Converting a 3-wire 240 Volt Circuit to a 4-wire 240/120 Volt Circuit

[Bob Deroeck]

Hi,

I have a question that's not related to woodworking, but could apply to shop wiring for a wood-working machine needing both 120 and 240 volt single-phase power.

I'm considering replacing an old gas range with a new dual-fuel range, using gas for the stove top and electricity for heating the oven. The stove manufacturer specifies a 4 wire outlet (two hots, a neutral and a ground). My understanding is that the current NEC requires separate neutral and ground wires for all stove installations, though in the past the NEC may have allowed a common neutral/ground wire in stove installations (3-wire NM).

I have an existing (but not currently used) stove circuit with double 50 amp circuit breakers and NM wire (6-6-G). The circuit length (one way) is about 60'.

I am considering converting the current 3-wire circuit into a 4-wire circuit by ending the current 6-6-G wire in an appropriately-sized box and running a new 14 gauge NM wire from the service panel to this new box. The hot 14 gauge wire would not be connected to anything in the service panel or the new box. It would just have a wire nut covering the wire at each end. The 14 ga neutral would be connected to the service panel neutral bus. Within the new box the two 6 hots and ground would connect to the two 6 hots and ground of a new NM 4-wire. The new 14 ga NM neutral would connect with the neutral of the new NM 4-wire in the box. The new NM 4-wire would go to a new 4-wire stove outlet to which the new stove plug would be plugged in.

The 120 volt load of the stove for the oven light, gas burner ignitors, and control panel can easily be handled by the 14 ga neutral. The 240 volts is only used by the oven heating element.

My question is "would this system violate the current NEC"? If "yes" what aspect of the NEC does it violate? I'm trying to avoid the cost and effort of running a new 4 wire NM cable from the service panel to the stove outlet. If what I have described violates the current NEC, can you propose another option that would allow me to use the existing 6-6-G wire for most of the run to the stove before it connects to a new 4-wire NM cable going to the stove outlet? For example, if using only the neutral wire in the new 14 ga NM cable violates the NEC, could I run just a single 14 ga wire in conduit from the service panel to the new box instead of running the new NM 14 ga cable?

All this wiring is easily accessible in my basement. Only the last 1-2' or so of the new 4-wire NM would go through the first floor subfloor to the new stove outlet.

Thanks.

Bob

[John Lanciani]

I hate to be blunt but that is just plain hack. Without bothering to pull out my code book there are several violations in your plan; conductor sizing, cable assembly requirements, listed uses, etc. 6-6-6-8 aluminum is not that expensive, just do it right or don't do it at all. It is ideas like this that keep tradesmen and firemen in business.

[Dan Friedrichs]

I think by "6-6-G" you mean "6-2G" (qty 2 #6 conductors and a ground)

The problem with your proposal is that nothing is protecting the #14 wire from over current. Say you had a failure in the circuit board that resulted it in drawing 45A. The breaker is rated 50A, the hot side is #6 (so it easily handles 45A), but the return is a #14 wire, which will melt and start your house on fire at 45A.

That said, I'm not an NEC expert, but I think you may be trying to solve a non-existent problem. I believe there is a NEC exception allowing you to use the 3-wire connection since it's existing. Often, ground and neutral are connected together at the stove in this situation. Are you sure the new range requires a 4-wire, or does it have a jumper somewhere to connect the ground to the neutral and use the 3-wire?

[Bob Deroeck]

Hi Dan,

Yes, 2 #6 conductors plus a ground.

I'm not sure which range will be purchased. I've checked the installation instructions for some GE ranges and they state "All new constructions, mobile homes, recreational vehicles and where local codes do not allow grounding through neutral, require a 4-conductor UL-listed range cord". The instructions do show how to do a 3-WIRE INSTALLATION (GROUND IS THROUGH THE NEUTRAL WIRE) as well as a 4-WIRE INSTALLATION. I will have to check to see if the Massachusetts allows grounding through neutral for existing construction.

I take your point about the 14 ga wire being overloaded. If Massachusetts does not allow grounding through neutral, what about installing a single 6 ga wire in conduit from the service panel to the new box to use as the neutral and then have the new 4-wire cable going from the new box to the range?

