Lets see here, what does not look right.
Overpacked conduit, sharp edges on cable, maybe some over heating caused by dense packed wires, or maybe over current.
The insulation seems a little rough, so they maybe a little old style.
Slag on the base of the conduit, could be arcing, or leftover from welding.
Seems to be clean around the conduit, was it swept, or natural blown?
Open weep holes, likely nothing, but not positive.
I've seen sharp conduit edges cut through many a conductor before. When they are first installed, the cable sheath comes out of the conduit, as it should, but over time the cable shield gets pulled down, and your conductors end up being exposed to the sharp edges on the conduit.
First thing that stands out to me is over filled conduit. If the load is high enough on each conductor they will over heat bundled like that. Understanding conduit fill and de-rating are essential.
Agree with mbrooke, looks like long-term thermal effects on the insulation. W-a-y too many conductors in that conduit.
Agree with comments about the general state of the installation too - something was going to go wrong, it was just a case of which fault happened first.
@cranky: this was the first view so no one cleaned the place.
We had the same conclusion, overpacked conduit, so it was just a metter of time after construction for the failure to happen and the fire that destroyed many small market stores. We found other cables still intact but damaged by sharp edges of other conduits, so it was just a matter of time...
On reconstruction we will change many design details and will supervise the construction as the codes state. Thanks for your observations!
Miniature Circuit breakers do not open based on the temperature of conductors, they open based on current. What protects those wires from over heating are de-rating and conduit fill. Which boils down to fewer circuits per conduit, and up-sizing the wire need be. This is not an electrical issue, its a thermodynamics issue.
There was overheating for a while, but once the insulation broke there was a short circuit and the slag shows that. You are right cranky, those holes should had never been left open, that won´t happen again
The code does not require sealing of weep holes. Punch outs, yes. But none the less if the environment is such things are getting inside you might want to consider a different NEMA enclosure. If someone can get conduit fill wrong good chance they got the enclosure wrong as well.
Derating according to the code that I work to.
The maximum size breaker allowed for #14 AWG wire is 15 Amps.
For 25 or more conductors the ampacity is 60%
A 20 amp breaker would protect a wire rated at 20/.6 = 33.3 Amps.
That would be a #10 AWG at 75 C. The ampacity of a #14 wire at 75 C will be 12 Amps.
For 43 or more conductors the ampacity is 50%
A 20 amp breaker would protect a wire rated at 20/.5 = 40 Amps.
That would be a #8 AWG at 75 C.
For 25 or more conductors the ampacity of #14 AWG is 12 Amps
For 43 or more conductors the ampacity of #14 AWG is 10 Amps
Bill
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"Why not the best?"
Jimmy Carter
.... So, this kind of breakers don´t detect arc faults and that´s why they didn´t protect this installation? The new change in the CODE requires AFCI to be installed in most 15 and 20 A circuits:
