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Old 12-09-2019, 09:03 PM   #1
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Default Thhn Insulation Melting

How many amps would cause the insulation on #8 thhn to melt ? I figured it would make it to about 70A.

A 40HP 480V motor shorted, and we thought the motor was fried, we meggered and it, and was good. But we found the wire from the control panel was #8 had melted !

This is on a Grinding machine, that the operator can increase the resistance by adding more pressure on the Metal when grinding.

The Ol's were sized properly, but didn't trip ??? wth ?

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Old 12-09-2019, 09:17 PM   #2
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52 amp x 125%=65 amps. #6 minimum if no derates.
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Old 12-09-2019, 09:29 PM   #3
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Motors are interesting, I tend to up size motors that are designed to be loaded on the heavy side of their capacity.

We have a project with some RTUs that require 108 MCA, I’m going to use #1 copper for them.
In a previous like, I would have used #2 aluminum.
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Last edited by Southeast Power; 12-09-2019 at 09:36 PM.
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Old 12-09-2019, 09:44 PM   #4
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52 amp x 125%=65 amps. #6 minimum if no derates.
I think it said 46 Amps on the nameplate
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Old 12-09-2019, 09:46 PM   #5
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Motors are interesting, I tend to up size motors that are designed to be loaded on the heavy side of their capacity.

We have a project with some RTUs that require 108 MCA, I’m going to use #1 copper for them.
In a previous like, I would have used #2 aluminum.

Yeah, I'm definitley going to upsize anything that's questionable.
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Old 12-09-2019, 09:52 PM   #6
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Originally Posted by dronai View Post
How many amps would cause the insulation on #8 thhn to melt ? I figured it would make it to about 70A.

A 40HP 480V motor shorted, and we thought the motor was fried, we meggered and it, and was good. But we found the wire from the control panel was #8 had melted !

This is on a Grinding machine, that the operator can increase the resistance by adding more pressure on the Metal when grinding.

The Ol's were sized properly, but didn't trip ??? wth ?
Did you test the OL ckt?
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Old 12-09-2019, 10:00 PM   #7
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Did you test the OL ckt?
No, it's an old Nema starter, and I did check the numbers on the heaters. How do you test that ?
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Old 12-09-2019, 10:03 PM   #8
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No, it's an old Nema starter, and I did check the numbers on the heaters. How do you test that ?
IDK. Try undersizing the the heaters just to see them drop the coil out.


edit...Was the breaker sized properly? How old is it? (will it trip?)
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Last edited by Bird dog; 12-09-2019 at 10:11 PM.
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Old 12-09-2019, 10:19 PM   #9
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IDK. Try undersizing the the heaters just to see them drop the coil out.


edit...Was the breaker sized properly? How old is it? (will it trip?)
This control panel has fuses, and yes the fuses went out. The wire was defintely undersized, but I just thought the insulation would handle at least 60A before melting. I think the OL's should have tripped at about 52-56

SQ D 87.7 I think they were
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Old 12-09-2019, 10:24 PM   #10
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I Have been told several times from wire manufactures that the ampacity of these wires are over engineered and that the table has to be followed but the wire typically can hold a lot more than was is required, I will say how ever when you placed your meg ohmeter on the motor what was the reading you received? is there maybe any weird conditions the wire has been subject to? is there perhaps any thing else on the circuit or is it in an hot environment? any other wires in the conduit?
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Old 12-09-2019, 10:25 PM   #11
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How was the wire melted? Maybe a failing termination?
Maybe a fluke? (no pun intended)
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Old 12-09-2019, 10:30 PM   #12
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I Have been told several times from wire manufactures that the ampacity of these wires are over engineered and that the table has to be followed but the wire typically can hold a lot more than was is required, I will say how ever when you placed your meg ohmeter on the motor what was the reading you received? is there maybe any weird conditions the wire has been subject to? is there perhaps any thing else on the circuit or is it in an hot environment? any other wires in the conduit?
Motor megged fine. No, and No

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How was the wire melted? Maybe a failing termination?
Maybe a fluke? (no pun intended)
Together in places, like a lot of heat. No terminations were good. Insulated taps in the pecker head
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Old 12-09-2019, 10:31 PM   #13
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ideally you should get a reading on a 480v motor of .3 megohms on your meter for a passed test,
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Old 12-09-2019, 10:32 PM   #14
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I Have been told several times from wire manufactures that the ampacity of these wires are over engineered and that the table has to be followed but the wire typically can hold a lot more than was is required, I will say how ever when you placed your meg ohmeter on the motor what was the reading you received? is there maybe any weird conditions the wire has been subject to? is there perhaps any thing else on the circuit or is it in an hot environment? any other wires in the conduit?
Motor megged fine. I figuered the same. No, and No

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How was the wire melted? Maybe a failing termination?
Maybe a fluke? (no pun intended)
Together in places, like a lot of heat. No terminations were good. Insulated taps in the pecker head
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Old 12-09-2019, 10:37 PM   #15
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Strange...anything out of the ordinary happen like lightning or could something else fault through the motor...any welding?
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Old 12-09-2019, 10:40 PM   #16
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Strange...anything out of the ordinary happen like lightning or could something else fault through the motor...any welding?
Don't think. like I said, I think the operator could have applied too much pressure driving the amperage up, but he does have an amp gauge to watch for this.
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Old 12-09-2019, 10:46 PM   #17
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Don't think. like I said, I think the operator could have applied too much pressure driving the amperage up, but he does have an amp gauge to watch for this.
Could it be possible the operator runs the machine in such a manner that it heats up the wire without the heaters tripping? Oversize the wire &/or undersize the heaters.
In other words, the wire cools slower than the heaters.
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Old 12-09-2019, 10:49 PM   #18
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Could it be possible the operator runs the machine in such a manner that it heats up the wire without the heaters tripping? Oversize the wire &/or undersize the heaters.
Rewired with #6, done
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Old 12-09-2019, 11:02 PM   #19
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if that doesn't work you can try to wire it up with TFE conductors I wire my street lighting up down here with it, it helps for high heat type situations,
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Old 12-09-2019, 11:06 PM   #20
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if that doesn't work you can try to wire it up with TFE conductors I wire my street lighting up down here with it, it helps for high heat type situations,
TFE never tried it
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