UL Approved Substandard Wiring?

Old 11-21-08, 06:39 AM
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UL Approved Substandard Wiring?

My wife and I are currently rehabbing a log house in southwestern Virginia. With the premature onset of cold weather here, we recently have been completely engrossed in providing adequate whole house heating. We have decided on a combination of mini-split heat pumps and resistance heat.

When installing a UL approved Marley baseboard electric heater, we used that company's bimetal, double pole, on heater thermostat (FTA2A). The instructions clearly state that the user can run a full 22 amps of current through this device using the amp ratings found on each heater. This would appear to allow the use of this thermostat when running a full 5000 watts of heater in parallel through this one thermostat. The cumulative amp rating for 5000 watts is 20.4.

Marley only uses 14 AWG wire in this thermostat. We already have questioned Marley regarding this. The company stated that their 14 AWG wire was of a "higher quality" than found in conventional wire.

We also noticed a similar discrepancy when installing a Chigo mini-split heat pump. The pump is rated at about 20 amps; but, the factory supplied wiring is strictly 14 AWG.

What is the real logic behind the two situations described above?

Thanks in advance.
Old 11-21-08, 06:52 AM
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Different insulation materials can affect the ampacity values for conductors.

Also motor loads are under a different sizing calculation for wiring sizing than other loads.
Old 11-21-08, 01:52 PM
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sounds perfectly fine...

You may be interested to know that it takes 166 amps in free air to melt 14 AWG copper wire. Any amount below that, it just warms up So, its really an insulation and voltage drop issue, if the wire is in a non-flammable space.
Does this help?
Old 11-21-08, 05:45 PM
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Internal wiring of appliances is not covered by the NEC, but by UL standards. It is common for UL to permit a smaller gauge of wire to be used than would be required by the NEC, and to permit types of wire that would not be allowed for branch circuit wiring under the NEC. Unfortunately, UL standards are extremely expensive to purchase.

Bear in mind that 14AWG type THHN wire has an ampacity of 25A according to table 310.16 in the NEC. You can't make use of that ampacity in branch circuit wiring because of the small conductor rule (240.4 (D)), but that doesn't apply to the internal wiring of a listed device.

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