Wall Oven 30 amp Circuit vs. 40 amp Circuit

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  #1  
Old 06-08-12, 08:04 AM
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Wall Oven 30 amp Circuit vs. 40 amp Circuit

Greetings. I have a malfunctioning wall oven that I want to replace. I have done simple wiring in that past (mostly light fixtures), but have not ever wired a 240 V appliance before. My line coming into the wall consists of two 120V hots, 1 neutral, and 1 ground. The wiring is aluminum (all the 120V circuits in my house I've seen so far are copper). The current oven is wired with the two 120V hot oven lines connected to the two 120V wall leads (black wiring). The existing oven's neutral AND ground are twisted together and connected to the wall's neutral (white wiring). I found this to be curious given that there is a ground wire available in the wall. I believe this approach was permissible and maybe the last installer just got lazy.

Based on some research I've done to this point, I've determined that I need to take extra care when wiring copper and aluminum together, making sure to add anti-corrosion material to the wiring and using a special Al Cu wire nut. There are however two concerns I have that I would like a little advice on.

(1) I'm a little nervous that the last installer ignored the ground wire in the wall and might have done this intentionally because maybe the ground really isn't grounded. I have a finished basement and so I really can't trace any wiring. Most likely the last installer was lazy, but I want to make sure that the ground I see in the wall really is grounded. How would I go about to test that?

(2) The replacement oven instructions indicate that:
models rated from 7.3 to 9.6 kW at 240 volts (5.5 to 7.2 kW at 208 volts) require a separate 40-ampere circuit. Models rated at 7.2 kW and below at 240 volts (5.4 kW and below at 208 volts) require a separate 30-ampere circuit.

The wattage rating for the replacement oven indicates that I will need a 30 amp circuit. When I look at my current oven circuit breaker, I see that the breaker switch is at 40 amps. Given that the current oven has been running for years on a 40 amp breaker, I believe the circuit itself is safe for 40 amps (I regret that I did not write down the wire gauge when I last had the existing oven pulled out to take a look at the wiring - the next time I do this I will note the gauge and verify it is safe for 40 amps). Assumming the circuit itself is OK at 40 amps, what are the consequences of keeping a 40 amp circuit breaker when the oven is expecting a "30-ampere circuit?" Will replacing the 40 amp breaker with a 30 amp breaker be necessary? I have never replaced a circuit breaker before. Is this a job for a professional?

I appreciate any advice given. If there is something else I should keep in mind, please let me know.
 
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Old 06-08-12, 08:49 AM
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You can check to see if the cable is grounded in the panel. Both the white and bare should be attached in the panel. If they both are connected the connection at the oven end should have been removed and the white attached to the white and the bare to the bare.

You will not find wire nuts that will fit a conductor that large. You will need to use something like a Polaris pre-insulated connector or a split bolt. Split bolts may be cheaper but are labor intensive and need to be wrapped with many layers of tape in order to insulate the connections.

You could change the breaker to a 30 amp from the 40. Luckily you don't need to go from a 30 to a 40. That would require a larger cable.
 
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Old 06-08-12, 09:14 AM
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Just a bit of a tech note: You wrote incorrectly:
My line coming into the wall consists of two 120V hots, 1 neutral, and 1 ground. The wiring is aluminum (all the 120V circuits in my house I've seen so far are copper). The current oven is wired with the two 120V hot oven lines connected to the two 120V wall leads (black wiring).
Actually you have the two sides of the 240 volts supplied to your house not two 120v lines. The blacks are the two sides of the 240v. The 120v in you house is derived from one of those hots and a neutral.
 
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Old 06-08-12, 09:54 AM
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@ray2047: Thank you for the clarification. I am obviously not an electrician by trade.

@pcboss: Thank you for your advice. I will look into the special connectors, most likely the Polaris. The oven currently is connected with what appeared to me to be large (purplish-blue) wire nuts.