[Dan Friedrichs]

The appliance, itself, is not subject to NEC, and the 3-wire connection was "legal" when it was installed. You aren't expected to re-wire your house when you get a new range. Use the 3-wire installation method specified in the manual.

[Jim Becker]

The appliance, itself, is not subject to NEC, and the 3-wire connection was "legal" when it was installed. You aren't expected to re-wire your house when you get a new range. Use the 3-wire installation method specified in the manual.

I feel personally comfortable with this recommendation as it's in alignment with my understanding of "in place" service. If you were moving the range to a different area, rewiring would likely be required, but installing it in the same spot as previous with the same electrical hookup should in most cases be "Grandfathered".

I had to deal with a similar situation when I was doing my kitchen renovation back in 2003, although it was plumbing related, rather than electrical. If I moved things around, I was forced to put in all new "stuff" and deal with permits/inspections beyond "hey, howya doin'" interactions. Keeping things in-place meant I could re-used existing without questions. This is not uncommon when it comes to renovation of existing and like-for-like is the plan.

[Peter Christensen]

Why would you want to have electric for the oven? We selected an all gas stove in our new house and couldn't be happier. It has a convection oven and is way better than any electric stove I've ever had.

[Dan Friedrichs]

Peter, many "high end" ranges are "dual fuel": gas for the stove and electric for the oven. Many people think electric ovens are superior to gas.

[Jim Becker]

Why would you want to have electric for the oven? We selected an all gas stove in our new house and couldn't be happier. It has a convection oven and is way better than any electric stove I've ever had.

For serious baking, electric tends to be preferred as it maintains temperature more keenly than gas fired. The only thing that kept me from doing dual fuel for our Thermador range was the added cost, quite frankly. The gas fired convection is pretty good with stability, but it still varies more than an electric oven would in the same configuration.

[John Lanciani]

Why would you want to have electric for the oven? We selected an all gas stove in our new house and couldn't be happier. It has a convection oven and is way better than any electric stove I've ever had.

For some of us it is the only real choice.

[Bob Deroeck]

Dan, Jim and John,

Thanks for your comments.

I buy into Dan and Jim's suggestions that using the existing electrical service is probably grandfathered since the range would be in the same location. But, I've done some more research. First I took apart the existing range outlet and found my previous assumptions were wrong. Instead of copper it uses aluminum wires. But, the 3rd wire is not insulated. Further research on the internet on sites that focus on electrical wire and code issues led to statements that 3-wire systems are grandfathered "unless the 3rd wire is not insulated". I don't know if that is correct or not, but it raises a caution flag.

I still haven't decided if I will purchase a dual-fuel range or a 100% gas range. The dual fuel is more expensive, but not prohibitively so. I've been happy with the "evenness" of baking on my previous gas ovens. But, I am tired of replacing the broiler and oven ignitors every 3-4 years on my current and previous gas ranges. Eliminating that chore is a factor for considering a dual-fuel range.

I also "buy" John's recommendation to either do it "right" and not do it at all. That was the reason I asked the question originally, since I do want to do it right. I hated the concept of just throwing out the old wire and was looking for a safe and code-allowed workaround to allow its continued use.

If I do decide to buy the dual-fuel range, I'll likely replace the existing 3-wire cable with a new 4 wire-cable. But, I'll probably take the easy way out and just replace the current stove with a gas-only model.

Happy Cooking.

Bob

[Ken Combs]

Hi Dan,

Yes, 2 #6 conductors plus a ground.

I'm not sure which range will be purchased. I've checked the installation instructions for some GE ranges and they state "All new constructions, mobile homes, recreational vehicles and where local codes do not allow grounding through neutral, require a 4-conductor UL-listed range cord". The instructions do show how to do a 3-WIRE INSTALLATION (GROUND IS THROUGH THE NEUTRAL WIRE) as well as a 4-WIRE INSTALLATION. I will have to check to see if the Massachusetts allows grounding through neutral for existing construction.

I take your point about the 14 ga wire being overloaded. If Massachusetts does not allow grounding through neutral, what about installing a single 6 ga wire in conduit from the service panel to the new box to use as the neutral and then have the new 4-wire cable going from the new box to the range?

