I'm about to start building my shop in the garage. I currently have an
old small subpanel in there with a bunch of 110 outlets scattered around
that the last owner installed. It's all pretty old stuff but still in
Many of the new machines I'm going to purchase can handle 110 or 220.
My question is should I rewire the garage with a new subpanel and make
all those machines 220 and scatter some new 110 outlets around as well
or just make everything 110? What's the benefit of running a machine
like a saw or shaper at 220 vs 110? Does it have more power? Can someone
explain the tradeoff?
Consider the current being draw through the wires as the air you breathe.
When you're working hard (running uphill or whatever) you need more air and
the faster the better, right? The same is pretty much true for your
equipment. Let's take your table saw. When it starts up it pulls a lot of
current (I think it's like 40-80amps) right off the bat to get the motor
spinning at full speed. That's one instance when a lot of current is needed
right away, another is if you're putting some wood through the blade and it
pinches a bit (hopefully you have a splitter -- but that's a whole 'nother
discussion), anyway the motor is now suddenly under load and needs more
juice to keep the blade spinning.
Ok, now to understand how 220 vs 110 helps with this -- imagine you're
breathing through a straw, a small one :) Now run up the stairs but only
breathe through the straw. Don't try this by the way -- you'd probably pass
out from a lack of oxygen, but that's the key -- you can't pull the air in
fast enough for your heart/lungs/brain to handle the quick load. On the
other hand if you were breathing through a large tube it would be pretty
easy right, might not even notice a difference.
Now the question is why this is a good/bad thing for the equipment. Here's
where it becomes a bit fuzzy for me. I believe you get a benefit from
spreading the voltage drop across 2 poles in your house (where most things
are 1 pole or 110) so you don't have a large drop on a single pole you have
1/2 the drop on both poles, kind of evens out the load on your house. That
has various benefits for delicate equipment like stereos, tvs, computers
etc. Also, I thought I read somewhere that it's better on your motor itself
and thus it should last longer (don't know if it was heat related or what).
Now, the real question is, is this fact or did I just pass along bad
information I've gathered over the years? For that you'd probably need to
ask an EE or electrician. If someone is one of those and has better info
I'd be happy to have it :) (or even happier to know I've more or less got
it right ;)
Man, thanks for taking all the time to answer my question. That all
makes sense. It seems to be more of an efficiency issue as opposed to
power. The motor for a saw is say 5HP and it'll always be 5HP if it's
running at 110 or 220. No more "power" with one or the other. It's just
more efficient to get the current to the motor via two poles (220)
instead of 1 (110). Maybe it'll be harder to bog down at 220 and maybe
the lights in the house won't dim as much if I'm running 220. I think my
question about rewiring the garage to 220 has been answered. It's better
to run them at 220. Thanks.
Mike in Idaho wrote:
Yes, but not quite in the sense you're thinking, I beleive. When
you run 110 you draw more current for a given amount of output
power, which causes "resistive" losses in the wires, etc (so you
have to draw even more current to make up for that). The problem
is in where those resistive losses go - which is into heating up
the wires, motors, and everything else in the circuit. Heat is
not a good thing; at best it shortens the life of the motor, and
at worst it sets the wiring on fire.
I think you are over rating the heat effect. If wiring overheats, the
breaker was oversized. That is why we have breakers. Properly sized wire
should have negligible resistive heating. Some people say a dual voltage
motor will last longer when run at 220v. I think the biggest motor
longevity factor is the quality of the motor itself.
Depends on what you mean by 'negligible'. To coin a "Clinton". :)
To deliver the same amount of power to the load requires *twice* as much
current at 120V as is needed at 240V. Losses in the wiring, etc. are
proportional to the _square_ of the current. Thus the losses in the
wiring are *four*times* greater at 120V vs 240v, for the same power at
In a properly designed system, these losses are "relatively small" at
either voltage. But, regardless, the losses are only 1/4 as large when
you run things at 240V.
EVEN WITH 'properly sized' wiring, the allowable voltage drop _in_the_wiring_
between the panel and the device can be several volts. A 5V drop in the
wiring, with a 15A load, and the _wiring_ is giving off as much heat as a
75 watt light bulb. And that's "within specifications" for a properly
You're free to think whatever you like. :)
Needless to say, the 'quality' of any given motor is *unchanged* by being
run at 240V instead of 120V. Regardless of whether it's a 'cheapie', or
a 'top of the line' unit.
Motors are, in general, more efficient when operated at higher voltages.
This means the internal "losses" are lower at higher voltages.
This means that there is, all else being equal, _less_ heat build-up in
the motor, when run at higher voltages.
