Identifying neutral/hot wire in 240V outlet

polaris_riderDecember 13, 2009


I'm in the process of installing an electric wall oven that calls for a 240v feed. There is a 240V outlet exactly where the oven will go, so far so good. After killing the power at the panel I removed the wall cover only to find three wires, two black and one uninsulated ground. My oven's electrical scheme has the neutral and hot wire specifically identified in the wiring diagram. My problem is that I have no idea which one of the two blacks in my wall is which. When I run a multimeter between the two blacks I get 240V. When I run the MM between one black and the ground I get 120V. Does this mean that it doesn't matter which black wire I use for the neutral or hot? If not how do I differentiate between the two?

Many thanks!!

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A straight 240v circuit does not have a neutral. What you have is two hots and a ground. As you determined with your meter, you get 240v between the two hots and 120v between either of the two hots and ground.

If your new oven calls for a neutral, you are going to need install new cable or wire depending on what you have now. Some ovens can be wired with three or four wires but only the instructions can tell you if three wires will be acceptable.

You also want to make sure the current circuit amperage matches that of the new oven.

    Bookmark   December 13, 2009 at 12:34PM
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to clarify what mike said, you do NOT have a neutral. you cnanot hook up your oven to this circuit unless and until the oven is rewired internally for no neutral. in some cases this is not possible at all, so you need to find out for sure.

and i know this sounds bad, but you do not know enough about AC circuits to safely do this alone. if you don't know how a 3 wire 240V circuit works you can easily start a fire. get someone who knows what they are doing, not your buddy who can hack it together either!

    Bookmark   December 13, 2009 at 12:42PM
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Crap. Sounds like I have to get an electrician. My oven accepts three or four wire power sources. Unfortunately for me though it is very clearly stated that one of the three wires has to be neutral. Will the electrician be able to use the existing wires or will he need to run new ones? I'm obviously hoping for the former because I would imagine that it will cost a lot more to have him crawl around in the atic snaking new Romex.

Thank you so much for your replies.

    Bookmark   December 13, 2009 at 1:07PM
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"to clarify what mike said, you do NOT have a neutral."

Not sure that your statement clarifies anything. Repeating is more like it.

    Bookmark   December 13, 2009 at 1:07PM
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What are all the wires on the oven manufacturer's connection diagram? I would think one diagram (modern connections) would have two hots, a neutral, and a ground. For some appliances it might just be two hots and a ground.

They may also give a diagram for older wiring with two hots and a wire that serves as both a neutral and sort of ground. That was an exception to the grounding rules allowed for many years.

In any of these cases, they would be assuming the hots are 120V each against neutral or ground, and 240V between hots.

You only mention one hot, which has me confused. Is this a product normally distributed in the US? If a direct import, the diagram could be for some country that uses one 240V hot, neutral, and ground, and things get trickier.

If the oven manufacturer gave you a choice of connection methods, and you are not redoing a lot of electrical wiring, but just replacing an oven, you might be legal in some jurisdictions with the wire you have, properly connected.

Is the breaker size what the oven manufacturer asks for? (You should also check the wire size is adequate for the present breaker, but chances are that it is.)

    Bookmark   December 13, 2009 at 4:49PM
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Polaris Rider,

You should probably get an electrician to look things over. It really depends on what the manufacturer of the oven calls for. If the oven requires a neutral, then you will have to run new cable.

Perhaps you could most the manufacturer/model number oven and we could take a look.

    Bookmark   December 13, 2009 at 5:01PM
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Way back when, it was legal to either send neutral current back through ground, or ground major appliances to neutral. I don't remember which it was, but it was legal - and now, in many cases, it's grandfathered.

If the instructions say it can be configured for 3-wire hookup, but the third wire is called "neutral", then they are talking about grounding to the neutral - or else not grounding at all, which I highly doubt is the case.

In either case, there is no logic supporting the idea that the oven's 3-wire option is any different from the old grandfathered 3-wire configuration. The original poster quite clearly has two hots and a "ground", and I am betting it's a piece of SE - which is so incredibly COMMON around my neck of the woods that I'd have a very hard time believing it wasn't originally legal (I know it's not now).

