2005 NEC


“III. Grounding Electrode System and Grounding Electrode Conductor 250.50 Grounding Electrode System All grounding electrodes as described in 250.52(A)(1) through (A)(6) THAR ARE PRESENT at each building or structure served SHALL BE Bonded together to form the grounding
electrode system. Where none of these grounding electrodes exist, one or more of the grounding electrodes specified in 250.52(A)(4) through (A)(7) shall be installed and used.”

“Exception: Concrete-encased electrodes of existing buildings or structures shall not be
required to be part of the grounding electrode system where the steel reinforcing bars or
rods are not accessible for use without disturbing the concrete.”

“250.52 Grounding Electrodes
(A) Electrodes Permitted for Grounding
(1) Metal Underground Water Pipe A metal underground water pipe in direct contact
with the earth for 3.0 m (10 ft) or more (including any metal well casing effectively
bonded to the pipe) and electrically continuous (or made electrically continuous by
bonding around insulating joints or insulating pipe) to the points of connection of the
grounding electrode conductor and the bonding conductors. Interior metal water piping
located more than 1.52 m (5 ft) from the point of entrance to the building shall not be
used as a part of the grounding electrode system or as a conductor to interconnect
electrodes that are part of the grounding electrode system.”

{Exception for certain commerical buildings}

This is one of the 250.52(A)1-6 that SHALL BE USED IF PRESENT.


“(5) Rod and Pipe Electrodes Rod and pipe electrodes shall not be less than 2.5 m (8 ft)
in length and shall consist of the following materials.
(a) Electrodes of pipe or conduit shall not be smaller than metric designator 21 (trade
size 3/ 4) and, where of iron or steel, shall have the outer surface galvanized or
otherwise metal-coated for corrosion protection.
(b) Electrodes of rods of iron or steel shall be at least 15.87 mm ( 5/ 8 in.) in diameter.
Stainless steel rods less than 16 mm ( 5/ 8 in.) in diameter, nonferrous rods, or
their equivalent shall be listed and shall not be less than 13 mm ( 1/ 2 in.) in

250.53 Grounding Electrode System Installation

“(D) Metal Underground Water Pipe Where used as a grounding electrode, metal
underground water pipe shall meet the requirements of 250.53(D)(1) and (D)(2).
(1) Continuity Continuity of the grounding path or the bonding connection to interior
piping shall not rely on water meters or filtering devices and similar equipment.
(2) Supplemental Electrode Required
A METAL UNDERGROUND WATER PIPE SHALL BE SUPPLEMENTED supplemented by an additional electrode of a type specified in 250.52(A)(2) through
(A)(7). Where the supplemental electrode is a rod, pipe, or plate type, it shall comply
with 250.56. The supplemental electrode shall be permitted to be bonded to the
grounding electrode conductor, the grounded service-entrance conductor, the nonflexible
grounded service raceway, or any grounded service enclosure.
Exception: The supplemental electrode shall be permitted to be bonded to the interior
metal water piping at any convenient point as covered in 250.52(A)(1), Exception”

This is from the NEC Handbook. It is not CODE. But is give explanation and background information.

“Section 250.53(D)(2) specifically requires that rod, pipe, or plate electrodes used to
supplement metal water piping be installed in accordance with 250.56. This
requirement clarifies that the supplemental electrode system must be installed as if it
were the sole grounding electrode for the system. If 25 ohms or less of earth resistance
cannot be achieved with one rod, pipe, or plate, another electrode (other than the metal
piping that is being supplemented) must be provided. One of the permitted methods of
bonding a supplemental grounding electrode conductor to the primary electrode system
is to connect it to the service enclosure.
The requirement to supplement the metal water pipe is based on the practice of using a
plastic pipe for replacement when the original metal water pipe fails. This type of
replacement leaves the system without a grounding electrode unless a supplemental
electrode is provided.”

“250.56 Resistance of Rod, Pipe, and Plate Electrodes
A single electrode consisting of a rod, pipe, or plate that does not have a resistance to
ground of 25 ohms or less shall be augmented by one additional electrode of any of the
types specified by 250.52(A)(2) through (A)(7). Where multiple rod, pipe, or plate
electrodes are installed to meet the requirements of this section, they shall not be less than
1.8 m (6 ft) apart.”

There is no requirement for testing the resistance. And unless it is tested and shown to be 25 ohms or less then an additional electrode (excluding a water pipe) is needed.

I can’t copy and paste this. But 2000 IRC says the same thing. This is the paraphrasing.

E3508.1 Grounding Electrode System

Where AVAILABLE electrodes speced in E3501.1.1 (metal underground water pipe), E3501.1.2, E3501.1.3, & E3502 SHALL BE USED.

E3501.1.1 Metal Underground Water Pipes. A metal underground water pipe SHALL BE SUPPLEMENT with other electrodes.

E3508 Resistance of made electrodes (which include rods).

A single rod that has a resistance more than 25 ohms shall be augment with one additional electrode.

The only difference between this and the 2005 NEC is that this implies that the water pipe can be used as the augmentation. Newer codes make it clear that it can’t.