Bill wrote:
Some of what I've learned since I read Lew's post the first time:
Evidentally, a load center is "Main Lug Only" when it doesn't have
it's own main breaker like the primary load center would probably
have.
Load Units, such as made by Eator Cutler-Hammer use descriptions such
as "12 spaces, 24 circuits".
If I learned part of my lesson yesterday, the only way you'd get 24
circuits would be to use 1 Hot
and a 1 Common for every circuit.
In 120v circuits you would always use one Hot and one Neutral, plus a ground
for every circuit, so if what you are trying to say is that you can only get
24 circuits by wiring only for 120v, then you would be correct. You get 24
branch circuits by using mini breakers instead of full size breakers.
Mini's will give you two breakers in the space of one. Each half of a mini
gets wired the same way as a full sized breaker - a hot wire to the breaker
and the neutral and ground to the respective buss bar(s).
Is it typical for someone put two
such circuits under a single breaker--probably
not in a shop environment, huh? Even "lighting" seem too important
to mess around with.
A branch circuit is the outlets/devices that are controlled by a breaker.
That circuit can run to multiple outlets, multiple devices, or it can be
dedicated to just one outlet or device. With that in mind, you don't want
to run multiple branch circuits off of one breaker. You calculate the load
that a given circuit will potentially exhibit to the breaker, and determine
how many outlets/devices to string off that breaker. Lighting is fairly
straight forward since you pretty much know the load of every light - you
know the wattage of the lights you'll be using. Outlets are less straight
forward because the many types of things one may plug into an outlet make
for an undefined load. People generally stick with some common techniques
such as 8 outlets (or less depending on preference) per branch circuit in a
shop type environment. There's no real science behind that though.
So you use 2 spaces for a 2P-30A branch For Each 240v stationary tool
in the load center. So with four 240v tools
one is basically left with room for 4 or 8 120v lines.
Correct.
This
raises the question: Which is better--two circuits with 1 outlet each
from 1 breaker or one circuit with 2 outlets on one breaker?
It is best to think about how you will be using the area. It really serves
no purpose to install dedicated outlets all around your shop. It's not
likely that you will be using tools simultaneously such that you need 20A at
each outlet. You'd be better off wiring branch circuits of 4-8 outlets per
circuit. However - you cannot run multiple branch circuits from one
breaker. So - think about your expected usage and then wire in branch
circuits accordingly. My guess is you'll find that wiring 4-8 outlets per
branch circuit works out just fine.
I
think the former--for the same reason Lew only wants one 240v tool on
each line. This creates a new question for me: in what sense does a
15A breaker support two different circuits? Specifically, does
it only support the sum of the amperages of the two circuits?
It does not.
--
-Mike-