Garbage Disposal and Dish Washer off Switched outlet?

murphysfOctober 28, 2012

Garbage Disposal and Dish Washer off Switched outlet?


I currently have a garbage disposal. There is a switched ac duplex outlet in the cabinet under the kitchen sink that the garbage disposal plug into. In my backsplash there is a wall mount switch that switches the outlet. This is the original installation. Home is on the San Francisco Peninsula, ranch style built in the mid 1950s.

The kitchen does not have a dish washer. I am going to cut out a cabinet and install a dishwasher.

The issue is the power needed for the dishwasher. The wiring is original knob and tube and I am not planning on opening the walls etc. The outlet is currently switched for the disposer however I need constant power for the dishwasher.

I was thinking if I could pull a wire off the hot side of the wall mounted switch that I could break the tab off the duplex outlet I could have one of the receptacles switched and the other constant. However I am not opening the walls and there is no conduit to pull a wire as it is knob and tube.

The only idea I have is to purchase an insinkerator counter top air switch. This way I could leave the wall mounted switch in the on position all the time to satisfy the dishwasher and also plug in the air switch and control the disposer from the air switches counter top switch. Is this my best solution?

Are there any tricks or cleaver ideas that would work that I am not aware of?


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As I posted in your duplicate post on the Appliance Forum, you cannot alter the knob and tube wiring. You will need to run a complete new circuit from the panel in order to be able to put in a DW. However, the larger issue is that your home still contains knob and tube, and you will need to address that sooner than later so you should seek to budget for that necessary upgrade before your insurance company demands that you do so.

    Bookmark   October 28, 2012 at 11:53AM
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The bigger issue here is whether your existing circuit can handle the dishwasher alone, must less the disposer and dishwasher at the same time. And is there anything else (electrical outlets, lights, other appliances) already on the same circuit as well? Check first.

What amperage is your circuit rated for? If it runs to a fusebox with Edison-type fuses (same size screw threads as a common light bulb), be sure it's of the original, intended amperage and not a larger one someone installed along the way.

Modern electric code, as well as most manufacturers, specify separate circuits for the dishwasher and disposer. I've seen them frequently on the same circuit in older houses, and you may be fine if you only run the disposer when the dishwasher's not running. But dishwashers typically use about 1400 watts or 12 amps (mostly for the heating element, which both heats the water and dries the dishes at the end), and disposers draw about 8 amps. You'll need a 15 amp circuit to run them alone, and a 20 amp circuit for both run together (though since the disposer is run only briefly, it may not trip your 15A circuit breaker or fuse even if run simultaneously with the D/W). Devices permanently attached to electricity are not supposed to draw more than 80% of the circuit's maximum rated power, so a dishwasher that drew 14 amps would specify connection to a 20 amp circuit.

Another issue - knob and tube wiring is inherently safe, perhaps more so than modern combined cables since the hot and neutral wires are run apart from each other, but it doesn't include a ground cable which your dishwasher and disposer should have. It seems possible, and easy, to add a ground wire to existing K&T installations but I don't know if this is allowed by code. Installing new knob and tube circuits is no longer allowed, but extending existing knob-and-tube wiring is specifically allowed by the National Electric Code. Many electricians don't seem to know this, and some, especially young ones who haven't worked on older homes, aren't familiar with K&T in general and are freaked out by the thought of electrical junctions made outside a metal box.

Insinkerator sells a "cover control" disposer, as well as an inexpensive batch feed adapter that fits any of their Evolution disposers, as well as an air switch. The air switch seems like a good idea, but run this by someone in the Wiring forum here, or an electrician to make sure what you're doing is safe and at least relatively legal. The break-off-the-tabs solution, in and of itself, should work, but there are other issues here that should be considered.

    Bookmark   October 28, 2012 at 12:19PM
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You CAN alter or repair knob and tube wiring under certain circumstances. Some insurance companies hassle homeowners who have K&T wiring, but the larger problems are incompetent repairs and additions made to K&T by DIYers over the past decades and people adding insulation around the wires, which is a no-no since K&T wires rely on airspace around them to keep cool. Properly implemented and maintained, knob and tube wiring is perfectly safe.

    Bookmark   October 28, 2012 at 12:26PM
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Sophie Wheeler

No reputable electrician will ever do anything but repair or replace K&T. Altering it or extending it? No way! If that (altering or extending) is the only way to get a DW into the space, then it's time to take that 19th century technology into the 21st and replace it. And replace all of it while you have the electrician there.

    Bookmark   October 28, 2012 at 12:29PM
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Alter as in replace damaged wiring or a bad splice. Nothing unreputable about that. Obviously replacing all the wiring and adding a new high amperage circuit panel is the best option, but the original poster obviously doesn't have the funds. I don't think ripping out the knob & tube behind the countertops (which likely is behind the drywall (plaster?) and probably not mucked with by incompetent DIYers) would significantly increase safety. I'd do it if the wiring is relatively accessable though, or if new wiring can be added. For my 1940s kitchen I'm working on, I decided to hire an electrician to install several new dedicated circuits to run appliances that the home didn't originally have, including a dishwasher, disposer, microwave, electric cooktop, and washing machine. Although the electrical panel is at the opposite end of the house, the new wires could mostly be run through the basement ceiling in one bunch to save labor costs, then poke though the kitchen floor where needed.

    Bookmark   October 28, 2012 at 12:44PM
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I'm pretty adventurous with electrical work. I've rewired most of my house, including the kitchen. But I wouldn't touch knob and tube wiring with an insulated ten-foot pole. I'd either run a new circuit for the dishwasher or I'd hire an electrician to do it.

Does your house have a crawlspace? I'm down in the South Bay, and most houses here have them, except for the Eichlers that are built on slabs. If you do have a crawlspace, how hard would it be to pull a romex cable through it from your fuse/breaker box location to under the sink?

If you can get cables to the right place, drilling up through the subfloor and into the wall shouldn't be super hard. Under the sink that's probably all you'd need to do -- after you've cut out a receptacle-sized hole in the wall it should be pretty easy to get the cable to it. I've done that at least a dozen times.

The fuse box may be harder. With mine it wasn't too bad, since the wall under the box wasn't covered with sheetrock. Once I got the cable up into the wall there I was home free. Fortunately I have circuit breakers and there was room in the panel for another breaker.

If crawling under the house doesn't sound like fun, hire an electrician!


    Bookmark   October 28, 2012 at 7:55PM
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