Did my garage heater's thermostat wiring overheat because it's undersized?

Question

Five years ago my electrician installed a Stelpro DRI0521W Dragon ceiling fan heater in my garage. It has a load of 20.84 amps. He installed a 30 amp breaker and told me that he used 12 AWG for the wiring.

Two days ago I caught the heater running endlessly and the garage smells like fire. The breaker did not trip.

I tripped the breaker manually, unscrewed the thermostat and, sure enough, the wiring had overheated and burned right through the insulation in multiple places.

I am not an electrician, but here is my understanding of what is going on...

• The heater draws 20.84 amps.
• Therefore, the wire should be rated to carry at least 20.84 * 1.25 = 26.05 amps
• The 30 amp breaker did not trip because the load did not surpass 30 amps.
• The load did not surpass 30 amps because the wire melted before it could do so.

To prevent a fire, I need to use a wire which is rated for at least 30 amps. Per my understanding, I need to either use a 10 AWG wire with a temperature rating of 60°C or a 12 AWG wire with a temperature rating of 90°C.

Given that the wire burned through the insulation and the breaker did not trip, it seems to imply that the 12 AWG wire does not have a temperature rating of 90°C. When I ran this line of reasoning by my electrician, and suggested that we might need to use a 10 AWG wire, he texted me back:

Yea but not for heating.

What am I missing here?

Could the overheating wires have been caused by a defective thermostat, and does not necessarily indicate a problem with the wiring?

Let's look at the photos...

On the one hand, the melted wire nut indicates a possible faulty connection:

On the other hand, there is melted insulation away from the wire nut:

Then again, the wire nuts were shoved into the back of the junction box. If it overheated while being folded in like that, it is quite likely that it was touching those parts of the wire and caused their insulation to melt.

Thoughts?

Answer 1

My money is on the "faulty connection" theory.

It's not true that 12 AWG copper automatically has ampacity of 20 A. Ampacity directly relates to the self-heating of the conductors due to the amps flowing through. Yes, NM-B cable with its 60 °C rating charts at 20 A ampacity. But 12 AWG copper with better insulation is rated higher: THWN with 75 °C insulation is 25 A; THWN-2/THHN with its 90 °C insulation has ampacity of 30 A. There are a lot of boxes that have to be ticked to actually use wire at its 90 °C limits, though.

Ampacity not withstanding, Code does limit the overcurrent protection (breaker) to not more than 20 A for 12 AWG copper in most situations. So you're correct that the circuit should have been wired with a heavier gauge.

In the photo it really doesn't look like you've got NM-B cable, but even if this were NM-B, it shouldn't have been operating so close to catastrophic failure. Something more was wrong.

It looks to me as if the connection in the wire nut failed. It likely began arcing. Copper conducts heat pretty well so the heat from the arcs easily traveled along the wire and melted its insulation some distance from the wire nut. With a little imagination as to how the wires might have been packed into the junction box it's easy to guess that the wire nut might have touched or at least been near the other conductor and caused the localized melting there too.

Footnote: the breaker didn't trip because there was no short circuit. The damage here shorted black to black, which bypassed the thermostat and caused the heater to run continuously, but it did not create an overcurrent condition which would have tripped the breaker. Also: is that junction box properly grounded? There's a chance the melting insulation could have created a wire-to-box short which would trip a breaker.. but only if the box is properly grounded.

Answer 2

To prevent a fire, I need to use a wire which is rated for at least 30 amps.

Yes, if the breaker is rated for 30 amps, the wiring must be rated to carry 30 amps as well. The breaker should protect the wiring. To do so, the breaker must be rated for less than the current carrying capacity of the wire.