Just for my better understanding, I would like to know why dropping down to a 30 amp breaker will be necessary. What are the risks with maintaining the 40 amp breaker? The only thing I can imagine is that the oven is wired internally in such a way that it cannot handle a sudden surge on the line. That is, the oven is depending on the breaker to trip above 30 amps. Assumming correct wire gauge, the house circuit itself should be OK, which leads me to believe it must be an oven-specific consideration. Why would the oven care? I apologize if I am missing something obvious. I have only a basic understanding of electricity and wiring.
 
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Old 06-08-12, 10:53 AM
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using a special Al Cu wire nut.
There aren't any wire nuts that are safe for protecting Al wire connections, let alone Al/Cu connections. You will need to use the Polaris connectors suggested by PCBoss.

I would like to know why dropping down to a 30 amp breaker will be necessary.
The manufacturer of the oven designed it to draw up to a certain amount of current. If it draws more than that, the reasonable assumption is that something has gone wrong inside it, and it needs to be disconnected from power until the problem can be corrected. The 40A breaker is small enough to protect the conductors feeding the J-box, but not small enough to protect the oven from an internal fault.

Just out of curiosity, can your new oven be plug-and-cord connected? If so, you could mount a receptacle in the wall, connected to the Al wiring, and avoid the whole Al/Cu headache.
 
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Old 06-08-12, 11:17 AM
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When you check the wire size you may not even have a choice if the original installer used #8 aluminum. There are cases where an installer more familiar with copper then aluminum used to small a wire.
 
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Old 06-08-12, 11:22 AM
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Never seen a wall oven that has a cord and plug. Do they have them?
I agree with Nash, size the oven per manufacture specifications. There is much science behind ampacity and protection of conductors and electrical equipment. If you research other sites, you will get all kinds of off-the-wall answers. Many saying it’s ok to install a larger breaker (Note: In some cases you are allowed to size a breaker larger)—as the load want draw over that amount etc etc.. The people misleading others aren’t fully versed on the other affects involved with protecting conductors and equipment. Size it per specifications and code, and be safe.
 
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Old 06-08-12, 12:18 PM
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I will certainly replace the 40 amp breaker with a 30 amp. I'm the type of person though that wants to know why I'm doing the thing that I'm doing! I just wanted to understand the reason behind why the 30 is necessary.

As for installing a plug and receptacle, I guess that's possible, but that requires work on both the oven end and wall end. And I'm committing myself to this same approach for future ovens. This may be more of a headache than the Cu Al connection.

Thanks to all for the comments. I appreciate it!
 
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Old 06-08-12, 12:27 PM
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When you check the wire size you may not even have a choice if the original installer used #8 aluminum. There are cases where an installer more familiar with copper then aluminum used to small a wire.
Ray, can you clarify what "choice" you are referring to? Is this the wire nuts vs. Polaris
connectors issue?
 

Last edited by flatworm; 06-08-12 at 12:29 PM. Reason: clarification
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Old 06-08-12, 12:38 PM
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The manufacturer of the oven designed it to draw up to a certain amount of current. If it draws more than that, the reasonable assumption is that something has gone wrong inside it, and it needs to be disconnected from power until the problem can be corrected. The 40A breaker is small enough to protect the conductors feeding the J-box, but not small enough to protect the oven from an internal fault.

But isn't it the oven's choice, so to speak, to decide how much current it will draw? If for whatever reason the oven decides to draw 31 amps, then the breaker trips and the oven ceases to work. If I had a 40 amp breaker, then the oven will potentially go into a fault mode. Either way, the oven is making a conscious choice to draw current above 30 amps when it shouldn't be. To me, the 40 amp breaker is really there to make sure the circuit is never overloaded, and so any rogue oven that wants to pull more than 40 amps and stresses out my wire needs to be shut off right away. Where is my logic going wrong?
 
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Old 06-08-12, 01:16 PM
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can you clarify what "choice" you are referring to?
A choice of breaker size. Ray is saying that
if the original installer used #8 aluminum.
then the conductors are only rated for up to 30A. The source I'm looking at says that's true for Types TW and UF, but that the other commonly used types are rated for either 40A or 45A.
 
