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Heat buildup in wiring

Discussion in 'Electrical Engineering' started by Terry, Mar 15, 2010.

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  1. Terry

    Terry Guest

    Hello:

    Just wanted to get the general opinions on the following that was
    being discussed:

    The statement:

    30 amps will develop 2 1/4 times more heat than 20 amps in a conductor

    The data to backup the claim:

    Most of us know that developed power is the product of the voltage and
    current, but voltage is equal to the current multiplied by the
    resistance, Hence:

    P=VI
    V=IR

    Substituting:
    P=(IR)I
    P=I(squared)R

    The resistance for the conductor hasn't changed but the current has
    changed from 20A to 30A. 30 is 20(1.5):

    P=(1.5)(squared)R
    1.5 (squared) = 2.25
    P=2.25R


    To me this is a wee bit simplistic, it doesn't take into consideration
    size of conductor, ambient temperature, length of run etc...
     
  2. daestrom

    daestrom Guest

    It may seem simplistic, but that's the answer.

    Now what isn't so simple is just how hot the conductor gets when
    carrying 30A vs 20A. Maybe that's what you're thinking of?

    With 2.25 times the heat generated, it should take 2.25 times the
    temperature difference between ambient if all other factors are the
    same. So if you had a 20C rise in conductor temperature over ambient at
    20A, you should expect 45C rise with 30A.

    But it actually is a bit worse than that because at 30A, with a higher
    temperature in the conductor, the resistance of the conductor is
    somewhat higher. So the power dissipated is slightly more than 2.25
    that at 20A.

    If the temperature rise at 20A is more because of insulation, conduit,
    embedded in a wall, etc..., than the rise for 30A will still be 2.25
    times as much, but that can be too much. For example if the temperature
    rise with 20A was 40C, than carrying 30A would make the rise 90C. And
    that's probably too high for the equipment and insulation.

    And of course if the ambient temperature is abnormally high, adding on
    the same rise (either 45C in the first case or 90C in the second case),
    it can still push the resulting temperature over some limits. That's
    why cabling is de-rated for high ambient temperature installations.

    daestrom
     
  3. Same effect with heating at contact and connection resistances too.
    That's why really good connections are essential with high current
    circuits. When heating does start, this often results in the quality
    of the connection changing for the worse, and the resistance rising.
    This generates more heat, and the two effects fuel each other, and
    you get a runaway, resulting in the connection grossley overheating
    and failing.
    and also (in UK at least, and probably most places) derating for
    cable grouping - running multiple conductors togther where mutual
    heating effects result in a higher temperature rise.
     
  4. Guest

    Guest Guest

    The assumption is that the same conductor is used at 30A as at 20A . You
    could do it, but in most jurisdictions, the codes take into account the
    changes in heating and dictate appropriate wire sizes.
     

  5. You're an idiot, and google will be killing this account soon as well.

    In fact, I am going to gather up ALL of the ip addresses you have google
    accounts with, and forward then all to google so they can see what you
    are about, and kill these accounts as well. The terms of google is that
    you do not lie about your identity and you have only ONE account. They
    will surely be interested in the fact that you post under several.

    They will be quite interested that you are back online, abusing their
    service yet again.
     
  6. You are an idiot, and YOU are the troll, dipshit.
    No, you have illegally opened multiple Google groups or google mail
    accounts and they need to know about your pathetic abuse.

    I am sure that they will also be interested in this claim, which I
    personally feel is total bullshit, because a retard like you doesn't have
    the brainpower to "route through" ANY other "account", you stupid fucking
    troll bastard liar.
    False accusations should get you in deep shit as well, but the current
    society we live in tolerates retarded fucks like you.
    Nope. every post I have ever made into Usenet in the past 5 years has
    been through THIS very account, you retarded piece of shit, and their
    logs would also show that NO other access of ANY type has EVER taken
    place from this account. Too bad that your false claims only make your
    position weaker. How your retarded ass could possibly be any weaker than
    it already is is surprising, but you take yourself lower with nearly
    every post you make.
    You are the menace. Especially when you claim to be getting access
    through other people's connection. That makes you a felon.
    Really? So, you have never listened to ANYONE when they tell you that
    your all caps horseshit is abusive. You ignore everyone, and now
    EVERYONE ignores you. Are you happy, dumbfuck?
    My authority is that of a Usenet user. Your lameness is that of a
    Usenet abuser. Unless you kill the all caps horseshit, that fact will
    never change.
    You wouldn't know, since you are about as far from God as an NYC
    retarded twit can get.
    I notice that you have cut out the offensive abuse. What's the matter,
    asshole? Did you get some fan mail from google groups administration?
    No, you haven't and you are so stupid that you wouldn't know.
    What? Are you going to block the gate? You're a fucking joke, boy. Or
    is it Roy?
    More folks to ignore you 100%, and laugh at your pathetic ass.

