Volts is volts. Watts is watts. Watts is volts squared. ;-)
I just bought a 2HP DC (came in a crushed box yesterday :(). I went
with 2HP because as long as I was going 240V anyway, might just as
well do it right. ;-) Still gotta run the line, so replacing the
crushed parts isn't an emergency.
During motor startup the current could be more like 40-60 amps. Breakers
are designed to allow for momentary inrush currents of that magnitude,
maybe even higher. What does that 6.8% figure change to when you
do your calculations on 40 amps instead of 10?
There are no stupid questions, but there are lots of stupid answers.
Larry Wasserman - Baltimore Maryland - lwasserm(a)sdf. lonestar. org
Okay, but let's use a more realistic wiring length of 30 feet. (20 feet
from the panel, 10-foot cord.)
Power loss in the wiring is:
Pc = I^2 * R
For 30' of #12 copper, R=0.048 ohm
At 120V, assuming 60A inrush:
Pc = 173W
At 240V, assuming 30A inrush:
Pc = 43W
So for a total inrush power draw of 7200W (60*120 or 30*240), at 120V we
lose 2.4% of the power to supply losses, while at 240V we lose 0.6%.
We get 1.8% more power delivered to the motor by switching to 240V.
On Wed, 2 Sep 2009 08:34:57 -0700 (PDT), " firstname.lastname@example.org"
My old Beaver table saw wouldstart like a bomb when plugged directly
into the wall, or on a short 14 ga cord. It often popped a fuse on
Putting it on a 25 foot 16ga cord it never popped a fuse, and took
about 3 times as long to come up to speed (this was a repulsion start
induntion motor - you could hear when the brushes came out)
I understand what your are saying. When on 120v the saw was on a
circuit with 4 outlets. The only other item plugged in was a cordless
There is a clear difference in start up. I am definitely not an
electrical professional, so I can't explain it.
OP: You have gotten a little good advice and a whole lot of very bad
advice. You problem is figuring out which is which. If I someday have
a death wish, I'll have to remember to ask such a question on the wreck.
I will probably just wire a separate cord for the 120v lamp. I do
like reading all the comments and thank everyone for posting. It all
helps me understand more than I did. It makes sense that once running
the motor has the same power (one hot leg at 15amps vs. two hot legs
at 7.5amps each).
Just remember those "7.5amps each" are not additive. (Robatoy, don't
look ;) ) It's the same amps in each leg - one's coming to the motor,
the other's returning it from whence it came. Just like 120v, one line
brings the amps in, the other line takes it back.
No. It runs at 15A @ 120V, or 7.5A @ 240V.
The amperages on the two legs of the 240V circuit don't add, because the two
legs are in series with each other. Current is the same at all points in a
Assuming the same horsepower motor, yes. If the motor had four
windings (instead of two, like the typical dual-voltage motor) wired
in parallel instead of series, yes. Remember 1HP ~= 750W (add for
With 120, 15 amps are running through the motor. 15 amps are coming in
one wire and 15 amps are going out through the other.
With 240, 7.5 amps are running through the motor. 7.5 amps coming in
through one wire and 7.5 amps going out through the other.
Two wires in each case. You could almost think of it like a
waterwheel, one wire pouring in the juice, one wire draining it out,
and the juice spinning the motor as it goes through.
OK, OK, I said "almost". It's just an analogy. (;-)
Well, from industrial customers, it goes back to the electric company
and into their settling ponds for waste treatment and sanitizing
before being recycled out to the residential customers.
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