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220v oven circuit, polarity and updating to 1998 NEC questions

220v oven circuit, polarity and updating to 1998 NEC questions

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  #1  
Old 11-15-10, 02:07 PM
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220v oven circuit, polarity and updating to 1998 NEC questions

Hi all--

Bought a house that was built in 1994. Just pulled out the wall oven. Not surprising at all to find that it has the typical for that time 3-wire 220 circuit--Stabiloy SE Type, two black wires and one bare. Now, my understanding is that the 1998 NEC has a grandfather clause that would permit me to hook up the new 4-wire oven, running both ground and neutral to the bare wire (the Stabiloy is the appropriate gauge). But I have two questions. 1) I realize it would be better to run a new 4-wire circuit, but how much better? We're talking a major project to get the cable to where it needs to be, will probably have to rip out some ceiling. The house hasn't burned down yet, and the oven never electrocuted anyone, how bad is the 3-wire method? 2) If I stick with the existing 3-wire, how do I determine the polarity? Both wires coming in are black, neither is coded to red. I do have a digital MM. Does correct polarity matter for an oven/micro combination unit? The diagram that came with it shows red to red and black to black of course, but if I were to reverse the polarity, would it harm the unit (for example, cause the microwave turntable to run backwards?) Thanks if an electrician can help me out here.
 
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  #2  
Old 11-15-10, 02:29 PM
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It is probably a 240v not 220v circuit. Polarity is not relevant on an AC circuit. I suspect you are asking which is the neutral. There may not be a neutral. Ovens are sometimes 240v only. There is no neutral on a 240v circuit. Just 2 hots and a ground. If this is a 120/240 oven then they may have used the bare neutral but that was not legal in 1994. If the oven is 240v you are ok. If it is 120v/240 the cable should be replaced. It was never legal foe 120v/240v.
 
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Old 11-15-10, 02:29 PM
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Originally Posted by Major Minor View Post
Now, my understanding is that the 1998 NEC has a grandfather clause that would permit me to hook up the new 4-wire oven, running both ground and neutral to the bare wire (the Stabiloy is the appropriate gauge).
All electrical work is grandfathered to the year it was installed as long as you do not alter it. The only exception would be if your oven manufacturer requires a four-wire circuit, although most allow a three-wire option. Check the installation manual.

1) I realize it would be better to run a new 4-wire circuit, but how much better?
I don't have any hard numbers, but the code making panel felt it a large enough risk to require four wire circuits since 1996 code. The safety issue occurs if the bare (neutral) wire in your three-wire cable is somehow severed or corroded, then the metal surface of the oven will become live. Neutral wires (particularly with aluminum cable like yours) do burn off occasionally, so pay close attention to good aluminum terminating technique if you choose to re-use your three-wire circuit.

2) If I stick with the existing 3-wire, how do I determine the polarity?
It doesn't matter; there is no polarity in 240V AC circuits.
 
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Old 11-15-10, 02:39 PM
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It is actually a 2 wire 240v with ground. This would be fine if the oven did not have any clocks or timers that ran on 120v and the oven ran on straight 240v. As it stands now, you can still use this for a similar style oven that requires only 240v and no neutral. HOWEVER, you may NOT use this for a 3 or 4 wire 240/120v oven. The bare wire may NOT be used for neutral under any circumstances. Even before 1996 (not 1998) this was a code violation. 3 wire circuits have two hots and a (WHITE) neutral, but no separate equipment ground. They were grounded through the neutral. This is what changed in the 1996 code (because if the neutral wire was broken or disconnected for some reason, the metal frame of the oven becomes live if there is no separate ground to blow the breaker), and as long as the circuit was unaltered, it remains grandfathered. But if you only have 2 hots and ground, you HAVE to change it to a 4 wire circuit to use a 3/4 wire oven. No way around it.

Oh, and there is is no 'polarity' in AC. Red or black doesn't matter, they're both hot. Only white and green for safety.
 
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Old 11-15-10, 02:55 PM
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Originally Posted by JerseyMatt View Post
The bare wire may NOT be used for neutral under any circumstances. Even before 1996 (not 1998) this was a code violation.
Many jurisdictions did allow a bare neutral within a cable assembly (mainly SE-U) for a range or dryer circuit. It was almost certainly an approved installation original to the home. Of course you could never install a new one like this, the old circuit is still legal provided that it was covered under the construction permits and inspections.
 
