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Electrical Question

newbuilder | Posted in General Discussion on March 7, 2009 01:55am

I’m getting into my main panel to run some wires to a sub panel in a new addition.  In making sense of the wiring that was set up YEARS ago and has worked without problems all this time I’m finding some things that make no sense to me.  Two examples:  The electric dryer clearly says on the back panel that it needs “a dedicated line with 30 amps available to it” — but the feed from the panel is clearly only providing 20 amps!  Another is a plug-in in the kitchen that was run shortly before we bought the house in ’94 that is for kitchen counter use.  We’ve run a 14 amp microwave and other things off of it with no problems for 15 years.  It turns out that it is running off of one 10 amp breaker. 

My question is ‘how can this be?’  How can a 30 amp drying run off of a 20 amp breaker and a 14 amp microwave run on a 10 amp line?  Is this common?  safe?  And how can it not trip the breaker?  If anyone has any clues on this I’d be most appreciative.

Thanks!

Aaron

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Replies

  1. sledgehammer | Mar 07, 2009 02:38am | #1

    Welp.... I hate to say it but.... if you can't figure it out, you probably shouldn't be running a sub panel either.

  2. USAnigel | Mar 07, 2009 03:20am | #2

    The rating is the max used by that unit. The circuit breaker is there to protect the wiring from over heating and causing problems like a fire.

    The wire must be the right size and protected by the correct breaker.

    An oversized wire with the correct breaker is fine. Undersized wire is a big "don't do.

    20amp breaker with #14 wire is a major "don't do"!

    15amp breaker with #12 is fine and might be the size needed due to the line length to reach the project.

    1. newbuilder | Mar 07, 2009 04:37am | #3

      Maybe I was somehow unclear.

      I'm not talking about wire size -vs- breaker -- I'm talking about two situations in which the appliance stated that the demand was much higher than the panel/wiring was feeding it.   In one, a clothes dryer that stated a need for 30 amps was only being fed 20 ... in another a line that carried 10 amps was being used by a 14 amp microwave. 

      How can this be?  How ok?

      Thanks -

      A

      1. USAnigel | Mar 07, 2009 06:00am | #7

        Barmil gives a good explanation of whats happening.

        See if that gives you a better idea.

  3. webted | Mar 07, 2009 05:31am | #4

    These devices work because they're not actually drawing the current that the circuit is protected at.

    Your "30 amp" dryer will work just fine on that circuit that is PROTECTED with a 20 amp breaker, as long as the dryer doesn't pull more than 20amps. Your dryer probably draws somewhere in the range of 14 - 15 amps, a little more on startup.

    Same logic carries on the microwave.

    But the other guys are right. If you aren't clear on the difference between current draws and circuit protection you should really be getting some help on your wiring project. Perhaps this forum will provide help in that regards - there are some real live electricians here (I AM NOT!) that often offer quite fine guidance.

    -t

    1. barmil | Mar 07, 2009 05:48am | #6

      An analogy. A car with a 200 horsepower engine is rarely delivering 200 horsepower. It only delivers the horsepower demanded of it in a particular situation. When more than 200 horsepower is demanded, it bleeds off RPM or explodes, depending. Same with an electrical circuit. I have an old 15 amp circuit in my kitchen that has a coffee maker, a coffee grinder, a toaster oven, and lights being fed by it. It never blows its circuit breaker because I know not to have them all on at the same time. I would never leave this in the hands of my daughter, because she believes from her experiences that there is unending electricity and hot water in a home, never knowing how things actually work. Without thinking, she would put a bagel in the toaster oven, turn on the coffee maker, and then plug her hair blower in, commending herself on her multi-tasking capabilities.  POP!

      1. DonCanDo | Mar 07, 2009 01:55pm | #11

        Nice explanation.  I particularly liked this line:

        "she believes from her experiences that there is unending electricity and hot water in a home"

        It's funny and has a ring of familiarity to it!  But I think I can understand the behavior of the youngsters who behave as if it's true since it's not terribly far from the truth either.

    2. newbuilder | Mar 07, 2009 10:35am | #10

      Hey ... thank you very much to all who responded with the great followups . . . MUCH appreciated! 

      In my wiring of the new addition, btw, I'll be closely followed by the inspector and not allowed to turn on the new juice until he has thoroughly gone over it.  It is true that I haven't done THAT much electrical work in the past, but what I'm doing is pretty straightforward and I proceed very carefully and seek an outside eye if EVER there is a question.  The things that I noted are things that were done by the LAST guy that worked on the wiring . . . a licensed electrician. 

      Anyway .. thanks again.  My questions were answered.

      A~

  4. k1c | Mar 07, 2009 05:40am | #5

    I am guessing so please don't take these as expert diagnosis. As mentioned, rating are maximum draw and maybe your dryer never needed the maximum. Your wires maybe running through the cooler parts of the house. You were very lucky. The older wires have thicker insulation but if you took it out, you may find the insulation deteriorating. 30 amp appliances, I believe, need 220 volt connections. You may want to go back to the outlet or the connections at the dryer and follow the wire back to the panel and follow each wire to how they are connected in the panel. It is possible you may have the connections confused. Don't trust the label unless you have confirmed the connection. If they follow and connect only to the 20 amp breaker, then you need to upgrade immediately. If I am wiring this house myself, I would change entire run of the wire connected this way because I would not know what kind of stress this connection had put on the insulation.
    Microwave is the same. Although it is rated for 14 amps, it does not mean it needs 14 amps all the time. Biggest draw is when the appliance starts. If your microwave needs 14 amps, maybe it has browning feature or somekind of convection cooking? Not using these features allowed you to use that circuit. To tell you I didn't even know that there were 10 amp breakers.
    Another point is, there is one name of panel manufacturer that often comes up as problem prone. Sorry but I can't remember the name now. Searching this site for defective panel probably would give you the name. If you have this panel, you may want to replace the panel before you start subpanel work. Hope this helps.

