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Any electricians on here?

Post in 'DIY and General non-hearth advice' started by charly, Jul 18, 2013.

  1. Joful

    Joful Minister of Fire

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    I'm getting confused now... can't keep track if you're responding to charly or Jeremy.

    Jharkin's problem is load balance, combined with a bit of under voltage (224V) at the mains. Charly's problem, at least the way he described one circuit dragging down a separate circuit, is also at the mains.

    In both cases, when checking mains voltage, try to get the probes right on the little bit of exposed main wire, right above the main breaker/knife switch lugs. Measuring on the lugs themselves will be affected by possibly loose or corroded lugs.

    A loose breaker or receptacle lug can be frustrating, as they may not get hot enough to trip the thermal mechanism in the breaker, particularly if its a cyclical load at the end of a long branch circuit. We had a window AC in a bedroom in the house where I grew up, and it would run about 12 hours before tripping the breaker. It took a while to figure out the problem was a too-loose receptacle, which took about 12 hours to transfer sufficient heat down the copper wiring to eventually trip the breaker. Potentially dangerous situation, but has nothing to do with either of the problems I read here.

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  2. oldspark

    oldspark Guest

    Joful, sorta responding to both of them but in the one case he has several high amp. items on one leg, I can check my voltage but I doubt if it is is supposed to be as out in the country. I have seen loose connections get the wire hot enough to melt the insulation and never trip a thing, loose connections can burn houses down.
    raybonz and Joful like this.
  3. jharkin

    jharkin Minister of Fire

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    I thread jacked :( Spark was helping me... I know what I need to do now.


    Back to Charlies issue.....
  4. oldspark

    oldspark Guest

    I thinks it's ok, close enough to the OP's problem I am sure he got some good out of it.
  5. jharkin

    jharkin Minister of Fire

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    Speaking of which, my problem is fixed it think. I mapped out my big loads (easy since I had mapped every outlet in the house when we had the new service installed 4 years ago) and figured which ones are on simultaneously typically. I.e half the ac's on each leg, fridge and basement freezer on different legs, etc.


    Turns out I was able to do 2 circuit pair swaps and get it all relatively balanced. Now with everything on my service reads 111.5 and 112.8 measured directly at the main lugs.

    Thanks Joful, oldspark.


    Update.

    I watched it through the day. Biggest variation was about 3v, until around 7pm when suddenly the readings were 118 and 110 with no significant change in usage, meter reading 3.7kw. Then I put the dryer on and the readings dropped to 110 and 104 :( at 8kw total draw.

    Correct me if I'm wrong, but i think a few houses share the same secondary winding on the tranformer right? could load from other homes be unbalancing the feed?

    Something is up. Might have to call NStar after all.
    Joful likes this.
  6. fbelec

    fbelec Minister of Fire

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    one other thing. if you had a bad connection with the copper and it was making heat you would be able to see it in the color of the copper. if at the connection the copper wire looks like it has a rainbow of color or purpleish blue then it has been getting hot and could be you problem. if you call the power company they will measure your voltage at the top terminals of your meter and if it is within there specs they will tell you have a nice day and you should call a electrician. so the next time you have a low reading go out to the meter and take it there also,
  7. oldspark

    oldspark Guest

    "Correct me if I'm wrong, but i think a few houses share the same secondary winding on the tranformer right? could load from other homes be unbalancing the feed?"

    Something is up. Might have to call NStar after all."
    I think they do in some places, so yes you might have to call them.
  8. Joful

    Joful Minister of Fire

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    Shut off your main breaker, and check the voltages. That'll tell you if that's the case.
  9. jharkin

    jharkin Minister of Fire

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    Yea, I thought of that as well. Going to try and find a chance to due that during a high load period.

    I wouldn't be surprised if the utility has issues. Its an old neighborhood, with correspondingly old infrastructure. I know this house was first wired for K&T, probably in the 20s and updated many times since, other houses around are of similar vintage. Certainly for many years most of these houses on my street where on 60amp or lower services and I have no idea if NStar has done much to update the lines and transformers as the loads have increased.
  10. Joful

    Joful Minister of Fire

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    It's probably been updated more than you anticipate. Most of those old transformers had pretty short service life, something like 15 years, before they required replacement.

    The funny thing is that I have all the same troubles with my under-sized 200A service and too-long run to the transformer, and I'm an electrical engineer! The cobbler's children have no shoes...
  11. raybonz

    raybonz Minister of Fire

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    That makes no sense at all.. I suspect a loose outlet caused a low voltage situation (brownout) which would make for a hard start of the AC compressor leading to a trip. Bad connections are the weakest link in any electrical system and can lead to fires.. Heat doesn't travel over a 12 hour period down to the breaker..

    Ray
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  12. raybonz

    raybonz Minister of Fire

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    Ran into a similar issue years ago as my power would drop a lot with just about anything turning on. The problem was out at the transformer caused by a bad connection at the power company's Insulug. I verified the connections were good at the main breaker and meter socket before N-star found the problem at the pole..

