Combo Microwave/Oven Wall Unit Wiring

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  #1  
Old 07-31-11, 02:34 PM
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Combo Microwave/Oven Wall Unit Wiring

Hi Guys,

Sorry if this post seems long but I want to include all of the relevant information.

I have a new 27-inch Kitchenaid Microwave/Oven combo unit that I'd like to put in. Here are the specs:

Kitchenaid KEMS378SSS05

The sticker on the unit inside the oven door says:
120/240V 120/208V
8.6 KW 7.2 KW

The instructions say: "Models rated from 7.3 to 9.6 kW at 240 volts (5.5 to 7.2 kW at 208 volts) require a separate 40-amp circuit". They also say that I should use #8 copper wire.

I just removed the old oven, which was gas. However, when the house was built in 1978 it was also wired for an electric oven at the same time -- there are unused #10 stranded wires ("10 AWG CU 600V UL OIL RESISTANT I E-13662") leading from under the old oven back to the breaker box (there is no sub-panel anywhere). There are 4 wires -- 2 blue, 1 white, and 1 green, all coated. The green and white wires are connected to the bus in the breaker box. The blue ones are completely disconnected, but through simple testing I've determined that the disconnected wires I'm seeing in the breaker panel are the same ones I'm seeing at the oven junction box. The wires are in flexible metal conduit. The breaker panel also has two completely unused breakers: one is 30 amp, one is 40 amp.

The oven comes with 4 wires in a pigtail: black, red, white, and bare copper. Red and black are #8, white is #10.

My question is this: Can I get by using these already-installed #10 wires, or do I have to use #8? I know that, typically, 40-amp circuits require #8 wire, and that is why Kitchenaid is telling me to use it. However, I also know that the distance the current has to travel makes a difference as well. The distance the current will have to travel from the breaker box to the oven is a maximum of 35 feet.

My calculations indicate that the voltage drop in these #10 wires will not be a problem over 35 feet for a 40-amp circuit. To me, this result is supported by the fact that there is a 40-amp circuit breaker in the box. There are no wires larger than #10 coming into the box, so the original electrician must have thought that the #10 wires were good to go with the 40-amp breaker over the relatively short distance to the oven.

If I were building the house now, I would use #8. But since the #10 wires are already there, I'd like to avoid the hassle and expense of putting in #8. Any thoughts? Thanks in advance!!
 
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Old 07-31-11, 03:04 PM
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Can I get by using these already-installed #10 wires, or do I have to use #8?
You have to use #8 on the 40a breaker.

The wires are in flexible metal conduit.
If this is really conduit you may be able to pull in new #8 using the old wires to pull it. If there is any paper packing around the wires this is most likely a metallic cable assembly and you will have to install new cable. Good news is you don't find two blue wires in cable so it probably is conduit.

Distance isn't a real factor on wire size in this case. You should always follow or exceed the manufacturers instructions.
 
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Old 07-31-11, 03:04 PM
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Regardless of your calculations, a 40 amp circuit requires #8 wires. If you had voltage drop, or a great distance, you would have to increase it to #6, but you can never go down in size. Hassle aside, do it right and sleep well.
 
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Old 08-01-11, 12:06 PM
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Hi Guys,

Thanks so much for the quick and thoughtful responses. I've checked out this doityourself.com forum many times in the past and I'm always impressed by the consistent high quality of information provided on this site.

OK, I'm convinced now -- I'll use #8 for the oven and be done with it. I'm still confused as to why there would be a 40-amp breaker in the box when there are no #8 wires going to the box, but I guess I'll never know.

So, given that I need to run #8, is it really possible to pull new wire through the existing flexible metal conduit? There is no paper packing, but the conduit is flexible (i.e., the spiral kind). This does not sound like fun, to say the least. Any tips, or is it even worth it? I can always run another direct line along the ceiling in my garage, because the kitchen is over the garage. Also, do all of the wires need to be #8, or can white and green be #10, like on my oven pigtail?

By the way, here's the main reason I was thinking that #10 would be OK, which led me to do the other calculations that I mentioned in my original post. It's a wire size calculator:

Wire Size Calculator

Thanks!
 
  #5  
Old 08-01-11, 01:09 PM
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Originally Posted by kaiserm98 View Post
I'm still confused as to why there would be a 40-amp breaker in the box when there are no #8 wires going to the box, but I guess I'll never know.
Some panels come standard with a 40A breaker if the electrician used an off-the-shelf service pack.

