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Thread: Neher-McGrath Calculation

  1. #1
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    Neher-McGrath Calculation

    How can this calculation I= Square-root of TC-(TA+ delta TD) divided by RDC (1+YC)RCA be preformed if according to Neher-McGrath the dielectric loss temperature rise (delta TD ) is insignificant to voltage's under 2000 when the calculation is in 310.15 ampacities for conductors rated 0-2000 or would i just use the zero in it's place? Thank you,Stafford Electric

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    Quote Originally Posted by stafelectric@yahoo.com View Post
    How can this calculation I= Square-root of TC-(TA+ delta TD) divided by RDC (1+YC)RCA be preformed if according to Neher-McGrath the dielectric loss temperature rise (delta TD ) is insignificant to voltage's under 2000 when the calculation is in 310.15 ampacities for conductors rated 0-2000 or would i just use the zero in it's place? Thank you,Stafford Electric
    Well if you don't have the specific "insignificant" value of delta TD, I surmise you'd plug in 0 for that value....
    I'll never get there. No matter where I go, I'm always here.

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    I assume you are performing this calculation under engineering supervision?
    "Just because you're paranoid, doesn't mean they're not out to get you"

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    Quote Originally Posted by Smart $ View Post
    Well if you don't have the specific "insignificant" value of delta TD, I surmise you'd plug in 0 for that value....
    I wouldn't. Speaking as an engineer, if I did not know what value to insert into a formula, I would use a value that I knew to be either higher or lower than what the actual value would be, and I would choose the higher or the lower based on which of those gave me the more conservative result. In this instance, a "conservative value" of ampacity would be low. That is, you do not want your calculated ampacity to be higher than it should be. So you don't use an assumed value for an unknown variable, if that assumed value gives you a higher value for ampacity. In the formula under consideration, if you use zero for deltaTD (i.e., the lowest possible value), that would tend to give you a higher answer for calculated ampacity than you would get by using a large number for deltaTD. So whatever else might be the best approach to this situation, I would not pick zero as the value for deltaTD.

    Actually, I would not use that formula at all. I think it is useless. If I had to calculate the ampacity for an underground ductbank, I would use a packaged software program.
    Charles E. Beck, P.E., Seattle
    Comments based on 2008 NEC unless otherwise noted.

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    Quote Originally Posted by charlie b View Post
    I wouldn't. Speaking as an engineer, if I did not know what value to insert into a formula, I would use a value that I knew to be either higher or lower than what the actual value would be, and I would choose the higher or the lower based on which of those gave me the more conservative result. In this instance, a "conservative value" of ampacity would be low. That is, you do not want your calculated ampacity to be higher than it should be. So you don't use an assumed value for an unknown variable, if that assumed value gives you a higher value for ampacity. In the formula under consideration, if you use zero for deltaTD (i.e., the lowest possible value), that would tend to give you a higher answer for calculated ampacity than you would get by using a large number for deltaTD. So whatever else might be the best approach to this situation, I would not pick zero as the value for deltaTD.

    Actually, I would not use that formula at all. I think it is useless. If I had to calculate the ampacity for an underground ductbank, I would use a packaged software program.
    You make a fair point.... but that is raising a value that has been authoritatively noted as insignifcant for the conditions to one of significant status.
    I'll never get there. No matter where I go, I'm always here.

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    Quote Originally Posted by Smart $ View Post
    . . . but that is raising a value that has been authoritatively noted as insignifcant for the conditions to one of significant status.
    What authority? The OP stated it, but did not cite an authority. For my part, I have never read the original paper by Messrs. Neher and McGrath. I think I was four years old, when it was first published.
    Charles E. Beck, P.E., Seattle
    Comments based on 2008 NEC unless otherwise noted.

  7. #7
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    The informational note in 310.60(D) says delta TD is negligible below 46 kV. I would include it for 12 kV, however, but use zero for LV cable.

    That being said, I agree with Charles in general about the usefulness of the equation as it stands. It is all the other factors besides delta TD that are the problem. There have been books written about how to determine them under various conditions. I have calculated ampacities using Neher-McGrath and it is complicated. Using a packaged software program is the way to go for sure, especially for youngsters like Charles (I was 11 years old).

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    Quote Originally Posted by charlie b View Post
    What authority? The OP stated it, but did not cite an authority. For my part, I have never read the original paper by Messrs. Neher and McGrath. I think I was four years old, when it was first published.
    Googling >"dielectric loss temperature rise" "insignificant for voltages below 2000"< I got several hits. Given the possible variations on textually expressing the idea, I'm sure there are many more.
    I'll never get there. No matter where I go, I'm always here.

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    Quote Originally Posted by charlie b View Post
    What authority? The OP stated it, but did not cite an authority. For my part, I have never read the original paper by Messrs. Neher and McGrath. I think I was four years old, when it was first published.
    Did we have the printing press at that time?

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    Quote Originally Posted by bob View Post
    Did we have the printing press at that time?
    Not yet. I had to learn to write using a chisel and a slate, and my teacher's name was Moses.
    Charles E. Beck, P.E., Seattle
    Comments based on 2008 NEC unless otherwise noted.

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