  #6  
Old 11-15-10, 06:22 PM
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hmmm, do I have a problem or not?

OK, the wall oven/microwave combination I took out (which was original to the house) had 4 wires, a read and a black, each of which went (separately) to an incoming black. The green and the white were pigtailed together in a wire nut with the incoming bare wire. Was this, or was this not, up to code back in 1994? And, whether or not, is it dangerous? Virtually every house in this subdivision was built by the same builder and has the same oven/micro in the wall, so I would assume they are all wired exactly the same way. Do I need to call the electrical inspector and sound the alarm, or do I just put the new oven in exactly like the old one and call it good. At a minimum, I need to mount the junction box. The slob that built this place just opened up the drywall behind the oven with a claw hammer, and threw the junction box inside the wall. Just hanging there. [edit--btw, I meant to say 240 in the original post, that was a mental lapse]
 

Last edited by Major Minor; 11-15-10 at 06:23 PM. Reason: forgot to say
  #7  
Old 11-15-10, 06:41 PM
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Running new cable and/or conduit may not be as hard as you think.
Is there an unfinished attic above or unfinished basement below?
Is it on an outside wall or have access to an outside wall through a cabinet.
 
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Old 11-15-10, 09:16 PM
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Originally Posted by Major Minor View Post
OK, the wall oven/microwave combination I took out (which was original to the house) had 4 wires, a read and a black, each of which went (separately) to an incoming black. The green and the white were pigtailed together in a wire nut with the incoming bare wire. Was this, or was this not, up to code back in 1994? And, whether or not, is it dangerous? Virtually every house in this subdivision was built by the same builder and has the same oven/micro in the wall, so I would assume they are all wired exactly the same way. Do I need to call the electrical inspector and sound the alarm, or do I just put the new oven in exactly like the old one and call it good. At a minimum, I need to mount the junction box. The slob that built this place just opened up the drywall behind the oven with a claw hammer, and threw the junction box inside the wall. Just hanging there. [edit--btw, I meant to say 240 in the original post, that was a mental lapse]
Ben brings up a point that as long as it was type SE cable, it would have been acceptable under pre-1996 code. However, legal or not, it's a dumb move for a few reasons (and I can't believe it was legal for so long). It is a current carrying conductor that is bare. That poses a shock hazard if the outer insulation is compromised. There is also the incredibly major issue of the pigtail from the oven (which is most likely copper) connected directly to aluminum conductors, most likely without the NoAlOx gel on the joint. That is a BIG no-no. Copper to aluminum joints are subject to deterioration and overheating and are a fire hazard if done improperly.

At the absolute least, put in a 3 wire receptacle that is rated for AL/CU conductors. Then use a 3 wire pigtail on the new oven. That at least takes the copper/aluminum joint out of the equation. But I would still recommend re-running the circuit properly with 4 wire.
 
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Old 11-16-10, 09:16 AM
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Originally Posted by Major Minor View Post
The green and the white were pigtailed together in a wire nut with the incoming bare wire. Was this, or was this not, up to code back in 1994?
It was probably up to code or at least would have passed inspection.

And, whether or not, is it dangerous?
I don't like the direct wirenut connection, but just the three-wire circuit is not some sort of imminent hazard. I think of it like older cars without airbags or shoulder belts -- less safe than the modern way, but not as good as it could be. If it was my house I would replace it, but it is legal for you to leave it if you choose.

do I just put the new oven in exactly like the old one and call it good. At a minimum, I need to mount the junction box.
Definitely a j-box and a three-prong AL/CU receptacle like Matt suggests. The new oven would get a matching three-prong cordset installed to the terminal block inside the oven wiring compartment. I'm almost certain that "Stabiloy" cable was all aluminum, but check it to make sure; scrape down into the bare wire to see fresh metal. If it is AL, you'll need to polish up and brush the wires with no ox grease prior to termination for a much more reliable connection.
 
  #10  
Old 11-16-10, 09:57 AM
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Is the cable #6 on a 40a breaker or #8 on a 30a breaker?
 
  #11  
Old 11-16-10, 02:24 PM
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answers to all (some?) of the questions and more info

It's #6 cable on a 40A breaker.

