Heating mat problem

McLintock

Senior Member
Location
USA
Occupation
Electrician
We just installed 2 25’ heating mats for a steal roof. Took AMP reading before they installed on roof, read 3.5 Amps. So we tied them both together on a 12 Awg wire. Turned it on today, as we got snow and ice last night, the breaker will trip after about 2 hours and it resets fine and will last another couple hours.

You guy have any thoughts?

It’s 240v, 20amp GFPE Breaker


“ shoot low boys their riding shetland ponies”
 

hhsting

Senior Member
We just installed 2 25’ heating mats for a steal roof. Took AMP reading before they installed on roof, read 3.5 Amps. So we tied them both together on a 12 Awg wire. Turned it on today, as we got snow and ice last night, the breaker will trip after about 2 hours and it resets fine and will last another couple hours.

You guy have any thoughts?

It’s 240v, 20amp GFPE Breaker


“ shoot low boys their riding shetland ponies”
Top of my head see NEC 2014 section 424 outdoor snow melting equipment but of course wait what others say here
 

SceneryDriver

Senior Member
Location
NJ
Occupation
Electrical and Automation Designer
The heating mats are likely self-regulating. They have a PTC (positive temperature coefficient) heating element material that has a lower resistance as temperature falls. This allows the mat to self regulate and create less heat as the ambient temperature rises. What temperature did you measure your 3.5A at? If warmer than when the snow covered the mats, they're likely working as designed. You likely need another circuit to handle the increased power at lower ambient temperatures.


SceneryDriver
 

kwired

Electron manager
Location
NE Nebraska
The heating mats are likely self-regulating. They have a PTC (positive temperature coefficient) heating element material that has a lower resistance as temperature falls. This allows the mat to self regulate and create less heat as the ambient temperature rises. What temperature did you measure your 3.5A at? If warmer than when the snow covered the mats, they're likely working as designed. You likely need another circuit to handle the increased power at lower ambient temperatures.


SceneryDriver
I'm guessing this is the problem. Maybe if this is a fixed mat vs a cable that would have rating that varies depending on it's length, it should have a max rating for the assembly that apparently wasn't taken into consideration when adding the second mat to the same circuit?
 

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Electrical Contractor
I am assuming you connected the mats in parallel at the start of the mats rather than a daisy chain. Is that correct?

If so then it seems like there is an issue with the install where there is some leakage that shows up when heated for some time. Hell to trouble shoot. Can you disconnect one set and see what happens
 

McLintock

Senior Member
Location
USA
Occupation
Electrician
The heating mats are likely self-regulating. They have a PTC (positive temperature coefficient) heating element material that has a lower resistance as temperature falls. This allows the mat to self regulate and create less heat as the ambient temperature rises. What temperature did you measure your 3.5A at? If warmer than when the snow covered the mats, they're likely working as designed. You likely need another circuit to handle the increased power at lower ambient temperatures.


SceneryDriver
Very possible, it was about 40 degrees out yesterday, but why pop the breaker then?

I tested it in the garage, 60 degrees,but the mat was warm the hour it was wired up.

What is frustrating is no paperwork whatsoever with the product, manufacturer says “ just hook it up to 20 amp GFPE Breaker

Here’s the junction of the 2 10’ mats and a 5’ section, my thought is the THHN wire needs to be protected, not just under the metal roof.

Not sure about this product yet, and boss just told me we have to wire up more on another house.


“ shoot low boys their riding shetland ponies”
 

kwired

Electron manager
Location
NE Nebraska
Very possible, it was about 40 degrees out yesterday, but why pop the breaker then?

I tested it in the garage, 60 degrees,but the mat was warm the hour it was wired up.

What is frustrating is no paperwork whatsoever with the product, manufacturer says “ just hook it up to 20 amp GFPE Breaker

Here’s the junction of the 2 10’ mats and a 5’ section, my thought is the THHN wire needs to be protected, not just under the metal roof.

Not sure about this product yet, and boss just told me we have to wire up more on another house.


“ shoot low boys their riding shetland ponies”
But if one mat draws more than 10 amps when cold enough that will put you over 20 if you connect two to the circuit. That is a lot of VA though unless this is a pretty large mat. No information on watt density of the thing or total VA rating of individual unit?

The mat likely draws less current the warmer it is. If it trips @ 40 degrees it is going to draw even more when it is 20, 10, or even below zero outside.
 

robertd

Member
Location
Maryland
Occupation
electrical contractor
>We just installed 2 25’ heating mats
>What is frustrating is no paperwork whatsoever with the product, manufacturer says “ just hook it up to 20 amp GFPE Breaker
Did you install two separate mats? If so and the instructions say to use a 20 amp breaker it may be that each one needs it's own dedicated breaker.
 

McLintock

Senior Member
Location
USA
Occupation
Electrician
We were out there all day Thursday, put an amp meter on it till it tripped, 6.5 amps the whole time. Called manufacturer he said that’s the right amps for two mats and that the mats should be able to run 24/7, and pull the 6.5 the whole time no matter the temp outside. Think water is getting in there somehow.


“ shoot low boys their riding shetland ponies”
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
>We just installed 2 25’ heating mats
>What is frustrating is no paperwork whatsoever with the product, manufacturer says “ just hook it up to 20 amp GFPE Breaker
Did you install two separate mats? If so and the instructions say to use a 20 amp breaker it may be that each one needs it's own dedicated breaker.
Are you using a GFPE or a GFCI circuit breaker? There is a big difference between the two when it comes to unbalanced trip current.
 

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Electrical Contractor
Here’s the junction of the 2 10’ mats and a 5’ section, my thought is the THHN wire needs to be protected, not just under the metal roof.
Nope.. Some floor heat comes with just a roll of wire and you fasten it to the board. When I do these I figure it out so both pieces will terminate at the T-stat so no connection is needed between the mats.
 
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