> that you are delivered one hot and one neutral and will get 220V between
> them. It is also a single phase. Don't UK wall outlets deliver 220V?

That's correct.  Many (most?) parts of the world outside of North
America use single-phase 220VAC for all household and office wiring.


-- 
								Greg A. Woods

+1 416 218-0098;  <gwoods at acm.org>;  <g.a.woods at ieee.org>;  <woods at robohack.ca>
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