GEC Sizing Calculator

What this GEC sizing calculator does

This page is limited to the grounding electrode conductor. It does not size the main bonding jumper, does not size feeder equipment grounding conductors, and does not estimate rod resistance to earth. Those are separate Article 250 tasks.

How the sizing basis works

1. Start with the largest ungrounded service conductor or the equivalent area of the parallel service-conductor sets.
2. Apply NEC Table 250.66 to get the base grounding electrode conductor size.
3. Check whether the reviewed connection qualifies for a sole-connection practical cap:
   • Rod, pipe, or plate electrode: not required larger than 6 AWG copper or 4 AWG aluminum
   • Concrete-encased electrode: not required larger than 4 AWG copper or 2 AWG aluminum
   • Ground ring: not required larger than the conductor used for the ground ring
4. If the reviewed connection is to metal water pipe, structural metal, or the full grounding electrode system, stay with the full Table 250.66 result unless another rule changes the installation.

Worked examples

Example 1: A service with 4/0 copper ungrounded conductors screens at a 2 AWG copper grounding electrode conductor from Table 250.66. If the reviewed connection is only to a rod electrode, the practical cap becomes 6 AWG copper.

Example 2: Two parallel sets of 4/0 aluminum service conductors have an equivalent area of about 423.2 kcmil aluminum. That still lands in the table row that screens at 2 AWG copper or 1/0 aluminum before any sole-connection exception is applied.

What this page does not do

This page does not confirm every electrode that must be bonded under 250.50, does not size the main bonding jumper, does not replace the full grounding and bonding layout, and does not estimate grounding resistance. Use the Grounding Calculator for the broader Article 250 conductor-family screen and the Grounding Resistance Calculator when the real question is rod performance in soil.