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!

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)

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)

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

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 and GMRc

This is my understanding, yes.