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Old 06-08-12, 01:16 PM
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When you check the wire size you may not even have a choice if the original installer used #8 aluminum. There are cases where an installer more familiar with copper then aluminum used to small a wire.
Ray, can you clarify what "choice" you are referring to? Is this the wire nuts vs. Polaris
connectors issue?

Ray is saying the circuit could have been sized wrong during the initial installation. Meaning: You are forced per code (depending on the year, and type of cable) to size the circuit based on NEC requirements.
 
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Old 06-08-12, 01:35 PM
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Where is my logic going wrong?
A couple of places. One is that the oven can't make a conscious choice about anything. It isn't conscious. It is a tool which you control.

Another is misunderstanding the term "fault." Fault, in electrical terminology, denotes a circuit fault. A ground fault, for example, denotes a condition in which less current is measured on the grounded conductor (the neutral) in a circuit pair than on the ungrounded conductor (the "hot" wire). As those are equal in a properly functioning circuit (under most conditions, but I'll skip the exceptions here), GFCI (Ground Fault Circuit Interrupter) technology has been developed to provide almost-immediate protection by interrupting the flow of power on the ungrounded conductor. We require that protection in those areas where another source of ground is present with which a human might be in contact - bathrooms, kitchens, pools, outside the house, etc.

A circuit fault in a large-draw heating appliance can be a fire in the process of happening. Safety lies in designing and installing the system to prevent that.
 
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Old 06-08-12, 01:44 PM
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@ flat, circuit breakers provide switching and overcurrent protection functions. The overcurrent protection is a key element in electrical safety, protecting conductors from damage due to currents in excess of their ratings and reducing risk of fire, property damage, and personal injury. So, if you decide to use over the required, then you are potentially allowing electrical equipment to operate outside of it’s specifications—which could cause a fire, property damage, or personal injury. I believe Nash said it correct (in part):The 40A breaker is small enough to protect the conductors feeding the J-box, but not small enough to protect the oven from an internal fault. Example (Not the greatest): Lets say you have a small pool with with a max water line of 2.25 feet. A child is in the pool, and the child is only 2.5 feet tall. You decide 2.25 feet is not enough, so you molded an extra 1 foot to the pool, and added water. What happens: as the water started to rise, the child immediately became paronoid—looking for someone to stop the rise of the water. But nooooooooo, you put your own specs on the pool, so it want stop until it reaches an extra foot. After the child sustained damage (or death), the water (breaker) finally stopped. Too late, the damage has been done.
 
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Old 06-08-12, 02:44 PM
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Correction of my original statement. I have always went by the rule of thumb of one wire size larger then copper for aluminum but a quick Google has shown me that may not always be true so my statement that #8 aluminum is good for only 30 amps may not be true depending on cable type, wire size, and temperature rating.
 
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Old 09-16-12, 11:44 AM
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Hi all. I had to put my oven replacement project on hold for a while but I am ready to get this finished now!

Ray and a couple of others had mentioned that I should be wary of my existing aluminum circuit - that it may have been sized wrong during the initial installation. I've pulled the oven out again to look for the existing gauge but could not find any labeling on the house wiring. I've taken a few photos:

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This is the junction box behind the oven.

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Here is an existing copper-aluminum connection.

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Here is a measurement of the width of the existing aluminum. I measured it at 3/16"

This is about all the data I can easily gather on the wiring behind the oven. Where should I go next to determine if this wiring is safe? Is there another place I should look to find the wiring info? The house was built in 1968 in Boulder, CO
 
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Old 09-16-12, 01:05 PM
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You should be able to see a cable size on the outside sheath of the cable near the panel.

You will need to use either split bolts or a Polaris connector on the copper to AL connections. Those wire nuts are not for use with AL.
 