So, given that I need to run #8, is it really possible to pull new wire through the existing flexible metal conduit?
Yes, it can be pretty simple to do. The basic process is that you tie a string to the old wires and pull them out of the conduit which then pulls the string in. Then you use that string to pull the new wires back in to the conduit. You apply a pulling lubricant to the wires as you pull them in and it eases the pull. It also helps a lot to have a helper - one person pulls on the string, the other pushes the wires into the other side.

Also, do all of the wires need to be #8, or can white and green be #10, like on my oven pigtail?
You could do #8s for the hots and #10 for the ground and white neutral. You get a little more future flexibility to do a #8 white for the neutral but it is not necessary for this particular unit.

By the way, here's the main reason I was thinking that #10 would be OK, which led me to do the other calculations that I mentioned in my original post. It's a wire size calculator:
I checked a few samples on that calculator and it is wildly wrong in all that I checked. It only accounts for voltage drop, not thermal heating. On a short wire the voltage drop will always be low but the wire might be glowing red hot.
 
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Old 08-01-11, 03:44 PM
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For runs less then ~150 feet a table like this is better. Houston Wire & Cable Company - NEC Table 310.16
 
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Old 08-01-11, 04:43 PM
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I have my doubts he will be successful pulling two #8'a and two #10's in BX (if that is what he has), if it is presently housing 3 #10's. Just an observation. If the BX is loose (doubtful) you could use the entire unit to fish wires back to the source.
 
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Old 08-01-11, 05:06 PM
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I think he has greenfield as he has 2 blues. Bx would be red/black.
 
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Old 08-01-11, 05:59 PM
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As Justin said two blues and I ask if there was paper and he said no but we don;t know what size Greenfield.

So, given that I need to run #8, is it really possible to pull new wire through the existing flexible metal conduit?
If the size is large enough yes. What size is it?
 
  #10  
Old 08-01-11, 07:18 PM
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Hi Guys,

Thanks again for the great responses.

OK, here's the full story on the flexible cable from the breaker box to the oven junction box. It actually has 6 wires in it:

2 #10 stranded blue, unused
1 #10 stranded white, connected to bus in box
1 #10 solid green, connected to bus in box
1 #12 solid black, connected to 20-amp circuit breaker
1 #12 solid white, connected to bus

The black and white wires are connected through the oven junction box to a 4 wire flexible conduit (there's an extra red wire in the smaller cable that is not used). This smaller cable goes on to some lighting fixtures. I omitted mentioning these in my original post because I figured it'd be best to simplify my request to just the oven wiring part. Anyway, it's one large cable sharing wires for use in two separate circuits: the unused #10 blue-wire circuit, and the #12 lighting fixture circuit.

The flexible cable is about 1.15 inches O.D., and it's connecting to the large size hole on the junction box.

I was figuring that, if it's possible to pull the new wires through at all, that I would end up having to basically replace all six wires in the cable, because I think it would be downright impossible to leave the #12 black and white in there while pulling the others through, and I am no glutton for punishment (my many do-it-yourself projects notwithstanding).

So, can 6 wires be pulled, three of them #8 and the other three #12? Also, if the original blue #10 wires were meant for the oven, that means that there are no intermediate junctions anywhere, right? Doesn't code say that this kind of oven wiring has to be an uninterrupted run back to the breaker box, assuming to sub-panel?

Thanks!
 
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Old 08-01-11, 07:59 PM
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Thats going to be just below the fill capacity for 1" FMC. It'll be a rough pull, and you'll definitely want a helper and a bottle of lube, but I think you can manage it. As for the code, well the code says a lot of things (like not to put a 40A breaker on #10 wire), doesn't mean they get followed if something was done without a permit. The circuit must be dedicated with nothing else on it, but doesn't need to be uninterrupted. I would give one of the blue wires a yank and see if it moves easily. If it doesn't then you're gonna have to go hunting for the junction box. And that won't be fun if it's been plastered over. Trust me, been there.
 
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Old 08-01-11, 08:16 PM
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How hard to wire the lights from another source. You said if you had to you could run a new 240 so a simple 120v #12 should be a cake walk. That would leave you plenty of room for the four oven wires.

Not sure about this so wait for the pros to weigh in but if the Greenfield is continuous and has a bonding strap you could do it with the Greenfield as your ground so long as the boxes are metal.
 
  #13  
Old 08-01-11, 08:44 PM
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Label all the wires at the circuit breaker panel and then disconnect them. Tie/tape a strong pull cord and then pull all wires out to the kitchen junction box. Remove the two blue wires and substitute two #8 wires and then pull the whole assembly back into the conduit. Reconnect the original wires. Use plenty of cable lube when pulling the wires back into place.

