NEC 250.50 · Homes and commercial

Bonding all grounding electrodes together

The code-listed grounding electrodes present at a building, such as a qualifying metal water pipe, a concrete-encased footing electrode or a ground rod, must all be bonded together into one grounding electrode system, with a narrow exception for existing buildings.

Cross-section of a house with its metal water pipe, Ufer footing and ground rod all bonded together at the service as one grounding electrode system. — Electrodes that already exist at the building count too: a qualifying metal water pipe or footing rebar has to be tied in, not just the rods that were driven. Diagram: Farmer Works Electric, free to share under CC BY 4.0 · Download PNG · How to credit

What the code requires

The code treats a building's grounding electrodes as one system. Every electrode type listed in 250.52(A)(1) through (A)(7) that is present at the building, such as metal underground water pipe, a metal in-ground support structure, a concrete-encased (Ufer) electrode, a ground ring, rods or plates, must be bonded together.

If none of those electrodes is present, one or more of the electrodes in 250.52(A)(4) through (A)(8) must be installed.

The bonding jumpers that join electrodes are covered in 250.53(C): they are sized from 250.66, installed under 250.64(A), (B) and (E), and connected under 250.70. Rebar may not be used to connect one electrode to another.

On the job

On new construction, a footing with qualifying rebar has to be bonded as a concrete-encased electrode, so the connection needs to be planned before the concrete is poured.
Installing only ground rods while ignoring a metal water service that qualifies as an electrode is a common correction.
The rule covers the listed electrodes that are present at the building. Each one that exists must be tied in, apart from the existing-building exception below.

Exceptions that matter

In an existing building, a concrete-encased electrode may be left out if its rebar cannot be reached without disturbing the concrete.

Related sections

Where this comes from

San Clemente · South OC to North County San Diego

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