# Thhn Insulation Melting



## dronai

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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## Cow

52 amp x 125%=65 amps. #6 minimum if no derates.


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## Southeast Power

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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## dronai

Cow said:


> 52 amp x 125%=65 amps. #6 minimum if no derates.


I think it said 46 Amps on the nameplate


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## dronai

Southeast Power said:


> 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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## Bird dog

dronai said:


> 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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## dronai

Bird dog said:


> 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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## Bird dog

dronai said:


> 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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## dronai

Bird dog said:


> 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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## anthonysolino

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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## Bird dog

How was the wire melted? Maybe a failing termination?
Maybe a fluke? (no pun intended)


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## dronai

anthonysolino said:


> 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



Bird dog said:


> 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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## anthonysolino

ideally you should get a reading on a 480v motor of .3 megohms on your meter for a passed test,


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## dronai

anthonysolino said:


> 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



Bird dog said:


> 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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## Bird dog

Strange...anything out of the ordinary happen like lightning or could something else fault through the motor...any welding?


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## dronai

Bird dog said:


> 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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## Bird dog

dronai said:


> 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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## dronai

Bird dog said:


> 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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## anthonysolino

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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## dronai

anthonysolino said:


> 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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## anthonysolino

Tetrafluoroetheylene had to find the spelling lol, I keep a roll of black red white green number 10 on my van, I use it for like kitchen equipment that gets extremely hot, my street lights that are HPS get super hot, the insulation can handle up to like 260C, it does cost a lot but ive found it lasts and never burns up in a situation like there where there might be constant high current- high heat being applied to the wire, the insulation can handle it,


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## Bird dog

anthonysolino said:


> Tetrafluoroetheylene had to find the spelling lol, I keep a roll of black red white green number 10 on my van, I use it for like kitchen equipment that gets extremely hot, my street lights that are HPS get super hot, the insulation can handle up to like 260C, it does cost a lot but ive found it lasts and never burns up in a situation like there where there might be constant high current- high heat being applied to the wire, the insulation can handle it,


That would be overkill. Would also need special terminations. Also known as hi temp wire, nichrome(?).


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## anthonysolino

Ive never used special terminations for it, it comes in all sized my street lighting I use those black high temperature wire nuts, ( the ones that come with the ballast kit that every one throws away to use their own because they think they are the "cheap kind") lol they're designed to be used in a high temperature environment, when I do like steam tables, I use it there also, you will notice when they come from the factory, they have these wires coming off the knobs and for all the heating elements because it holds up and lasts, im a huge fan of the stuff, its kind of expensive yes, but if the environment calls for it, use it. it almost looks like that good ole, cloth wiring were all used to seeing y'all probably have seen it countless times, I just get city electric to order me a few rolls of it, it comes in handy, you can charge a premium for it, and it ends up quality and lasts forever,


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## micromind

If the operator doesn't like his job, he may very well have applied some sort of torch to the conduit.........


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## macmikeman

I know, I know, Teacher please let me answer this one......



It was from harmonics.....................:devil3:


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## dronai

micromind said:


> If the operator doesn't like his job, he may very well have applied some sort of torch to the conduit.........


Nah, He's a nice guy, Maybe the graveyeard shift lays on the pressure.

This was liquidtight from the enclosure to the motor. Not a blister in that


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## micromind

dronai said:


> Nah, He's a nice guy, Maybe the graveyeard shift lays on the pressure.
> 
> This was liquidtight from the enclosure to the motor. Not a blister in that


Rats......another perfectly good theory shot pieces.........

I think 8s are too small for a 40HP, you did the right thing; 6s will hold just fine.


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## 460 Delta

If this is a grinding machine to refinish rolls for mills or something such, a chart recorder logging current might be a good idea. You may be shocked to find an operator is doing a too heavy hogging pass and a light cleanup. If he is smart he may have figured out how to jumper the overloads or jam the reset button in so it cannot trip.


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## splatz

You can rule out it wasn't melted from some past incident that nobody checked?


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## 99cents

This is really weird. I have seen insulation melt but that was on short circuit with a breaker. By the time the breaker reacted, the insulation had bubbled and burned. You would think that a fuse element would have a lower melting point than nylon insulation.

If the wire is undersized, I could see the insulation weakening and discolouring over time but melting is something different.

Those are just my thoughts but, then again, I’m just a dumb pipe kinker and wire yanker  .


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## HackWork

dronai said:


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


There is no set temperature. 70A sounds awfully low to melt the insulation off of #8, however, if that 70A was allowed to flow for hours it just might. 

In connector tests that I have seen it take some time for 100A to melt the insulations off of #12 wire.


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## 99cents

splatz said:


> You can rule out it wasn't melted from some past incident that nobody checked?


It does seem like a one time, separate event to me. Considering the safety factor built into wire sizing calcs, I don’t see how #8 would melt and #6 wouldn’t.


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## HackWork

99cents said:


> It does seem like a one time, separate event to me. Considering the safety factor built into wire sizing calcs, I don’t see how #8 would melt and #6 wouldn’t.


