Water heater internal wiring type

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LarryFine

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
I have a customer with an electric water heater that overheated the internal wiring. He asked me to re-wire it. He already bought two new T-stat/over-temp units.

Is regular THHN/THWN/2 adequate, or is something with a higher temperature rating better? I know they come with #12 internal wiring, but should I use #10?
 
I have replaced/rewired water heater thermostats and elements before. I used #12 THHN/THWN-2 for this. Is it the same that came from the factory? I don't know. I do know that if I don't exceed 90 degrees C. (194 degrees F.) that I won't exceed the temperature limitations of the conductor. I've never set a water heater temp at higher than 125 degrees F. I used that rationale for doing what I did. I've never had a call back on any that I have done. Hope this helps.
 
I don't see how it could overheat the wiring. My guess would be corrosion at the terminals.

THHN insulation is good for 90 deg C with is almost the boiling point of water. I suspect the PRV on the water heater would trip before that happened.
 
Rick 0920 said:
I do know that if I don't exceed 90 degrees C. (194 degrees F.) that I won't exceed the temperature limitations of the conductor.
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petersonra said:
THHN insulation is good for 90 deg C with is almost the boiling point of water.
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Sure, but the ampacity of a 90C insulated conductor at 90C ambient is 0 amps.

Say the water heater is set to 140F (high), and say its insulation is awful, so the conductor is operating at 140F ambient (very unlikely). Then the ampacity correction factor is 0.71 for a 90C insulated conductor, and a #12 Cu conductor still has an ampacity of 30 * 0.71 = 21.3A.

Cheers, Wayne
 
wwhitney said:
Sure, but the ampacity of a 90C insulated conductor at 90C ambient is 0 amps.

Say the water heater is set to 140F (high), and say its insulation is awful, so the conductor is operating at 140F ambient (very unlikely). Then the ampacity correction factor is 0.71 for a 90C insulated conductor, and a #12 Cu conductor still has an ampacity of 30 * 0.71 = 21.3A.

Cheers, Wayne
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Absolutely correct. A lot of "what if's" there but they are possible. By using 90 C. rated conductors, if the ambient was 125 F., the correction factor would be .76 30A x .76 = 22.8A still ok to operate a 4500 watt element which is 18.75A The manufacturer obviously did not have to consider this appliance as a continuous load which is why they can wire it with #12 and we have to use #10.
 
I don't think I would want to rewire something that (supposedly) got that hot. Especially if it is an older tank. I would just replace it be done with it. If you do rewire it, they make high temperature wire that I've used to rewire stove tops.
 
I am going to see it today, and will base my decisions on what I see there.

I may well recommend replacing it, which I can also do if he likes my price.
 
LarryFine said:
I am going to see it today, and will base my decisions on what I see there.

I may well recommend replacing it, which I can also do if he likes my price.
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By the time you rewire all of that the price for either will be close to the same. and they will have a new water heater.
If they go with that price.
 
Thre internal wiring might be fixture wire or other high temperature type.
To be sure what temp is needed, check the terminal temperature specs for the heating elements.
Thermostat end is very unlikely to overheat.
 
Looks more like you had something shorting out. Look for some skeletons. The internal wiring is usually a higher temperature than typical circuit wiring. A couple of mfg sources indicate 105 oC is used for the internal wiring. I would say just put in a new heater unless you want to take the liability.
 
Okay, I'm back. The damage looked worse than it really was. The front of the lower T-stat had blown off.

I pulled out what was left of the old one, put in the new one, cut and re-stripped both wires, all is good.
 
LarryFine said:
Okay, I'm back. The damage looked worse than it really was. The front of the lower T-stat had blown off.

I pulled out what was left of the old one, put in the new one, cut and re-stripped both wires, all is good.
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Did you check the wires at the upper stat, and did you also replace the upper one?
 
Little Bill said:
Did you check the wires at the upper stat, and did you also replace the upper one?
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Not really a bad idea, those thermostats as well as the high limit are not really all that expensive and might save from having to come back sometime soon when the one you did not replace fails.
LarryFine said:
Okay, I'm back. The damage looked worse than it really was. The front of the lower T-stat had blown off.

I pulled out what was left of the old one, put in the new one, cut and re-stripped both wires, all is good.
Click to expand...
I was going to ask before seen this post, is the wire totally melted or just near a termination? Near a termination is what I've seen often. Water in the tank doesn't get hot enough to be much of a contributing factor to conductor insulation problems, bad terminations however get much hotter but normally only effect a couple inches of conductor at the most. Doesn't matter if it is a thermostat or an element terminal, they get much hotter than the water in the tank if there is a problem.
 
kwired said:
I was going to ask before seen this post, is the wire totally melted or just near a termination? Near a termination is what I've seen often.
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The body of the lower T-stat was blown open, and about one inch of each wire was melted.
 
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