I have to wire for a 3hp cabinet saw. The motor is a Baldor and the tag on
the motor housing says it is 12.5 amps at 230V. (It also says it will
operate on a minimum of 203v at 13.7amps. It has an over-heat shutoff.)
Does this mean I can use a 20amp double pole breaker? What difference
would it make if I used a 30amp? I think I'll use #10/3 wire (romex-type).
I need to wire a plug-lead from the saw's mag switch. I reckon a 20amp/230v
male plug with recepticle to match would be correct. Again: what
difference would it make if I used a 30amp setup?
Oh yeah, something else: I have two (unused) 15amp breakers. If I gang
them with a tie bar, does this make the setup function as a double pole
I will appreciate comments and recommendations.
No problems with anything until you came to the two 15 amp breakers, you
need a double 30 amp breaker for this setup.
Myself, I would go with the 20 amp breaker 12 gauge wire, and an appropriate
20 amp plug, socket, and cord, but if you want to up it all to 30 amp
breaker, 10 gauge wire and 30 amp socket, plug and cord I would not have a
problem with that either. Assuming the length of run of the circuit is not
The breaker is there to protect the wire, not necessarily what is connected
to the circuit.
Because, it is designed to protect the wiring. If you want to protect what
is connected you should buy the tool with it's won built in protection.
If the breaker was designed to protect the tool, you would only be able to
plug "1" tool into a circuit at a time. Many tools only pull 7 amps. Are
you going to put in 7 amp breakers and what if you want to plug in 2, 7 amp
tools into the same circuit? What if you want to run a dust collector and a
TS on the same circuit? You can't have a breaker that will protect both and
not trip when both are on.
I know what it designed to protect, but why the heck not put a 15a breaker
on the circuit instead of a 20 or 30? Doesn't cost anything so you pick up
protection for free. I am not saying it required, or even necessary, but
that doesn't mean it isn't preferable.
If he wanted to run a DC and a TS on the same circuit he would wouldn't have
asked about wiring a 12a TS. And if he did want to run both, the 20a
circuit wouldn't be adequate either!
In the case of power tools, the rated current is usually less than the
startup current, so you don't want the circuit to be *too* close to
the rated current, or you risk occasionally tripping the breaker when
you start the tool.
Hence, read the manual and do what they recommend.
No. A _smaller_ breaker only limits the circuit load to less than what
it could otherwise be used for.
I see no sane reason to limit the breaker size to less than what the
circuit wiring is rated for, either. One never knows what may come down
the road as a desired addition, so having to go back and replace the
breaker to take advantage of the circuit's full capacity is simply an
unnecessary pita and cost (although not huge, another breaker for no
reason other than undersizing it originally just makes _no_ sense).
What is the point??? The circuit breaker is not a good protection for an
appliance, it is not finely tuned enough for that purpose and it has no idea
if the tool is getting too hot or not since the wiring near it is not
getting too hot. A tool can get damaging hot and not draw too many amps to
trip the breaker.
Ok, you asked why shouldn't it also protect what is connected.
I explained why it shouldn't, not that it could not be done. It shouldn't
be done because the breaker is used as a last resort to protect you home
wiring. It shouldn't be done because of the reasons I mentioned.
It can be done, but it shouldn't.
Additionally, The protection needs to be located at or very near the motor.
The farther away it is the slower it will react to an overload especially if
the tool that is overheating is doing so slowly.
Breakers tend to not work very quickly. I regularly run a 1100CFM dust
collector, a 15 amp router, fan, and radio on a 15 amp circuit. No problems
as long as the DC and Router are started and brought up to speed 1 at a
time. Breakers are designed to take overloads and not trip immediately and
are usually rated at less than what the wiring in the house will handle.
That may not be fast enough to save a tool. Thermal protection built into
a tool tends to work at the threshold.
Consider a 20 amp circuit. Beyond 20 amps, the wire may melt and
cause fire. Below 20 amps, all is safe.
Consider what you plug into a 20 amp circuit. It could have *any*
current rating, from a tiny lamp cord to a power tool. The breaker
can't protect what's plugged in unless everything you plug in is
capable of carrying 20 amps safely.
So, devices plugged into a circuit which desire overcurrent protection
must provide a *suitable* protection themselves.
Note that in the UK every device has a suitable fuse in the plug,
which protects the device.
To recap, a circuit breaker...
* CAN protect the hardwired circuit
* MAY protect some plugged-in devices
* CANNOT protect ALL plugged-in devices
By definition, a circuit protective device, protect the insulation on the
conductors of the distribution system.
Overload devices provide the protection for the device.
About the only thing they have in common is protection, but they of
necessity, approach it from different directions.
1. The circuit breaker has no way of knowing what is plugged into it,
might be a 5 watt night light, might be toaster oven, might be an
arcwelder. Generally there are multiple sockets tapped off each branch,
with who knows what plugged into them. You don't want the circuit
breaker to pop until the current is enough to overheat the wires.
2. Electric motors draw enormous amounts of current getting started. To
avoid popping circuit breakers, you want to furnish the motor plenty of
current to start with. The starting load only lasts a few seconds while
the motor is coming up to speed. Your 12 amp motor might draw 30-40
amps at startup, and then drop down to a couple of amps after to getting
going. If you put it on a 12 amp breaker, it will pop that breaker on
3. The house circuit breaker is supposed to prevent house fires.
Without protection, a short circuit will heat the wire, running thru
the wooden studs of your wall, up to red hot. Then the studs catch fire
and your house burns down. Circuit breaker size is set by the branch
wire size. Rule of thumb: 14 gauge wire = 15 amp breaker, 12 gauge wire
= 20 amp breaker, 10 gauge wire = 30 amp breaker. Doesn't matter what
you plug into the branch circuit, choose a breaker that's right for the
4. Stationary saw motors always have a thermal protection switch built
into them that shuts the motor down if it gets too hot. They don't need
any more protection. Speaking of which, should the thermal overload
pop, you want to be aware that it might spontaniously reset, starting
the saw up again. You want to keep your hands clear of the blade until
you unplug the tool.
Was it me, I'd run your 220 volt saw on a 12 gauge branch circuit
protected with a pair of 20 amp breakers, one in each hot lead. Use two
conductor plus a ground wire cable. I'd make sure the iron frame of the
saw was connected to the green ground wire. Back at the terminal box
make sure the green ground wire goes to ground.
That is why I said, "but if you want to up it all to 30 amp breaker, 10
gauge wire and 30 amp socket, plug and cord I would not have a problem with
that either." My intent was everything was to be 30 amp. I suppose it could
be translated otherwise.
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