  5. brucet9 | Mar 07, 2009 06:56am | #8

    Since you are suspicious of those circuits, I'd recommend checking the wire size while you are at it. Does that dryer line have 10ga wires? For 30A it should. If they are 12ga, you can't install a bigger breaker than 20A.

    Are the wires on the 10A circuit 14ga? If so, the guy who set up the panel originally may have stuck a 10A breaker in just because that's all he had left in the truck that day. If the line length is not excessive, you could install a 15A breaker instead.

    Two of the problem panels mentioned by another poster are Zinnsco and BullDog PushMatic. They are known for having had a high failure rate - breakers not tripping when they should and, in Zinnsco's case, arcing at the connection to the bus. There have been fires caused by those panels.

    BruceT
    1. gfretwell | Mar 07, 2009 08:59am | #9

      The only place that it is really legal to use 10a breakers is on an alarm panel. There is an official intrepretation out there somewhere that came about because someone tried to circumvent the AFCI rules by using a 10a breaker in the bedroom.All branch circuit wiring also has to be 14ga or larger.

      1. brucet9 | Mar 07, 2009 11:50pm | #12

        "All branch circuit wiring also has to be 14ga or larger."You mean to say that 50 feet of 16ga zip cord supplying a detached garage where two families of illegals are living is not up to code? :)BruceT

        1. gfretwell | Mar 08, 2009 08:26am | #13

          "All branch circuit wiring also has to be 14ga or larger."You mean to say that 50 feet of 16ga zip cord supplying a detached garage where two families of illegals are living is not up to code? :)Nope The IAC (Illegal Alien Code) says that has to be an orange cord ;)

          1. User avater
            Jeff_Clarke | Mar 08, 2009 08:36am | #14

            But they'll all be wanting to get green cords ... ;o)Jeff

        2. JTC1 | Mar 08, 2009 03:18pm | #15

          >>50 feet of 16ga zip cord supplying a detached garage<<

          Zip cord is legal as long as it is used as an overhead service to the detached building.

          Not allowed for direct burial, would have to run through 1/2" CPVC plumbing pipe, clear plastic tubing or an old garden hose.

          16 ga is overkill though, could have used 18 ga in this application, but better to be oversized!

          :)

          Jim Never underestimate the value of a sharp pencil or good light.

          1. jimjimjim | Mar 08, 2009 08:54pm | #16

            Jim,

            Thanks for the info, I didn't realize this was allowed.

            I'd like to bury my wires in the PVC pipe (white, right?) - that will make for a much better install.  How deep must it be - I can go down about 3" since there are lots of rocks.

            I've learned that the color of the wires is important - do I need to use white or brown zip cord?

            !!!

            Jim x 3

             

          2. JTC1 | Mar 08, 2009 10:50pm | #17

            To answer your questions:

            How deep? = As long as it is mostly out of sight - OK if it shows at some rocks. And try to get it so the lawn mower won't hit it too often.

            What color? = If you are using white PVC pipe the zip cord should be white of course - so it matches!

            If used as an overhead service, then use brown (or black if you can get it) as this speeds ice melting in the winter due to solar heat gain.

            OTOH, if overhead and run through / supported by a bunch of bushes / trees - try and get the green for a low key installation.

            JimNever underestimate the value of a sharp pencil or good light.

          3. brucet9 | Mar 08, 2009 11:20pm | #18

            "16 ga is overkill though, could have used 18 ga in this application, but better to be oversized!"I went oversize just to be safe because there are two hotplates, a crock pot, a big TV and an electric space heater in there. Does weaving the zip cord along the top of a chain link fence and up through a hole in one of the frieze blocks count as "overhead service"?BruceT

          4. JTC1 | Mar 09, 2009 12:06am | #19

            If that chain link fence is rusty - I'd definitely go with the brown zip.

            If not rusty, you could try getting grey zip but I don't think you will have much luck; maybe some plastic model type spray paint in battleship gray will be your best chance for a color match.

            Actually most of the zip cord I have removed was fastened with office type staples to the baseboards or casing and painted with latex to match the trim.

            JimNever underestimate the value of a sharp pencil or good light.

  6. EricGunnerson | Mar 10, 2009 07:25am | #20

    Breakers don't trip instantly at their rated current. If you look on a website, you will see that they are rated to trip at a specific multiple of their rated current after a specific time period. I don't remember the specifics, but I do remember that with a small overage - say 1.5x the rated current - it takes quite a long time to draw enough current to trip.

    This allows motors to draw more than their rated current for a short amount of when they start up.

    The dryer baseplace will tell you what the actual rating is. It may be quite a bit less than 30 amps.

     

    1. newbuilder | Mar 10, 2009 08:08am | #21

      huh.  weird.  never heard this before.

      thanks -

      A~

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