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Thread: Old conventions

  1. #1
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    Aug 2011
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    Columbus, Ohio, United States
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    Old conventions

    Hello!

    I work for a company who does industrial installations, sometimes outdoors.

    We use a lot of rigid metal conduit and thhn/thhw wire.

    My boss for the longest time would refuse to use smaller wire or conduit for 480V wiring. As a rule of thumb, he would never allow us to spec. anything smaller than #12 wire for 480V power wiring and never anything under #14 for 120V or DC wiring. He also would never allow us to use anything smaller than 3/4" rigid metal conduit.

    When we asked him why, we never got a good reason. It was always, "This is how we've always done it, it's not going to change!"

    Well, he has left the company and now I'm the decision maker. Do these old conventions have any reasoning behind them? I would like to be able to use #14 thhn/thhw wire for 480V power feeds and 1/2" conduit where allowed.

    Does anyone know where these conventions originated from?

    Thanks!

  2. #2
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    Sounds to me like “The ham bone story.” A man watches as his wife cuts the end from a bone-in ham. She tosses the end away, and prepares the remainder for cooking. He asks why she did that. She replies that that is how her mother did it. So they call her mother and ask why she did that. The mother replies that that is how her mother did it. So they call grandma, and ask the same question. Her reply: well I only had this small pan, you see.

    If the load on the 480V circuit is under 15 amps, you can certainly use #14. If you stay within the fill limits, you can certainly use 1/2” conduit. That said, every set of project requirements I have come across from any client I have worked with have said the minimum wire size for power circuits is #12. Many have set 3/4" as the minimum conduit size. I never heard a specific technical basis for these requirements. Perhaps that is how their mother did things.
    Charles E. Beck, P.E., Seattle
    Comments based on 2017 NEC unless otherwise noted.

  3. #3
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    inside Area 51
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    > > > >

    minerman,

    If you DO use the smaller sized conductors & conduit, ...just be
    able to provide documentation from your adopted NEC, of the
    exact, applicable Article & Tables [ if asked for the "Why" ].

    < < < <

  4. #4
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    Apr 2006
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    Simi Valley, CA
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    It's always funny as an inspector, how guys will rant and rave about costs and how they are losing bids and then, on their own, run up the cost of the job.

    a lot of time, while it may be a spec on the plans, you can call the engineer and have it revised.

    I had a plan check the other day and they were calling for #2 on a 30 amp breaker. I called the client and asked her about it and she said that she had questioned the engineer about it and he asked what the problem was as he was sure it would pass. Well yeah anytime you over size the conductors x5 it should pass.

    Now could you imagine how that call would have gone, if I had asked for #2 for a 30 amp circuit?
    I can build anything you want if you draw a picture of it on the back of a big enough check.

    There's no substitute for hard work....but that doesn't mean I'm going to give up trying to find one.

    John Childress
    Electrical Inspector
    IAEI / CEI / C10
    Certified Electrical Inspector

  5. #5
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    Thanks for the commentary. I'm glad you all seem to agree. LOL at the ham story.

  6. #6
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    My guess is for industrial locations with RMC, 1/2 and 3/4 FS boxes are the same size and about the same price. Very little difference in labor between 1/2 and 3/4. So the 3/4 gives a bit more future as RMC is more expensive to replace.
    Wire, well, I don't like 14 for general circuits its a bit small. I'll use it for control wiring.
    Moderator-Washington State
    Ancora Imparo

  7. #7
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    I would keep the 3/4" limitation for most things. As pointed out the cost difference is minimal.

  8. #8
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    Quote Originally Posted by charlie b View Post
    That said, every set of project requirements I have come across from any client I have worked with have said the minimum wire size for power circuits is #12. Many have set 3/4" as the minimum conduit size. I never heard a specific technical basis for these requirements.
    Somewhat related.....
    For control circuit wiring in the panels we make, we use 1.0mm2 wire as a minimum. In terms of current rating, it is more than adequate. But there are a number of advantages in using just one size as a standard.
    The drawings don't need to specify a size for every wire. "Unless otherwise specified all wiring is to be....."covers it.
    Standard wire can be stocked/used. And mostly just one colour - black.
    Standard wire markers can be stocked/used.
    Standard crimp (compression) wire terminations can be stocked/used.
    Standard DIN rail terminals can be used to interface with the outside world.

    Now and again we get specifications that set a larger minimum conductor size or different colours for different voltages - grey for 24Vdc for example. And we can get dispensations for comms cables and for smaller wiring to PLCs

  9. #9
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    Indiana
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    Urban legend time:
    According to my colleague's second cousin's college roomate's uncle on his adopted father's side:
    Once upon a time in industrial plants no one wanted to wait on ladders. The practice was to step up on anything that would support a man's weight. Sometimes that would be electrical conductors that would yield unexpectantly.
    So a brilliant "engineer strung several conductors between 40' bay columns and stood on each to test them. He decided that #14 couldn't be expected to support a man. Therefore all conductors run outside an enclosure must be #12 or larger.
    A rose by any other name is tax deductible [1978 Wayne Wilcox]
    People who read too many books get quirky. [2000 John Taylor Gatto]

  10. #10
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    If I have to spec it, I also spec #12 as the smallest wire that can be pulled in conduit. I do not care what voltage it is. It is far more resistant to being damaged than #14 or smaller. Cost wise, there is not any real difference.

    As for conduit size, I also spec 3/4 as the smallest size that can be used. Again, mostly because of the potential for damage to wires being pulled in smaller conduit is much higher.

    These days, I generally avoid having anything to do with spec'ing wire sizes or conduit sizes as over the years I have discovered ECs often ignore what I put on the drawings and do as they please anyway. If it is not on the drawings, it is not my problem to deal with.
    Bob

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