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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!

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    @P2000, I believe that 80% derate is only for continuous loads. Intermittent loads (which this saw is almost certainly considered) are allowed to use the full 15A, which is why you can find tools with nameplate draw ratings of 1800W using a standard 15A plug.
    – Nate S.
    Aug 5, 2020 at 19:56
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    @NateS. Maybe you are right. Canada and NEC are different and I scrapped my comment, as it is not what the OP asked.
    – P2000
    Aug 5, 2020 at 20:20
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    If you ever get the chance, pull a 20A circuit as well for power sockets, and leave the 15A for lights and so on. If you can do any work to make later work easier (ie lay a conduit when doing something else) then do so. If you ever think of having an EV, consider its charging needs and run something suitable, or install condit to make this a one-time problem.
    – Criggie
    Aug 6, 2020 at 3:14
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    @Criggie I have been strongly considering this but as a complete rookie I have approached the idea of running a new circuit myself with extreme caution. Are there any resources you recommend for me to run a new circuit safely and with adequate future proofing? Moreover, do you have a strong opinion about whether someone should even attempt it if they haven’t done it before? I consider myself to be pretty smart and pick up on things easily but the idea of miscalculating current and burning my house down is horrifying.
    – Pierce
    Aug 6, 2020 at 3:28
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    @pierce why not post that as a question. Include a drawing: mark location of panel, highlight types of walls (thickness, material, structure), any bulkheads, elevation differences, firewalls (e.g. garage), and mention your jurisdiction (country/state). Lots of professionals and weathered DIYers here.
    – P2000
    Aug 6, 2020 at 14:28

4 Answers 4

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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.

enter image description here

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.

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  • Thank you so much for this detailed answer! The other answers were great (thanks everyone!) but this fully makes me feel better about this effort. More importantly it gives me something to show the lady when her inevitable look of disapproval arrives. I will definitely have my fingers crossed when I start it the first time. Really great to know that using a long extension cord could offer enough resistance to make the difference in a pinch. I will keep a close eye on temperatures as well, great thinking. Thank you!
    – Pierce
    Aug 5, 2020 at 21:05
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    Oh oh, I was always worried about someone reading my comments and then getting in trouble with the inspector... but the risk of trouble with the lady is a whole new concern now! Good luck and happy cutting.
    – P2000
    Aug 5, 2020 at 21:51
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    Haha indeed! She’s pretty cool about most things :) just when it comes to the health and safety of her cat (even if the thing in question only threatens said cat’s safety by multiple tangents) she draws a hard line. I suppose some of it is also my wanting to avoid any future potential “I told you so”’s, which are more damaging than any table saw could ever be 😜
    – Pierce
    Aug 6, 2020 at 3:32
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    Inrush current can be a factor. I used to have a Makita circular saw rated at 1750W, which tripped the breaker (rated for 3500 W, 16A at 220V) with some regularity.
    – Hobbes
    Aug 6, 2020 at 7:12
  • I like the seventh point on your list!
    – Pavel
    Aug 6, 2020 at 9:43
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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.

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    Oh, that's a very good point: "if the breaker does trip while the saw is running, you've created a safety issue" Safety lighting might not switch on fast enough for that first fraction of a second, and lighting on a separate circuit is certainly good advice! In my case, if the breaker pops at all, it's at start-up, not stall. With a stall I have some time to reach for the power switch.
    – P2000
    Aug 5, 2020 at 17:22
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    Agreed that a fast-starting emergency light would be needed, and lighting on a separate circuit is even better. And yeah, the startup inrush is more than the stall current, but it's possible for the saw to stay in the stalled state much longer than the inrush current duration (possibly several seconds, vs milliseconds), so in theory, either one could trip the breaker, and which is more likely will depend on the saw and load.
    – Nate S.
    Aug 5, 2020 at 19:53
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    @NateS. that is a fantastic point, thank you! I will absolutely be doing this now, no question. I didn't realize until I read your answer how horrified I am of the thought of having a moving blade kick up a piece of hardwood directly at me right as the lights go out. Yeah, no thanks.
    – Pierce
    Aug 5, 2020 at 20:58
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    Upvoted for calling out the lights-out hazard. Aug 6, 2020 at 4:46
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    The lights-out hazard is real and important, but the OP should be able to test this safely enough in daylight.
    – alephzero
    Aug 6, 2020 at 10:38
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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.

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  • That is an excellent point! I have discovered so many delightful "surprises" since buying this house last September that I would be shocked if they weren't doing it the cheapest, easiest conceivable way. At the very least I will ensure the outlets are properly installed (not "back stab"-bed), and will definitely upgrade the outlet if it looks super chincey. Thank you!
    – Pierce
    Aug 5, 2020 at 21:01
  • @Pierce The way things are going I'm surprised you haven't been shocked yet. Aug 7, 2020 at 6:10
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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.

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  • Hello, and welcome to Home Improvement. Thanks for the answer; keep 'em coming. And, you should probably take our tour so you'll know the details of contributing here. Aug 8, 2020 at 13:44

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