Correct wiring method? Wires overheating and burning up!

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The short version- 108- #8 THHN cu wires, 6-#8 THHN cu gnd(480v), 18-#10 THHN cu, 6-#10 THHN cu gnd (120v control) in a 6" x 6" metalic wireway.

The long version- These wires feed from a control cabinet containing heating coil contactors and controls TO (6) electric duct heater sections; each section is 157kw which is broken down into (1) 120v control circuit & (6) 480v 3 phase power feeds (26.17kw ea.) All wiring is in a 6" x 6" wireway (metal).

The wires are burning up inside the wireway; at the 90 bend. I believe this installation is not in acceptance to NEC 300.34 or NEC 300.20 :confused:
 

kingpb

Senior Member
The conductors are taking up about 13% of the area of the raceway.

400.34 does not apply as it is for OVER 600V.

The raceway is considered ferrous, so 300.20 applies but you say the conductors are burning up, which could be different than the conductors heating the surrounding surface by induction.

You say it's heating at the 90 bend, I would venture to guess you have exceeded the allowable bending radius of the manufacturer.
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector
300.34 is for over 600 volts... is this ?
I don't see where 300.20 is applicable.

In less I'm overlooking something, 376.22 should apply and you would need to derate. Unsure of the layout, but if any cross-section has more than 30 conductors and it sure looks like it would, the you are at 35% of the 310.16 ampacity or less than 20 amps for your #8s.
May not be the reason, but sure looks suspicious.
 

iwire

Moderator
Staff member
Location
Massachusetts
The short version- 108- #8 THHN cu wires, 6-#8 THHN cu gnd(480v), 18-#10 THHN cu, 6-#10 THHN cu gnd (120v control) in a 6" x 6" metalic wireway.

The long version- These wires feed from a control cabinet containing heating coil contactors and controls TO (6) electric duct heater sections; each section is 157kw which is broken down into (1) 120v control circuit & (6) 480v 3 phase power feeds (26.17kw ea.) All wiring is in a 6" x 6" wireway (metal).



Do I have this right

108 - 8 AWG copper making up 36 three phase circuit with each one carrying about 31.25 amps (26.17KW/480/1.73)

It seems like you have a serous derating issue right there without even including the other circuits.

2008 376.22(B) applies to wireways, but you are well past 30 current carrying conductors.

(B) Adjustment Factors. The adjustment factors in
310.15(B)(2)(a) shall be applied only where the number of
current-carrying conductors, including neutral conductors
classified as current-carrying under the provisions of
310.15(B)(4), exceeds 30. Conductors for signaling circuits
or controller conductors between a motor and its starter and
used only for starting duty shall not be considered as
current-carrying conductors

Looking at 2008 310.15(B)(2)(a) we see that anything more than 41 current carrying conductors together gets a .35 derating.

Under those conditions 8 AWG CU THHNs are only rated to carry 19.25 amps each.

If the conductors are running heating equipment with a load of 31 amps you would need at least 4 AWG and I think maybe a 3 AWG.

The wires are burning up inside the wireway; at the 90 bend.
Literally or figuratively?
 
Do I have this right Yes

108 - 8 AWG copper making up 36 three phase circuit with each one carrying about 31.25 amps (26.17KW/480/1.73)

It seems like you have a serous derating issue right there without even including the other circuits.

2008 376.22(B) applies to wireways, but you are well past 30 current carrying conductors.




Looking at 2008 310.15(B)(2)(a) we see that anything more than 41 current carrying conductors together gets a .35 derating.

Under those conditions 8 AWG CU THHNs are only rated to carry 19.25 amps each.

If the conductors are running heating equipment with a load of 31 amps you would need at least 4 AWG and I think maybe a 3 AWG.



Literally or figuratively?
Literally
 
300.34 is for over 600 volts... is this ?
I don't see where 300.20 is applicable.

In less I'm overlooking something, 376.22 should apply and you would need to derate. Unsure of the layout, but if any cross-section has more than 30 conductors and it sure looks like it would, the you are at 35% of the 310.16 ampacity or less than 20 amps for your #8s.
May not be the reason, but sure looks suspicious.
Thank you, this helps
 
The conductors are taking up about 13% of the area of the raceway.

400.34 does not apply as it is for OVER 600V.

The raceway is considered ferrous, so 300.20 applies but you say the conductors are burning up, which could be different than the conductors heating the surrounding surface by induction.

You say it's heating at the 90 bend, I would venture to guess you have exceeded the allowable bending radius of the manufacturer.
Is my understanding of 300.20 that the "grouping together" of the phase conductors and grounds are...all A phases grouped, all B phases grouped...etc?
Because currently it is one mass of wires tie wrapped together.
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
Is my understanding of 300.20 that the "grouping together" of the phase conductors and grounds are...all A phases grouped, all B phases grouped...etc?
Because currently it is one mass of wires tie wrapped together.
Grouping wouldn't apply if all of the conductors are within the same raceway since they are essentially all grouped together.
 

kwired

Electron manager
Location
NE Nebraska
Is my understanding of 300.20 that the "grouping together" of the phase conductors and grounds are...all A phases grouped, all B phases grouped...etc?
Because currently it is one mass of wires tie wrapped together.
If all conductors of each circuit are contained within the same wireway and are in close proximity to one another you have met 300.20 requirements. If each circuit were to have its the conductors of that circuit bundled together you may have a little better installation but I think you have a serious deration issue.

