It happened to me! Overloaded 30amp cord

[Insist on live music]

Yep. My bad! I was using 2 portable electric heaters (low setting) and electric "fireplace" to provide heat in cold South Carolina weather early January (2025). My "rig" is wired for 50amp (though I have only 1 roof-top A/C). You all know how heavy and cumbersome 50amp cables are. . . . I switched to 30amp, buying used cable & surge protection. Mistake! Thank the Lord I was present when this cable burned up-quickly turn off breaker with no subsequent damage. Lesson learned . . . 50amp more efficient and would have prevented possible tragedy.

 

[Soundsailor]

Thanks for posting this cautionary tale.

 

[WhittleBurner]

I would send a pic of that to camco. Shouldn't it have shut you down once sensed an overload? or at least a blown fuse in the trailer? 2nd hand or not that could have been tragic if you hadn't caught it.

 

[coglesby]

Were your plugs were left out in the weather and got wet and then corroded adding significant resistance in the plug area?

 

[jkwilson]

I would send a pic of that to camco. Shouldn't it have shut you down once sensed an overload? or at least a blown fuse in the trailer? 2nd hand or not that could have been tragic if you hadn't caught it.

Likely no overload occurred, though the power defender doesn’t monitor for current. An overload should have tripped the breaker at the pedestal. Likely a poor connection overheated. I’ve seen it happen with 50-30A dogbones. Corrosion or wear on the pins is the most likely issue

 

[coglesby]

Likely no overload occurred, though the power defender doesn’t monitor for current. An overload should have tripped the breaker at the pedestal. Likely a poor connection overheated. I’ve seen it happen with 50-30A dogbones. Corrosion or wear on the pins is the most likely issue

Yep, I had it happen with no dogbone and no extension cord due to corrosion

 

[Riverbug]

Too many house and mobile home fires start from electric heaters. Might be a good case for sucking up the cost of propane and using the furnace. The wiring and receptacles inside these trailers are not nearly as robust as residential. Something to think about...

 

[dryfly]

Likely no overload occurred, though the power defender doesn’t monitor for current. An overload should have tripped the breaker at the pedestal. Likely a poor connection overheated. I’ve seen it happen with 50-30A dogbones. Corrosion or wear on the pins is the most likely issue

Wouldn't corrosion causing a bad connection at the pedestal increase the resistance in the circuit causing an increase in current causing an overload? If this is the case do you have any idea why the pedestal breaker didn't trip? Faulty breaker??

 

[coglesby]

Wouldn't corrosion causing a bad connection at the pedestal increase the resistance in the circuit causing an increase in current causing an overload? If this is the case do you have any idea why the pedestal breaker didn't trip? Faulty breaker??

higher resistance causes less current, not more. It would cause less current but more heating at the connection due to the increased resistance. A breaker won't help in this scenario.

 

[CedarCreekWoody]

Were you plugged into a 30 amp pedestal or a 50 amp pedestal with a dogbone adapter?

 

[dryfly]

higher resistance causes less current, not more. It would cause less current but more heating at the connection due to the increased resistance. A breaker won't help in this scenario.

You are right. I need to have a second cup of coffee to get my brain going before I think about Ohm's Law.

 

[jkwilson]

You are right. I need to have a second cup of coffee to get my brain going before I think about Ohm's Law.

If you are running an AC motor, low voltage causes higher current since the motor is constant RPM. That’s why low voltage is so dangerous to air conditioners. For a resistive load, like an electric heater, Ohm’s Law applies. In the case of a poor connection, Joule’s Law applies as well:

“The amount of heat generated in an electric conductor is proportional to the square of the current and the resistance of the conductor.”

Meaning bad connections get hot.

I lost 5 points on a EE104 exam when I answered the question “What determines the amount of heat generated in an electrical conductor according to Joule’s Law?” I answered current and resistance. Dr. Baxter gave me zero points and could not be persuaded otherwise. I still say I was right, and my first thought when I read his obituary was that he stole 5 points from me.

 

[Scott'n'Wendy]

higher resistance causes less current, not more. It would cause less current but more heating at the connection due to the increased resistance. A breaker won't help in this scenario.

A higher resistance connection causes a voltage drop and heat. For example an appliance that runs at a nominal 120vac and draws 10 amps..1200 watts. Lets say that high resistance connection causes a voltage drop of 15 volts. So the appliance only 'sees' 105 vac. For that 1200 watt appliance to run it will demand more current. Ohms law. So now it is drawing 11.4 amps. Higher amperage through that bad connection. Which causes more heat..which causes a bigger voltage drop, which causes the appliance to demand more current....yada, yada, yada... vicious circle until the connection fries.

Best thing to do to help keep those cord plugs in good shape is to use dielectric grease on them. Helps reduce corrosion.

 

[coglesby]

If you are running an AC motor, low voltage causes higher current since the motor is constant RPM. That’s why low voltage is so dangerous to air conditioners. For a resistive load, like an electric heater, Ohm’s Law applies. In the case of a poor connection, Joule’s Law applies as well:

“The amount of heat generated in an electric conductor is proportional to the square of the current and the resistance of the conductor.”

Meaning bad connections get hot.

I lost 5 points on a EE104 exam when I answered the question “What determines the amount of heat generated in an electrical conductor according to Joule’s Law?” I answered current and resistance. Dr. Baxter gave me zero points and could not be persuaded otherwise. I still say I was right, and my first thought when I read his obituary was that he stole 5 points from me.

What was his "correct" answer?

He wanted time in there as well?

 

[Scott'n'Wendy]

I still say I was right, and my first thought when I read his obituary was that he stole 5 points from me.

Time to let it go John...lol

 

[coglesby]

A higher resistance connection causes a voltage drop and heat. For example an appliance that runs at a nominal 120vac and draws 10 amps..1200 watts. Lets say that high resistance connection causes a voltage drop of 15 volts. So the appliance only 'sees' 105 vac. For that 1200 watt appliance to run it will demand more current. Ohms law. So now it is drawing 11.4 amps. Higher amperage through that bad connection. Which causes more heat..which causes a bigger voltage drop, which causes the appliance to demand more current....yada, yada, yada... vicious circle until the connection fries.

Best thing to do to help keep those cord plugs in good shape is to use dielectric grease on them. Helps reduce corrosion.

Then why didn't the breaker trip?

 

[sande005]

Then why didn't the breaker trip?

Because the breaker is supposed to be the weakest link, but in this case it was the connection point and not the breaker that "tripped".

 

[MtnGuy]

If he was plugged into the 50 amp breaker with a 50 amp to 30 amp dog bone, then that probably would not trip the breaker. A 30 amp breaker should have tripped.

 

[coglesby]

The current drops because you now have two resistors in series with 120V across the entire circuit, the two resistances being the heaters and the resistance in the plug. The voltage across the combination of the heaters and plug doesn't change and remains constant at 120V. V=I*R so the current has to decrease if the resistance increases to satisfy the equation. Since P=I^2*R there is now power being dissipated at the plug because without the corrosion, the resistance at the plug would be near 0 so power dissipated would be near 0 so no heat at the plug under normal conditions.

 

[coglesby]

If he was plugged into the 50 amp breaker with a 50 amp to 30 amp dog bone, then that probably would not trip the breaker. A 30 amp breaker should have tripped.

Based on the OP, there was most likely a dogbone at both ends of the extension cord and I wasn't thinking about that so 50A breaker at both ends so no 30A breaker in the circuit so I agree. My mistake.