0

I'm getting a new cooktop installed that requires 240V 50A wiring. The old cooktop only needed 40A so I need to replace the old 8/2 wiring with 6 AWG wiring and change out the breaker to a 50A breaker.

Both old and new cooktops use the same wiring scheme: two hot legs and a ground. My question is: when running a new wire should I run 6/2 or 6/3 wiring? I've seen some resources that imply new versions of NEC require 6/3 but I'm not sure what section(s) of the NEC I can reference to verify this.

Is there anything else I need to take into consideration when upgrading a 40A circuit to 50A?

  • 1
    Heating elements (cooktop or oven) typically just need 240V = 2 hots. When there is other stuff - control panel, fans, etc. - then you often need a neutral - and therefore 6/3 instead of 6/2. Going up to 6/3 may or may not be required by code but should future-proof for a replacement appliance that needs a neutral. – manassehkatz Oct 4 '18 at 2:09
  • No they don't. The old cooktop used two hots and a neutral, with no ground at all. They used the ground wire in 8/2 for the neutral, which was illegal (it was legal to use the bare wrapping in SE cable as the neutral [in a service entrance, the bare is the neutral], and this made people feel entitled to do the same with the cheaper NM cable). To "ground" the cooktop, they bootlegged ground off neutral. if the neutral wire had a problem, it electrified the cooktop surface! And that's why we use /3 cable now. – Harper Oct 4 '18 at 7:30
  • @Harper Trust but verify. See images.homedepot-static.com/catalog/pdfImages/ab/… - the manual for a basic GE coil cooktop. For the actual connection, it references red & black to hots and ground to ground (or neutral with a warning, etc.) but does NOT actually talk about connecting a neutral wire. No clear diagram, so I can't say 100%, but I am fairly certain this model (a GE basic coil cooktop with NOTHING special) does NOT actually use neutral itself. The manual implies there should be neutral in the box, but does NOT actually to connect it. – manassehkatz Oct 4 '18 at 14:39
2

Running 6/3 is future-resistant, but only a conduit is future proof

While many modern cooktops are designed for worldwide usage, running all controls and power electronics on 240V as that's what's guaranteed to be there no matter where you are, some North American market cooktops chintz out and rely on a neutral being present to run their controls, and the next owner may wish to rearrange things and put a range in, or tap an oven off this circuit for that matter -- both of these things typically require a neutral in North America.

So, running a 6/3 cable is more future-resistant than a 6/2; however, if you truly want a future-proof installation, you'd want to run a 3/4" conduit of some flavor to the cooktop location and then pull stranded THHNs through there -- ENT aka "smurf" is the cheapest option for this, just be mindful of the bending limits when installing it. The conduit option also has the benefit of being able to run the wire at 75°C, which allows you to run 65A over 6AWG copper. This is plenty of current, even for the largest cooking appliance setups.

2

Most people who approach ranges and dryers for the first time see 3 wires, often one is bare, and they presume "240V, 2 hots and ground". No sir. The third wire is combined neutral+ground. It's there to be neutral, and it's being overloaded to be ground, or to use the proper term, bootlegged. The appliance industry got an exception in Code notched out to make this permissible. The vast majority of dryers, ovens and ranges do use that franken-wire as a neutral. If the wire breaks, it electrifies the chassis of the appliance. Very bad design.

This was deprecated starting in the 1970s and finally outlawed in I believe 1989. Now the standard is /3 cable with separate neutral and ground. That is fine.

There's no escaping the need for neutral, eventually somewhere in a well equipped kitchen you're eventually going to have an oven, and only incandescents work for an oven light, and the obvious choice is common-as-dirt incandescent 120V bulbs, which necessitates neutral. It's common for cooktop and oven to share the same circuit.

Now it's possible you've got some ultra-modern Eurodesign that really does use only 2 hots and ground, that's particularly the case on a cooktop only that isn't an oven. Europe has only 240V, and on our side of the pond that becomes hot-hot-ground. 6/2 would suffice for that, however it would also paint you into a corner if you ever replaced the unit with another one, or shared the circuit with an oven. The vast majority still use neutral, and realistically you'd wind up horking it together with old-style bootlegged neutral-ground just to get the job done, damn the fact that it's been illegal for 30 years. Because that's the way things work in the appliance business.

In fact it wouldn't surprise me if your local inspector had a big problem with installing 6/2 and required the future-proofing of the 6/3, simply to avoid the scenario above. Same logic as needing neutral in switch loops; easy to do it now, prohibitive to do it later.

  • As I noted in a comment above, in at least some situations (which may be only the most basic of coil cooktops), there is the ground-ground vs. ground-bootleg-over-neutral issue but, unlike clothes dryers, the cooktop does not appear to connect or use neutral EXCEPT as a bootleg ground. I agree about NOT using neutral-as-ground any more (and the manual I referenced says code has prohibited it in new construction since 1996), but as far as I can tell there are still at least some cooktops that only actually use 2 hots + ground and do NOT require a neutral. – manassehkatz Oct 4 '18 at 14:42
  • Great information that I wasn't aware of. The old cooktop from 1997 has a manual that recommends connection of appliance ground to white neutral in a 3 conductor branch circuit "if codes permit." The new cooktop (which is a higher end brand versus the old) specifically says to cap off the white neutral if present, so that makes me confident it's doing things the "right way." In any case, sounds like the consensus is to run 6/3 or equivalent through a conduit. – CodingHero Oct 4 '18 at 15:40
  • @CodingHero "Cap the neutral" makes it clear that they expect there to be a neutral (i.e., use 6/3 and not 6/2) but don't actually use it. I would expect the neutral to actually be used if (a) a combination cooktop/oven (e.g., a drop-in-range) which would typically use the neutral for oven controls, (b) a cooktop with built-in downdraft fan (which I've seen but never used - seems very inefficient) or (c) a cooktop with timers or other extra features. Most of the cost of running wire in a house is labor (whether your time or an electrician's) so go with 6/3 and you're set for the future. – manassehkatz Oct 4 '18 at 16:02
  • @CodingHero -- the choice is either a) run a 6/3 cable, or b) run 6AWG THHN (hot, hot, ground) in a 3/4" conduit – ThreePhaseEel Oct 4 '18 at 22:22
  • 1
    Because with conduit, it's pretty easy to add the neutral later. – Harper Oct 4 '18 at 22:43

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.