Have you ever measured the operating temperature of this installation when it is at its highest loading?

Insulation will break down first then at some point you will haveone conductor fault to another someplace that may rapidly spread to others once the arc flash starts since insulation is probably deteriorated and exposed conductors are everywhere.
 
If all conductors of each circuit are contained within the same wireway and are in close proximity to one another you have met 300.20 requirements. If each circuit were to have its the conductors of that circuit bundled together you may have a little better installation but I think you have a serious deration issue.

Have you ever measured the operating temperature of this installation when it is at its highest loading?

Insulation will break down first then at some point you will haveone conductor fault to another someplace that may rapidly spread to others once the arc flash starts since insulation is probably deteriorated and exposed conductors are everywhere.
Temperature inside the control panel is 180 degress F measured. Thanks for clarification on 300.20
 

kwired

Electron manager
Location
NE Nebraska
Temperature inside the control panel is 180 degress F measured. Thanks for clarification on 300.20
That is about 82?C. I'm suprised you are not having trouble with what is most likely 75?C terminals too.

I bet the temperature in the center of the bundles of conductors gets a lot higher.

Properly derated conductors should bring the temperature down some.

IMHO this is a very poorly designed installation.

Insulation is failing at the bend because that is where the insulation is being stretched from the bend and it is operating at or above the temperature rating of the insulation. This point is weaker because of the stretching imposed on it by the bend and is the first point to fail. It is not necessarily any hotter than other points along the conductor.
 

Cow

Senior Member
Location
Eastern Oregon
Yep, you definitely need to do something different!!

Whether it's several gutters, tiered cable tray, runs of conduit, etc. Somebody needs to be paying attention to 310.15(B)(2)a when this trainwreck gets redesigned.
 
When rewiring, you can divide the raceway

When rewiring, you can divide the raceway

If the conductors are burning up and failing, you must be rewiring the installation.
We often install in excess of 30 current carrying conductors in raceway (gutter), and comply with code by installing barriers withing the wireway. You can divide the 6x6 into (3) 2 x 6 raceways by adding barriers. Then rewire it and treat it as 3 different raceways and keep it under 30 current carrying conductors per "slot". The wire fill in each slot must remain less than 20%. Without a sketch, it's hard to tell where each of the 6 heater feeds drop out of the raceway, but you can probably make it work.

Also, the worst possible way to install the conductors is to "bundle" them with tie wraps. That guarantees the conductors in the middle of the bundle will build up excess heat. Install them without bundling.
 
Oops-you'd need to divide into 4 slots

Oops-you'd need to divide into 4 slots

Considering the 108 power carrying conductors (and ignoring the #10 control wires) you would need 4 slots, not 3. This can still be done, but a 1.5" x 6" slot (9 sq. in. x .20 allows 1.8 sq. in usable cross section) seems pretty small to fit 27 #8 (plus the proportante #10) into, but it is legal. 48 #8 THHN would be legal as far as 20% fill calc. You have said how hard it would be to add a 2nd 6x6 divided in half, but I would if at all possible. Do not bundle the conductors.
 

kwired

Electron manager
Location
NE Nebraska
If the conductors are burning up and failing, you must be rewiring the installation.
We often install in excess of 30 current carrying conductors in raceway (gutter), and comply with code by installing barriers withing the wireway. You can divide the 6x6 into (3) 2 x 6 raceways by adding barriers. Then rewire it and treat it as 3 different raceways and keep it under 30 current carrying conductors per "slot". The wire fill in each slot must remain less than 20%. Without a sketch, it's hard to tell where each of the 6 heater feeds drop out of the raceway, but you can probably make it work.

Also, the worst possible way to install the conductors is to "bundle" them with tie wraps. That guarantees the conductors in the middle of the bundle will build up excess heat. Install them without bundling.
Although I see what you are doing and agree that it will be code compliant, I am curious about how much heating effects actually change. You still have the same conductors that should be giving off the same heat located within the same space.

The barriers could provide some heat sinking effects but ultimately they will still have to transfer that heat to the outer enclosure that was already there to dissipate whatever heat it was sinking in the first place.

Not bundling or smaller bundles would help with temperature developed inside the bundle(s).

If OP did this it may be code compliant but is there any gurantee he will not still have heating problems within the wireway?
 

boxs21

Member
by reading this thread, is that mean wire have limited electric transfer? I thought all are the same, but I am thinking why a lot of wire size in market
this is new knowledge for me since I am blink for electric think
 

John120/240

Senior Member
Location
Olathe, Kansas
by reading this thread, is that mean wire have limited electric transfer? I thought all are the same, but I am thinking why a lot of wire size in market
this is new knowledge for me since I am blink for electric think
Yes, AWG wire size, ambient tempature of the installation, length of run from

source to load, will all affect the conductors ampacity. Not really sure what your

question is ?
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
I am betting the real problem is the bundling. Cut the tie wraps and open the bundle up, especially at bends where the insulation is stressed anyway.

that will likely cool it off some until you can fix it properly.
 
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