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  1. #1
    Join Date
    May 2010
    Location
    New Mexico
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    35

    Question Temperature correction factors

    I'm running several new circuits through 1.5" EMT to a large junction box where I split off to 3 - 3/4" EMT runs. All of this is external to the building. Most of this is under eaves and shaded, but the 1.5" is in direct sunlight for a good part of the day and gets very hot to touch. I have not tried to measure the exact temperature.

    According to Ugly's Electrical References (Not NEC, I know, but a good reference book), I need to derate conductors according to ambient temp and # of current carrying conductors in a run. Ugly's says a 12 AWG THHN is rated to 25 AMPS, but at an ambient temp of 150F (which I'm assuming the EMT is capable of getting to on a hot summer day) I have to derate the conductor to 58%. So 25 x .58 = 14.5 Amps.

    Now I need to further derate the conductor based on # inside the conduit. I'm running 8 circuits. Not counting grounds, I'm assuming I need to figure 16 current carrying conductors (8 hot + 8 returns). Ugly's tells me that for 10-20 conductors, I need to further derate to 50%. Now I'm down to 7.25 amps for a 12AWG THHN wire.

    All runs are below 100 ft., so I'm not worried about voltage drop.

    Does this all sound right? Am I missing something? This sounds like I'm going to need to run bridge cables just to run a few light bulbs.

    One thing I'll do is paint the EMT so it will collect less solar heat. I don't want to run a bunch of 1/2" EMT conduit when I already have a 1.5" going to where it needs to be, and a 5 amp circuit breaker isn't going to cut it for the load on some of these circuits.

    Help?

  2. #2
    Join Date
    May 2010
    Location
    New Mexico
    Posts
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    Default Re: Temperature correction factors

    So... I do not have to derate the conductors because the conduit is not running over a roof. I can run 12 AWG THHN on a 20A breaker and sleep easy. I'll still paint the conduit just because it looks ugly.

    Shared return: I understand why you can share returns on two circuits on different phases. When one phase is at peak, the other is essentially off. The wire never carries excess current. The part I don't like is when I come across one of these after killing power to the circuit I'm working on, and see sparks when I separate the neutrals, knowing that the other "neutral" now has 120 volts on it.

    The old building we're in tends to have bad grounding issues. It shows up as noise in our sound and video systems. I want to keep things as clean as I possibly can.

    Thanks for the clarification!

    John

  3. #3
    Join Date
    May 2010
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    New Mexico
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    35

    Default Re: Temperature correction factors

    I made a mistake in my initial calculations. Looked in the wrong column for conductor "ampacities". 12 AWG THHN copper has a max rating at 30C in raceway of 30 Amps, not 25.

    So, 30 Amps derated to 50% for # of conductors equals 15 Amps. I'm installing 15 Amp circuits, so I think I'm okay. Actual load will be less. Heaviest use will a 10A lighting circuit. I want to make sure I use the right breakers though.

  4. #4
    Join Date
    May 2010
    Location
    New Mexico
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    35

    Default Re: Temperature correction factors

    Quote Originally Posted by Ernie_Fergler View Post
    Just wondering if you have explored the possibility reworking {when possible** the grounding system by an upgrade to an isolated ground system?
    We just had that done, actually. Electrician came in and installed a new main panel with two ground rods (2 required in this area, 6 ft apart). He reworked all the grounds to, and in, the subpanel, with no actual connection to ground. Originally had no bonded ground at the main disconnect and all the grounds and returns were tied together at the subpanel. A lot of the old circuitry used the conduit as the ground. Running actual ground wires as we touch things. Sound and video are newer circuits with dedicated ground wires.

    The grounds rods were driven through the asphalt parking lot (runs all the way up to the building). I'm thinking that soil has not seen moisture for decades and is probably not a great ground. I'm thinking of knocking some asphalt away from the ground rods so rain (or a hose) can soak the soil. Think that may help?

    John

  5. #5
    Join Date
    Mar 2009
    Location
    wisconsin
    Posts
    159

    Default Re: Temperature correction factors

    i wouldnt sweat the ground rod thing. your getting your best ground from utility anyways. the rods are mainly for lighting, and a supplement. If you really want to go to the hastle of busting up asphault and wet the soil, knock your self out. Just seems a bit excessive to me! Did the electrician tie the ground wire to your water line??

    About the part about not de-rating due to not running over the roof. Still de-rate, if its a 150 degrees its a 150.

  6. #6
    Join Date
    May 2010
    Location
    New Mexico
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    35

    Default Re: Temperature correction factors

    Quote Originally Posted by sparky1 View Post
    i wouldnt sweat the ground rod thing. your getting your best ground from utility anyways. the rods are mainly for lighting, and a supplement. If you really want to go to the hastle of busting up asphault and wet the soil, knock your self out. Just seems a bit excessive to me! Did the electrician tie the ground wire to your water line??
    We have incredibly dry and porous soil here in the southwest. I've actually seen this make a difference with electronic equipment in a factory I worked at. The noise had been taken care of, but showed up again when temps got to 100F.

    Ground did get tied to the water pipe at the water heater at the subpanel (indoors) while the main panel (outdoors) is close to the ground rods.

    Incoming power goes through the meter to a 200A inline fuse disconnect (where the ground is connected and tied to neutral), to a 200A Sq D Homeline panel (outdoors) (ground/neutral separate). 200A panel also feeds a 100A subpanel indoors. 100A subpanel was originally the only panel for the building. Subpanel also feeds another panel right next to it, because they ran out of space for circuits at some point.

    200A panel is brand new and is where I'm adding the additional circuits.
    Last edited by jvande7471; 06-25-2010 at 05:53 PM.

  7. #7
    Join Date
    May 2010
    Location
    New Mexico
    Posts
    35

    Default Re: Temperature correction factors

    Ernie, how much could a 200A power conditioner possibly cost?!

    I think I'll hunt around for some loose ground screws and wirenuts first.

    Cutting out some asphalt and moistening the soil around the ground rods SEEMS to have at least helped. But then again the temp dropped down into the 80's so it's hard to say if that was it, or it's a loose connection somewhere. Still hunting.

  8. #8
    Join Date
    May 2010
    Location
    New Mexico
    Posts
    35

    Default Re: Temperature correction factors

    Closing feedback: Although cleaning up grounds has helped a lot, it didn't solve the problem.

    Noise was being injected onto the power source from switching controllers for stage lighting that had been installed. Dimming is controlled through pulse width modulation, and all that high amperage switching was causing noise on other circuits.

    Installed EMI filters on sensitive equipment (sound board, video projector, computer, etc.) and things are working well now.

    One thing I did notice is that there is no ground conductor running from the new Sq-D panel to the subpanel. It seems to be relying on the conduit, and the subpanel is tied to the water line. There is definately a difference in the grounds at these two panels. We'll take care of that when we have the Federal Pacific sub-panel replaced. Many thanks to people posting in this forum for the heads-up on FP. I would not have known to be concerned otherwise.

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