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-   -   Two 120v Nuheat mats wired in series for 240v circuit? (http://www.diychatroom.com/f18/two-120v-nuheat-mats-wired-series-240v-circuit-69784/)

chrispy35 04-25-2010 01:34 AM

Two 120v Nuheat mats wired in series for 240v circuit?
 
Hi there,

I just realized (a little late) that the two Nuheat mats I installed in my bathroom are 120 V mats and the old circuit for power I'm using from the baseboard header is a 240 V circuit.

My proposed solution to this is to wire the mats in series to the thermostat rather than in parallel. Each mat would have ~120 V across it and both mats would have the exact same current which would in theory give more even heating on the floor. The power to each mat would be approximately the same as if they were each connected directly to a 120 V circuit as intended.

Does anyone see any issue with my proposed solution? I can't think of problems with it but it's late and I want to make sure I'm not overlooking anything.

Thx,

Chris P.

frenchelectrican 04-25-2010 01:44 AM

My answer is no it is not good idea due couple reason .,,

one is that you will never able get both mats have the excat same size

Second thing if one fail the other heater mat will not work at all

Third I am pretty sure your code do required RCD { GFCI }

How many watts do that mat heater do require ?

And is that heater is it own circuit if so you can able reconfreaged from 240 to 120 volts by looking at one of the double pole breaker and find the white conductor and move it to the netural bussbar but due you are in BC Canada area so I will say a 50/50 shot that you do not have white conductor in your cable if so you will not able remark it for 120 volts.

One of our members from Canada he will give you the straght answer with BC codes.

so hang on he should chime in so he will look it up for ya.

Merci,Marc

thefatpigeon 04-25-2010 02:09 AM

french electrician speaks the truth. You should have a two pole breaker protecting your heater circuit. under the screws you will have a black and a white wire( you should unless the elctrician was doing something funny with 3 wire) remove the white wire and connect it to the neutral busbar. you should then replace the breker with single pole breaker of the correct rating.

AllanJ 04-25-2010 06:55 AM

Quote:

Originally Posted by frenchelectrican (Post 433185)
... but due you are in BC Canada area so I will say a 50/50 shot that you do not have white conductor in your cable if so you will not able remark it for 120 volts.

Sometimes (more often in Canada compared with the U.S.) 240 volt circuits are wired with 2 conductor cable with red and black conductors.

Daniel Holzman 04-25-2010 08:59 AM

I have NuHeat mats also, and I am pretty sure you will immediately void the warranty by stringing the mats together in series. Not sure how you plan to make the connection between the two mats, but however you do it, NuHeat is not going to like it, not to mention the connection could be a fire hazard. Those leads they give you on the mats are designed for attachment to the thermostat only.

Yoyizit 04-25-2010 09:26 AM

Check that the voltage across each mat is close enough to 120v.
For heaters using Nichrome wire this will probably work, electrically.
NEC, etc., is another issue.

tpolk 04-25-2010 09:57 AM

isnt he saying he wants to use each leg of the dble pole as a seperate feed to each matt even tho he calls it in series?

chrispy35 04-25-2010 12:48 PM

Thanks for the quick replies all, I have a few follow-up statements and questions:

Quote:

Originally Posted by frenchelectrican (Post 433185)
My answer is no it is not good idea due couple reason .,,

one is that you will never able get both mats have the excat same size

I'm not sure I understand your statement regarding both mats being the same size. Do you mean same power rating? Mat #1 is 120 W @ 1.0 A (120 ohms) and mat #2 is 90 W @ 0.8 A (160 ohms). Wired in series, these two would create a 280 ohm load that would produce 205 W of heat on a 240 V line which is marginally less than if they were wired in parallel to a 120 V line.

Quote:

Originally Posted by frenchelectrican (Post 433185)
Second thing if one fail the other heater mat will not work at all

I'm not worried about one mat failing because, if one failed with a parallel connection, half the floor would be cold and I would probably disconnect the other mat in that case and go back to a baseboard heater as it was before (the baseboard heater wiring is being used for the mat so it would be trivial to go back).