All of this is predicated on all the original wiring having type THHN insulation. If it has any other designation (except THHN/THWN)then all will need replacement.
 
  #14  
Old 08-02-11, 04:32 PM
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Hi Guys,

Well, I gave a tug on one of the blue wires, but nothing much moved. So I'm thinking that there is probably at least one junction box between the breaker and the oven. Not that there are any circuits connected to the blue #10 wires (i.e., it is probably still dedicated), but there could easily be connections that branch the smaller #12 wires.

I am disinclined to search for any junction boxes under the drywall. BUT -- I can see a junction box on my garage ceiling that has 1-inch flexible cable coming from it. That will be my last hope as far as pulling the wires. If I'm lucky, it'll be my blue-wire circuit, and it would even make pulling the cables easier, since it the entire length of the pull would be broken into two parts. But if the junction box is not on the blue-wire circuit, I'm just going to run another cable.

I'll check it out tonight and keep you posted!
 
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Old 08-02-11, 09:22 PM
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Thumbs up

Hi Guys,

UPDATE: No luck! The junction box I thought might save me contains wires for a different circuit altogether. Looks like I'm running new cable with #8 wires!

Thanks again to everyone who contributed -- all were useful posts and I learned a lot. I also really liked the "you-can-do-it" attitude inherent in all of the comments. Long live the electical guys on doityourself.com!!
 
  #16  
Old 08-02-11, 09:43 PM
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Since the blue wires are not connected you can pull on one of them with quite a bit of force to see if it moves. A slight tug may not cut it. Start by removing the cover from the service panel and straightening the blue wires so that they won't "fetch up" on anything when pulled from the box in the kitchen. Be sure to cut off any bare ends and a single wrap of tape over the end is a good idea. Go back to the kitchen and see if it does move. If yes, then pull it out all the way. If it is definitely stuck after pulling a ways then simply cut it off right where it enters the box. If you get one of the blue wires out then pull the second one as well.
 
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Old 08-03-11, 09:53 AM
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I agree with Furd -- give it one more shot. Sometimes old wires get stuck in there with a layer of "crud" and you really need a good heave-ho to get them moving. Sometimes I use vice grips to get a good grab on a wire that I know I won't be reusing. If you have enough wire exposed in the box a couple wraps around a hammer handle can make a good pull bar.
 
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Old 08-03-11, 11:52 AM
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And squirt in plenty of lube at the end opposite where your pulling. You might even try to push a well lubed fish tape through to break up any gunk and get some lube in the conduit.
 
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Old 08-03-11, 05:54 PM
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Hi Guys,

OK, I've got about a foot of wire to work with at the breaker, so I'll give it one more shot tonight and let you know how it goes!
 
  #20  
Old 08-03-11, 06:54 PM
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Originally Posted by ray2047 View Post
And squirt in plenty of lube at the end opposite where your pulling. You might even try to push a well lubed fish tape through to break up any gunk and get some lube in the conduit.
No, waste of time and lube when trying to pull wires OUT of conduit. Excellent idea when pulling wires IN to conduit.
 
  #21  
Old 08-04-11, 02:32 PM
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Hi Guys,

UPDATE: Let me just say before I begin that I'm sure there are some jobs where giving yourself a hernia or pulling your arm out of its socket is an acceptable price to pay to get it done, but this job isn't one of them. The upshot: I tried, but not to the point of injury.

In the end it just wasn't happening. I tried yanking, pulling, and even ratcheting, but the most I got was a loud "thuuunnnnnggggggg" as everything vibrated when I yanked on the wires.

I am now fairly convinced that there must be some kind of junction between the breaker box and the oven. I think this is perfectly possible, because the flexible cable itself contains wires for two different circuits -- the second circuit being a normal 20-amp one. I think the #10 blue wires are dedicated, but the 20-amp circuit branches somewhere between the breaker and where I can see it at the oven.

So, I'll be running another cable with #8 wire along my garage ceiling. No big deal -- it'll just require a bit of drywall patching when I'm done, and boy am I familiar with that job!

Thanks again for all the help. I am glad I checked with you all before moving forward!
 
  #22  
Old 08-04-11, 03:47 PM
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Yup, if it isn't coming with what you have done then the new cable is the answer.

I kind of doubt that there is a "junction" in between but there very well may be a "pull" box or conduit fitting such as an ell in between.
 
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