    Ray
  13. charly

    charly Guest

    I'm finding my dimming lights are most likely going to be because of the wire run at the poles.. My drop service comes from down the street from the one transformer pole to another pole by my house, probably 4-5 hundred feet of drop service run, then from the pole to my house, another 100 plus feet.. Electrician is coming shortly and going to talk to someone he knows about the service run from the poles.. Seems awful long and my neighbors service entrance wires are twice as big and only a 50 ft run... Be interesting to see what caused the dimming of my lights.. Breaker box checked out fine.. Installing a new meter and entrance cable and getting rid of my two 200 amp fuses and knife switch box before my main panel... Basically going to clean up any faults..
    raybonz likes this.
  14. Joful

    Joful Minister of Fire

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    A residential circuit breaker has two trip mechanisms, magnetic and thermal. The thermal trip mechanism is driven by a bimetalic element, thru which the load current is drawn, causing heating of the element. A given load at or over rating will trip the breaker in a given time, and there can be trouble with these elements, as they are somewhat sensitive to heat.

    Now, as you've already indicated a loose connection can drive temperatures easily exceeding the ignition temperature of wood. This would mean at least 300C, under most conditions, and typically much, much hotter... as there will be significant temperature drop between the actual heat source (loose connection) and any wood outside of the receptacle box.

    Copper is a very good thermal conductor, roughly 400 W/m°K, and AWG 12 has a diameter of 2.053 mm. Completely ignoring the heat generated by drawing current thru the wire (which can be significant, but is an intended component of the system), and just focusing on the heat transfer from a hot loose screw in a receptacle to a "cold" breaker, 20 meters of this wire would have a thermal transfer of 23 milliwatts, from receptacle into wire.

    Now, consider that little bimetallic element in the breaker, which is designed to heat as current is drawn thru the breaker, and trip when it generates sufficient heat. These elements are designed to dissipate minimal power, so as to not cause a voltage drop in the system, so they are typically designed to operate on microwatts of power themselves. Now, you're pumping an addition 23 milliwatts of unintended heat into the wire, some portion of which will directly reach the breaker, after loosing some heat out thru the insulation (1000x poorer conductivity than the copper wire). What do you think is going to happen, after a period of time?
  15. fbelec

    fbelec Minister of Fire

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    joful so what your saying is heat can now travel down not only up. i change out receptacles all the time. thats my job. under normal use, volts and amps a breaker can get warm to the touch and run that way for ever. now you say that the heat that is generated at a receptacle that usually goes bad way before anything gets back to the breaker or fuse can trip the breaker or fuse before it melts the insulation on the wire and either shorts hot to neutral or hot to ground or shorts to the metal of a box, which is ground. and so what if it's a plastic box it would still short the wires. when i get to the house that i get a call for a outlet that is not working the insulation is all burnt off and the copper wire got so hot that when i move it it broke or was already broken. way before it even gets to a wood stud. i must have missed something not only in school but in the real world experience i get daily. what ray said a few up is what happens. the more hot and cool cycles and arcing is made the more carbon on the wire and screw to the receptacle and it snow balls until that receptacle and the ones down stream shut down. the breaker is still on. what charly is seeing, the length of the service drop and a long distance from transformer to his house across a wire that is aluminum and size 1/0 is his problem. all those houses along the wire is to much for the wire and is draging down the voltage. if there were a problem with a main breaker or fuse holder you would be able to smell it long before something happen.
    raybonz likes this.
  16. jharkin

    jharkin Minister of Fire

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    It always has. and sideways too. Heat will travel in any direction moving from hot to cold until temperature equilibrium is reached in a material. Its only "hot air" which naturally wants to rise above regions of cold air, due to hot air being less dense.


    Not to say that this is not all theoretically possible, but without doing the math itself it sounds like a bit of a stretch?? Did you actually find the wire hot near the load center? Are sure that the loose connection in that receptacle for your AC wasn't just causing enough resistance to push the steady state circuit draw over the limit by a tiny fraction (say to 15.00000001 A or something) which took a long time to trip the coil....
  17. Joful

    Joful Minister of Fire

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    yes. This is how the problem was discovered. Realized this wire was getting warmer than the rest, feeling around in the panel, so I checked the connection at the breaker, which was fine. Swapped breaker, and new one did the same. Then thought to pull the receptacle, and found the problem.

    Now, I have an MSEE (actually 75% complete on my Ph.D.), and do thermal simulations and calculations as a part of my job. Back when this happened, I was 14 or 16 years old, and just trying to keep mom's house running.
  18. raybonz

    raybonz Minister of Fire

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    No what I am saying is your breaker will never see this heat however your wire at the bad connection will and your house will be on fire long before your breaker ever trips.. I have been a licensed electrician for 25 years and have worked industrial for 35 years plus and I do have an education as well. FBelec is just like me and we have been there and done that. The OP like FB said is experiencing voltage drop which gets worse with loading to the point that his service cables essentially become heaters.. It's a vicious circle that will not end until his service wires are upsized. BTW I fully understand how breakers work along with many other things..

    Ray
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  19. fbelec

    fbelec Minister of Fire

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    hot air rises but also with heat on metal like a heat sink pulling heat away from the heat maker and it's usually at the top
  20. fbelec

    fbelec Minister of Fire

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    sorry charly didn't mean to hijack the thread.

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