In your case it's just semantics, but since there is no neutral in you wiring, the neutral through the ground. and, that's the issue the new 4 wire requirement was intended to fix. when replacing a pure 240 appliance with one that uses 120 also, the 120v circuit's return current flow is thru the uninsulated ground wire.

But, as others have stated, bonding the two at the range is permissible on a replacement. That's what I would likely do.

It is not permitted to have wires to one location take different routes: ie, all must be in the same sheath or conduit. However, there is an exception for adding ground wires to an existing circuit with no ground. That is normally used to add grounding to an existing 2 wire convenience outlet. While not exactly what you want, you may be permitted by local rule to stretch the definition to add a neutral in solution you mentioned with a separate #6.

Local rules are unique, in my location the new aluminum 4 wire solution mentioned would not be allowed, no AL inside the structure, not even from the weatherhead to panel. Born from the 70s issues with AL caused fires.

[Ken Combs]

Dan, Jim and John,

Thanks for your comments.

I buy into Dan and Jim's suggestions that using the existing electrical service is probably grandfathered since the range would be in the same location. But, I've done some more research. First I took apart the existing range outlet and found my previous assumptions were wrong. Instead of copper it uses aluminum wires. But, the 3rd wire is not insulated. That is normal for most 240 only installations Further research on the internet on sites that focus on electrical wire and code issues led to statements that 3-wire systems are grandfathered "unless the 3rd wire is not insulated". I don't know if that is correct or not, but it raises a caution flag. Maybe by local rule, but not a Nec requirement AFAIK

I still haven't decided if I will purchase a dual-fuel range or a 100% gas range. The dual fuel is more expensive, but not prohibitively so. I've been happy with the "evenness" of baking on my previous gas ovens. But, I am tired of replacing the broiler and oven ignitors every 3-4 years on my current and previous gas ranges. Electric is not a sure cure for failures: I've replaced the temp sensors in both my top and bottom ovens, 4/5 years old. Eliminating that chore is a factor for considering a dual-fuel range.

I also "buy" John's recommendation to either do it "right" and not do it at all. That was the reason I asked the question originally, since I do want to do it right. I hated the concept of just throwing out the old wire and was looking for a safe and code-allowed workaround to allow its continued use.

If I do decide to buy the dual-fuel range, I'll likely replace the existing 3-wire cable with a new 4 wire-cable. But, I'll probably take the easy way out and just replace the current stove with a gas-only model.

Happy Cooking.

Bob

See my comments in red.

[Dan Friedrichs]

One final comment: every code and safety decision is a balance between cost and utility. At the point that 3-wire hookups for ranges were allowed, the costs of wire and rubber were high, and when weighed against the marginal benefits of having a separate ground wire, it was deemed to be a poor investment. Today, that situation is different. That's not "right" or "wrong", per se - the calculus just changed.

Personally, I think the risk of a shared neutral/ground is extremely small - the risk of it causing any problem in your life is probably less than the risk you incur by getting into your vehicle and driving to the store to buy a spool of 6/3G wire (to say nothing of the risks you incur doing that wire replacement work - or worse, doing something "wrong" while doing that work). I'd also think that the hundreds of dollars you spend on the wire could probably positively benefit you in a much more meaningful way than reducing a fairly trivial risk to an even more trivial risk.

But that's just my opinion

[Jim Becker]

If you have the access to be able to replace the three wire setup with a four wire setup and can afford the material costs, that's not a horrible idea. And if you go with an all-gas range, you're still going to need 120v power to that range for the igniters, etc. I re-provisioned a 240v #8 wire circuit for that when I did my kitchen renovation. It was one of two circuits in place for the previous electric JennAir cooktops that the previous owner used. Since access under our kitchen was seriously constrained in that area (crawl space with almost no headroom in the area of the range), re-utilizing the wire made sense. But all the conductors were insulated. I will tell you that getting that number 8 wire transitioned to work with a normal 20a 120v outlet was "fun" and included an extra j-box that just squeezed behind the range's lower lip... The breaker end was a lot easier.