    You have a hard time activating your welfare check, boy. You couldn't
    activate anything more than that if you tried.
    A threat? Since it comes from a retarded little pussy punk like you, I
    have no fears.
    No, you are the retard Roy, and Google should kill every account you
    have ever opened and every account you currently have, and they should
    block your retarded, pathetic ass from ever opening any more ever again.
    You are a pathetic little cry baby retarded twit, and nothing more.
     
  7. I have not lied, Roy, you fucking total retard.
    I have not threatened anyone either. Telling you that you NEED a
    sledgehammer upside the head is NOT a threat, idiot. It is an opinion.
    I have broken no laws. You have broken laws, and you will again after
    the sting from the last complaint fades from your feeble brain.
    You should remember those words, idiot. They will bite YOU in the ass.
    False complaint? Let's see which post I complained about and do a poll
    as to whether or not it is abuse. These quotes are what you are about.
    They show the world just how pathetic you are.


    Quote from the abusive bastard Roy, who posts as "PROTEUS":

    YOU ARE EITHER A REDNECK FUCKTARD OR A TROLL
    A SALIVA DRIVEN DEVICE
    GET A LIFE
    OR AT LEAST TELL YOUR DOCTORS TO CUT BACK ON THE MIND ALTERING DRUGS

    I AM PROTEUS

    Yep, that one looks pretty abusive. Let's see:

    THE WIZDUMB IS JUST OOOZING OUT OF YOU HERE ISN'T IT DONKEY ?

    I AM PROTEUS

    Yep, that one makes you look like an immature little abusive shithead
    as well. Those two were one right after the other. That also says
    something about just how tragic your case is. Let's try one more:


    YOU ARE JUST A RELFECTION OFTHE TOTAL FAGGOTY FUCKTARD YOU WERE
    NEVERTHELESS I WILL LEAVE MY DEEP ANAL PROBE IN YOUR RECTUM AS A
    REMINDER JUST IN CASE YOU DECIDE TO CONTINUE HARASSING THE FINE FOLK
    THAT VISIT ALTERNATE.ELECTRICAL.ENGINEERING

    I AM PROTEUS

    That one is about as abusive as it gets. I didn't catch that one last
    time, but I can surely issue a nicely drafted complaint for that one too.
    You deserve to lose everything, and you most certainly should.

    How about I post it all in a nice new Google group specifically meant
    to show google where their abusive posters are at? I'll bet that your
    accounts would not last long after that, and I'll bet that the group
    gains a HUGE popularity among folks trying to purge the net of abusive
    little retarded bastards like you.
    My company is separate from my access to Usenet, you fucking total
    retard.

    Now take your retarded ass to a tall bridge and jump off of it, ROY!
    Feeding the fishes is all your pathetic hulk would be good for.

    Sad though, they wouldn't go near your stinking corpse.
    You are Roy, and absolute total retard. The MS owned WebTV service
    even TERMINATED your abusive, retarded ass. That should tell you
    something, dumbfuck.
     

  8. At 153rd and Morris. That is one location from which you abuse this
    group.

    230th and Broadway is another.

    Looks like your pathetic street beggar ass gets around the slums of
    Harlem.

    Where else do you commit your pathetic abuse from?

    Hahahahah... it figures.. it is called "Hicksville, NY"!

    Bwuahahahahahhaha!

    Let's see if these guys like how you post through google groups:



    I will track you down, and I will report your abuse, BOY!
    You can count on it!
     
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