Electric Floor Heat

[Bus Driver]

Upon further reflection, I suggest installing a double pole GFCI breaker for this circuit in the panel. The heating elements will then be properly protected. The only downside to having the breaker GFCI upstream from the thermostat GFCI is that it makes for more possible confusion in the case of tripping. I would make this change even if the cause of the previous tripping is found and corrected.
The wire gauge of the cables in not mentioned. At 18 amps and with a heating load, this is addressed by NEC Article 100.1, Definitions, as a Continuous Load ( the NEC uses definitions that may differ somewhat from those of Webster). Article 210.19 (A)(1)(a) would require #10 gauge conductors for copper and Article 210.20(A) requires a circuit breaker of at least 25 amperes with 30 amps being acceptable. 25 ampere breakers are often difficult to find, but they are available.
If the existing conductors are 12 gauge, then do not use a breaker large than 20 amperes.

[Bill Hudson]

My head hurts.

Bill

[Mr E]

My head hurts.

Bill

Me, too.

[Bus Driver]

Hey, we didn't yet get into joules, Coulomb's Law, henry, farad, etc.

[Mr E]

I'd sure like to tour the home with the two of you, so I could ask a few questions.

[Bill Hudson]

I'd sure like to tour the home with the two of you, so I could ask a few questions.

I think I'll pass.

Bill

[Mr E]

I'd sure like to tour the home with the two of you, so I could ask a few questions.

I think I'll pass.

Bill

I'd only ask one or two, Bill.

[John *.?-!.* cub owner]

I'd sure like to tour the home with the two of you, so I could ask a few questions.

I would too, but do not think as fast as I used to, and would need to draw a diagram as I went.

[Jim Becker]

This project is beginning to make me feel like I was trying to sort out somebody's 12-volt conversion on a tractor. Bus Driver, your comments have really helped me out. I had forgotten the 125% rule for continuous loads. At first look, a 20 amp circuit seemed right and I never questioned it. The entire system is sized for 20 amps, with the single exception of the thermostat at 15 amps. Specifically, 20 amp breaker, 12 gauge wire etc. The 20 amp contactor is actually rated for 25 amp resistive load, but that is irrelevant. I don't know about the capacity of the power company's components for the curtailment system. But that makes no practical difference because it is a 12 gauge wire from it anyway.

Around noon today, I connected the control line (#3) directly to what had been connected to the contactor output (#4). This removed the contactor from the circuit. I then went to the box where #4 connects to the heating elements and disconnected two elements, leaving the one large element where the thermostat sensor is. This "simplified" configuration now matches the manufacturer's recommendations and puts the connected element in a circuit that is covered by the GFI. It has been running that one element for 9 or 10 hours now, no problems. Tomorrow I can hook in each of the other elements (one at a time) to see if it still behaves. At this point, I think it will.

The problem now is to decide on a permanent fix. While I was reconfiguring the elements, I took a resistance reading on each one individually, which is the only way I have to determine what is actually buried under the tile. The 2 large measured at 30 ohms, which matches the reading for the manufacturers longest heating wire (which they rate at 7 amps). The smaller measured 64 ohms, between 2 lengths (50 to 63 ohms and 4 amps) (67-83 ohms and 3 amps). The current draw calculation will depend on just what voltage you pick. My earlier quoted current draw came up a little higher than their numbers. Whatever the exact load, running all 3 elements properly will require either adding a second complete circuit for part of the load or upgrading the existing circuit. Upgrading the existing circuit essentially requires a complete replacement. Either option would require pulling the power company in to some extent or another.

The other option is to reduce the load down to what the existing circuit can support. The two larger elements together heat a roughly rectangular area in a room. The smaller heats a hallway. If I leave the smaller one disconnected, the two larger will draw 14 to 16 amps. At 125%, that is in bounds for the existing circuit. The manufacturer says the 15 amp thermostat can handle the two larger elements without added relays etc. The boss has OKed killing the heat in the hallway. In fact, as I spelled out the situation, she suggested it before I got that far.

Unless I discover something else (like a surprise as I test with both large elements connected), I am going to reconfigure it to only use two elements. This requires:
1) Remove the contactor and contactor box.
2) Place an electrical box where the contactor was, connect the original input (cable 1) to the cable feeding the thermostat (cable 2). Run a new cable from the output of the thermostat to the box where cable 4 is connected to the elements. Connect elements 1 and 2 to the new cable.
3) Cables 3 and 4 and element 3 will all be abandoned in place ( and tagged for future reference).

With these changes, it will be wired according to the manufacturers instructions and the elements that are in use will be protected by the GFI. (Yes, I understand the protection is actually for the people standing on/near the heated floor.) In some ways, I'd rather have a GFI in the breaker box. But with this configuration I should be up to code.

The only question I see right now is whether the thermostat/GFI will continue to function properly. Since it will be installed per the manufacturer, if it doesn't, it needs replacing anyway.

[Barnyard]

And all these years I thought figuring out amperage, circuits and timers for all these Christmas lights was an adventure! Wow, you guys have me confused.

[Jim Becker]

I plowed my drive today. When I got done, the Cub still had half a tank of amps.

[Mr E]

I plowed my drive today. When I got done, the Cub still had half a tank of amps.

[Winfield Dave]

I plowed my drive today. When I got done, the Cub still had half a tank of amps.

[Scrivet]

........The problem now is to decide on a permanent fix. .........

I'm thinking wood stove at this point.

.......Unless I discover something else (like a surprise as I test with both large elements connected), I am going to reconfigure it to only use two elements. This requires:
1) Remove the contactor and contactor box.
2) Place an electrical box where the contactor was, connect the original input (cable 1) to the cable feeding the thermostat (cable 2). Run a new cable from the output of the thermostat to the box where cable 4 is connected to the elements. Connect elements 1 and 2 to the new cable.
3) Cables 3 and 4 and element 3 will all be abandoned in place ( and tagged for future reference)......

Are you SURE you didn't accidentally copy this from some instructions for a toy you assembled Christmas Eve?

The only thing this thread is lacking is pictures. I'm thinking a giant bottle of aspirin would be appropriate.

[Bus Driver]

There is another possibility to operate all three on a 20 ampere circuit. The hallway would have it's own thermostat as an addition. That thermostat would receive it's power supply from a replacement contactor. The new contactor would be double-throw in that it has one set of points normally closed (NC)and a set normally open (NO). The hallway supply would be through the NC contacts and could operate as that (new) thermostat operates UNLESS the thermostat for the larger area closes the NO contacts to supply power to the larger area heating elements. Thus they are sequenced and the load at any moment is less than 16 amperes.
The elements could thus be said to be interlocked.
It would require a GFCI breaker in the panel.