Burned Marettes

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  #1  
Old 06-20-13, 09:19 PM
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Burned Marettes

Air conditioning unit went down 2 days ago. Repairman came tonight I thought for sure it was the compressor but after fiddling for a few hours we discovered that it's the 24VAC that feeds the controller that wasn't getting any voltage. Repairman left and I started troubleshooting on my own and I found 2 marettes completely burned and disintegrated to ashes in the J box all that is left is the copper cone inside the marette. Here's some pix, not the best quality but it's quite dark in there and there's not much to see anyways. My question is what the heck happened? Short or a surge?
 
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  #2  
Old 06-21-13, 12:08 AM
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We would need to know exactly what that box is feeding. It does not look like 24 volt wiring.

It looks like it may be the 120 vac that feeds the air handler/furnace.
What size breaker is feeding that junction box ?

It would appear that there is a heavy load connected there and I have a feeling the circuit may be overfed (to big a breaker)

If that was a short.....a properly protected circuit would trip the breaker.
Surge...kind of an iffy word. Lightning surge.....possible but doubtful.
 
  #3  
Old 06-21-13, 05:49 AM
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Hard to see much detail in the pics but another issue I have seen is a loose wire nut connection that starting arcing. Arcing created heat which melted the wire nut and wire insulation pretty far back. Didn't throw the breaker, noticed the initial problem because the digital thermostat lost power and the unit stopped working.

--edit-- this was on a 10+ year old unit and it took that long to fail...
 
  #4  
Old 06-21-13, 09:04 AM
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That doesn't look like 24V control wiring, and 24V AC is not likely to melt down a wire nut and fry insulation - at least not as likely as 120V or 240V is. The likely cause appears to be a loose splice similar to the problem Ward experienced.

Check the wire size against the breaker size and the load size, as PJ suggested. If all of that checks out, repair the damaged insulation with good quality tape or liquid tape or heat shrink. Strip the wires so that +5/8" of bare conductor is showing and make a good tight splice - at least three clockwise twists, starting with the ends of the insulation even. Trim the tip of the splice and protect it with a new wire nut (Marette). Make sure that no bare copper is showing at the back of the wire nut. Fold the wires into the box and position them so that the tip of the wire nut is pointing upward. Cover this box and continue troubleshooting.

all that is left is the copper cone inside the marette
Tech Note: Wire nuts have a steel threaded cone or spring inside - not copper. Hard to tell when it has been well-burned, though.
 
  #5  
Old 06-21-13, 07:45 PM
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I think it goes 120VAC stepped down to 24VAC The breaker is 15A. I was just reading about wires getting lose in marettes and arcing.
 
  #6  
Old 06-21-13, 08:58 PM
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I think it goes 120VAC stepped down to 24VAC
As PJ implied earlier, this looks like 120V wiring, and the damage looks like 120V damage. The control wiring and voltage should be created somewhere downstream from here.

The breaker is 15A.
Then the wire here needs to be #14 AWG or larger.

I was just reading about wires getting lose in marettes and arcing.
Splices are made by twisting wires together. Wire nuts are used to protect splices, not to create them.

Tech note: While not all "fridges" are made by Frigidaire, all Marrette® wire nuts are made by Thomas&Betts. It's a corporate brand name.
 
  #7  
Old 06-21-13, 09:01 PM
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Tech Note: Wire nuts have a steel threaded cone or spring inside - not copper. Hard to tell when it has been well-burned, though.
Older wire nuts had copper springs.
 
  #8  
Old 06-21-13, 09:03 PM
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The breaker being 15A is good.

Loose wires can only arc if there is a load running thru them. A small load will usually not cause that kind of damage. If that's a 15A circuit then there is something connected to it that loads the circuit pretty heavily. A plug in electric heater or a blow dryer would cause the wires to burn like that.

That box looks to be 120vac only. That 120v may feed a transformer somewhere down the line but just a 24v transformer would not cause that to happen.
 
  #9  
Old 06-21-13, 09:06 PM
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Older wire nuts had copper springs.
Really? The oldest ones I've seen - the black Bakelite ones from the '30s - have steel coils.
 
  #10  
Old 06-22-13, 07:35 AM
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Splices are made by twisting wires together. Wire nuts are used to protect splices, not to create them.
I have to disagree with this staement. 3M states on their instructions that the wires may be twisted or not twisted before applying a 312 connector. A splice is required to have mechanical pressure which is provided by the zinc plated steel spring in the wire connector. If a splice was made just by twisting, we'd still be twisting and taping splices (which was common at one time).
 
  #11  
Old 06-22-13, 09:03 AM
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If a splice was made just by twisting, we'd still be twisting and taping splices (which was common at one time).
But you need to solder after twisting, then use rubber tape and wrap friction tape over the rubber tape.

http://www.doityourself.com/forum/el...uts-not.html#b

http://www.doityourself.com/forum/el...uestion.html#b

http://www.doityourself.com/forum/el...re-nuts.html#b

http://www.doityourself.com/forum/el...uestion.html#b
....and many more.
 
  #12  
Old 06-22-13, 09:55 AM
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3M states on their instructions that the wires may be twisted or not twisted before applying a 312 connector. A splice is required to have mechanical pressure which is provided by the zinc plated steel spring in the wire connector.
3M can say whatever they want. When you've failed an inspection because one of the wires came out of the wire nut "splice" when the inspector tugged on them, you learn to twist the wires into a splice and trim it before you put the wire nut on it.

Especially when the inspector, if he or she decides to be cute, schedules the re-inspection for two days later and your daily payroll on the job is maybe $5,000 and you need everybody there to do the grunt work of checking every @#E$%^ splice in the building.

