adding a switch to an outlet with two circuits

jeffpeter

I'm trying to do some under cabinet lighting. I found the 15 amp circuit that runs the kitchen and foyer lights. I added up all the wattage and it can handle more lights. I was thinking I could add another switch to the existing outlet to control the under cabinet lighting. I've been following this tutorial on The Family Handyman

But, my setup isn't exactly the same because I want to tap into an outlet that controls a kitchen light. This outlet also controls the garbage disposal (on a different circuit that also has the dishwasher). See my diagram.

http://t.co/qxgxDQ9R

Circuit A - holds the dishwasher and disposal (20 amp)
Circuit B - holds the kitchen light and is the circuit I want to tap into

Assume everything is grounded. I just didn't draw it.

So, my question is how would I add a new line into the box on circuit B? Just add a new line and connect the blacks before it goes into the switch?

Missouri Bound

I take it this is a MWBC?

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jeffpeter

Multiwire branch circuit? Not sure. The switches do all share the same neutral. However, everything is grounded inside the outlet box. The switches are in the same box, but are two different circuits.

Missouri Bound

The switches share the same neutral? Hmm...what neutral? If you have two circuits and they share the same neutral then it's a MWBC...if it's NOT a MWBC then you have a problem. How many breakers turn off everything...just one?

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jeffpeter

Two breakers are required to turn off everything. One controls the disposal & dishwasher (20 amp). One controls the lights (15 amp). The switch on the right in my diagram controls the disposal. The switch on the left controls the light.

I don't know what neutral they share. Are you asking if the shared neutral goes back to the 15 amp or the 20 amp? I have no idea. Both? I thought it was weird that they'd share the same neutral when I opened the box. All five white wires in the box are connected with one wingnut.

http://t.co/qxgxDQ9R

Missouri Bound

I don't want to complicate your issue, but if both breakers share a neutral it's a MWBC. And if those breakers aren't tied to gether with a common trip bond it's an illegal installation. You may want to get that taken care of before your remodeling project.

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jeffpeter

How would I tell if the breakers were tied together with a common trip bond?

k_buz

That still isn't mandatory in some places. That code was put into effect for the 2008 code cycle. There is no requirement to replace the breaker if it wasn't required when the circuit/s were installed.

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jeffpeter

House was built in 2000 in NC. I hear we have very relaxed building codes...

hammerlane

Not as dire as Missouri makes it out to be. Just be sure to turn each of the breakers off when working on either of the circuits because current on the neutral conductor can bite also. Post a note on/in panel that circuits X & Y are part of a MWBC as a reminder to turn off both breakers.

As long as each of your hots are attached to a breaker on separate buses of the panel...meaning the breakers are put in next to each other as the photo below..... the 2 hots can share the same neutral. You may have to make sure that the breakers are tied together with a handle-tie....like the photo below.

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hammerlane

Here are 3 ways of doing it based upon whether you run power to the switch location first or to the light location first. Diagram #1 takes power to the switch location first. Diagrams #2 and #3 take power to the light location first. Also diagrams #1 and #3 have a neutral conductor at the switch location and diagram # 2 does not.

No ground wires shown for ease of drawing.

Attached Thumbnails

     

Missouri Bound

OK OK...I'll quit yelling "the sky is falling"

But it's imperative that the circuits be on opposite "phases" of the panel or the neutral can be overloaded. That's one of the purposes of the two pole breakers. The other is to assure that both circuits are energized or off at the same time.

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hammerlane

jeffpeter

This is what my breakers look like. They're not right next to each other... Am I still okay to share a neutral?

Attached Thumbnails

 

Jim Port

Those breakers are on the same leg of the panel. If they are sharing a neutral there is a potential to overload it.

I would question whether this is a MWBC based on the 15 and 20 amp breaker sizes.

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Answers based on the National Electrical Code. Local amendments may apply. Check with your local building officials.