[Build] Next: Dirty Shop

Sean Glang seanglang at gmail.com
Tue Jun 29 13:23:16 PDT 2010


Hi Jim (et al.)

I'm going to intersperse my comments, but also edit out some of the
noise, so our conversation is a bit easier to read.


>> its not a good idea to put all your heavy loads (like power tools) on
>> one phase and all your light loads (like lighting... heh) on the
>> other.  This creates load imbalance, can cause load on your neutral,
>> and violates electrical code.
> JS: power tools, while drawing a lot of current, are used
> sporadically, probably fair to say rarely. lights consume
> a fair amount of current and are on for hours at a time.
> other receptacles are used mainly for powering computers
> or running electronic equipment (soldering irons, scopes...).

For the purposes of phase balancing, looking at what happens over time
is not helpful.  You need to know about the balance of electrons right
now.  There's no way for the wires to store up a balance they then
spend later.  When you balance out your circuits, you do so by adding
up the wattage on either phase, then seeing how close they are to
equal.

If your lights and power tools are anything at all like the ones I've
seen before, your power tools are going to pull WAAAY more wattage
than the lights.  There is little doubt that you should have your 110v
power tools with dedicated outlets on alternate circuits.  Also, your
convenience outlets (where someone could plug in a drill or soldering
iron) should alternate.

This is basically covered by NEC 250.53 (in the example you can see
that they have 1/4 hp motors on separate phases)

Here's an example that might be closer to the actuality:
Let's say phase one is power tools and convenience outlets, and phase
two is lights.
Also, let us assume that the lights have a load of 4000 watts for the
whole site, and that there are up to 8 power tools in use at any given
time, each of which runs at 1000 watts.  (Of course, these numbers are
simply made up to provide easy examples.

So, Fox decides to come into NB.  He shouts "Let there be light" and
turns on all the lights.
Phase A has zero load.  Phase B has 40 amps of load.  The neutral is
carrying 40 amps of load.

Then, Fox realizes that he's using up too much power on the lights.
Being a good responsible citizen, he turns off the lights he's not
using.
Phase A still has zero load.  Phase B now goes down to 10 amps.  The
neutral is carrying 10 amps of load.

Now, Fox wants to take some noise samples to upload into an MP3 that
plays in the background of pictures of people using power tools.  So,
Fox turns on all of the power tools.  Fox leaves the lights as they
are.
Phase A has 80 amps of load.  Phase B has 10 amps.  The neutral is
carrying 70 amps.



The way I have typically wired up a building is like this:
Half of the lighting circuits are on one phase, half are on the other
(note, this is a code requirement for commercial buildings - not for
load balancing, but for public safety.  If one phase burns out, you
don't want to be stuck in the dark.)
The fixed appliances (dryers, heaters, power tools, etc) are on
alternating circuits.  This is the easiest way, though it is also
likely I'd wire up two power tools next to each other on the same
circuit.  People tend to not use two large power tools next to each
other at the same time.  I feel this is a subtle point, but I also
figure anyone reading at this point might enjoy some of the nuances.
The convenience outlets alternate circuits.  This is another code
requirement, and it makes sense.  If you have 4 duplex outlets that
people might plug 8  250 watt soldering irons into at any given time,
you can have a neutral load of 20 amps, or a neutral load of 0 amps.


However, I should also point out that in the last 10+ years of doing
electrical work, I've only seen a neutral load become an issue twice.
Both times involved people running lots and lots of high wattage
appliances (I suspect they were growing weed) for long hours in an
imbalanced manner.  In real world situations, like I expect NB to
have, I expect it to be very unlikely for there to be any real safety
issues.




>   the electrical wiring currently in place is as i described
> (black for receptacles, red for lights and appliances), so
> the proposal for the shop just extends that convention. the
> reasoning for sticking with that convention is consistency:
> in a hacker space, people may have the moxie to try things
> they don't know much about (hacking); consistency gives them
> a fighting chance of succeeding without bringing down the
> house.
>   as to load imbalance, generally a black, white, and red
> have been pulled together (black always from the "left"
> side of the transformer, red always from the "right",
> therefore out of phase). the white is a common return from
> devices on both the black and red.

Is there a transformer in your system?  If so, why?
When you pull power from a load center (also know as "the circuit
breakers" it's important to remember (at least in modern load centers)
that the phases are stacked.  So, the top two breakers are on the same
phase.  I know this isn't what you described, but I thought I'd throw
that out there for anyone who might be doing some wiring in the
future.


>   for example: if just devices on the black are running,
> the white returns the current delivered on the black. if
> some devices on the red are also running, the current on the
> white is reduced to the difference between the current on
> the black and that on the red. assuming no overload on any
> circuit, there is no problem with an imbalance between red
> and black power use. the system (and the electrical code)
> allow (actually assume) this.
>
>
>> To keep your lighting safe from tripping along with your radial arm
>> saw, simply put it on a seperate circuit.
> JS: the convention of red for lights and black for
> receptacles ensures lights will be on different
> circuits (the primary reason for the convention--
> again, the rule is to help ensure that hackers will
> do the right thing).


I'm currently up in Portland, and my girlfriend assures me it's time
for me to stop being on the computer and go do something.  So, I can't
quite get as super geeky as I'd otherwise be.  However, if you have
some time when I get back, I'd enjoy having a conversation with you in
person, ideally at NB, since it's so much easier to discuss these
things real time with potential examples right in front of you.

Also, I hope I didn't come off sounding snooty or whatever.  I
apologize if I did, I blame all the stoned hippies up here.

Love,
Sean is.


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