That said, there are some splices I twist and some I don't twist. I always twist same-size and close-to-same-size solid-to-solid splices (#12 to #10, for example). Trying to twist a #18 or #20 solid fixture wire with a #12 circuit wire is frustrating at best, and usually counterproductive. And I've learned that twisting stranded conductors into a "rope" and then twisting that to a solid conductor seems less effective than untwisting the stranded conductor, laying it against the solid conductor, and letting the wire nut make the splice.

The wire-nut-only "splice" may work better with vermicelli (#14) than with real spaghetti (#12). It seems like it might but IDK. I've never tried it. But the above is why I insist that splices are made by twisting the conductors together in most cases, especially for DIYers who aren't experienced at lining up the wires where the insulation ends, or testing the splice by pulling on the wires, or any of the other tricks we pick up with experience.

If a splice was made just by twisting, we'd still be twisting and taping splices (which was common at one time).
Probably not. Wire nuts are not only faster, they also provide much better mechanical and electrical protection than most tapes do. We don't solder splices anymore either, or make them in the open, and both of those were standard practice at one time.

I do still tape a splice every once in a great while, and I carry three tapes to do that with. When a couple of conductors are spliced with a split bolt, they get a double wrap of rubber tape, covered by a double wrap of plastic tape - preferably Scotch® Super 33+™ from 3M - covered by a double wrap of friction tape. It's expensive and time-consuming to do, but it's sometimes preferable to using a cold tap when space is tight.

I have to disagree with this staement.
That's OK. This is one of the ways we keep each other on our toes.

Note to general readers: The day you don't catch a couple of electricians "arguing" about the "better" or "proper" way to do something, or the "better" or "proper" way to interpret the code, is probably the day when their only assignment is to wait for the inspector to show up to walk them through for final inspection - and they're 99.44% sure they've nailed the work. Or one of them is asleep.

Take it with a grain of salt. Read it for the insights and information it contains. Laugh if you want to - no problem. It's another part of learning about doing this work in the safest and most effective manner. At least, it can be.


ECHO, ECHO, Echo, echo, [SUB]echo...[/SUB] Ray typed faster than I did.
 
  #13  
Old 06-22-13, 02:12 PM
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My point is that a wire nut is not just to protect a splice, but an integral part of the splice. A wire nut not only provides the required mechanical compression for a tight connection, but also protects the splice, no taping needed.
 
  #14  
Old 06-27-13, 02:15 PM
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Somehow the connections inside the wire nut became loose, or oxidized in a fashion that the conductors no longer made good contact with one another. Then it does not take very many amperes of current flow to cause severe overheating. Eight or nine amps may have been enough to burn things up as you have now.

Wire nuts that don't require that the wires be twisted have the inside spring made of a square steel wire instead of round wire. The inside of the spring then bites into the conductors. You must not have so many conductors inside that one or more are "in the middle" and not bitten into by the spring.
 
  #15  
Old 06-27-13, 06:51 PM
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Allan, I think you're serious. If so, this is interesting, but I'm not sure how useful it is.

Let me tell a quick story as an example. I was asked to go help out a fellow foreman so that we could get his job finaled and make our final draw. What he needed me to do was install the devices - all the switches and receptacles. There may have been 5 or 6 different devices when you added up the 3-way and 4-way switches and all of the special receptacles, which all had to be put where the plans called for them. Double duplex (quad) boxes and multi-gang switch boxes with a mix of switch types in each, as well as single device boxes.

It took me exactly 10 8-hour days to install the 800 devices. the foreman whined or grumbled a bit about half-way through because he had budgeted for 100 devices per day, not 80, but that was as fast as I could to get them done, even though I was trying to pick it up all the way through.

In part, it was because I wasn't accustomed to doing devices in production mode. In part, it was because he hadn't protected the boxes before the finishers came through. Not only were the wires painted, most of the boxes were partially mud-filled. Filled-in holes for the device mounting screws were more common than not.

Can you imagine what my pace would have been if I'd taken the time to assess whether or not I actually had to twist the wires or not for each splice? As soon as I could excavate the box, re-tap the mounting holes and clean the wires enough to see the colors, it was strip 'em, trim 'em, cap 'em, mount 'em, cover 'em and move on.

I make up splices the way I described in post #13 because it minimizes both the time required and the likelihood of failure. To me, a wire nut is the best piece of material available to protect a well-made splice. And I'm also realizing, as I write this, that I particularly want to impress this on DIYers who don't do this work every day.

An ounce of prevention, etc.
 
  #16  
Old 06-27-13, 06:51 PM
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Although the zinc plated steel spring is a conductor, it is not intended nor is it designed to be a load carrying part of the circuit. If it does become a part of the circuit and is carrying load, it would be somewhat normal for it to overheat and burn up. This happens a frequently when aluminum and copper wires are joined with a wirenut. The whole idea of mechanical pressure onj the conductors is to keep them held tightly together so the path of the current travels directly from one conductor to the other.
 
  #17  
Old 06-27-13, 07:59 PM
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most of the boxes were partially mud-filled. Filled-in holes for the device mounting screws were more common than not.
I know EXACTLY what you're talking about. I was on a similar job recently and I made a comment to the job super on his finishing crew. He just gave me a shoulder shrug. It's a complete lack of craftsmanship.
 
  #18  
Old 06-28-13, 11:01 AM
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I know EXACTLY what you're talking about. I was on a similar job recently and I made a comment to the job super on his finishing crew. He just gave me a shoulder shrug. It's a complete lack of craftsmanship.
Thank you for the sympathy, PJ!

It is. Our foreman, however, had not protected the boxes nor made an agreement with the finishers and painters about protecting them, so the fault was partially ours. Given that, and since it wasn't my job, I only commented to the foreman we had on that job. VERY frustrating though.
 
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