According to the Alcan website, Stabiloy, even back in 94, is a "new generation" aluminum that isn't supposed to have all the problems, even though using an anti-oxidant is still recommended. The connections that were in the junction box to the old oven were in purple wire nuts (don't know if they were the special ones for AL/CU or not) and looked good--no signs of overheating or oxidation on the wires, even though they had been in use for 16 years.

The new oven, just like the old, has Greenfield flex conduit coming off the back of it, and is designed to be wired directly into a junction box. There is no terminal block you can open up and install an oven cord to.

As I see it right now, if I use an anti-oxidant compound and purple wire nuts, I don't think I would ever have an oxidizing/overheating problem. And I've figured out there is room behind the drawer below the oven to surface mount the j-box, so I could always open it up and inspect every so often.

But here is the problem/danger I see, if I am understanding all of this correctly. If the bare wire from the feed inside the junction box were ever to come into contact with the box itself, both the box and the oven frame (since the Greenfield from the oven is connected to the j-box) would become live (if the breaker didn't trip) killing my beautiful wife, correct? Yikes.

Last, running new cable is going to be difficult. The breaker panel is on the outside wall of the garage, cement slab floor. The oven is at the opposite end of the house, first floor. The existing cable goes into an attic above the garage, then into the dead space between the first and second floor, then drops down into the wall behind the oven. It will not be difficult to drop some new cable down through the floor behind the oven into the crawl space. The trick is going to be finding a way to get that cable out of the crawl space and across the garage to the breaker panel.
 
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Old 11-16-10, 02:35 PM
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The purple wire nuts are not large enough to use with the #6.

The bare wire should only carry current in the event of a fault.
 
  #13  
Old 11-16-10, 02:50 PM
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The ones that were on there were big enough, and looked purple to me. I could take a picture. Is a special wire nut needed for AL-CU connection if using Stabiloy, or is using no-ox grease and a regular wire nut allowed/sufficient?

Also, JerseyMatt said, regarding the bare wire: It is a current carrying conductor that is bare. That poses a shock hazard if the outer insulation is compromised.

You are saying this is not true even if the white from the oven is connected to it, and is true only if there is a fault in the circuit?
 
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Old 11-16-10, 02:56 PM
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Originally Posted by Major Minor View Post
is a "new generation" aluminum that isn't supposed to have all the problems
It's better than plain pre-1972 aluminum, but it is still substantially more likely to fail than copper. All aluminum wire requires special termination.

As I see it right now, if I use an anti-oxidant compound and purple wire nuts
Purples aren't big enough, and they don't work any better than regular wirenuts. You should use a mechanical fastener with set screws or bolts to join the copper and aluminum; it should be rated "Cu/Al" on the package to indicate its use with both metals. Before terminating the aluminum, shine the conductor up with some emory paper and put on a good coating of no ox grease.

But here is the problem/danger I see, if I am understanding all of this correctly. If the bare wire from the feed inside the junction box were ever to come into contact with the box itself, both the box and the oven frame (since the Greenfield from the oven is connected to the j-box) would become live (if the breaker didn't trip) killing my beautiful wife, correct? Yikes.
Not quite. This can only happen if the bare wire becomes broken or burned off between the oven and the main panel. That's why it's particularly important to get the connections secure.
 
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Old 11-16-10, 05:48 PM
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You could also use a tin plated split bolt connector approved for Al/Cu connections with a spacer to prevent direct contact between the copper and aluminum conductors, but you'll need a larger junction box.

Product Results - 1HPW
 
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Old 11-17-10, 02:39 PM
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I spoke to a neighbor last night, house built at same time as ours, same builder-installed wall oven/micro combo unit. He recently had a new wall oven installed by a licensed electrician. The electrician did NOT run a new circuit, and did not mention any problem whatsoever regarding the wiring. I would love to know if he just threw the j-box back loose inside the wall like I found mine.
 
  #17  
Old 11-17-10, 02:44 PM
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The electrician who submits the $600 bid rarely gets chosen over the one who submits the $150 bid, regardless of the quality of the work. Using the existing circuit was probably a legal, least-cost option, so he took it.
 
