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Background: New construction, 2x6 load bearing wall that will contain insulation, installing individual cooktop modules in the kitchen and the run from the breaker box is 40'. There are 4 cooktop modules (these are Miele combisets if you are curious what I mean by cooktop modules) each requiring 240V and a 15A circuit. I am a little confused by 334.80 and 310.15 of NEC code. My County does not impose additional requirements other than the NEC.

My plan: I would like to run (4) 14/2 cables from the breaker box to the cooktop destination using the neutral wire as a second hot wire (black-striping them at the ends).

Question: what is the correct way to route the cables from the breaker box to the cooktop(s)? All 4 in 1-inch stud holes? Two cables each in two 3/4" stud holes? Each cable in its own 1/2" stud hole throughout the 2x6 studs? Also, given that these cables will be carrying 240v, if I combine cables in stud holes, will they be derated and would it be smart to switch to using 12/2 wire instead of the 14/2?

godeatgod
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  • Is there a reason you're running individual cables here? Are all these cooktop modules being put near each other, or are they scattered throughout the kitchen helter-skelter? – ThreePhaseEel May 24 '20 at 05:53
  • Where in the world are you located ? Part of question sounds U.K. and part sounds like US . It makes a difference, – Ed Beal May 24 '20 at 07:41
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    it sounds entirely US to me. UK does not have 2x6 (it has 6x2 or now in millimeter sizes) does not have 14/2 (it uses mm sizes) does not have NEC, does not have white neutral (it is black or blue) and neutral would not need to be striped over (as it would be present on a 240V appliance.) for county read AHJ. – Jasen May 24 '20 at 08:53
  • Perhaps look at: https://diy.stackexchange.com/a/96975/18078 and https://diy.stackexchange.com/a/36334/18078 – Ecnerwal May 24 '20 at 11:17
  • What model(s) are you installing? – NoSparksPlease May 24 '20 at 13:23
  • Yes, this is U.S. I think the cooktop modules are European. The 4 modules are being put right next to each other. The reason I was doing individual cables is because I did not want to have to have a subpanel in the kitchen. I know a 60A subpanel is an option, and I would do it if most of the expert advice told me that is the clear cut answer, but then I need to deal with the subpanel "accessibility" issue which becomes a PITA. The models I'm installing are the Miele combiset 12" cooktop, 15" electric grill, 15" teppan yaki, and 12" deep fryer. – godeatgod May 24 '20 at 17:22

4 Answers4

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You're in the clear, as described

For 310.15(B)(3)(a), this boils down to a very simple rule for circuits 15-30 amps: No more than four.* So you're G2G for any of your plans. You could even run all 4 of these in conduit (as individual wires please...**)

Also look to see if the instructions allow 2 cooktops on a 30A circuit.

However... plan for the future

You have the walls open and easy access. Now is the time for provision for the future. And this is expensive, high-end stuff, so I don't want to hear about chintzing out :) Do it right, now.

Between ranges and ovens, ONE of your cables needs to be ready for 50A w/ neutral -- so a 6/3 cable. And another of your cables needs be 30A-ready -- so a 10/3 or 10/2 cable; I recommend 10/3 because it makes you more stupid-proof.

Why? Because that makes you ready for a future installation of more pedestrian fare: either a 50A range+oven, or separate range and oven of 30A each.

You really don't want to be 10 years down the road, modernizing to a different setup, and discover the only thing in the walls is a bunch of #14.

Different deal in conduit. With conduit it is very easy to change wires, so just go ahead and pull #14. You can easily swap it out for different wires later. 1/2" conduit should suffice. (since you can use #8 for 50A).

* Why? First, in split-phase (not 3-phase), all circuits have exactly 2 conductors that matter to the 310.15(B)(3)(a). Four circuits means 8 conductors means a 30% derate. This derate is off the 90C column for NM-B and UF-B and most other wires. Separate to that, NEC 240.4(B) already has statutory limits on #14-#10 wires, and those limits amount to 30%-ish derate. off the 90C numbers.

** And the way you color-code four 240V circuit in conduit is circuit 1: Black x2.... circuit 2: brown x2... Circuit 3: red x2... Circuit 4: blue x2. Or whatever. It's perfectly fine for both hots to be the same color.

