AC outlet voltage at 78v.... that's just weird

Discussion in 'General Technical Questions and Answers' started by W7UUU, Nov 13, 2017.

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  1. K8MHZ

    K8MHZ Ham Member QRZ Page

    I have pulled burnt receptacles that failed just because they were cheap. They weren't back stabbed.

    burnt receps.JPG
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  2. KA3TJI

    KA3TJI Ham Member QRZ Page

    Where did I say anything about receptacle boxes? Either way he open the panel and check voltage there first. Sure he should have noticed the voltage increase on the other leg, provided he measured the other leg. Even pros overlook stuff, it happens.
  3. KA3TJI

    KA3TJI Ham Member QRZ Page

    Back stabbing should be outlawed!!! Many causes for the burnt receptacle you've shown. Lime you said, junk.
  4. N2EY

    N2EY Premium Subscriber QRZ Page

    Years ago, a friend reported odd electric behavior in their downstairs bathroom. Turn the light on, and it was fine, but if you turned the fan on, it got dim, and if you turned the heater on, the light went out. Other odd behaviors too.

    I went to the main panel and started looking around. Modern breaker panel with the usual 200 amp main and lots of circuits, nothing suspicious. Removed the front cover panel and looked at the wiring....nothing out of the ordinary.

    Found the bathroom circuit and for some reason I put a screwdriver (insulated handle) on the screw that connected the black wire to the breaker....and it was loose. Tightened it about a half turn!

    Started checking the other screws in the box....some weren't tightened all the way. Went over every single screw in that box and tightened them all down - black, white, red, ground. About 1/3 weren't tightened all the way! No obvious arcing or burning.....but after the job was done, all the odd behaviors stopped.


    I agree that using a high-impedance DMM can cause all sorts of odd behaviors. 78 volts open-circuit? Could be some sort of capacitance/inductance coupling - what does it read with a good old fashioned 100 watt incandescent bulb load?


    IANAE, but, IMHO, "backstab" wiring should be forbidden in the NEC. There's just no reason for it.
  5. K8MHZ

    K8MHZ Ham Member QRZ Page

    I won't do BS connections, I think it's crappy workmanship. That being said, as much as I dislike BS receptacles, millions of them are still working fine after being in service for decades. If installed properly, only 14 AWG can be used (it's printed on the back of the receptacle) which also limits the circuit to a 15 amp breaker. Many of the failed BS receptacles had 12 stuffed in them.

    I think they should outlaw 29 cent receptacles. Those burnt ones in my picture were really, really crappy. Slater was the brand name.
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  6. KA3TJI

    KA3TJI Ham Member QRZ Page

    This is the best advice so far. Let's start at the panel.
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  7. K8MHZ

    K8MHZ Ham Member QRZ Page

    I see it all the time. In fact, 78-80 volts is a pretty common reading on an unloaded incomplete circuit using I high-impedance DMM. It's from capactive coupling. It's even possible the receptacle in question is on a switch somewhere which is open. That would show DMM voltage, too. I know putting a load on it would drop the reading to zero. For me, getting concerned about the 78 volt reading is a waste of time.
    You need to take that up with UL. The NEC allows that which is listed by an NRTL. UL is the most used and they allow the design.
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  8. KA3TJI

    KA3TJI Ham Member QRZ Page

    I am sure it works fine in lots of installs, but using it is poor workmanship. the whole concept of having basically a knife blade of contact patch instead of being under a screw is a recipe for failure. The question is why do it when your reputation is on the line? I don't want callbacks for something that is avoidable.
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  9. K8MHZ

    K8MHZ Ham Member QRZ Page

    I am with you. Most don't realize the NEC is a set of minimum standards and following minimum standards makes for the worst installations allowable by law.
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  10. KA9JLM

    KA9JLM Ham Member QRZ Page

    I Agree. Builders are cheap.

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