Lights Dim When I Turn On My Saw

[jonmnelson]

I just got a Jet 708100 and set it up in my garage. When I turn it on, the overhead light dims and then comes back. If I run my Shop Vac at the same time that also slows and then seems to come back to power. Obviously the electrical system isn't what it should be, but does this sound like a long-term problem? I don't want to be tripping circuit breakers (I haven't yet), but I also don't want to do any damage to my tools, or have them operating at less than full power. BTW, the house is a rental so I don't know that I want to upgrade the electrical system for my landlord's benefit.

Thanks for any input.

[crokett]

The outlet you are using is on the same circuit as the lights. Most likely this is a 15 amp circuit. You can check this back at your breaker box - the breaker will have an amperage rating on it. Your saw would be happier to be on a 20 amp, if you have one available. At the very least I would move your shop vac to an outlet that is on a different circuit from your tablesaw.

[LCHIEN]

when induction motors come on they typically draw 4-10 times the maximum current for just a brief moment, less than a second.

Which means a 15A saw might draw 60 or more amps as it starts.
Circuit breakers are designed to allow this momentary startup surge and not trip; otherwise we could never start our tools and appliances.

There is however a limit to the circuit wiring in your house... a surge of 60A or more might cause a 15-V or more drop in voltage that is reflected by the lights dimming and TV pictures shrinking, etc. Incidentally, running a 15A tool at full power will probably drop your circuit voltage by a few volts, maybe 4-5; this is normal, too.

if you have a 15A saw, that will draw maybe 6A running but up to 15Amps when its cutting full power, you shouldn't be running your shop vac on the same circuit breaker/line. You'll trip the breaker when the combination goes over 15A. I'll bet your shop vac pulls at least 6 running which only leaves 9 for your saw so it will never reach full power.

[LarryG]

Adding to what's been said:

The receptacle powering the saw isn't necessarily on the same circuit as the lights (although it may well be). A typical residential panel has two legs, with 120V fed to each, for a total of 240V. The dimming-lights problem can also occur if the heavy motor load is on the same leg as the lights. I initially had this problem in my previous shop. After I put in a new sub-panel, the fluorescents would momentarily go out every time I switched on my dust collector. The DC was on a dedicated circuit, but it was on the same leg of the panel as the light circuit. Physically unsnapping the breaker and moving it to a blank space on the other leg instantly cured the problem. Total cost: zilch. Time required: less than five minutes. If you have any blank spaces in your panel, and know what you're doing (or have a friend who does), this might solve your problem too.

But if you don't want to do anything at all to change the electrical status quo in your rental, you might try plugging the saw into a different outlet, if available, that's on a different circuit tied to the other leg. This might require using an extension cord, which can introduce problems of its own; but if you keep the cord short and use the proper wire gauge, you might be better off than you are now.

Alternatively, as a long shot ... I see that your saw can be wired for either 120V or 240V operation. If a 240V outlet is available, you could (probably should) rewire the motor for that voltage, as that will cut the amperage requirement for it in half.

[jonmnelson]

The dimming lights occurs whether or not the shop vac is running, but it is a little more pronounced with both running. Since the shop vac presumably uses less power, perhaps I could run that off another plug using an extension cord and at least minimize the power drain? I'm more concerned about the saw bogging down when cutting or, worse, tripping a circuit while cutting. I haven't run any wood through the saw yet because if it isn't going to work in my space it'll have to go back.

[Mr__Bill]

This sounds like a tool gloat to me!!

[jonmnelson]

This sounds like a tool gloat to me!!

It's a little too early for that. Assuming the power and space requirements aren't too much, and once I actually do cut some wood with it, then I might have something to gloat about. It sure is quiet and seems very solid, though.

[LarryG]

Yes, moving the plugs around to different outlets is the place to start. Obviously the other outlets have to be on different circuits, otherwise you're accomplishing nothing. Try as many combinations as you can, see what happens. Smaller loads like the shop vac will be less affected by a longer extension cord (although you should keep any cord as short as possible and make sure the wire gauge is matched to the load).

Do the lights dim momentarily when you start the saw and then return to full brightness? Or do they dim and stay dim? The former is not ideal but is usually tolerable. The latter, however, isn't good ... you really don't want to operate that way.

[jonmnelson]

Yes, moving the plugs around to different outlets is the place to start. Obviously the other outlets have to be on different circuits, otherwise you're accomplishing nothing. Try as many combinations as you can, see what happens. Smaller loads like the shop vac will be less affected by a longer extension cord (although you should keep any cord as short as possible and make sure the wire gauge is matched to the load).

Do the lights dim momentarily when you start the saw and then return to full brightness? Or do they dim and stay dim? The former is not ideal but is usually tolerable. The latter, however, isn't good ... you really don't want to operate that way.

The lights dim on startup and then seem to come right back to full power. I'm concerned that the power requirements when cutting wood might cause a similar drain on the system, which I agree is not a good thing. Has anyone see this happen -- the lights dim on startup and then again when cutting anything difficult, such as hardwoods?

[radhak]

The dimming lights occurs whether or not the shop vac is running, but it is a little more pronounced with both running. Since the shop vac presumably uses less power, perhaps I could run that off another plug using an extension cord and at least minimize the power drain? I'm more concerned about the saw bogging down when cutting or, worse, tripping a circuit while cutting. I haven't run any wood through the saw yet because if it isn't going to work in my space it'll have to go back.

You need to run the shop-vac on a different circuit : so you'd need to do some checking for that. eg, you could plug it in and turn on a couple of lamps, then cut power at the board one circuit at a time to see which outlet is not on the same circuit as your TS, and then use it. I am sure using an extension cord for the vac should be okay too.

I own a much less worthy saw - which means it's less efficient in cutting, and thus a bigger risk of bogging down while cutting. It draws the same 15amp as your Jet, but there have been times it has slowed down, and even tripped while cutting - it's been an education how to sense it before it actually happens, and re-adjust the feed rate to avoid it . Once you separate your shop-vac to a different circuit, I don't see any reason you'd have to worry about it - other than learning the optimal rate of feed which is after all a nice art to acquire in itself.

[LarryG]

The lights dim on startup and then seem to come right back to full power.

I think you're probably okay, then. As Loring explained, an induction motor can and will draw several times its rated amperage momentarily on start-up. But this is virtually instantaneous, over almost before it begins (otherwise, again as Loring says, the breaker would trip and we'd never be able to use our tools). Even in my new shop, with twice as many amps available as in the old (60A now vs 30A then), I still sometimes get a faint ripple of the lights when I turn on my cabinet saw. It is, at most, an extremely minor annoyance, and doesn't affect the way the saw runs in the least.