# Geometric Mean Radius



## Byk (Jan 8, 2021)

Can someone please explain when do use any of the GMR formulas listed below?

All of the problem that I solved involving GMR in the equitation (typically looking for  capacitive reactance or inductive reactance) do not use any of the formulas for GMR listed below.

Typically, they use r'.

I just want to make sure I do not make a mistake on a simple problem where all you need to do is plug numbers.

Thanks for all the help!


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## jd5191 (Jan 8, 2021)

The GMR formulas you've listed are used when multiple conductors are used in a single phase of a transmission line. I haven't seen many practice problems that are anything but single conductor per phase.




A side note based on one of your statements: the inductance formulas use GMR (0.7788r) and capacitance formulas use GMRc (r)


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## DarkLegion PE (Jan 12, 2021)

Someone correct me if I'm wrong here:

If you're using subconductors (multiple conductors per phase) you''ll use you the formulas you listed when solving for XL or L

GMRc is used for finding C and XC. It's the same formula except you use you're actual radius instead of r' (0.7788r)

When you just have one solid conductor (like in the NCEES exam) you can just use the radius of that cable for GMR (radiusx.7788) and GMRc (the actual radius)


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## jd5191 (Jan 12, 2021)

DarkLegion said:


> Someone correct me if I'm wrong here:
> 
> If you're using subconductors (multiple conductors per phase) you''ll use you the formulas you listed when solving for XL or L
> 
> ...


This is my understanding, yes.


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## DarkLegion PE (Jan 12, 2021)

jd5191 said:


> This is my understanding, yes.


I made a mistake in my last sentence, I edited to reflect the change. Wanted to let you know!


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## jd5191 (Jan 12, 2021)

DarkLegion said:


> you can just use the radius of that cable for GMR (sqrt(radiusx.7788)) and GMRc (the actual radius)


There's no sqrt in the formula for r', but yes, 0.7788r for GMR of single strand


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## DarkLegion PE (Jan 12, 2021)

jd5191 said:


> There's no sqrt in the formula for r', but yes, 0.7788r for GMR of single strand


Woops thanks!


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