This professional tool determines the minimum Grounding Electrode Conductor (GEC) size per NEC 2023 Table 250.66. It supports service/feeder and separately derived systems, parallel conductors, copper or aluminum options, and automatically applies electrode-specific caps from 250.66(A)–(C).
Select the material of the largest ungrounded conductor(s) feeding the service or SDS.
The tool reports the minimum size for both materials, then applies electrode caps to your chosen material.
Primary Reference: NFPA 70 — National Electrical Code (NEC), 2023 Edition, Article 250, specifically Table 250.66 and Sections 250.66(A)–(C), 250.30(A)(5), and 250.62(A). Access via NFPA: NFPA 70 (free access portal).
All calculations are strictly based on the formulas and data provided by this source.
For parallel ungrounded conductors of one phase, compute the equivalent circular mil area:
$$A_{eq}=\sum_{i=1}^{n} A_i$$
Determine the minimum GEC size by locating the row in NEC Table 250.66 that matches your ungrounded conductor material and equivalent area. Conceptually:
$$\text{GEC}_{Cu}=\begin{cases}
8\ \text{AWG}, & A_{eq}\le A_{Cu}(2\ \text{AWG})\\
6\ \text{AWG}, & A_{Cu}(2\ \text{AWG})
Similarly for aluminum/copper-clad aluminum thresholds (1/0, 3/0, 4/0, 250 kcmil, 600 kcmil, 1000 kcmil, 1750 kcmil):
$$\text{GEC}_{Al}=\{6,4,2,1,1/0,3/0,4/0,250\ \text{kcmil}\} \text{ per matching row.}$$
Electrode-specific caps are then applied:
$$\text{Rod/Pipe/Plate}: \text{GEC}\le \begin{cases} 6\ \text{AWG Cu}\\ 4\ \text{AWG Al} \end{cases}\quad \text{CEE}: \text{GEC}\le 4\ \text{AWG Cu}\quad \text{Ring}: \text{GEC}\le 2\ \text{AWG Cu}.$$
Scenario: Service with two parallel 350 kcmil aluminum ungrounded conductors per phase, connecting to a driven ground rod. GEC material desired: copper.
Use the physical size (AWG/kcmil) of the largest ungrounded conductors, or the equivalent area for parallel conductors. GEC sizing is independent of OCPD ampacity.
Use the largest ungrounded conductor(s) of any phase. When in parallel, sum the areas of the parallel conductors for that largest phase.
No. NEC 310.10(H) requires parallel conductors to be the same length, gauge, material, insulation, and termination. This calculator assumes one material per phase.
Aluminum or copper-clad aluminum grounding conductors are not permitted in direct contact with earth or concrete, or in corrosive locations (250.62(A)). Many outdoor GEC runs to electrodes therefore must be copper.
Yes, but it applies to the GEC that connects to that electrode. If a single GEC interconnects multiple electrodes, the largest required size among them may govern the common portion.
This tool is focused on the GEC. For bonding jumpers, see 250.66, 250.102, and 250.104 as applicable, and confirm with your AHJ.