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wfischer 12-28-2007 07:46 PM

Simple problem that requires a complex solution.
I recently purchased a house which was built in 1938, and have been doing a number of things to improve it. For Christmas, I got a new mitre saw, and set it up on the workbench in the basement. The first time I ran it, my wife noted that it caused most of the lights in the living room to dim. I did a little checking, and sure enough: half the living room, the basement workbench outlets and lights, and 4 of the basement light fixtures are all on the same circuit. I'm assuming the only way to fix this is to put the workbench on its own circuit, which is something I wanted to do anyway... but I used the last available spot in my breaker panel when I installed the dishwasher. As I see it, I have 2 options: Install a larger panel, which would require the inconvenience of having the electric company shut off my power for the better part of a day, or install a sub panel next to the main panel. Is there another option? Is one of my two options safer, or better in some way that I'm missing? Also, what codes would I need to be aware of for both options?

AllanJ 12-28-2007 08:01 PM

Do the lights dim for just a moment, or all the while the saw is cutting?

It is not unusual for lights to dim for just a moment when the saw starts up even though the saw draws much less than 15 amps while it runs. It just draws a lot more upon startup. If you are not blowing the breaker you probably do not have an overload problem for the time being.

You could see if your panel accepts a double breaker (for two circuits and has two handles) that fits in the same space as one original breaker. Then you can run a separate circuit.

Even with a separate workshop circuit, it is possible that some dimming of lights will still be experienced. This depends on the stability of the electrical system out to and including the pole transformer.

You are more likely to have dimming problems if receptacles, switches, etc. are wired using the back stab holes instead of the screw on terminals.

Andy in ATL 12-28-2007 08:06 PM

Mr. wf,

Installing a subpanel is the easiest option of the two you describe. If you search previous posts on this subject, you'll find tons of info. How large is your current service?


helpless handyman 12-28-2007 08:18 PM

1 Attachment(s)
This is what a tandem breaker looks like, they fit in one slot, and gives you 2 circuits.

wfischer 12-28-2007 09:00 PM

Thanks for the info on tandem breakers! That looks like the easiest (and least expensive) way for me to go.

AllanJ: The only thing I've cut with that saw so far is 11/16" quarter round, which takes literally less than a second... so I couldn't really say. ;) And while I haven't blown a breaker yet, I'm planning some upgrades for the basement area that I wouldn't trust to be on the existing circuit, as it stands.

Drennen 12-29-2007 02:14 AM

twins is definitely the way to go but it wouldn't hurt two check the load distribution on the house and put your twin on you less demanding leg.

HouseHelper 12-29-2007 06:00 PM

Be advised, the twin breakers may not fit in your panel. Some panels accept them, others don't. And even those that do may only allow them in certain spaces.
What type of panel do you have and what is the model number?

wfischer 12-29-2007 08:33 PM

According to the sticker inside the door, it's a Square D QO Load Center, Cat. No. QOC-16M Series L1

frenchelectrican 12-29-2007 11:53 PM


Originally Posted by wfischer (Post 83790)
According to the sticker inside the door, it's a Square D QO Load Center, Cat. No. QOC-16M Series L1

If i am reading this catalog number but i doubt it will match right but you have 16 space breaker box.

it should have a second number below the main catalog #'s

but i will forewarn you about the SqD QO series they do have twinner verison but two diffrent format.

one verison will have a cam below of the breaker [ i will provide a photo a sec ] and other verison dont have a cam below of it.

you need to read the panel box cover for details and a notch in case they do allow it.

this is one type of QO breaker can used in the QO panelbox.

but i try to get the second photo unforetally it will not paste it here if Speedy Pete or Stubbie can come up a idea ya' all guys are welcome to help here

Merci, Marc

wfischer 12-30-2007 02:45 AM


Originally Posted by frenchelectrican (Post 83827)
If i am reading this catalog number but i doubt it will match right but you have 16 space breaker box.

Not counting the main, there are indeed 16 spaces.

Andy in ATL 12-30-2007 09:37 AM

How big is the service? 60A? 100A?

wfischer 12-30-2007 04:14 PM


Originally Posted by Andy in ATL (Post 83880)
How big is the service? 60A? 100A?

I'm not sure. The spot on the sticker where that was supposed to be written in is blank.

wfischer 12-30-2007 04:16 PM

I take that back... the main breaker is 100A.

wfischer 12-31-2007 01:04 PM

Okay, here's an update: Last night I took the front cover off the panel, and discovered a sticker inside that includes additional wiring information. According to this sticker, if I'm reading it right, tandem breakers can be used in spots 13-16. 14 and 16 are the ones that power almost the entire house anyway, so that works out perfectly.

On a related topic, is there any reason to use a breaker larger than 20A for a household 120V line, aside from dedicated appliance circuits?

Andy in ATL 12-31-2007 02:46 PM

WF, I'm scratching my head and can say the only 30A 120V circuit I've installed recently is for a whole house vacuum system.


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