kitchen remodel; derating

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  #1  
Old 08-16-06, 12:07 PM
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kitchen remodel; derating

First- this is a great resource. I have learned a lot from reading this today. Thanks to all of the experts who answer questions.

I am installing three new dedicated circuits in my kitchen, one each for range, vent hood and microwave. The range requires a 30 amp 240 V circuit. I'll install 6/3 NM cable for this, with a 30 amp breaker (being sure to cut the ground strap between the unit and the terminal block on the range). The vent hood and microwave each require 15 amp 120 V circuits. I'll use 12/2 NM cable for these. I plan to run all three cables (one 6/3 NM and two 12/2 NM) along the edge of the 8' ceiling of the garage inside conduit, back to the circuit breaker panel. My town follows the NEC.

My questions are: Considering derating, can I run all three cables inside the same 3/4" EMT conduit? Max ambient temp is 110 C. Could I instead use PVC conduit? Is THHN/THWN used inside conduit, instead of NM, just because it is easier to pull? Any other considerations for the type of conduit to use?

Thanks,

Daniel
 
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Old 08-16-06, 12:11 PM
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Why do you want to use conduit? Is the ceiling of the garage finished?

Generally, NM is not used inside conduit except when needed for protection, and then generally only where protection is needed. Please describe the run in the garage a little better.

As for the circuits...

30 amps doesn't sound right for a range. Are you sure this is correct? If it is 30 amp, then the 6-3 is overkill, but I like that, since the range may some day change.

Make the circuits for the microwave and range hood 20 amp, since they can be.
 
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Old 08-16-06, 01:37 PM
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Thanks for your reply, racraft.

Yes, the garage walls and ceiling are finished. The wires will run ~ 40 feet in the garage. They will start out near the ceiling (coming out of the floor above) and run along part of two edges of the garage ceiling (turning one corner), and then go down into the circuit breaker panel.

The range is a Wolf 36" dual fuel range (gas for the burners, electric for the oven), so I expect that is why the electrical requirement is lower than all-electric ranges. I agree the 6-3 is overkill, but am installing it, as you said, in case of future changes.

I'm not sure what you mean by 'Make the circuits for the microwave and range hood 20 amp'. My original plan was to use wire rated for 20 amp, but to put 15 amp breakers on. Are you suggesting using 20 amp breakers?

Thanks,

Daniel
 
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Old 08-16-06, 02:14 PM
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Now the 30 amp electrical requirement for the range makes sense.

I am suggesting 20 amp wire and breakers for the range hood and microwave. There is nothing wrong with this.

I would not use conduit for that long a run of NM. that much cable won't pull through 3/4 inch conduit.

Personally, I suggest running the cable in the walls and ceiling.
 
  #5  
Old 08-16-06, 02:32 PM
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Thanks for clarifying.

It will be very hard to run through walls and ceiling in this location. The garage walls are cement block and the room above has a sliding glass door in it. It might be easier to build a chase around the NM cable. Is this OK?

Thanks,

Daniel
 
  #6  
Old 08-16-06, 02:42 PM
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There is no possible way that you can pull 12/2, 12/2, 6/3 in one 3/4" conduit. The 6/3 alone won't pull in 3/4" conduit for more than a few feet with no bends.

For conduit fill calculation, calculate the cross-sectional area of each cable at its widest point. For example 12/2 is about 0.5" diameter, yeilding 0.1962 sq. in. The 6/3 cable is about 0.75" diameter, yeilding 0.4415 sq. in. This brings the total cross-sectional area of your cables to 0.4415 + 0.1962 + 0.1962 = 0.8339 sq. in.

Your cables can occupy no more than 40% of the cross-sectional area of the conduit; therefore you must use a conduit that is at least 0.8339 / 0.4 = 2.08 sq. in. The conduit that qualifies is 2" or larger.

Overall, I second racraft's suggestion that you do not use conduit for the NM cables. Instead, install a running board like a 2x4 and run the cables "in the ditch" beside the 2x4 for protection.
 
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Old 08-16-06, 06:47 PM
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Thanks, ibpooks and racraft.

Can you give me some more detail on what is required to make the running board to NEC? Do I need to completely cover the cable on all sides? Since the walls and ceiling of the garage are finished, I was thinking of screwing the 2" edge of one 2x4 to the wall 4 inches below the ceiling, place the cable on it, and then screw the 4" edge of a second 2x4 to the first 2x4 to make an enclosure. Is it OK to leave a gap between the 2nd 2x4 and the ceiling so that cable can be added and removed from this tray? What does the doe require for NM cable in a garage?

Thanks a lot for your help,

Daniel
 
  #8  
Old 08-17-06, 07:58 AM
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The cable does not need to be enclosed, but you may do so if you like. When running along a ceiling, the running board doesn't need to be anything special -- even a 1x2 would qualify. It's just there to keep you from accidentally hitting the cable with something tall or to keep someone from using the cable as a clothsline. Whereever you put it, the cable should be stapled to the running board at minimum every 54".

Technically, the 6/3 is substantial enough to not require a running board, but since you're installing one anyway for the 12/2, might as well protect the 6/3 with it also.
 
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