Jane wrote in message
...
I just read Black & Decker's Home Wiring Guide in an effort to
understand how the service panel distributes electricity, but they use
the word "circuit" to mean the wall socket / receptacle sometimes, and
alternately to mean circuit *breaker* and it isn't clear to me when
they mean one or the other. Consequently I am not clear on the
following concept:

A circuit is actually a complete loop through which electrons can flow.
Nobody in residential wiring really means that when the use the word
though.. It typically refers to the wiring hooked up to an individual
breaker, at least around here.

Simplistic Hypothetical: ------------------------------------------

Circuit Breaker #1 on the service panel is a 20amp 120v circuit.

It's wired to *four* wall circuits [recepticles].
Each recepticle is now a 120v 20a outlet.
Confusion point:
The book states that a full load for a 120v 20amp circuit is:
120v x 20a = 2400 watts x 80% (safe capacity) = 1900 watts

The 80% rule is for resistive loads, and even then it's only applied to
special circuits (heating, water heater, other dedicated resistive load
circuits). For everything else there is a "12 box rule". That is you can
have any combination of light fixture and receptacle boxes (excluding boxes
for switches and splices only) as long as the total of that combination is
no more than 12. Most electricians like to keep it at 8 or 10. i.e.:

Circuit 1: 4 recessed light fixtures in the bedroom plus 5
receptacles = 9
Circuit 2: 6 receptacles in the living room, 2 in the hallway, 8 in
the den = 16

Circuit 1 is ok because it only has 9 "outlet boxes", however circuit 2
is a violation, because it has 4 too many boxes.

The reason being is because you have no idea what someone will plug in
some day. Someone might plug in two 1200 watt heaters. If the electrical
installation is sound a 20A circuit will handle it

Does that mean each one of those recepticles can have up to a 1900
watt load on them *simultaneously* without tripping the breaker, or
that *collectively* at any given time their load in summary shouldn't
exceed the safe capacity of 1900 watts total?

For branch circuits it's always collectively. A 20A circuit can run 20A
collectively. ie: 10A on one outlet, plus 10A on another. 20A on only and
10A on the other would be pulling 30A through a wire (12gauge) that's only
meant for 20.

The reason I ask is this... the book also says that microwaves and
large appliances should have a "dedicated circuit." Do they simply
mean a dedicated wall outlet, or a dedicated circuit *breaker* --
meaning no other outlet is wired to that circuit breaker but the one
the appliance is plugged into...??

Because the way I read it, is I need a separate *circuit breaker* for
my washer/dryer outlet... one for my 1700watt treadmill (which can
also run a TV and lights in the garage, but then reaches the 1900watt
safe mark)... one for my garbage disposal and one for my microwave...

Unless your treadmill is hardwired in (no plug) or attached to the house
it doesn't need it's own circuit. The reason being is because it's
portable, and will probably be moved around.

and the circuit breakers just keep adding up...!

They do so blazingly fast! When I bought my 200A, 40 position Square D
panel a year ago for our 2200 sq.ft house I never dreamed I would get close
to filling it. Yet I am now close-- at 5 spaces left.

Every motor load circuit will need it's own breaker (code). This
includes the microwave, refrigerator, dishwasher, washing machine, furnace,
garburator, AC units, pumps, central vac, etc. Most small fans (ceiling
fans, range hood, bathroom exhaust) can be placed on regular lighting
circuits. If you have a lot of cool toys(like a garburator, hot tub or
central vac) you'll fill up a panel quick.

The stove, dryer and electric water heater will all need separate
breakers as well.

Most panels come as a 'kit' with a bunch of breakers included. It's
almost a sure bet that you'll be buying extra.

If you are replacing an existing service, and using the existing wiring,
the rules are often relaxed (sometimes even ignored) in many localities, you
should call your inspector. (And for a service upgrade you WILL need to be
inspected, the utility won't touch you otherwise).


Or do they mean a single circuit breaker can *share* outlets with
major appliances because each outlet it services has a *simultaneous*
capacity for a 1900 watt safe load?

Nope, major appliances and outlets that have a purpose to serve things like
microwaves and washing machines need their own dedicated breaker. That's
the code.

-- Steve

p.s. the 12 box rule is based on my knowledge of the CEC, I understand
it's the same for the NEC but I may be corrected.