Re: AC polarity

[Jim Green <Jim.Green@Snow.edu>]

A>        My conception now is that the long/left hole and the short/right
> hole both have a 117V potential w/rt ground, and that they are 180 degrees
> out of phase with each other.  That would mean that measuring the maximum
> potential across the left and right holes would give 230v.  This doesn't
> sound right, so please correct me where wrong.

No.  If the house is correctly wired, the long hole is always white ie at
"ground".  The short hole is either red or black depending which way the
electrician chose to wire the receptacle.  That wire (red or black) goes to
a breaker in the breaker box and then to a "hot" buss (red or black) and
then to the transformer's.  Correctly wired all the reds from the
transformer to the red buss in the breaker box through one of the "red"
circuit breakers to the short hole.in any of the various receptacles are all
the same potential, 117V.  And likewise all the various blacks go from the
transformer's black side to the service entrance/breaker box black buss
through a black breaker to the short hole in *other* receptacles.  IE the
short hole can be *either* black of red. But there is not *both* black AND
red connected to the same receptacle UNLESS 230V is needed (like for a
kitchen range) -- in which case the receptacle has differently oriented
holes so line can not plug a 117V device in a 230V receptacle.

The long hole is *always* white = neutral = ground.

Between red and white is 117V -- between black and white is 117V -- BUT the
phase difference for the red's 117V and black's 117V is 180o.  Such that
across red and black there is 230V. 

> My follow-up question would be:  "What's the difference?"  I can see
> how two speakers connected the two ways would be out of phase with each
> other and give destructive interference.  But why does it matter to my
> electric pencil sharpener or my box fan which way the plug goes in?  

To the appliance it makes no difference at all. But the electrician can do
some tricky things with the red and black combinations.  For example the
size of the wire has to be able to carry the current. Thus for a 20A load a
12AWG wire is needed.  Now suppose that the service entrance is for 100A.
That would mean that there could be five 20A circuits on the black side and
another five 20A circuits on the red side.  Each circuit can have up to six
receptacles. (It is assumed that the sum of the currents used by all the
receptacles on one circuit would not exceed 20A. -- or the breaker trips
(:-))  Now the kitchen has to have two 20A circuits by code.  That would
mean that there needs to be two red or two black or one red and one black
circuits AND there would need to be two whites to carry the return for two
20A circuits.  BUT look if one red and one black are used, the red circuit
and the black circuit will be 180o out of phase.  If 20A is in use in each
circuit, the corresponding white wires will also be 180o out of  phase. SO
if the two whites were connected together ther would be NO CURRENT in the
connected white wires -- the two return currents cancel each other.  So I
don't need the white wires!!!!  Well this is if the red current and the
black current are exactly equal.  If they are not, then the return current
will be somewhere between zero and 20A.  HENCE only one 12AWG wire is needed
for the two circuits.  If the electrician (or the architect) is smart
enough, the house should be balanced between red circuits and black
circuits, so that (1) the least white wire is used (wire is dollars) and (2)
neither the red side or the black side carries more than its share.
Remember in our example the red side gets a maximum of 100A and the black
likewise.  So to get the use of the entire service entrance there needs to
be balance.

I can't believe I wrote al thhis!!!


 


Jim.Green@Snow.edu                             

Victory goes to the player who makes the
next-to-last mistake.  Savielly Grigorievitvh