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My dishwasher has a heavy wire (hard to bend, not stranded) coming out of the wall into a junction box, and then connects to my dishwasher.

The wire from the wall has heat-damage:

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With corresponding damage to the dishwasher cover:

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The junction box looks like this:

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And has a warning about temperature like this:

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Now, my question is:

Did the wire heat up, and damage the dishwasher, or did the dishwasher heat up and damage the wire?

How could the wire get so hot? (The warning on the junction cover mentions 75 deg Celsius. Is that normal for a dishwasher wire?) How many Amps are we even talking about here?)

I am really confused that a thick copper wire like that could even heat up? Isn't the rule that the thicker the wire, the less heat?

NOTE: The dishwasher is a Bosch SHS63VL5UC and it gets fed hot water, so I don't even expect it to draw that much power?

I am in Canada, on 110V.

UPDATE

The black gunk does come off with alcohol, but the discoloration remains. The outer mantle was not penetrated.

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    Get some isopropyl alcohol and try wiping the burn mark as if it were a stain. Does it wash off like a stain? Jan 24, 2022 at 5:00

2 Answers 2

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It looks to me that the dishwasher’s case gets hot due to drying and the hot water. Certainly when I open my dishwasher after it’s completed a cycle, it and the dishes are steaming hot.

But here’s the thing: that wire is the wrong wire (and it isn't clamped correctly either). You can’t use nylon sheathed wire (called NM or sometimes “romex”) in that way— where it comes out of the wall and into the dishwasher junction box. You have to use armored cable — in Canada we usually call it AC90 for armored cable rated at 90 degrees Celsius. If the correct armored cable were in place, you wouldn’t have the melting of the wire’s nylon sheath.

Hire an electrician to replace the wire with the correct type and secure it so that it’s not resting or running against the dishwasher.

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  • Thanks! Can this armour be applied retroactively? Like add a metal sheath over the plastic one?
    – Bram
    Jan 24, 2022 at 5:32
  • @Bram no, but I’ve seen licensed electricians do it. You can’t put NM cable in conduit of any sort. Jan 24, 2022 at 5:50
  • Thanks. Wow. So the builder screwed up big time, then. I wonder how the electrical permit got though? This is the original wiring from a 2012 build in Vancouver. Was the inspector sleeping?
    – Bram
    Jan 24, 2022 at 5:56
  • @Bram inspectors are human and besides making straightforward mistakes, they often see something wrong that’s relatively minor and they look the other way. I recently had an electrical inspector pass me without realizing that something was wrong with what I had done. Jan 24, 2022 at 5:59
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    @Bram I believe you are wrong about NM inside conduit, at least in the US. AFAIK, you just can't put NM in outdoor or buried conduit (because those are considered "wet" locations). You do have to increase the conduit size vs. conduit for individual wires.
    – DoxyLover
    Jan 24, 2022 at 7:30
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That's not melting. Least not from the wire.

That is chemical staining due to interaction between the Romex jacket and the insulation material, possibly fueled by heat from the dishwasher but unlikely from the Romex. (but possibly the two added together; NEC 310.15 has a table for that.)

That's why I suggested wiping it off with isopropyl alcohol (or vodka). With a stain, 90% of the contaminant will wipe off. With thermal damage nothing would wipe off but a little bit of the char.

I've seen it before, where Romex leans against tar-paper or insulation vapor barrier. Definitely not thermal there. It's a chemical reaction between Romex sheath and certain materials.

But you do have a thermal problem.

In fact, the labeling calls it out.

NM cable, or common Romex, is rated at 60C thermal for normal current carrying (but 90C thermal is used for certain "too many wires in a bundle" type overheat calculations). NEC 334.80.

Your labeling calls for 75C wire. Every modern wire type is 75C or 90C - most are 90C - except for NM and UF, the only 60C orphans.

Most of the 75/90C wires need to go in conduit. So a flex metal conduit is an option. However you can also grab any type of cable that isn't NM or UF - so, AC/MC cable would work, which is armored metal cable. You need to use "redhead" grommets to protect the wires from the nasty sharp cut edge, and you need connectors to go into that 1/2" knockout.

You could also use flexible metal conduit (FMC) again with connectors made for FMC, and then run THHN individual wires through them. Black white green. This will seem prohibitively expensive only because of the ludicrous "small-reel" prices at the big-box stores. Buy it by-the-foot at better stores... preferably a competitive lumber yard, but even a boutique hardware store will be the laughable small-reel prices at the box store.

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