Scorched space heater outlet

Discussion in 'Electrical Forum discussion & Blog' started by jvstevens, Jan 10, 2014.

  1. jvstevens

    jvstevens Member

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    This might be a dumb question, but what is the technical explanation as to why an electric space heater (two prong) plugged into an outlet (also two prong) would cause the heater plug and outlet itself to show signs of overheating? That is, the heater plug shows signs of slight melting of the plastic and scorch marks on one prong, and the receptacle looks a bit scorched as well at the same corresponding location. The outlet is as old as the house...perhaps 50 to 60 years, and the heater is almost brand new. The heater has a max power of 1500 watts, and the outlet, though old, should be able to accommodate a 1500 watt load even if its only a 15 amp circuit, yes? Is it maybe because the old outlet has lost its ability to "grip" and so the metal contact area has decreased? I plan on replacing the outlet, but would like to hear the voices of experience on this. Would it be a good idea to put a new plug on the heater as well (it still seems to work OK as-is).
  2. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    Two things...if the spring tension on the receptacle isn't adequate, it acts like a resister and gets hot on its own. Second, using one of those space heaters for a long period is not a good idea, either. If there's anything else on that circuit, it would try to draw more amps, making things worse.
  3. jvstevens

    jvstevens Member

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    You're saying the heater will draw more amps? I don't see how that happens. The amount of current the heater draws is only dependent on the internal resistance of the heater. It shouldn't matter what else is on the circuit. Or am I missing something?
  4. Reach4

    Reach4 Active Member

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    Replace the outlet. That is probably enough, but monitor that plug for a bit. I expect it will not run hot, but checking seems worthwhile.
  5. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    When the connection is loose, it can act like a resister, i.e. a resistance in series with the load (same reason why wires get hot). Also, when it's loose, it can arc, and that is probably what caused it to char. This is one reason why AFCI were instituted...do it enough, things get hot enough, and you can start a fire.

    But, no, it wouldn't increase the total amps drawn, but the resistance will generate heat, as will arcs.
    Last edited: Jan 10, 2014
  6. ActionDave

    ActionDave Electrician

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    Colorado
    Funny thing is there has been no evidence that an AFCI will trip under such circumstances.
  7. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    Location:
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    Depends on how loose the connection is.
  8. ActionDave

    ActionDave Electrician

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    Location:
    Colorado
    No it doesn't. Check out this link, tell me what you think. http://www.combinationafci.com/
  9. jwelectric

    jwelectric Electrical Contractor/Instructor

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    Location:
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    A 1500 watt electrical heater plugged into a 15 amp receptacle will fail every time and the type or age of the receptacle will not stop the failure.

    Use a little common sense here, if the cord cap is melting then it needs to be thrown away and never used again.

    Arc Fault does not protect overcurrent it protects arcs
  10. jvstevens

    jvstevens Member

    Messages:
    46
    And this is why I'm confused. Shouldn't we expect an outlet that's rated for 15 amps to be able to accomodate a 1500 watt load? And shouldn't a name brand heater that's presumably tested (UL and otherwise) be expected to perform per its specs without self destruction of the plug? Would plugging the heater into a 20 amp circuit help things?

    Is it advisable to cut off the old plug and put a new one on, or should the (almost new) heater be discarded?

    The other thing is I don't think the heater is generally, if ever, operated at full 1500 watts. That's just its maximum capacity. It's generally operated at around half of this most of the time.
    Last edited by a moderator: Jan 12, 2014
  11. jwelectric

    jwelectric Electrical Contractor/Instructor

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    Location:
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    The maximum cord and plug load that can be plugged into a 15 amp receptacle is 12 amps. The 15 amp receptacle is tested at 12 amps for a period of more than three hours.

    1500 watts divided by 120 volts will equate to 12.5 amps. All resistive heating elements are calculated at 125% just because of the physics of this type load. 12.5 times 125% equates to 15.6 amps which will destroy a 15 amp receptacle is short manner.

    Yes the heater should be plugged into a 20 amp receptacle that is on a 20 amp circuit. A 20 amp circuit with a 15 amp receptacle is not going to help very much.

    The heater is tested using a 20 amp receptacle on a 20 amp circuit and should always be used with this type of supply but alas this is not pointed out and even if it was people would still burn down their homes using such dangerous set ups.
  12. jvstevens

    jvstevens Member

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    Great! Thanks for the fine explanation. Its good to know the technical background on such things.

    I guess I'm a little surprised by the fact that a 15 amp receptacle really is not rated or tested for a full 15 amp load. If you have two devices plugged into a single 15 amp receptacle (say using a triple tap or powerstrip) that results in a total of, say, 14.5 amps, you could be at risk of overheating the outlet (but not tripping the breaker) and possibly causing a fire! Isn't this something that could be worrisome?
  13. Reach4

    Reach4 Active Member

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    If what you say were true, don't you think the heater would have come with a NEMA 5-20P plug?
  14. jwelectric

    jwelectric Electrical Contractor/Instructor

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    Every ounce of my post is fact. The NEC in Table 210.21(B)(2) and the rest is found in the UL standard that the fine folks who make the heater has to follow. Ohm’s law is used to find the math parts but someone with an electrical education would know this.
  15. bluebinky

    bluebinky Member

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    Location:
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    I gave up my IEEE membership almost 20 years ago, and am starting to miss it. If the claims are really true, than heads should roll.
    I am probably in the minority (maybe the only one) wishing for a AFCI/6mA GFCI combo breaker. About $900 worth of high-tech breakers in my new panel, and only one nuisance trip in two years.
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