Can I run a 15A table saw on a 15A circuit if there are also shop lights on the same circuit?

Question

I apologize if this is a tired question, but none of the other similar questions I found are close enough to what I really need to know.

I am hoping to buy a DeWalt table saw for use in my garage. The only documentation I can find shows that the saw draws 15A, 120VAC. For reasons that I will never understand, the garage was wired for just 15A instead of 20A. I recently installed 4 40W LED panel lights with a total power draw of just a couple of amps (I haven't pulled out the multimeter and actually measured this).

My question boils down to this: Can I run the table saw with the lights on?

More specifically, is the table saw going to draw 15A constantly and definitely trip the breaker? Or is that a "burstable to" or "maximum" rating, with a constant power draw of fewer amps?

In theory I would only need to run it for a few minutes at a time, so it might fall into the overcurrent "grace period" I have seen mentioned in other similar answers. I am just not wise enough to know.

Another option, of course, is to find a 20A circuit and run an extension cord to the garage. That seems hacky and gross, not to mention inconvenient, but it is an option if necessary.

I appreciate any insight you can offer in my dilemma!

Answer 1

Your usage scenario is quite typical, and whether your 15A breaker will trip or not depends on how long you are cutting, how heavily the saw is mechanically loaded, and the magnetic & thermal trip characteristics of your breaker.

Let's say you have the lights on and you are doing some heavy cutting. You are past the inrush (startup) and the motor is not stalled.

Here's a sample calculation for your nominal cutting conditions:

1. The motor for a dewalt 7480 is spec'ed at 1850W or 15.4A at 120V, with maximum load before stall.
2. Your lighting is 4 40W LED panel lights, 1.3A.
3. Total current 16.8A
4. Cutting Time 100s
5. From the "trip curve", a "Square D" will definitely trip at 2x the rated current, i.e. 2x15A = 30A. And definitely not trip at 1.2x rated, i.e. 1.2 x 15A = 18A
6. The total is just below the "definitely not" trip point. Turn down your garage stereo.
7. Happy cutting.

Load time (seconds) vs Maximum current as a factor of the rated (e.g. 15A) current. To the left of the grey band: definitely not trip, below thermal trip. To the right of the grey band: definitely will trip, above magnetic trip.

Breaker trip chart from https://download.schneider-electric.com/files?p_enDocType=Data+Bulletin&p_File_Name=0600DB0105.pdf&p_Doc_Ref=0600DB0105

You can check the stall current by measuring the resistance of the motor, and calculate I = 120/R.

It's usually not listed, and likely well above the 15A rating. Your breaker will not trigger immediately if the current is briefly above 15A. A GFCI breaker may behave differently and trigger faster.

The inrush current is typically much higher than the stall current, and more likely to trip the breaker if the circuit is already heavily loaded.

Your wiring, especially any extension cords, will resistively limit the current and provide you with more cutting time, albeit at a slightly reduced voltage and thus power. For short term use this is not a practical problem not a problem, but do be watchful of any heat-up in the wire or at the socket & plug. If it is too hot to touch you are approaching unsafe levels.

Answer 2

Good question. Your table saw, like any electric motor, will vary how much power it actually uses based on the load connected to it -- if it has to work harder to turn, such as when you're cutting a difficult material, it'll use more power. It'll also take extra power briefly when it's first starting up, and also if the blade binds and the motor stalls.

However, it's 15A rating may not be its actual maximum power draw -- motors generally don't count the startup current (and possibly stall current) as part of their normal usage ratings. So it's possible that the motor may draw 15A much of the time while its running, and burst a bit above that from time to time. It should be designed such that it doesn't draw more than 15A for long enough to trip a 15A breaker, but it's possible your lights might push it over the edge.

All that said, there's a good chance it might usually work. Assuming that the breaker can handle the initial startup (and it won't hurt anything to just give that a try), and you can avoid binding the blade, it'll probably work just fine. Personally, I'd just give it a try before I spend money re-wiring or do a hacky extension cord thing.

There's one other issue if you do this: if the breaker does trip while the saw is running, you've created a safety issue because now you have a spinning blade but you've just lost your lights. It could be even worse if this happens because the blade binds to something and tries to throw your workpiece across the room in the dark. I'd highly recommend you install some battery-backup emergency lights if you attempt to go this route.

Answer 3

The draw of a motor is directly related to the amount of work it's doing. When your table saw is just turned on and not cutting it's not going to draw anything near 15 amps. It might get there when cutting hardwoods at the maximum rate the saw can handle. So you might be over 15 amps total for a while, but as others have mentioned here, breakers have "trip curves", meaning you can draw more than 15 amps for a short period (depending upon how much additional current you are drawing, that could be a few minutes to under 1 minute).

Since you'll be using a fairly heavy duty tool, you should check to make sure the outlet(s) in the garage are not wired using the "back stab" wiring method. Given the fact this is a 15amp circuit, I'd bet they used cheap outlet(s). You may want to replace them with spec grade or commercial grade.

Answer 4

My 1st time posting. Be gentle. This is just an alternate solution, not an expert answer. I'm just trying to alleviate the OP's concern of tripping his breaker from a different perspective.

My humble advice that to help you avoid max amp draw when doing heavy cutting, thereby greatly lessening your chance of binding or stopping the motor, also thereby avoiding drawing max-plus amps and tripping you're breaker.

The binding issues usually arise when rip cutting difficult stock. For example, ripping 8/4s or 10/4s white oak. This can be done all day long on a 15A/120v saw but you first need to slow your feed rate a great deal. Often, as you approach the middle-to-end of the rip cut, the wood's memory (yes that's a real thing) will cause it to try and squeeze back together. If this happens, it will pinch closed on the blade. If you're lucky, this will only trip you breaker. This is also the perfect recipe for violent kickback, so you have two reasons to avoid it.

Simple solution. When you setup your new saw there will be procedures outlined to square the mitre gauge to the blade and then set the fence parallel to the mitre gauge slot and the blade. This sets your rip fence 100% parallel to the blade from start of cut to finish. Sounds better than it is.

This adjustment can easily contribute to blade pinch and the tripping of your breaker, many times during the course of a busy day. Follow the manufacturer's setup instructions, except set the measurement between fence and the rear of the blade, 1/32" wider (further away) than the measurement at the front of the blade. This will widen the kerf, as you go along, just a tiny bit. The effect on accuracy will be all but unnoticeable and you will give your saw an extra bit of clearance to avoid the dreaded pinch. Happy and safe cutting.

Answer 5

Gentlemen/ladies Just hire a electrician, to replace the wires to handle 20 amps , and install a 20 amp breaker. The cost is cheaper than risking a cut finger, loosing a eye from kickback in the dark, and worst of all damaging your ego. retired electrician and woodworker.