Table saw trips circuit breaker

Question

My Craftsman table saw, model number 137.218073, will occasionally pop a breaker when I start it if other appliances are active on the same circuit. However, if I manually turn the blade before I start the saw, it starts without tripping the breaker. I've since switched the saw onto another circuit, and although it starts without rotating the blade, it still seems to be drawing too much current initially. The saw makes accurate cuts, but tends to labor a bit when cutting hardwoods like oak.

Source: Sears Table Saw Parts: www.searspartsdirect.com

Answer 1

One spec on a motor is LRA or Locked Rotor Amperage. This is the amps the motor will pull on startup, because it is stopped when you start it. This spike runs typically 4-10 times normal running amps and lasts for dozens of milliseconds until the motor starts to turn a bit and start developing back EMF. Which is what you're accomplishing by using a stick to get the blade turning.

A typical circuit breaker will "instant-trip" above around 800% of rated amps. This is happening magnetically and is time-insensitive, i.e. it's not using the thermal trip function which has a time factor. It's not exactly 800%; the manufacturing tolerance is 600% to 1050%. This overlaps LRA! So occasional trips are to be expected.

NEC Article 430.51 et.seq. say on a dedicated circuit that serves only a saw (or other large motor load), the circuit breaker may be oversized to stop LRA from overlapping the instant trip band. It defers to UL to set the value for any given motor which UL is listing, and that will be on the motor nameplate. If it's not, you go to 430.52.

Why is this safe? The motor which allows this has onboard thermal overload protection (#1 of the 3 protections a breaker provides). This does a better job because it knows the temperature of the motor, it's not guessing it based on the temperature of the bimetal strip in the breaker. Certain usage patterns will allow that bimetal strip to cool off even though the motor is quite hot (or the reverse; ask anyone whose panel is in direct sun in Arizona).

Protections #2 and #3 are short-circuit (hot-neutral) and ground-fault (hot-ground) dead short protection, and a somewhat larger breaker is not going to impede that protection significantly.

So the ideal answer (and the instructions may even tell you this) is a dedicated circuit for the appliance, with circuit breaker sized according to motor nameplate or 430.51 etc. However most people, if they're going to a 30A breaker, they just go ahead and install 30A wire (10 AWG) and a 30A socket (NEMA 6-30). In this day and age, that's also a good idea due to EV charging adding value to a home.

Answer 2

Here is the manual for your saw.

This saw is a typical contractor's worksite table saw and has a universal motor with brushes, but no starting or running capacitors.

The indications you describe sound like the brushes are worn out (or had manufacturing defects if the saw is new). One or more of the motor's armature commutator pads could also be damaged or missing. It could also be possible that a bearing is bad.

To make these checks, unplug the saw. You'll be touching the blade and the saw's electrical parts.

See the manual to find the brush caps on the motor, then remove the brushes. These will be long black carbon-like blocks, with a flexible stranded copper wire coming out one end, encased by a spring, and capped by a piece of metal. The other end that goes into the motor will be curved. The black carbon part should be decently long, and the curved end should be smooth and unbroken. Web-search for replacement brushes for the saw and compare the size with your saw's brushes to see if they need replacement.

Also, look down one of the brush holes into the motor and carefully spin the blade, so the commutator pads pass by for inspection. Use a borescope or mirror to see the pads. Look for heavy scratching (there will likely be light scratching) or for missing pads. Damaged or missing pads will require a new armature.

It could also be that the bearings in the saw are bad. Typically the bearings don't need lubrication, as they are sealed. But bearings do go bad.

Answer 3

I just discovered an old extension lead I had couldnt handle my drop saw and it was tripping the circuit breaker, i put it on another less powerful device and it was fine, so the moral of the story the problem can be in many places

Answer 4

All electric motors are essentially a dead short across the line when they start. This causes a large amount of inrush current until the motor begins to spin and develop something called counter EMF or Electro Motive Force.

Basically a motor is just a dead short circuit until it begins to spin. Check the resistance of the windings of a motor and you will get just a few ohms and at 120 volts that could be 30 to 40 amps of inflow current at the time of starting.

However, once the motor begins to spin it creates a counter force that pushes back on the incoming current and reduces it to its normal running current that is on the nameplate.

All, large saws will dim the lights and labor when they start. When you put a heavy load on them they will draw their rated current and maybe slightly more.

To compensate for this, if possible convert the saw to 240 volts. Make sure it is on a 20 amp circuit and nothing else is running on that circuit. Make sure it is plugged directly into the circuit or you are using a #12 gauge extension cord as short as possible. These things could reduce the drag that the saw is putting on the circuit.

Good Luck!

Answer 5

It sounds like you are overloading the circuit. What size breaker is it that trips? Another possibility if you are running an extension cord to the saw is the length of cord and the size of the wire in the cord. I would recommend adding another circuit for power tools and equipment regardless and if necessary to use an extension cord make sure it's at least a number 12 wire and not extremely longer than necessary to reach equipment. No matter if the saw is 5 ft from the outlet if it's plugged into a 100 ft cord it's going to be a voltage drop.

Answer 6

https://www.searspartsdirect.com/manual/1uynkmg3cb-000247/craftsman-137218073-table-saw-parts

Ampere...... 15 amp

kinda comical how there's 5 other answers talking about... whatever. no one even attempted to RTFM.

also might want to consider, sears is all but completely out of business. That doesn't happen by making/selling well designed problem free products. unless we're talking about something built in like the 1950's sold by them.