Wiring Size in Electric Heater

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I have a 5000W electric heater at 240VAC being fed with a double pole 30A breaker and #10 copper wire. The wiring diagram inside the unit lists some data about the heater:
Heater Amps: 20.9
Min. Circuit Ampacity: 26.1
Max. Fuse Size: 30
Min. Wire Size 75°C Copper: #10

It has a built-in thermostat mounted on the unit, which makes it difficult to access since it is mounted approximately 8' off the floor. I am planning to install a remote thermostat and I have a wiring question that I can't seem to find the answer to. The internal wiring connecting the built-in thermostat to one of the 120V lines coming in is 14AWG stranded copper. How is this not a code violation and can anyone reference the section of the code that states this is acceptable. Table 310.16 of the NEC states that the maximum ampacity of a 14 AWG conductor is 20A, therefore I don't see how this is allowable. I've attached a picture of the wiring schematic and the printing on the internal cable jacket for a reference. Any help on this would be greatly appreciated.
 

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iwire

Moderator
Staff member
Location
Massachusetts
If I was to do this I would have to use 10 AWG for the external wiring.

The internal wiring does not have to nor will it comply with the NEC.
 

GoldDigger

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Location
Placerville, CA, USA
Occupation
Retired PV System Designer
The AWM, Appliance Wire Material, designation is for Recognized Component wire internal to listed products. The 105C designation is probably necessary to attach to the heater terminals.
This wire inside the product is covered by UL, not NEC rules.
To remotely mount the stat you must follow NEC rules instead.
See http://www.southwire.com/products/MtwTewAwmIA.htm for one type of AWM wire.
 
So if any of the internal wiring is modified it must be #10?
Maybe the better route for mounting a remote tstat would be to install an electric heater relay and then use 18/2 wiring to the tstat from the heater relay.
 

GoldDigger

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Location
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So if any of the internal wiring is modified it must be #10?
Maybe the better route for mounting a remote tstat would be to install an electric heater relay and then use 18/2 wiring to the tstat from the heater relay.
Actually, it is not clear just what field modifications, if any, you can make to the product. I would check for supported options first.
 

kwired

Electron manager
Location
NE Nebraska
What is inside the listed appliance is not covered by code. What you field install is.

Since this thermostat is carrying full heater current it needs to be able to be rated for the load (conductor and the thermostat). Some heaters that are designed for remote thermostat do have a control circuit that doesn't carry full heater current and often is only 24 volt class 2 circuit, but don't look like you have that option on this unit.
 

iwire

Moderator
Staff member
Location
Massachusetts
So if any of the internal wiring is modified it must be #10?

Strictly speaking you are not supposed to modify the internal wiring of listed equipment beyond what the manufacturer condones.

Is this a duel element heater?

I ask because if it is it will have more than a simple single pole thermostat.

Also the operating temperature thermostat often contains the safety high limit which needs to remain operational.
 

ActionDave

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Licensed Electrician
The internal wiring connecting the built-in thermostat to one of the 120V lines coming in is 14AWG stranded copper. How is this not a code violation and can anyone reference the section of the code that states this is acceptable. Table 310.16 of the NEC states that the maximum ampacity of a 14 AWG conductor is 20A, therefore I don't see how this is allowable. I've attached a picture of the wiring schematic and the printing on the internal cable jacket for a reference. Any help on this would be greatly appreciated...
What others have said about fiddling with internal wiring is true. Many heaters will have an option and instructions for a remote t stat though. Every one I have done recently has anyway.

As far as the fourteen wire inside the heater... Like others said it's not covered by the NEC, also the insulation is rated at 125°C so it can withstand more than load than thhn rated 90°C.
 

JFletcher

Senior Member
Location
Williamsburg, VA
Mfg wiring is not subject to the NEC. I dont know of any appliance that uses the same size wiring inside as the branch circuit feeding it.

Ive seen that 125*C wire in ovens too. Must be cheaper than using larger conductors at lower temps.
 

kwired

Electron manager
Location
NE Nebraska
Strictly speaking you are not supposed to modify the internal wiring of listed equipment beyond what the manufacturer condones.

Is this a duel element heater?

I ask because if it is it will have more than a simple single pole thermostat.

Also the operating temperature thermostat often contains the safety high limit which needs to remain operational.
Did you look at the schematic provided by OP?

It has multiple element segments, and you change overall watts by how you configure them. All wattages still have single pole thermostat in L1 and separate single pole high limit in L2.
 

kwired

Electron manager
Location
NE Nebraska
Placing a thermostat or thermostat controlled contactor in the supply line until you consider the fact it will not allow extended fan run time for cool down after setpoint temp is reached.
 

kwired

Electron manager
Location
NE Nebraska
Placing a thermostat or thermostat controlled contactor in the supply line until you consider the fact it will not allow extended fan run time for cool down after setpoint temp is reached.

I left out a part of what I intended to say in there for some reason. See bold print.

"Placing a thermostat or thermostat controlled contactor in the supply line seems like a good alternative until you consider the fact it will not allow extended fan run time for cool down after setpoint temp is reached."
 
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