Heated Floor Horror!!! - 240v voltage on a 120v cable


Old 02-01-12, 08:48 AM
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Heated Floor Horror!!! - 240v voltage on a 120v cable

Hello All,

My contractor installed heated floor cable (DFT-1059 120V Cable Kit). When the project was complete we tested the floor a few times and noted that the heated floor took a long time to get warm (more than an hour to go from 64 degrees to 72 degrees). We called the contractor and he simply switched the voltage to 240v. We then noticed the floor heated up very quickly and surpassed the temp on the thermostat very quickly which also seemed wrong. The contractor told me it was not an issue since the cable could be used on 120V and 240v interchangebly. Something did not sound right to me so I asked for the original instructions (DFT-1059 120V Cable Kit) for the heated floor cable. Low and behold - it's rated only for 120v. I cannot begin to express my anger, concern over the risk he took with my family, etc. BTW - The voltage was changed to 240v post my final inspection hence this is something the building inspector did not even know about.

So, my question to you - the cable had 240v run through it a couple of times. Certainly not more than 5 times. Can we just go back to 120v or has the cable been compromised at this point?

Last edited by pcboss; 02-01-12 at 10:00 AM. Reason: changed current to voltage
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Old 02-01-12, 10:02 AM
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I would suggest you contact the manufacturer. They would have the most knowledge.

In Floor Heating, Heated Floor, Electric Radiant Floor Heat
Old 02-01-12, 01:15 PM
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Thanks! I actually spoke with the gentleman from the site. Bottom line - he gave me great advice, to test the ohm resistance on the wire to check continuity and integrity. based on that i will determine next steps. i like it.

thanks guys.
Old 02-01-12, 03:26 PM
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I am not sure how great the "gentleman from the site's" advice is!
The total wattage of that heater at 120 volts would be about 768 watts.
By applying 240 volts it would have forced the heater to operate at about 1500 watts which could have pushed the operating temperature to the point that the insulation could be degraded.

Call the "gentleman" back and ask if you described the conditions the heater operated under to either UL or CSA if it would still meet approval standards and I think you know what the answer would be!

Another thing is I see is that the website advertises that their products are approved but do you see any approval stickers on the components themselves?
Regardless of an electrical component having an approval mark on the packaging it must be affixed to the component and if it doesn't it is not approved for sale or use.

I personally would consider that there is a good chance the cable has suffered some damage.

Maybe others can comment on this idea because I have never used it to test this type of cable but a high voltage meg-ohm meter or megger can test motor and wire insulation.
Perhaps this test applied to the heater might indicate something.
These testers are a high resistance ohm meter that apply 500 volts or higher when performing the continuity test and will jump across broken or damaged insulation to read electrical leakage.
Old 02-01-12, 03:44 PM
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I have never heard of a heat cable that can go either 120v or 240v. They are either one or the other but not both. Surely your thermostat will be toast and possibly the heating cable as well. The ohm test should tell you if the cable is ok. IDK if a Megger is really necessary. IF the insulation is compromised it would short and trip the breaker or GFCI device. Your floor heat is protected by a GFCI device correct? It is required by code.

I would be guessing that your "contractor" is not an electrician. I do not think any electrician would do such a bone head move. In floor heat takes a long time to heat up tiles (and the rest of the mass). 8 degrees in one hour is pretty quick in my book.

Last edited by Tolyn Ironhand; 02-01-12 at 05:37 PM.
Old 02-01-12, 03:54 PM
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I would be suggesting that it might be possible for the insulation to be overheated just short of being totally compromised.
IOW the insulation might not have totally failed but possibly be brittle much like the wrong type of wire being used in a high temperature fixture which I have seen all to many times.
I didn't actually get too far into the product info to know if this is a series connected resistance element or solid state.

The sad part is that in the case of floor heat you really can't inspect the cable without pulling up some flooring.
Old 02-16-12, 11:33 AM
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If the power wasn't applied for too long a time, it might be OK, but if it was left on very long, it could have seriously degraded the individual wire insulation. I don't see how you would know except to tear up part of it, remove the braided cover, and examine it.

When I laid my Warmwire heat cable, the instructions pretty clearly described the delay that warming ceramic tile would take. The thermostat (assuming a "smart" timer thermostat) takes that into account and starts the warming process early enough that the floor is at the expected temperature by the time set on the thermostat. I set mine for a low temp of 60 and a high temp of 75, and the thermostat determines how much earlier than the expected time to start applying power. Mine's been in place for 1 1/2 years and works perfectly.

Incidentally, this was my first tiling project other than a couple of tile repair jobs. Not perfect, but plenty acceptable. It's an open screened porch, converted to a 4-season sun room, with the floor built up over a concrete slab with 6" of fiberglass insulation between the slab and the floor.


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