The OP needs to configure his oven for a 3-wire installation and connect the third wire, whatever he chooses to call it, to the not-black wire, whatever he chooses to call that.

I haven't seen one of these yet that isn't a matter of turning some screws, and most of the ranges and clothes dryers I've encountered come with instructions intended to be followed by the person who bought it.

I have learned a LOT from online forums and similar resources. It irritates me to think that nobody like me will ever learn this stuff again, because all of the resources are slowly being replaced with big signs that say "Call someone who already knows."

No, I won't, and I won't tell others to either.

I'm not arguing that the OP is misled - quite clearly he thinks one of the hots is neutral (YIKES!), but he also clearly knows that he needs correcting, or else he probably would have never asked the question. It's three wires. Given proper instructions and a little bit of education to go with them, it's pretty hard to screw up.

    Bookmark   December 13, 2009 at 6:59PM
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If what you have is Type SE cable where the bare wires are wrapped around the two hots, and if this circuit originates from the main panel (not a subpanel), then you may use these wires... you are allowed to ground the appliance to the neutral for an existing three wire circuit.

The ONLY three wire circuits you are allowed to continue to use are THREE insulated wires or Type SE cable, These are the only ones that were ever legal... xx/2 NM cable is NOT allowed in this situation.

    Bookmark   December 13, 2009 at 7:13PM
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Lots of smart people here, thank you kindly for taking the time to post.

In no particulat order:

1) I don't think one of the hots is neutral, I'm merely confused by the omission of a neutral. I've never dealt with "straight" 240.

2) I'm not asking for you to configure my oven for me but since someone asked for diagrams I included the installation instructions in the link below. Page five has the electrical hookup info. Again, all I have coming out of my wall is two black heavy gauge wires wrapped by an unsheathed ground.

3) The breaker will need to be changed as my oven calls for 30 amps but the existing breaker is rated for 40.

Here is a link that might be useful: Wiring diagrams on pages 5-6

    Bookmark   December 13, 2009 at 8:09PM
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Can you see the cable in the wall? Can you see, written on the outer insulation, what type of cable it is?

If it's SE, treat the uninsulated "ground" as being both ground and neutral.

The diagrams for your oven are among the simplest I've seen, and translate to: "If you don't have a fourth wire, hook ground and neutral together."


Hook ground and neutral together, and to the uninsulated "ground" -- if, and ONLY IF it turns out to be type SE cable in your wall.

If it's not SE, your installation is illegal (and was to begin with), and the cable must be replaced.

    Bookmark   December 13, 2009 at 8:14PM
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The wrap (outer insulation) reads, "Olin C Type SE Cable". I'm guessing the "SE" is what you are referring to? If so, and following your guidance, I'd want to twist the ground and the neutral from the oven together and attach them to the ground coming out of the wall.
Now obviously one black from the wall ties to the black on the oven. What about the other black fom the wall? Does it go to the red on the oven or does it also get attached to the black from the oven?

Sorry for being so obvious (to you at least!).

    Bookmark   December 13, 2009 at 9:01PM
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black from oven to one black from wall, red from oven to other black from wall, white and bare from the oven to the bare from the wall.

    Bookmark   December 13, 2009 at 9:30PM
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Cool, Thanks!

    Bookmark   December 13, 2009 at 9:53PM
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why is it "safe" to ground your oven (which likely has 120 volt components) to the neutral wire, or use the ground wire as a neutral depending on how you look at it. But if some one on this forum were to suggest using a ground wire as a neutral for a light fixture they would be crucified!!

The way I heard the story, which may not be entirely true, three wire 240 volt circuts were made legal during WWII as a way to save copper which was in short supply. 240 volt appliances of that era rarely had any 120 volt components, so it was two hots and a ground. no neutral needed. No problems. Due to inertia, and a home building industry reluctant to add costs that would not increase selling prices, the three wire circut remained legal long after the scarcity of copper was rectified. And Appliances with 120 volt electronics became common.

    Bookmark   December 19, 2009 at 9:35PM
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qwertyui, that sounds somewhat believable.

Another theory...

If you look at some old 120V appliances in metal cases, you will indeed find the case "grounded" to neutral. This is one of the reasons polarized plugs are so important. Grounding the case of your electric range to neutral is conceptually the same thing.