I completely agree. If the insulation is melting off of #8, then the insulation on #6 would still be getting severely damaged int he same situation.


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## 99cents

HackWork said:


> There is no set temperature. 70A sounds awfully low to melt the insulation off of #8, however, if that 70A was allowed to flow for hours it just might.
> 
> In connector tests that I have seen it take some time for 100A to melt the insulations off of #12 wire.


The insulation does have a set melting temperature but I know what you’re saying. Here’s how I see it: If you overdrive the conductor, you’re turning it into a heating element. If you don’t reach the melting temperature of the insulation, it won’t melt. The characteristics of the insulation may change (discolouration, etc.) but what bothers me is that it actually melted. I can’t see #8 melting even if it is undersized. Besides that, the fuses blew.

I’m thinking there was a separate event aside from motor operation.


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## Cow

dronai said:


> No, it's an old Nema starter, and I did check the numbers on the heaters. How do you test that ?





Bird dog said:


> 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?)


Are the heater elements those bimetallic style that start with a B---?

What is the part number for the heater elements?

I've seen A LOT of starters over the years with the overloads sized/set to 125% of the motor nameplate, even though they should be sized to 100% of the nameplate since they are set to trip at 125%. Nobody usually notices the error until later, when something burns up. Sometimes years down the road.

It sounds as though this machine has the potential to be overloaded by operator input fairly easy, so it's definitely worth verifying it's correct in my opinion.


EDIT: A CC87.7 bimetallic looks like it could be 1-3 sizes over the size it should be for a 46 amp nameplate motor depending on which column you should be using on this chart:

https://www.google.com/url?sa=t&rct...Brochure.pdf&usg=AOvVaw0r-vtDAPf9kR3YbtDMffQL

Take a look at the bottom of this pdf. It mentions how to do some of the sizing for Sq D's overloads.


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## Wirenuting

Are the overloads the correct trip class for the machine?
A class 30 will allow an operator to bear down and grind like heck long enough to melt the wires over time and yet not trip the starter. 

Our machine shop here did that with our bigger grinder, 10hp. The operator had leaned over the years just how long he could go before tripping. 
All internal wires were high temp, except from the newer starter. They melted in the whip. 
The original grinder wiring had a contactor inside the base. When that died 20+ years ago a starter was mounted on the wall. The grinder is from the 1940’s.


https://www.gt-engineering.it/en/Insights/nema-classes-vs-ansi-tripping-curves


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## Wirenuting

dronai said:


> No, it's an old Nema starter, and I did check the numbers on the heaters. How do you test that ?


An older Square D? 
Tap it with a screw driver and Klein’s. lain:
Don’t try it unless you have a spare on the truck.


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## John Valdes

Dorian. Do you remember my story about bypassing my electric meter with number 10 some years ago?
Ran the complete apt for two weeks with zero wire damage. Thats with range, HVAC ect....
I think number 8 could stand many more amps than 70. Way more.


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## macmikeman

Wirenutting nailed this. The repeated bogging down of the load is probably the answer. I recently was amp checking a box manufacturing operation for a moving company. They build big hauling boxes onsite for moving furniture and the like and are pretty much one time use so these guys build seventy or eighty big boxes a day. The table saws were tripping often, not a voltage drop problem. It turned out to be the operators- they would bog the saw blades during ripping plywood sheets constantly. Blades would stop. #10 wires were melting. Sometimes the breakers didn't trip till stuff started smoking. Would not have mattered if I ran #6 to the saws, the operators are the problem. They are under pressure to hurry all the time. Very hard to communicate to them, they are all from an island chain out there where there are French Electricians and no speaky Englisky.


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## just the cowboy

Was it a surface Grinder? One that would take a pass, lift the head, move the table back, drop the head then repeat. These would do that to motors and wire because they allowed the heater and motor to cool between passes on heavy cuts. Think of starting the motor over and over, only motor don't stop just the heavy cut acts like starting current.


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## CoolWill

macmikeman said:


> Wirenutting nailed this. The repeated bogging down of the load is probably the answer. I recently was amp checking a box manufacturing operation for a moving company. They build big hauling boxes onsite for moving furniture and the like and are pretty much one time use so these guys build seventy or eighty big boxes a day. The table saws were tripping often, not a voltage drop problem. It turned out to be the operators- they would bog the saw blades during ripping plywood sheets constantly. Blades would stop. #10 wires were melting. Sometimes the breakers didn't trip till stuff started smoking. Would not have mattered if I ran #6 to the saws, the operators are the problem. They are under pressure to hurry all the time. Very hard to communicate to them, they are all from an island chain out there where there are French Electricians and no speaky Englisky.


Racist.


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## CMP

*SqD Heater Elements Sz3*



dronai said:


> 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



NEMA heater tables already include the service factor of 1.15 for 1.15 service factor motors, you are supposed to size them for the motor nameplate value or deduct 15% for 1.0 service factor motors.


Table for a Size 3 starter, using CC type overloads, remember that the table values show nameplate values, but give you another 15% as standard.


