derating questions

[james_mcquade]

Hi all,

i was at a site last week in which the wiring in the conduit melted.
asking questions as one would i told them that you had to derate the ampacity of the wire due to the 14 conductors (#4).
derate due to the heat and that they also had to use the 70 deg c wire rating due to the lugs and breaker rating.
they didn't like my answer. they put the thhn wire back in and let it go, then the wire melted again.

turns out that they removed several feet of earth that covered a pipe to allow for molten slag to flow better.
the air temp comming out of the pipe was 200 degrees.

#1 did i miss state something?
even thhn wire cannot withstand temperatures in the 300 plus range and i told them that. they said thats what the conduit was for, to protect the wire.

also, i also read on another post that the 90 deg column was for wire derating only.
i thought that if you had 90 deg breaker, lugs and wire you could use that column to do your derating,
but if your lugs, breaker, or wire was rated 70 deg, then use that column?
Would someone please clearify these items.

thanks,
james

 

[suemarkp]

Only thing you mis-spoke was 70C. The temp columns in the code are 60C, 75C and 90C most commonly. Never seen a 70C.

With 14 conductors in a raceway, what termination temperature you can use isn't going to be relevant because you'll have so much derating just due to bundling.

But if that raceway only had a few conductors, you could use the 90C rating at the devices if the lugs are rated for it. Wire nuts and many splices are 90C rated, so you could splice on a larger gauge pigtail if needed near the breaker or other terminations. I've never seen 90C lugs on a breaker. I've seen some lugs stamped AL9CU that are 90C, but I believe the whole assembly has to be rated for 90C and you just can't throw in some random 90C lug and be good to go.

Raceways are protection from damage (abrasion, snagging) not from heat...

 

[Jraef]

...

Raceways are protection from damage (abrasion, snagging) not from heat...

And in fact, they TRAP heat and THAT's why you have to de-rate for fill!

Sounds like "they" just want to keep justifying their mistakes by ignoring not only the rules, but the REASONS why the rules are the rules!

 

[augie47]

You didn't mention the load, but with the numbers you quote it's almost irrelevant.
90?C insulation would be, I believe, 194?F so much above that and you would have insulation failure at zero load.
(14) #4 THHN in a conduit would have a derating factor of 50%, a 159?F ambient would call for a 0.41 multiplier on a THHN conductor so you are looking at your #4 being good for about 20 amps if the ambient didn't exceed 176?F.

 

[charlie b]

turns out that they removed several feet of earth that covered a pipe to allow for molten slag to flow better.
the air temp comming out of the pipe was 200 degrees.

I don't understand this part. When you say "pipe," do you really mean "conduit"? If so, then what to what does "molten slag" refer, and why would then want to make it easier for the molten slag to flow? And how would removing the dirt above a pipe (or conduit) impact the ability of a fluid to flow? This part just doesn't sound electrical to me, so I don't know how it impacts the installation. Is there a hot plumbing pipe nearby the electrical conduit that had the melting wires? Is the 200 degree measurement related to the electrical conduit, or is that from a plumbing pipe?

I think there must be more to the situation than you have described so far. A #4 wire is normally good for 85 amps. So if the circuit has an 80 amp breaker, and if because of conduit fill and ambient temperature, the ampacity is (as Gus suggested) closer to 20 amps, I am not convinced that that would be enough to cause the insulation to melt. I wonder if the conduit is too small for the number of wires installed in it. That would make the situation worse. Also, did the breaker(s) trip on the circuit(s) installed through this conduit?

 

[Jraef]

I don't understand this part. When you say "pipe," do you really mean "conduit"? If so, then what to what does "molten slag" refer, and why would then want to make it easier for the molten slag to flow? And how would removing the dirt above a pipe (or conduit) impact the ability of a fluid to flow? This part just doesn't sound electrical to me, so I don't know how it impacts the installation. Is there a hot plumbing pipe nearby the electrical conduit that had the melting wires? Is the 200 degree measurement related to the electrical conduit, or is that from a plumbing pipe?

I think there must be more to the situation than you have described so far. A #4 wire is normally good for 85 amps. So if the circuit has an 80 amp breaker, and if because of conduit fill and ambient temperature, the ampacity is (as Gus suggested) closer to 20 amps, I am not convinced that that would be enough to cause the insulation to melt. I wonder if the conduit is too small for the number of wires installed in it. That would make the situation worse. Also, did the breaker(s) trip on the circuit(s) installed through this conduit?

I interpreted his "molten slag" comment to be sarcasm, meaning the idiots thought that by simply putting less dirt on top, the wire would not melt, which proved to be wrong as he said. Maybe I was wrong though.

In theory you could get 14 x #4 THHN conductors in 2" RGC. I wouldn't want to, but it is technically possible. Was it smaller than that?

 

[james_mcquade]

Sorry for the confusion.

the plant I am talking about melts steel into ingots, i-beams, rods, plates. slag is the waste product created on top of the steel as it is melted and
Is dumped off into a pit before another process can take place.

I did mean conduit instead of pipe.

what happened was the electrical contractors buried an electrical conduit and wiring under the drainage area for the steel slag pit.
and this depth has worked for years.
the plant upgraded the melter to a larger unit which created other problems.
the solution was to dig down several feet of earth in order to let the slag flow out of the way.
now, the earth which was not heated previously has melted scrap steel on it and is heating the conduit.

I found out a little while ago that the conduit has had to be re-routed and several people have had to explain things to the plant mgr.

thanks for your comments.
I needed to make sure I wasn't making incorrect assumptions.

james.

 

[eHunter]

In the words of an immortal American...
"And now you know -- the rest of the story."
"Paul Harvey ... Good Day!"