I am now running my table saw and dust collector on the same 20a circuit,
but will be buying a larger dust collector. Eventually I will put in a new
240v circuit, but for the moment I can change the TS back to 120v. Problem
is the magnetic switch as a 240v relay in it.
Is the relay likely to work properly on 120v? When my 120v relay burnt out
and I switched the whole thing over to 240v it was only like $10 for the
relay, but it was a lot of work that I would just as soon avoid.
Three possible problems
The coil won't work on 120V. (You can likely buy a 120V coil)
The starter ratings may too low at 120V - the motor current will double.
If the starter has "overloads" that cause the starter to trip on
overcurrent they will be the wrong size (they can probably be replaced
with the correct size ones). Instead the motor may have built-in thermal
protectors that disconnect it if it overheats (it will then be marked
Hmmm, the fact that it take half the current at twice the voltage to
generate the same number of watts doesn't seem to me to mean that the
current will double if the voltage is cut in half.
It seems to me that if the voltage is cut in half, the current will
also be cut in half also (E=IR), and the wattage will be only a
quarter of what it was. (I'm not sayihng that wouldn't work. I don't
Is there any particular reason I"m wrong (AC current?)?
You are right about the current to the coil; since it will be pulling a
quarter of the power, it might not work.
But the big problem is with the current running though the relay contacts.
It will be twice the current at half the voltage; and will likely burn out
or fuse the contracts.
Relays are rated by the size of the motor they will start without failure;
typically they will start twice the hp at 240 as at 120v. Unfortunately.
Since I don't want to replace the relay, I will have to put in the new
circuit sooner than later.
Considering you will have to change the motor wiring, the power cord
or plug on the end of it. and still have to deal with this relay,
wouldn't it be easier to just run some 12-3 or 10-3 romex from your
panel and put a 220 outlet on the end of it? Unless you are lacking
space in your breaker box or have to fish the wire thru a whole bunch
of stuff, it just seems easier to run the 220 now. You only need 3
parts, the wire, the breaker and the 220 outlet. (plus screws and
wire staples) If it's just a clear run across the basement ceiling of
about 50 feet max., you're talking 2 hours labor. You'll spend more
time than that on all that other stuff.
For the relay, you probably could buy a small step up transformer 120
to 220, but it would probably cost as much as the stuff to rewire.
And while this may not be legal, you could make up a rubber coated
long extension cord an just put the 220 outlet right under the breaker
box. If you own the house, I'd wire it permanently, but a renter
might prefer the ext. cord. (of course that rubber cord costs a
Well, two reasons; I have no more space in the box and will have to get a
quad breaker, and 50 joists to drill through. But yes, since the relay
won't work, I have to do it now.
BUT, why do you say 12-3 or 10-3? Why three rather than two?
Yeah, the old full box issue. Of course there are add on boxes that
will hold 4 or more single breakers. Sometimes the box is cheaper
than those special breakers. All you need is a couple inches of
conduit and some heavy cable to put these boxes right next to your
present one. If this is an unfinished basement, you can always nail a
1x4 to the joists and nail the cable to that. No drilling that way.
For something thats only 240, you only need 2 wires +gnd. Things
like a kitchen range use some 120v things inside such as the light
bulbs and clock, need 3 wire cable +gnd.
It's a common question. What is not realized is that the motor or other load
has to be changed or rewired so that its impedance (basically AC resistance)
is reduced to one-fourth of its 240V value. This is commonly done by
switching from two coils in series to two coils in parallel. So you get
twice the current with one-half the voltage. If you do not change the load
the current IS reduced to one-half and the power IS reduced to one-fourth.
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