CC87.7 ultimate trip point is 61.41 with the 15% included.


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## dronai

Cow said:


> Are the heater elements those bimetallic style that start with a B---?
> 
> What is the part number for the heater elements?
> 
> I've seen A LOT of starters over the years with the overloads sized/set to 125% of the motor nameplate, even though they should be sized to 100% of the nameplate since they are set to trip at 125%. Nobody usually notices the error until later, when something burns up. Sometimes years down the road.
> 
> It sounds as though this machine has the potential to be overloaded by operator input fairly easy, so it's definitely worth verifying it's correct in my opinion.
> 
> 
> EDIT: A CC87.7 bimetallic looks like it could be 1-3 sizes over the size it should be for a 46 amp nameplate motor depending on which column you should be using on this chart:
> 
> https://www.google.com/url?sa=t&rct...Brochure.pdf&usg=AOvVaw0r-vtDAPf9kR3YbtDMffQL
> 
> Take a look at the bottom of this pdf. It mentions how to do some of the sizing for Sq D's overloads.





CMP said:


> NEMA heater tables already include the service factor of 1.15 for 1.15 service factor motors, you are supposed to size them for the motor nameplate value or deduct 15% for 1.0 service factor motors.
> 
> 
> Table for a Size 3 starter, using CC type overloads, remember that the table values show nameplate values, but give you another 15% as standard.
> 
> 
> CC87.7 ultimate trip point is 61.41 with the 15% included.
> 
> 
> 
> View attachment 138752


Thanks, I didn't know that you are supposed to use the nameplate value without adding 125%. I didn't have time today to look again at those heaters, and verify the number. Do recall 87.5 I thought, but the chart looks like it's 87.7 I thought the boss said it should be protected at 52A and that it was right. Definitley looks oversized on the chart above


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## dronai

Cc 81.5


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## CMP

For your future knowledge, read the sizing information, at the bottom of the charts, in the link given previously. It's good info to keep in your knowledge base, for all NEMA starters.


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## varmit

My guess would be some combination of problems to cause the wire to melt. 
Possibilities:
Overloads oversized
Overloads bypassed
Excessive loading of the motor
Previously damaged insulation allowing a high resistance fault to the raceway
Mechanical problem(s) with motor or machine
Motor starting under high load repeatedly
Ground fault on ungrounded system

There are always other odd things that no one thinks of to cause you grief.

There is never a dull day fixing old dilapidated industrial equipment. Some of the junk, that I repair, should have been thrown away years ago.


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## micromind

Any chance of a chemical attack? Like cleaning solution or something along those lines?


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## John Valdes

Anyone up for an experiment?


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## just the cowboy

I do know from experience from when I was young and dumb that #10 THHN will melt almost right away and fill a pickup truck cab with smoke if shorted across a car battery with no fuse. I had a set of fog lights I hooked up too the wrong side of a fuse block, the wire went thru the firewall and shorted to it from rubbing. I had two girls in the front seat when it shorted and the truck filled with smoke and stopped running, you should of heard them screaming FIRE. Me I reached down and ripped the wire out, burnt my hand it was so hot, all in about 15 seconds.


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## 99cents

just the cowboy said:


> I do know from experience from when I was young and dumb that #10 THHN will melt almost right away and fill a pickup truck cab with smoke if shorted across a car battery with no fuse. I had a set of fog lights I hooked up too the wrong side of a fuse block, the wire went thru the firewall and shorted to it from rubbing. I had two girls in the front seat when it shorted and the truck filled with smoke and stopped running, you should of heard them screaming FIRE. Me I reached down and ripped the wire out, burnt my hand it was so hot, all in about 15 seconds.


Two girls? That’s greedy. You don’t believe in sharing?


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## Gnome

just the cowboy said:


> I do know from experience from when I was young and dumb that #10 THHN will melt almost right away and fill a pickup truck cab with smoke if shorted across a car battery with no fuse.



Could have been a couple hundred amps on that wire. Current basically limited by the resistance of the short to the body.


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## brian john

dronai said:


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


Thermal OLs are crap tested many and had many fail.


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## gpop

99% of the time ive seen the wires melt together in a non metallic flex it due to a overheated ground wire. Got to love working with welders.


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## Southeast Power

I think I read every post and didn’t see one response questioning the distance or possible voltage drop.


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## joe-nwt

How long of a run to the motor? Possibly a voltage drop problem?


:biggrin:


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## dronai

Southeast Power said:


> I think I read every post and didn’t see one response questioning the distance or possible voltage drop.






joe-nwt said:


> How long of a run to the motor? Possibly a voltage drop problem?
> 
> 
> :biggrin:


Nah on the voltage drop, 480V 40hp #6 tray cable I recall, at maybe 100'

The grinding wheel has a long lever with lots of leverage, that the operator uses to apply pressure to grind the ingot. I watched him, and there is an amp meter for him to watch what he's doing. 
I think some of these guys on another shift just lean on that thing, and almost lock the rotor.
Once I replaced the 8 with 6, no issues since.


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