[Read!] Damn Near Burned The House Down - Space Heaters

[woodchucker]

No excuses here, I'm running all 20A outlets, rated wire, and breakers on my 110v circuits, but why isn't it a good idea to run a 20A receptacle on a 15A circuit? I get that someone could plug a 20A appliance in, thinking it's a 20A circuit, and heat up the breaker and/or wiring. But aside from the need to communicate that limitation to prevent the assumption of the user, what could go wrong? I'm speaking strictly in terms of capacity, nothing else. Just curious. I never really thought an overrated receptacle could cause any problem beyond said operator error.

Just because the outlet conveys with the side ways T , that it is 20amp and the ckt is 20 amp.
Even if you alert everyone in your house, suppose a guest comes over, or you pass away... the new owner wouldn't know.

 

[ErichKeane]

No excuses here, I'm running all 20A outlets, rated wire, and breakers on my 110v circuits, but why isn't it a good idea to run a 20A receptacle on a 15A circuit? I get that someone could plug a 20A appliance in, thinking it's a 20A circuit, and heat up the breaker and/or wiring. But aside from the need to communicate that limitation to prevent the assumption of the user, what could go wrong? I'm speaking strictly in terms of capacity, nothing else. Just curious. I never really thought an overrated receptacle could cause any problem beyond said operator error.

Just that. Folks tend to plug in stuff into whatever outlet fits.

 

[John281]

I've been considering getting a thermal imaging camera (e.g., Flir One, Seek) so I can do spot checks for overheating circuits and other things. Has anyone used one of these cameras for this?

 

[pontiac428]

Yes, they are very effective. I've only used FLIR products and maybe an Exetech once. You have control over the range, so you can detect large or small differences. I borrowed one when I installed my RPC. Everything was good, but now I know where to check hot spots with an IR thermometer if I need to.

 

[MikeInOr]

Failure analysis: So how did this happen?

Facts:

• About 2 weeks ago the original receptacle was showing signs of overheating
• Receptacle was replaced
• Cord and plug was inspected
• The fire started a few seconds AFTER the heater was turned OFF.

The cord is the standard over molded 2 conductor. Probably due to the loose receptacle connection and subsequent overheating, the plastic in the plug started to carbonize internally thus creating a current path between the terminals, but not visible externally. As long as the heater was on, the current path was through the heater with very little current flowing between the terminals of the plug due to the relatively low resistance of the heater vs. the current path resistance between the plug terminals. (see Ohms Law for parallel resistors). Once the heater was turned off, the only current path was then between the terminals. And this immediately caused the plug to become a heater. Interestingly enough, the current stayed low enough not to trip a 15 Amp breaker.

Conclusion: The cord was damaged due to overheating. Likely due to the loose receptacle connection over time.

Prevention: In the future if a receptacle needs to be replaced due to signs of overheating, the mating over molded plug needs to be replaced also. You can't see the hidden damage inside of the plastic plug. I would recommend a good quality back wired plug. Don't plug space heaters or other high current draw devices into loose receptacles. I would recommend replacing the cheap over molded plugs on high current draw devices with a quality plug right out of the box. I will be doing this from now on.

If you put an ohm meter between the remaining prong and the nub of the missing prong on the heater plug does it show a short or an open? ...I am guessing it will show open. What do the two sides of the burnt receptacle look like? ...any signs of arcing?

I purpose that the problem was in the receptacle. When the heater turned off there was already a fire in the receptacle which caused the heater to loose power and turn off. The fire in the receptacle was not noticeable until a few seconds after the fire started in the receptacle and the heater lost power. The fire then spread from the receptacle to the plug which I believe is plausible looking at the significant fire damage to the receptacle. I do not believe the receptacle damage would have been so severe and deep if the fire originated in the plug.

 

[MikeInOr]

If you really want a very high quality receptacle hospital grade is much more stout than commercial grade in my experience. The biggest problem with hospital grade is that they are often orange and it is difficult to find them in beige. When I wired my shop I found a deal on a bulk supply of USED hospital grade receptacles and used them through out my shop. Even used the hospital grade receptacles are MUCH more beefy than anything I could find at a home improvement store. Hospital grade receptacles also have a MUCH firmer grip on the prongs of the plugs inserted into them.

 

[markba633csi]

Apparently the charred plastic in the plug became conductive enough to cause the fire. I totally believe that could happen. Avalanche effect.

 

[MikeInOr]

No excuses here, I'm running all 20A outlets, rated wire, and breakers on my 110v circuits, but why isn't it a good idea to run a 20A receptacle on a 15A circuit? I get that someone could plug a 20A appliance in, thinking it's a 20A circuit, and heat up the breaker and/or wiring. But aside from the need to communicate that limitation to prevent the assumption of the user, what could go wrong? I'm speaking strictly in terms of capacity, nothing else. Just curious. I never really thought an overrated receptacle could cause any problem beyond said operator error.

The 15amp breaker is in the breaker box to protect the wiring and should pop BEFORE there is any damage to the wire if a 20amp appliance were plugged into a 20amp receptacle on a 15amp circuit. I believe it is not good practice to use 20amp breakers receptacles on 15amp circuits but there should be harm in doing so. I do not believe the internal paths in the receptacle are necessarily smaller in a 15 amp receptacle than a 20 receptacle the major difference being the extra hole in the face for a horizontal prong used on 20 amp devices.

Edit: Typo. Definitely NEVER use a 20amp breaker on a 15amp circuit! I had meant to say 20 amp receptacle!

 

[ErichKeane]

The 15amp breaker is in the breaker box to protect the wiring and should pop BEFORE there is any damage to the wire if a 20amp appliance were plugged into a 20amp receptacle on a 15amp circuit. I believe it is not good practice to use 20amp breakers on 15amp circuits but there should be harm in doing so. I do not believe the internal paths in the receptacle are necessarily smaller in a 15 amp receptacle than a 20 receptacle the major difference being the extra hole in the face for a horizontal prong used on 20 amp devices.

20a breakers on a 15a circuit can be bad because the 15a wiring might not be up to snuff.

 

[extropic]

What if you have a fire in your shop and the insurance investigator finds a 20 amp receptacle on a 15 amp circuit? How's that going to effect your claim?