Quote:

Originally Posted by frenchelectrican (Post 433185)
Third I am pretty sure your code do required RCD { GFCI }

The NuHeat thermostat (Solo) is GFCI and is rated for 240/120 V operation. Is there something else I need?

Quote:

Originally Posted by frenchelectrican (Post 433185)
How many watts do that mat heater do require ?

And is that heater is it own circuit if so you can able reconfreaged from 240 to 120 volts by looking at one of the double pole breaker and find the white conductor and move it to the netural bussbar but due you are in BC Canada area so I will say a 50/50 shot that you do not have white conductor in your cable if so you will not able remark it for 120 volts.

The total output of the mats would be 210 W in parallel or 205 W in series.

The wiring into the junction box that has the thermostat has white/black/bare wires coming in from the breaker and another set of the same going out to the load. As long as the wiring has a white conductor it is OK by code to have it re-wired for 120 V as the breaker panel?

Quote:

Originally Posted by Daniel Holzman (Post 433261)
I have NuHeat mats also, and I am pretty sure you will immediately void the warranty by stringing the mats together in series. Not sure how you plan to make the connection between the two mats, but however you do it, NuHeat is not going to like it, not to mention the connection could be a fire hazard. Those leads they give you on the mats are designed for attachment to the thermostat only.

According to the Nuheat installation instructions and their tech support, it is OK to shorten the NuHeat leads and connect them to existing wiring for power as long as the connection is in a junction box with an accessible cover panel. You do need to re-attach the 'do-not-remove' tags to the shortened leads though.

Chris P.

Scuba_Dave 04-25-2010 01:06 PM

Call NuHeat & ask them
You may very well burn them up
And...this came up a while ago...even tho the thermostats are GFCI, it appears from code that the circuit must be GFCI protected
Strange as it may seem
And the cost of a 240a GFCI breaker will most likely change your mind

Yoyizit 04-25-2010 01:28 PM

Each mat must be the same resistance for this to work.

chrispy35 04-25-2010 01:45 PM

Quote:

Originally Posted by Yoyizit (Post 433385)
Each mat must be the same resistance for this to work.

I don't understand why having the mat resistance matching would have an impact. The series loads are cummulative and the series circuit would result in both mats drawing 0.85 A (240 V/280 ohms). One mat would be putting out slightly less than it's rated output and one slightly more.

Scuba, thanks for mentioning the GFCI breaker in addition to the thermostat. I will have a look into either getting that upgraded or having the circuit rewired for 120 V as Marc suggested.

Chris P.

Yoyizit 04-25-2010 01:57 PM

I = 240v/(120Ω + 160Ω) = 0.857A
0.857(160Ω) = 137V so the power into this mat would be (137v/120v)^2 = 1.3x the rated value of wattage. It will overheat unless it is rated for this voltage. This will probably shorten its life in any case.

brric 04-25-2010 02:54 PM

Quote:

Originally Posted by Scuba_Dave (Post 433380)
Call NuHeat & ask them
You may very well burn them up
And...this came up a while ago...even tho the thermostats are GFCI, it appears from code that the circuit must be GFCI protected
Strange as it may seem
And the cost of a 240a GFCI breaker will most likely change your mind

Can you show us what article requires supply circuit ahead of the TS to be GFCI protected?

AllanJ 04-25-2010 03:32 PM

If the loads (the mats) are not the same, then they should not be wired in series to connect across a 240 volt power source.

The lower wattage mat has a higher resistance. It will get more than 120 volts in the series circuit.

Since the resistance varies with the exact temperature, it is not easy to predict exactly what the voltage across each mat is and what the total current draw is.

You may not connect an ohmmeter's probes to a circuit with the power still on. It is customary to compute the resistance of a load in the circuit indirectly by measuring the voltage across the load and measuring the current flowing through the load. Resistance equals voltage divided by current.

joed 04-25-2010 06:52 PM

Wiring is series is just not a good idea. The mats not being the same wattage makes it an even worse idea.
Do it right the first time or do it over.


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