Learn how to calculate the GMR and Radius for a bundle (more than one conductor per phase) for both single-phase and three-phase transmission lines.

NCEES® Reference Handbook variables:
r is the radius of the individual sub-conductor.
r' is the self GMR of the individual sub-conductor.
GMR is the GMR of the overall bundle.
GMR_C is the overall radius of the bundle.

Video Table of Contents (click each timestamp to advance):

0:45
1. No bundle - one (1) conductor per-phase:
r' = 0.7788r for Solid Conductors
GMR_C1 = diameter/2

For named stranded conductors (like Blue Jay ACSR), the GMR has to be looked up in a table or given in the problem.

2:02
2. Two conductor bundle - two conductors per-phase:
GMR = √(r'·d)
GMR_C = √(r·d)

7:19
3. Three conductor bundle - Three conductors per-phase;
GMR = ³√(r'·d²)
GMR_C = ³√(r·d²)

12:38
4. Four conductor bundle - four conductors per-phase;
GMR = ⁴√(r'·d³·√2)
GMR_C = ⁴√(r·d³·√2)
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