Excess heat _is_ one of the biggest enemies of longevity.
winding and that the heating is a result of I^2 * R.
0.25 ohms ( [R1 * R2] / [R1 + R2].
ohm ( R1 + R2 )
There isn't any difference noticed in the motor itself. As you note, the
motor itself sees the same voltage and current on it's windings. The losses
are generated from the wiring which supplies the motor. All the way from
the main electrical panel, through any sub panels, to the outlet and though
any extension cords.
Good, we agree on that. So the 20A 120v motor will need to be fed from a 30 A
breaker using #10 wire while the 10A, 240V motor will
need to be fed from a 15A breaker using #14 wire. Givint this installation, there
will be negligable difference in the performance
of the motor when the system is examined in total.
30 A breaker using #10 wire while the 10A, 240V motor will
installation, there will be negligable difference in the performance
I agree. I'm not one that thinks the losses from wiring is a big deal, I
was just sumarizing the other side of the argument. Truth be told, I like
240volt runs mainly because I can run smaller wire which is cheaper and
easier to use.
Nobody who cares about I^2 * R losses and voltage drop is going to
take the trouble to run a motor on 240V and then turn around and cheap
out by using #14 wire just because it's legal to do so. Most people
would use #12 wire or better (I personally would never use #14 for
anything, given a choice; the stuff is a gift to the power companies.
Feel a piece of #14 romex when it's carrying 10A or so sometime - it
is noticeably warm).
And if you really want to go overboard, you could even use #10 for
your 10A, 240V motor, in which case your I^2R losses would be one
fourth of what they would be in your example for the same #10 wire
at 120V, 20A. Bottom line is, if you've got room for the double pole
breaker, there is no good reason not to use 240V for your motors, it
can't hurt and will always same some amount of energy over time, even
if it isn't huge.
Two things - one is that these are AC motors, and so there's
inductive effects (so I^2 * R isn't accurate, you need to consider
the impedance and the phase angle); and secondly because you're
drawing more current at 110, the drop in the circuit to the motor
is greater, in order to produce the same output power you need more
than twice the current than the 220 motor draws.
and 2.0HP @ 240V. with a similar 'mis-match' in current draw
at the two voltages.)
With the same power in, and lower _mechanical_ power out, it should be obvious
that some power is going 'somewhere else'. Which it is. into heat.
On Fri, 06 Feb 2004 09:48:13 +0000, firstname.lastname@example.org
(Robert Bonomi) wrote:
Which rule is that?
If the windings in a dual voltage motor are split and are wired in
series at 240V and parallel at 120V, how is the motor to know to
produce less shaft horsepower when the voltage (and current) in the
individual winding is the same for either source?
There shouldn't be a difference.
Master Woodbutcher and seasoned termite
Shamelessly whoring my website since 1999
The 'one with the typo', isn't it obvious? <grin>
I was doing the math right, just got the wrong symbol in the formula.
thanks for the catch.
I don't know all the theoretical underpinnings, I can state it is "observed
fact" from having motors on a test stand.
The electrical characteristics _are_ different, depending on whether the
windings are in series or parallel. If the windings are not _exactly_
identical, you get different voltage/current relationships depending on the
type of inter-connect. In series, the current through both windings is
identical, but the higher resistance winding will have a larger voltage drop
across it. OTOH, in parallel, the voltage across both windings is identical,
but the lower resistance winding has more current flowing through it.
"In theory, there is no difference between theory and practice.
In practice, on the other hand ....."
If you want a _real_ mind-bender, try and figure out why motors run at
1725 RPM or 3450 RPM. The 'obvious' speeds are 3600 RPM, and even
sub-multiples thereof (1800, 1200, 900, 720, 600, etc.), depending on
The number of 'poles' in the motor design. Yet, 4+ percent 'slow' is
nearly universal -- pretty much independent of any combination of
manufacturer, HP, or operating voltage.
It's only a mind-bender if you've never studied how an induction motor
works. There are many different ways to build a motor. The most common
design that you find in shop machinery is what's called an induction
The electric field created by the stator windings rotates at 3600 RPM
(for a 2-pole motor). The rotor rotates a little slower than that (say,
3450 RPM). The difference, 150 RPM or 2.5 Hz, is what induces current
to flow in the rotor windings (hence the name). There are no brushes.
As the load increases, the rotor slows down and the slip frequency
increases, causing the rotor current to increase. It's a very clever
One of the drawbacks of induction motors is that the rotational speed is
not constant. As load increases, the rotor speed drops. For many
applications, this is not a problem.
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