Remember that our regular 120V circuits have followed a similar sequence of evolution: we originally had just hot and neutral. The third ground prong was a later addition. Same deal with our 120/240 stuff. Long ago we had the minimum necessary to make it work: two hots and a neutral. Ground was added later.

Two-prong 120V outlets are grandfathered. Same with three-prong 120/240 outlets.

    Bookmark   December 20, 2009 at 1:14AM
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Ron Natalie

It's frowned upon in modern code safety, but grounding the case to the neutral is better than nothing and a relic from the days when hardly anything was grounded. For stoves, the neutral use is minor, maybe the clock and a lightbulb, but all the load is across the hot legs directly.

    Bookmark   December 20, 2009 at 9:45AM
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So grounding the case to neutral is better than no ground. Therefore someone with old 2 prong outlets would be "safer" installing 3 prong outlets and connecting both the ground and neutral to the neutral wire?? Or maybe this is "safe" only if the load is less than some small amount , say 100 watts??
This makes no sense to me. I cannot see how having the neutral current carrying wire connected to the appliance case is safe no matter how you stretch the definition of safe. I think any appliance that has "any" 120 volt component must have a case ground wire separate from the neutral wire.
The code change that made 3 wire 240 legal was a balance between saving copper for the war effort and consumer safety. If a few consumers died, it was ok because it helped defeat the natzi's. Well they have been defeated for quite a while now, It's time to rewire with 3 insulated conductors and a ground.

    Bookmark   December 20, 2009 at 8:56PM
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"It's time to rewire with 3 insulated conductors and a ground."

That's what I ended up deciding to do, replace the old aluminum 2 wire and a ground with copper 3 wire.

    Bookmark   December 20, 2009 at 9:43PM
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Ron Natalie

I didn't say I advocated the connection of grounds to neutrals, the question was why it was allowed. Yes it is archaic (though I dispute the assertion that it was related to the war effort).

    Bookmark   December 21, 2009 at 7:35AM
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"Therefore someone with old 2 prong outlets would be "safer" installing 3 prong outlets and connecting both the ground and neutral to the neutral wire?? Or maybe this is "safe" only if the load is less than some small amount , say 100 watts??"

The code acceptable way to replace 2 conductor receptacles without re-wiring the circuit is to use a GFCI.

They provide a very high level of protection, with the only thing they cannot detect being if you are across the hot and neutral and ALL the current returns on the neutral.
If ~5 mA goes anywhere else, the GFCI will trip off.

The 'No equipment ground' sticker is required on the GFCI device, and regular 3-prong receptacles downstream need the 'no equipment ground' & 'GFCI protected' stickers on them.

GFCIs do not require a grounding conductor to operate correctly.

    Bookmark   December 21, 2009 at 8:13AM
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So let me get this straight, prior to world war two, 120/240V circuits such as a range or oven had to have a seperate wire for the nuetral and ground? Interesting.... and yet I've worked in numerous house wired up to the 60's that had no grounding wire on 120V circuits let alone 120/240V circuits. I find it highly dubious that from 1900 to 1942 circuits had grounding wires,especially since I've yet to see a ground installed as a part of knob and tube wiring. It was changed just for world war two and then not changed until nm cable started to have grounds in the '60s and then, oh by the way, we forgot to change back from world war two copper rationing and the 1996 NEC changes 120/240V circuits to 3 wire with ground? Ideally should a 3 wire with ground be installed if the old wire does not have one? Yes, absolutly, its the safest way to do it. Is it legal to hookup an old SE cable to the appliance if the wire hasn't been moved/extended? Yes. To compare such a setup to a general use 120V circuit where there are switches, light fixtures, outlets, power strips, appliances, in essence a number of places where things can go wrong, is asinine.

    Bookmark   December 21, 2009 at 8:35AM
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The code acceptable way to replace 2 conductor receptacles without re-wiring the circuit is to use a GFCI.

Or to replace them with new 2-prong outlets. They're still available.

I don't think it had anything to do with the war. I don't think 4-wire connection was ever required until the "recent" change, just like I don't think 3-wire 120 was required at any point.

