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220v, existing 3-wire circuit, new 4-wire cooktop (I understand that white and ground on cooktop get connected to bare ground on incoming line).

BUT, the 3-wire incoming line first goes into a junction box which feeds a 220v, 3-wire undersink tankless water heater. That same 3-wire line then goes to the new cooktop where the white neutral and ground are tied together. The cooktop has an integrated fan so I’m sure that the neutral and ground will be part of an active (i.e. “live”) 120v circuit.

QUESTION: is this dangerous? Logically if it was dangerous, any instructions suggesting tying the neutral and ground together would also state that the neutral and ground can ONLY be tied together on a dedicated circuit, which the instructions did not state.

So logically, this setup should be safe. Having said that, “logic” has gotten a lot of people killed!

Any comments would be greatly appreciated!

  • Hello, and welcome to Home Improvement. Interesting question; let's see if one of our pros has a good answer. And, you should probably take our tour so you'll know you'll know the details of contributing here. – Daniel Griscom Apr 2 at 20:37
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    It's easy to be swayed by the interpretation that is convenient. You're right to be skeptical. Those instructions are not written in a vacuum; it isn't the job of the instructions to restate the entire Electrical Code. Therefore if part of the Electrical Code is not mentioned in the instructions, that should not be construed as an exemption. – Harper - Reinstate Monica Apr 2 at 20:55
  • Can you post photos of the insides of the junction boxes in question? Also, can you post specs, nameplates, or make/model info for the appliances you're talking about here? It could very well be that the cooktop fan runs on 240V, or low voltage DC for that matter... – ThreePhaseEel Apr 3 at 1:07
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The exception that allows grounding your range, ovens, cooktops and clothes dryers via the current carrying neutral (which is called "the grounded circuit conductor" in the NEC) does not include a water heater and so grounding your water heater that way is a code violation.

As you recognized the safety of those circuits that were legal to be installed that way is compromised, so much so that new circuits are not allowed to be wired that way. The exception applies only to existing circuits, the exception allows you to not update the wiring when connecting a new appliance or replacing a damaged receptacle.

Additionally connecting the water heater to the range circuit will not satisfy the overcurrent protection required for the water heater and is another violation of the NEC.

250.140 Frames of Ranges and Clothes Dryers. Frames of electric ranges, wall-mounted ovens, counter-mounted cooking units, clothes dryers, and outlet or junction boxes that are part of the circuit for these appliances shall be connected to the ground (equipment grounding conductor) in the manner specified by 250.134 or 250.138.

Exception: For existing branch-circuit installations only where a ground (equipment grounding conductor) is not present in the outlet or junction box, the frames of electric ranges, wall-mounted ovens, counter-mounted cooking units, clothes dryers, and outlet or junction boxes that are part of the circuit for these appliances shall be permitted to be connected to the neutral* if all the following conditions are met.

  • (1) The supply circuit is 120/240-volt, single-phase, 3-wire; or 208Y/120-volt derived from a 3-phase, 4-wire, wye-connected system.

  • (2) The neutral* is not smaller than 10 AWG copper or 8 AWG aluminum.

  • (3) The neutral* is insulated, or the neutral* is uninsulated and part of a Type SE service-entrance cable and the branch circuit originates at the service panel (service equipment).

  • (4) Grounding contacts of receptacles furnished as part of the equipment are bonded to the equipment.

* NEC's legal term for Neutral is "grounded conductor", which is very confusing. For clarity, it's been changed to "neutral" here.

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This is a tour de force of NEC violations, all due respect.

220v, existing 3-wire circuit, new 4-wire cooktop (I understand that white and ground on cooktop get connected to bare ground on incoming line).

Noooo, that's never true. Neutral is not ground. 4-wire cooktops are looking for neutral. 3-wire range connections provide hot-hot-neutral. A legal 3-wire range connection does not provide ground.

When a 3-wire range ties neutral to ground, what it is doing is supplying neutral in the normal way, and then bootlegging ground. This is a "legal bootleg" owing to an exception in Code at 250.140. The rationale is that these connections are rarely disturbed, and this will avoid interrupting appliance sales to underequipped locations. Sadly many of those locations are grounded but the installer does not recognize the metal conduit wiring method, so bootlegs.

BUT, the 3-wire incoming line first goes into a junction box which feeds a 220v, 3-wire undersink tankless water heater.

OK, a 3-wire feed to a 240V water heater is Hot-Hot-GROUND. Can you use that for a water heater? You betcha. Any NM-B type cable will be fine for that; just re-task the white wire to Hot L2, and use the bare ground for ground.

So stop right there. You're all set.

That same 3-wire line then goes to the new cooktop where the white neutral and ground are tied together.

Nope, can't do that. The circuit is oversubscribed. The total loads are too much for the circuit. You will need to run a separate cable for the cooktop. In some cases the cooktop and oven can share a cable. This cable MUST be /3. You would be well advised to use #6 or at least #8.

The cooktop has an integrated fan so I’m sure that the neutral and ground will be part of an active (i.e. “live”) 120v circuit.

Then it needs Neutral. Neutral specifically! Ground will not do.

Now, if you look at 230.140 Exception Condition 3, you can see that only insulated neutrals are allowed. With an exception for the webbed neutral found wrapped around the hots in SE cable. Your "Last Guy" is trying to play it both ways: use a grounded 2-wire Romex cable, call the bare ground a "neutral" even though that is specifically prohibited, then turn around and use it for a ground also. Can't do any of that.

QUESTION: is this dangerous? Logically if it was dangerous, any instructions suggesting tying the neutral and ground together would also state that the neutral and ground can ONLY be tied together on a dedicated circuit, which the instructions did not state.

It's always dangerous when neutral and ground are mixed. This was only permitted because the appliance manufacturers projected that there wouldn't be enough fatalities to justify slowing appliance sales. A simple wire break shouldn't make a circuit dangerous; however if a stove or dryer is "legally" wired, a neutral wire break will energize the chassis of the range or dryer. Because of this damn fool commingling of neutrals and grounds, a ground wire break will energize all the grounds in the area. And suddenly light switch screws are biting you.

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