Article 250 Grounding Electrode System: A Visual Guide

The six grounding electrode types (250.52)

• Metal underground water pipe in direct contact with earth for 10 ft or more (250.52(A)(1)). Must be supplemented by another electrode.
• Metal in-ground support structure (250.52(A)(2)). Building steel that’s buried.
• Concrete-encased electrode (Ufer) (250.52(A)(3)). Bare copper or rebar in the foundation footing.
• Ground ring (250.52(A)(4)). 20 ft or more of bare 2 AWG copper buried 2.5 ft or more.
• Rod and pipe electrodes (250.52(A)(5)). 5/8 in × 8 ft, 90% bonded to earth.
• Plate electrodes (250.52(A)(7)). 2 sq ft of metal surface, buried 2.5 ft or more.

The "use all that are present" rule (250.50)

If a building has more than one electrode type from 250.52(A)(1) through (A)(7), all of them must be bonded together. You don’t pick one. You bond them all to form a single grounding electrode system. If none of those exist, you must install one or more from 250.52(A)(4) through (A)(8).

Practical example: A new building with a metal water service, building steel, and a concrete-encased electrode requires all three to be bonded. A residential remodel with no Ufer and no building steel requires a rod or pipe electrode to be installed.

GEC sizing (Table 250.66)

The GEC connects the system to the electrode. Table 250.66 sizes it based on the largest ungrounded service conductor:

• 2 AWG copper or smaller service → 8 AWG GEC
• 1 AWG or 1/0 → 6 AWG
• 2/0 or 3/0 → 4 AWG
• Over 3/0 to 350 kcmil → 2 AWG
• Over 350 to 600 kcmil → 1/0
• Over 600 to 1100 kcmil → 2/0
• Over 1100 kcmil → 3/0

The electrode-type caps

Table 250.66 references three caps for specific electrode types that override the table:

• For rod, pipe, or plate electrodes (250.52(A)(5), (A)(7)): GEC need not exceed 6 AWG copper.
• For concrete-encased electrode (250.52(A)(3)): GEC need not exceed 4 AWG copper.
• For ground ring (250.52(A)(4)): GEC need not be larger than the ring conductor itself.

These caps catch candidates. The full Table 250.66 might say "2/0 copper" but if the only electrode is a rod, the GEC to that rod need only be 6 AWG.

Rod electrode resistance (250.53)

A single rod or pipe electrode that doesn’t achieve 25 ohms or less must be supplemented by an additional electrode at least 6 ft away. In practice, most installers default to two rods because measuring resistance takes special equipment.

Common exam patterns

• List a service size, ask for GEC size to a specific electrode type. Trap: applying full table value when a cap applies.
• Describe a building with multiple electrodes, ask "which one do you bond." Answer: all of them per 250.50.
• Specify a single rod and ask for the next required action. Answer: measure resistance or add a second rod.
• Single buried metal water pipe. Trap: need a supplemental electrode per 250.53(D)(2).