Grounding Electrode Calculator
Size the grounding electrode conductor (GEC) for your electrical service per NEC 250.66.
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How It Works
The grounding electrode conductor (GEC) connects your electrical system to the earth through a grounding electrode. This provides a path for lightning and fault current to safely dissipate into the ground, protecting people and equipment.
The Formula
GEC size is determined from NEC Table 250.66 based on the largest service entrance conductor size, with maximum sizes limited by electrode type.
Variables
- GEC — Grounding Electrode Conductor -- wire from service panel to grounding electrode
- AWG — American Wire Gauge -- the wire sizing standard
- kcmil — Thousands of circular mils -- used for wire larger than 4/0 AWG
- 25 ohms — Maximum ground resistance for a single ground rod per NEC 250.53(A)(2)
Example
A 200A service with 4/0 AWG copper service entrance conductors: GEC = #2 AWG copper per NEC 250.66. If using a ground rod, GEC is limited to #6 AWG copper maximum per NEC 250.66(A).
Tips
- Ground rods are limited to #6 copper GEC because the rod itself is the weak link -- a larger wire would not improve grounding.
- Ufer (concrete-encased) grounds are the most effective and lowest-resistance grounding method.
- Metal water pipes alone are no longer acceptable as the sole grounding electrode -- they must be supplemented.
- Never use aluminum for a grounding electrode conductor that is in contact with earth or concrete.
- Two ground rods are required if a single rod has resistance above 25 ohms (most inspectors require two regardless).