I'm also not necessarily convinced that having the stove grounded to neutral is any safer than having it not grounded at all, except in the case that one of the internal hot wires becomes connected to the case. I believe this is the ONLY hazard intended to be "resolved" by grounding to neutral - and for parallel comparison, yes, the same type of fault is resolved in a 120V appliance by grounding it to neutral.

Grounding a 120-only device to neutral creates its own safety issues, and the same ones are created in grounding a 120/240 device to neutral.

There IS a parallel and the comparison is not "asinine". The same dangers exist, and the same type of fault (internal short hot-to-chassis) is resolved. The difference is historical.

    Bookmark   December 21, 2009 at 4:33PM
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The worst is if that neutral comes lose on the 3 wire circuit. Then the next person that touches the dryer and washing machine at the same time can complete the circuit for the timer and light bulb to work, or the sink and the oven. Having the neutral come lose basically turns the chassis into a hot wire at that point. I try to sell all of my customer's on changing out their 3 wire 120/240 circuits. About half say, "Not very interested at this time" The same with when I tell them their FPE service panel is waiting to burn their house down. "Havn't had any problems with it yet but we'll keep that in mind" Yeah sure you will! lol. I can't believe it took until the 90's to quit allowing new installations of 3 wire 120/240.

    Bookmark   December 21, 2009 at 6:32PM
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I'm not sure about the WWII copper shortage. That's just the "story" I heard. Doesn't change the safety issue.

Here's an excerpt from a website that attributes the 3 wire "legalization" to the post war housing boom. A little dubious because 1947 seems to fast after WWII for a housing boom.

Circuit grounding was one of the more hotly contested topics in the early history of electrification. In the early 1890Âs, the New York Board of Fire Underwriters had condemned the practice of grounding the neutral as a dangerous practice, especially in a 3-wire Edison (120/240 Volt) system. The Edison utility companies, on the other hand, found just cause to ground their supply systems,
even as others thought the utilities were doing this to just save copper and money at the cost of an increased fire risk. The great debate continued for over a decade, but in 1903 the Code was revised to recommend that these circuits be grounded, and finally in the 1913 Code a mandatory circuit grounding requirement was included for circuits like the popular residential Edison 3-wire system.

The permission for neutral grounding, the practice of using the neutral conductor as an equipment grounding conductor, was first permitted in the 1947 Code for electric ranges. At around that time many electric utilities were promoting the use of residential 240 V cooking for the post WWII housing boom, and many were even offering to install an upgraded service to older homes at no charge. However, there were no NM cables available at the time with conductors of sufficient ampacity to handle these higher amperage branch circuits. There were, though, service entrance cables of sufficient size, but they had a bare neutral conductor. This special Code permission allowed the frames of these large appliances to be grounded through the uninsulated grounded neutral conductor of the Type SE service entrance cable used to supply the branch circuit. The use of neutral grounding was also extended to electric clothes dryers in 1953. However, almost 50 years later, this special permission for neutral grounding was taken away in the 1996 Code for all but existing branch circuit installations.

    Bookmark   December 21, 2009 at 10:18PM
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"3) The breaker will need to be changed as my oven calls for 30 amps but the existing breaker is rated for 40."

I glanced through most of the posts, and I don't believe anyone has pointed out that his previous 30amp wiring is probably 10 ga wire. His new stove requires 40 amps and therefore 8 ga wire.

    Bookmark   January 6, 2010 at 10:47AM
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... IIRC, electric ranges and ovens (except for small hotplates, waffle irons, etc.) were virtually non-existent prior to WW2. NatGas and propane were kings of cooking.

And yes, the POCOs were on an expansionist rampage after WW2, and for tract home developers it was MUCH easier to just run larger wires, than it was to lay underground natgas piping... a nice little collusion for the "advantages" of "all-electric living" was the result. The consumer got stuck with the bill.

And yes, a housing boom was well underway (though not at peak) by 1947, certainly in Cali, Great Lakes, and Eastern Seaboard.

And yes, DURING WW2, males in essential defense jobs back on the homefront (think aircraft industry, So-Cal), were "taking care of" all the lonely wymins... above and beyond... ;')

    Bookmark   January 24, 2010 at 3:39PM
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