  #18  
Old 11-30-10, 10:25 AM
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oven is wired -- one last question

Well, Thanksgiving with no oven was not fun. But, I would like to thank all of you, especially ibpooks, your help has been tremendous. After speaking to two local electricians who confirmed that code here allows a new oven to be hooked to the old wiring I have, I decided to go ahead. (In fact, they told me that if [retailer] had installed the oven, they would probably have put some anti-oxide grease on the aluminum and used the big blue wire nuts that had been in there for 15 years.) By the way, the new oven came with the green and white wires taped together, and said to hook both to your white or bare incoming wire if allowed by local code. Perhaps some day in the future I'll run new wiring from the panel to have both a dedicated ground and a neutral, but here's what I did for now.

First, used a deeper 4" J-box. Second, got 3 Polaris black IT-4 connectors, which are dual-rated and insulated (to avoid having to put umpteen layers of three different kinds of electrical tape on the two hot split bolts). The problem with that, though, is that they are not recommended for fine-stranded copper, which is exactly what the new oven has. So also made three jumpers out of #8 copper, stripping the insulation off one to make it bare. Each of the jumpers went to the copper oven wiring using a new big blue wire nut. Then the jumpers went into the Polaris connectors (which have a-ox grease in them), and the aluminum incoming (which I trimmed and cleaned) went into the Polaris connectors. I do not have a torque screwdriver, but I made the set screws tight, but not too tight, so as not to break the aluminum. I used the jumper running from the incoming bare to oven green/white connection to ground the J-box. I tugged on the green/white/jumper wires in the wire nut good to be sure none are loose. Turned on juice, and all appears well. Now, here is the last question. When I touch my DMM between the J-box or the oven frame, and ground (either the floor or a nearby grounded outlet cover screw) I get a reading of zero that occasionally fluctuates to 1-volt. I'm assuming this is just "stray" voltage, and OK, but if I have a problem, please let me know. Again, thanks for all the invaluable help. EDIT: I wired oven last night. Just went to put load of clothes in electric dryer, and found its breaker tripped. Just re-set breaker, everything seems OK, but this also seems more than just a co-incidence. That breaker has never tripped before. The dryer is on its own 240v circuit, but it of course has the same wiring issue as the oven.
 

Last edited by Major Minor; 11-30-10 at 10:48 AM. Reason: additional info
  #19  
Old 11-30-10, 06:04 PM
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First, used a deeper 4" J-box.
Was it a metal box? If so, did you ground it?
 
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Old 11-30-10, 10:43 PM
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CasualJoe, as I said: I used the jumper running from the incoming bare to oven green/white connection to ground the J-box. To be absolutely clear, what I've got is the bare #6 AL coming from the service panel going into one side of the Polaris IT-4 connector, the #8 copper jumper going into the other side of the Polaris, that jumper looped around the screw holding the box down, and then going into the wire nut connecting it to the green/white pair from the oven.

By the way, the original metal junction box was NOT grounded (except possible incidental contact between the bare wire and the box) which maybe is why they threw it inside the wall behind the oven where nobody could see or touch it. It was not accessible without taking out a 200 lb. wall oven. Since the new oven is not as tall, leaving a cavity beneath it which is concealed by a false front, there is room behind that front, under the oven, for the J-box so I can get to it if I need to.
 
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Old 12-01-10, 07:47 AM
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that jumper looped around the screw holding the box down,
To be code compliant it must go to an approved ground screw in a tapped hole in the box. On newer boxes there are usually pre-tapped holes. Wrapped around a mounting screw is generally no longer accepted.
 
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Old 12-01-10, 01:53 PM
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yup, I saw the tapped hole and knew it was for a ground screw, but couldn't find one in my box of odds and ends. I'll fix that this weekend.

What I'd really like to do is eliminate the jumpers (except for the one to the box ground screw) and have the oven wires going directly into the Polaris connectors. Is there some type of copper compression sleeve or ferrule that I could put on the ends of the stranded copper from the oven, and then into the Polaris, that would be code compliant?
 
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Old 12-01-10, 05:05 PM
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Originally Posted by Major Minor View Post
yup, I saw the tapped hole and knew it was for a ground screw, but couldn't find one in my box of odds and ends. I'll fix that this weekend.
It is a 10-32 screw. The green ones are the correct size.
 
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