Did wrong size wire cause fire or loose terminals?

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carnes51214

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We were called out to a school where a Air handler disconnect had started melting. This is actually the 2nd disconnect that had this problem. Here are the facts. There is #6 wire ran to J-box, then changes to #8 wire thhn to disconnect then down to furnace unit. The furncace unit has 10kva heat strips. The max fuse rating is 60amps. It operates on one leg being 120v and a high leg around 200v. So 240v air handler. I know that #8 is good for 50 amps if using the 75 degree column for the terminals. There were 60 amp fuses in disconnect. Would the smaller wire have caused the fire.
Also when the maintence man pulled out the pull out on disconnect on of the stab blades from pull out was left in the disconnect because it was melted off. So one leg still connecting possibly.
So my question is what could have caused the fire. It was cold that morning around freezing. The first disconnet that had this problem I replaced last year, I just diagonsed the issue as loose terminals. This year it was fine no sign of melting. So what could be the problem. This installation was about 7 years ago.
 
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DARUSA

Senior Member
Location
New York City
What is the the full load amps showed on the nameplate of the unit?
Is this unit lacated on the roof?
Are you considered the derating ampere for roof temperature?
How did you splice the 6 and the 8 wire? Wire-nuts ?
Assuming that the unit is outside you should use a THHW or THHW-2
If im not wrong THHN is rated 60deegres in a damp location.
 
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BILLY101

BILLY101

Member
Location
Telford, Pa
Is this 3 phase 208/120 volt system feeding the furnace or is 240/120 volt?
How many 10 KW heaters?
What feeds the fan?
What is breaker size protecting the feeder?
What is on rating plate on the furnace?
One example would be 2- 10Kw heaters @ 240 volts is 83 amps. If this is true the entire feeder is undersized. And that doesn't include the fan.

BILLY
 
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augie47

augie47

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Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
without the other data (fan, etc) we don't know the actual load, but I would be hard pressed to think the #8 was the cause of any fire in this situation.
You have an actual heater load (assuming actually 9.6kw) of 40 amps + the fan motor...
I would be more suspect of poor connection on the pull-out type disconnect.
Either way it turns out as a good example of how a Code violation can be
a likely culprit in any liability situation.
 
Cavie

Cavie

Senior Member
Location
SW Florida
My moneys on loose conections of the pull out blades. Never did like pull disconects of any kind. Fused or not. Never installed on on any job I had control of.
 
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suemarkp

Senior Member
Location
Kent, WA
Occupation
Retired Engineer
I think your wire is marginal. Strip heaters are continuous loads, so you need 125% of 10KW. That gets you right near 50A. Now you add the fan and controls. I would run #6 copper, as the larger wire will help carry some heat away from the disconnect. Is there an MCA nameplate value on the air handler?
 
jumper

jumper

Senior Member
Location
3 Hr 2 Min from Winged Horses
DARUSA said:
What is the the full load amps showed on the nameplate of the unit?
Is this unit lacated on the roof?
Are you considered the derating ampere for roof temperature?
How did you splice the 6 and the 8 wire? Wire-nuts ?
Assuming that the unit is outside you should use a THHW or THHW-2
If im not wrong THHN is rated 60deegres in a damp location.
Click to expand...

Assuming THHN is a typo. THHW is 75 degrees in a wet location.
 
Dennis Alwon

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
Regardless the #8 didn't melt the disconnect as Gus stated and I agree with Cavie- bad connection as the culprit. Although probably not compliant the #8 would work fine. If anything it would more likely melt the insulation on the conductor rather than melt the disconnect but I even doubt it would do that.
 
roger

roger

Moderator
Staff member
Location
Fl
Occupation
Retired Electrician
I lean towards the loose connections too, but location of the disconnect and the conductors acting as heat sinks could be a possible contributor. As mentioned earlier, it would be nice to know the FLA and / or minimum circuit ampacity, I'm not sure the #8 is wrong with out more information.

Roger
 
reddy kw

reddy kw

Member
Location
Mudville, upstate New York.
carnes51214 said:
We were called out to a school where a Air handler disconnect had started melting. This is actually the 2nd disconnect that had this problem. Here are the facts. There is #6 wire ran to J-box, then changes to #8 wire thhn to disconnect then down to furnace unit. The furncace unit has 10kva heat strips. The max fuse rating is 60amps. It operates on one leg being 120v and a high leg around 200v. So 240v air handler. I know that #8 is good for 50 amps if using the 75 degree column for the terminals. There were 60 amp fuses in disconnect. Would the smaller wire have caused the fire.
Also when the maintence man pulled out the pull out on disconnect on of the stab blades from pull out was left in the disconnect because it was melted off. So one leg still connecting possibly.
So my question is what could have caused the fire. It was cold that morning around freezing. The first disconnet that had this problem I replaced last year, I just diagonsed the issue as loose terminals. This year it was fine no sign of melting. So what could be the problem. This installation was about 7 years ago.
Click to expand...

Replace AWG 8 with AWG 6. Eliminate the splice and run all new wire, #6 all the way. Use high-quality disconnect switches, not cheapo pullouts. Here's why: Second service call for same problem? At a SCHOOL?! The SAFETY OF THE OCCUPANTS is more important then saving a few bucks on materials. Explain the danger of the existing installation to customer. Tell them they need to do this the right way to protect the lives of the children. If the customer wants it done cheap, not right, WALKING AWAY is better then being responsible for an unsafe job that could result in property damage and death. The NEC is only the minimum requirement. It's not against the law to do a job BETTER THEN CODE REQUIRES.
 
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