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Old 09-16-12, 01:12 PM
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That wiring isn't safe, but not necessarily because it's run in aluminum from the panel to the J-box. Rather, it's because there are direct Cu-Al splices "protected" with wire nuts.

There is no way to directly connect copper wires to aluminum wires safely. As a separate issue from conductor size, breaker size, circuit size, etc., you need to make these connections with isolating connectors. AlumiConn is one example.
 
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Old 09-16-12, 02:16 PM
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The Alumi-conns are only for the smaller #12 and #10 AL.

AlumiConn | KingInnovation

The OP will need split bolts or Polaris.
 
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Old 09-16-12, 02:49 PM
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I will use a Polaris connector when I wire in the new wall oven. Right now, the circuit is not live and will not be live until the new oven is installed.

I could not locate a cable size printed on the outside sheath. I just don't have enough cabling visible. Is there a way to tell by the 3/16" diameter?
 
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Old 09-16-12, 03:39 PM
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What size breaker is installed?
 
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Old 09-16-12, 03:50 PM
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A 40 amp breaker is currently installed. If you look back at some of the other posts to this discussion, a few folks (ray2047) mentioned that I may have no choice but to install a 30 amp breaker if I have 8 gauge aluminum. To me, it looks like 6 gauge or less aluminum, but that is based solely on it's measured diameter with a tape measure.

Shy of the labeling on the wire sheath, is there any way to know for sure?
 
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Old 09-16-12, 03:54 PM
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You could compare it against a piece of wire of a known size.
 
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Old 09-16-12, 04:14 PM
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The Alumi-conns are only for the smaller #12 and #10 AL.
Oh well. Learn, or wither on the vine.
 
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Old 09-16-12, 04:18 PM
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Shy of the labeling on the wire sheath, is there any way to know for sure?
The wires are in a cable. Look for the information on the cable jacket.
 
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Old 09-16-12, 04:25 PM
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It's #6AL which is ok on a 40A breaker.. But the whip coming out of the oven is #10Cu, which is only rated for 30A. That's why you have to drop the breaker to 30A.


But I can't believe y'all missed what really makes that junction dangerous - the EGC and the neutral nutted together!!

Flatworm, you can NEVER EVER EVER have the ground and the neutral connected together like that in a junction box. The two only come together at the main service disconnect, and must be separate everywhere downstream.

Your best option is to just put a 4 prong 30A cord and plug on that. The cord won't be as troublesome as you think, as there is normally an access panel on the back of the oven which exposes the terminal block.
 
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Old 09-16-12, 05:04 PM
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The OP will need split bolts or Polaris.
I wouldn't use a split bolt for this. Besides putting the CU and the AL in direct contact they require a lot of taping to properly insulate (I use rubber tape, then friction tape, then Scotch 33+ for a split bolt).

Polaris connectors, I've read here before, are bulky. If so, I'm wondering if four small clear taps, such as the Ilsco ClearChoice connectors, might fit better. Same principle: Solid mechanical and electrical connection without placing the CU and AL in direct contact with each other, and they are also insulated, like the Polaris connectors, so no need for taping in either case.

That's if the OP would prefer cleaning up the splicing over installing a receptacle.
 
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Old 09-16-12, 06:50 PM
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@JerseyMatt: The ground / neutral connection is the way I found the existing oven wired when I pulled it out. If you look towards the beginning of this thread, you will see that this issue has already been addressed.
I did not wire it that way! All I can do is blame the previous owner - I will not be repeating that mistake. Why do you advocate a plug over just hard wiring?
 
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Old 09-16-12, 06:59 PM
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I wouldn't use a split bolt for this. Besides putting the CU and the AL in direct contact they require a lot of taping to properly insulate
They may not be right for this application, but aluminum to copper split bolts are an acceptable means of splicing aluminum conductors to copper conductors and don't put the aluminum and copper in direct contact; the separator keeps that from happening. The entire split bolt connector is usually made of aluminum, but it is tin plated and acceptable for copper contact.