Harper - Reinstate Monica
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  • Range loads don't need the continuous derate, see the footnotes to table 220.55 – ThreePhaseEel May 24 '20 at 16:19
  • You need a 40 or 50A circuit for the cooking appliance tap rules to kick in, but yes, two 8/3 cables might be a better option....also, 1/2" ENT will just barely fit a range circuit. (2 8AWG THHN hots, a 10AWG THHN neutral as permitted by 220.61(B), and a bare 10AWG ground) – ThreePhaseEel May 24 '20 at 17:05
  • All of these answers and discussion have been helpful. I do have a 40A double wall oven on the same wall and have already run an 8/3 cable for it. – godeatgod May 24 '20 at 18:03
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I would run a conduit instead of a bank of cables here

While your plan is OK as far as the derate rules go, as with 14-10AWG, you can go up to 9 current-carrying conductors before the derates start to become an issue, I would not do it this way. Running a 3/4" ENT ("smurf tube") instead of the bundle of cables provides far more flexibility for the next installer, who may wish a range or a conventional cooktop instead of the multi-module arrangement you describe. Inside this ENT, you can simply run 9 14AWG THHNs (2 each of 4 different "hot" colors for the 240V circuits, and a green 14AWG ground) to a large box (such as a deep 120mm (aka "5S") square box) mounted somewhere convenient, then distribute out to the individual cooktop modules with 14/2 NM cables.

If this is in an island, I would run two 3/4" ENTs to the island, even; the other ENT can then be used for the island's general receptacle circuits. This is especially useful if you are doing slab-on-grade construction, where having to chisel cables out that are directly buried in concrete is a major barrier to any sort of upgrade down the road.

ThreePhaseEel
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Not sure what exactly what issue you are having? NM-b is 90°C wire, #14 90° is good for 25A, 70% of 25A is 17.5A.

Would I run all 4 cables through a single hole? Probably not. Wouldn't be any fun or save any time or effort trying to keep it tidy, and I would expect that one of the cables might get twisted out of order and increase the possibility of insulation damage. Cables tend to have more friction than the wood, so all those cables get difficult to slide. Attracts unnecessary attention from Inspectors.

I guess you could argue that you think that one 1" hole is easier to drill than two 3/4", but even that is an unresolvable debate since it is opinion.

NoSparksPlease
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Subpanel

Right now you are running 4 circuits with 15A each. You can just as easily, and almost as cheaply (just the difference between 14 and 12 wire) have them ready for 20A each. But this is an unusual setup. Most people don't have 4 separate cooktop modules. They have either a combination oven/cooktop or a separate cooktop and oven. Depending on the types (excluding gas, of course, but I assume that isn't an option or you would likely be doing that now), those are typically 30A to 50A each. Code, as cited by Harper, requires you to have the big circuits available in a kitchen anyway.

Put in a subpanel, minimum 60A. 60A is enough for your current (pun intended, of course) 4 cooktop modules all going at once. It is also enough for pretty much any other typical combination of ovens & cooktops. It will cost more for cable (6 AWG minimum, hot/hot/neutral/ground) but you only have to run one of them. The subpanel will cost some money, but a small panel doesn't cost much. Don't get too small - extra spaces are always good and since your walls are open, you can wire up extra circuits for countertop receptacles and other built-in appliances using the same subpanel, which will save you on more of those 40' runs back to the main panel. Depending on how many circuits you add, you may want to upsize the cable and the main panel breaker serving the panel (the subpanel doesn't need a main breaker but can have one as long as it is >= the main panel breaker feeding it) to increase capacity.

Keep in mind that the subpanel needs to be accessible, which determines height min/max and that it can't be blocked by stuff. And the usual requirements that the experts are always listing here apply: torque everything appropriately, separate grounds & neutrals, etc.

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manassehkatz-Moving 2 Codidact
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  • I wouldn't bother with a subpanel here; subpanels in kitchens tend to collide with cabinetry in most unfortunate (and Code-violating!) ways – ThreePhaseEel May 24 '20 at 16:36
  • Normally I would agree, and I do note the need for access. But 4 circuits just for cooktops alone is a bit unusual. – manassehkatz-Moving 2 Codidact May 24 '20 at 16:38
  • There's also the possibility that one could run a fat cable and then use tap rules, given that those are in play for kitchen appliances....but not all manufacturers are keen on that – ThreePhaseEel May 24 '20 at 16:47
  • Might look odd, but do the usual NEC rules allow for a subpanel between counter and upper cabinets? Cabinets would overhang the panel but straight-on access would be normal. – manassehkatz-Moving 2 Codidact May 24 '20 at 16:50
  • No, it would not, the lowers and uppers both would be obstructing clear working space. You'd need a fridge-sized hole in the cabinetry, or to mount the subpanel inside a cabinet so that the front edge of the sub is flush to the cabinet front.... – ThreePhaseEel May 24 '20 at 16:53
  • I was afraid of that. But still could work in some setups - all depends on the layout of the kitchen. If these cooktops are on an island then could be on one of the end panels. – manassehkatz-Moving 2 Codidact May 24 '20 at 16:58
  • Realistically you want to mount panels in places which due to the layout must inevitably be pathways/hallways - where nobody will stack stuff because they need to get through. – Harper - Reinstate Monica May 24 '20 at 17:06
  • That's why I was thinking island end-panel - need to be able to walk all around the island or it isn't much use for cooking. Of course, based on the other requirements, you probably couldn't have a countertop overhang there, which is very common to increase usable counter space. – manassehkatz-Moving 2 Codidact May 24 '20 at 17:08