Why a cable is overheating?

[vallesj]

I just noticed the box where my solar cables enter the garage for my Aoi system looks like over heated.
Opened it and couldn't find anything abnormal, the cables come from the panels and here is not any connections, it's just the cables. They are 10 AWG and the Max current is less than 8A. Measured the cables at the panels as well inside the garage and they weren't as warm as on this corner spot.







It looks like the black one is the warmest there.
I pulled a little bit the red one, and tried to move a little the cables, after some minutes the area cooled down. My system has a year old and is the first time I noticed the box on this condition, very evident.

So what could it be? A pinhole? I didn't remove the corner plastic conduit to observe more in detail if a scratch on the cable. Maybe later today.

 

[skyking1]

kinked and damaged. A few broken strands are impossible to detect visually. It is odd they are that hot with that load.

 

[wpns]

It looks charred on the outside, but it might be just discolored, is it black on the inside as well?

 

[cs1234]

What do we think that PVC box is rated for, 60 to 75 C ?

 

[Tulex]

Physical cause seems odd if it's been contained and running for a year with no issue.

 

[Glock24]

Maybe the jacket is perced somewhere and humidity corroded the cable.

 

[Hedges]

How long is wire to the indoor side? If it lands on a terminal a short distance away and has poor contact, that heat source would conduct back through the wire.

45C vs. 37C, a gradient, tells me which way thermal energy is flowing.

Wires in lugs seem tight, but after a season of temperature changes causing it to expand and contract (or causing plastic conduit to do so), the wire wiggles and gets loose. So I wiggle in rotating direction, tighten more, repeat.

 

[RCinFLA]

Is the 8 amps actually measured on wire with clamp-on amp meter or a reported number by inverter? If actually measured on wire also check AC current reading with meter.

You could have AC leakage current to ground via a conductive leakage path from PV pos line and frame of PV panel.

Many AIO high frequency inverters are not galvanically isolated from inverter AC output to PV +/- terminals. Both pos and neg PV terminals must not have significant leakage to frame ground of panels or there will be AC leakage current from AC output of inverter.

This would be detected if inverter has ground fault detection on PV lines. Not all HF inverters do.

 

[LydMekk]

This isn't alu cable?

 

[robbob2112]

One side note - this is one reason the NEC requires all conduit with PV wires inside it are made from metal when in or on a structure.. Won't melt or distort like PVC will. Especially when it looks like it is 6 inches from a gas line --

Most gas lines are required to be at least 12" from any electrical wires unless inside metal conduit

Is this the sunny side of the house? Meaning does that box get sun and also heat from the wires? that combination alone might account for the melty bits... And in that situation I would put small spacers under the box to allow for airflow... doesn't take much.

If that continues as PV for any length inside the house it should be switched to metal.

If just the other side of that wall is the inverter then I am with @Hedges on it being a bad connection at that end....

But with the way it looks like the cables were cork-screwed in there I am actually also thinking internal wire damage is a high possibility.

note - all of these are just theories and none are critiques of what was done...

 

[skyking1]

that's a soldered copper line, most likely a TPR line from the water heater.

 

[Backyard]

Your box looks only burned or darkened at the outside top. Hard to see but the inside doesn't look chard. I'd expect if the heat source was from the inside it would show much more damage and burning then the outside. And because the darkening doesn't extend down the side where the connector is, I'd say the burning was on the exterior

Is there by chance a time of day when the sun is at just the right angle where a window, solar panel or other reflective source can concentrate sunlight down on top of that box? I've see something similar where we have a window with a slight concave shape and at a certain time of day during summer you can stand in the right location and feel the intense heat. Step on foot over and nothing.

 

[OhYou_]

it's a red herring. That the wires are warm is just a coincidence.
Grey PVC stuff does that when exposed to the sun for years.
Even the melting of the box is from the sun, do you live in Nevada or Arizona or something with giga-hot summers?

10awg at 8amps will get warm with no air cooling(in a conduit). That is why ratings exist.

 

[fmeili1]

This was how my PVC looked (before I've repaired it) after just one summer in the Mohave desert... maybe it's just "PVC sunburn"...

 

[vallesj]

Thanks for the feedbacks! I had the cables just crossing the wall without any conduit, so I will install a metal one. There's no gas line at all, the copper is just for the water heater over pressure valve. The wall is on the east side, and just a few feets from fence, so the sun is mostly in the morning, despite is only black outside, it is curious that it gets like that with just mornings sun for a year. I measured the cables temperature inside, just like a feet before a connector, and another feet to the inverter, they weren't warm neither the connectors. It's interesting the theory of the grounds, but I would expect the heat on other area rather than just on this area of the cables.

 

[vallesj]

I finally was able to replace the PVC box with a metal one. No damage on cables, and the temperature I measured was because the sun in the morning. Measured the PVC box of the optical fiber Internet on the same area, and similar temperature on top of it




The charred look is only outside meaning it was the sun. Cheap PVC box from Amazon, it came with 2 boxes, the other box was below my solar tracker, so no sun on it and looks fine.


Thanks for letting me know about NEC requirement for metal conduit when going inside a structure, below is the replacement for the PVC box.


Thanks God the issue was not related to the cabling meaning a potential fire risk.

 

[robbob2112]

If you spray paint that shiny silver and grey conduit white it will cool it off considerably -- The grey is an ok color for absorbing solar heat, but white reflects a lot of it.