Morris Products 90424 Split Bolt Connector With Spacer,Dual Rate For Copper and Aluminum Conductors,350 AWG,350 -350 Max Run To Max Tap,1/0 - 4 Min Run To Min Tap,350 - 4 Max Run To Min Tap,650inlb Torque: Amazon.com: Industrial & Scientific
 
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Old 09-17-12, 02:41 PM
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Originally Posted by flatworm View Post
@JerseyMatt: The ground / neutral connection is the way I found the existing oven wired when I pulled it out. If you look towards the beginning of this thread, you will see that this issue has already been addressed.
I did not wire it that way! All I can do is blame the previous owner - I will not be repeating that mistake. Why do you advocate a plug over just hard wiring?
I understand that, I didn't mean to imply that you did it, I was just making sure you knew that it was improper and dangerous so you didn't take it as the way you should install your new one.

Basically I say install the cord and plug as a proper way to eliminate the aluminum to copper connection. With the exception of Polaris connectors and Copalum (which can only be done by an electrician who has been certified by Tyco), there is no safe way to do a copper to aluminum joint in a high-current circuit. And considering that the four Polaris connectors are going to run you about double what a pigtail and a receptacle are going to cost, cord and plug is the least expensive option for you.
 

Last edited by JerseyMatt; 09-17-12 at 03:20 PM.
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Old 09-17-12, 02:47 PM
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pcboss: I couldn't reply to your PM.. I know you connect the appliance neutral and ground together if the feed is 3 wires. But look at the pic again - the feeder is 6/3+G. There are four wires going into one nut - neutral and ground from the feeder, and neutral and ground from the oven whip.. That is dangerous in more ways than one.
 
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Old 09-17-12, 03:10 PM
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I see the white now JM. Thanks.
 
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Old 09-17-12, 03:50 PM
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aluminum to copper split bolts are an acceptable means of splicing aluminum conductors to copper conductors and don't put the aluminum and copper in direct contact; the separator keeps that from happening.
I've seen split bolts with that spacer before and didn't "get" why it was there. Thanks, CJ, a new one for the virtual toolbox.
 
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Old 09-17-12, 06:06 PM
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Hey all - I just wanted to give a big thank-you to all who gave me advice. The uninitiated like me really appreciate it!
 
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Old 09-26-12, 07:25 PM
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Hi all. I got my new oven and have a couple of questions about the wiring. Take a look at the pictures.

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This is a closeup of the red wire and the grounding wire from the replacement oven.

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This is a view of the 4 wires coming from the replacement oven.

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This is the labeling on the black wire coming from the replacement oven.

I was wondering if anybody knows what the two hots and neutral are made of? The ground wire is clearly made of copper - but the other three appear to be made of something else. Do you know what that material is and does it affect how I need to wire the oven to the aluminum wire in the junction box? With copper, I planned on using the Polaris connectors. But are these copper?
 
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Old 09-26-12, 07:32 PM
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I was wondering if anybody knows what the two hots and neutral are made of? The ground wire is clearly made of copper - but the other three appear to be made of something else. Do you know what that material is and does it affect how I need to wire the oven to the aluminum wire in the junction box? With copper, I planned on using the Polaris connectors. But are these copper?
Those wires have stranded copper conductors that have been tinned for durability/strength after the insulation was stripped. Treat them as copper.
 
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Old 09-26-12, 08:16 PM
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That said, then I will still look into the Polaris connectors. I've never worked with these connectors before. Can someone tell me which type of Polaris connector to buy? I believe I have 6 gauge AL wire coming into the junction box and 12 or 14 gauge CU coming from the oven. A specific part number would be great. Also, any pointers to instructions that explain how to install the connectors would be helpful also.
 
  #38  
Old 09-27-12, 06:53 AM
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Sorry, make that 16 gauge wiring coming from the oven.
 
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Old 09-27-12, 09:32 AM
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Nevemind. I called Polaris and got it figured out.
 
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