Clarification About 25 Ohms —
The View from NFPA 70: National Electrical Code
By Richard Kithil, President & CEO, NLSI
This paper discusses language in NFPA 70: National Electrical Code (NEC) as it pertains to low-resistance grounding for lightning protection systems, covering such areas as the following:
- "The radio tower specs call for grounding of 10 ohms and the footprint is all sand. What do we do?" —Contractor
- "The job is on a mountaintop. It's all granite. The wind turbine vendor wants 2 ohms. How can we do this?" —Design engineer
- NEC gives some guidance. See NEC 250, Grounding and Bonding. Read carefully.
- Earth resistance of 25 ohms or less is not required except under specific circumstances. NEC 250 defines these. The NEC 2011 handbook is used as a guideline for the following discussion.
2. Relevant NEC 250 Sections
The two relevant sections of NEC 250, 2011 version, are listed below:
Part 250.52, Grounding Electrodes
A) Electrodes Permitted for Grounding
- Metal Underground Water Pipe
- Metal Frame of Building or Structure
- Concrete-Encased Electrode (Ufer)
- Ground Ring (Counterpoise)
- Rod and Pipe Electrodes
- Other Listed Electrodes
- Plate Electrodes
- Other Local Metal Underground Systems
B) Not Permitted for Use as Grounding Electrodes
- Metal Underground Gas Piping
Part 250.53, Grounding Electrode System Installation
A) Rod, Pipe and Plate Electrodes
- Below Permanent Moisture Level
- Supplemental Electrode Required
- Supplemental Electrode
B) Electrode Spacing
C) Bonding Jumper
D) Metal Underground Water Pipe
- Supplemental Electrode Required
E) Supplemental Bonding Electrode, Connection Size
F) Ground Ring
G) Rod and Pipe Electrodes
H) Plate Electrodes
3. NEC's Reference to 25 Ohms
Where in NEC are references to 25 ohms and what does the NEC have to say? Only in two places in 250.53, Installation, are there references to 25 ohms:
Part 250.53 (A.2), Supplemental Electrode Required
A single rod, pipe or plate shall be supplemented by an additional electrode of the type listed in 250.52 A, 1-8 (above). NEC provides an exception: If a single rod, pipe or plate electrode has a resistance to earth of 25 ohms or less, the supplemental electrode shall not be required.
Part 250.53 (D.2), Supplemental Electrode Required
A metal underground water pipe shall be supplemented by an additional electrode of a type listed in part 250.52A, 1-8 (see above list). The supplemental electrode must be installed as if it were the sole grounding electrode. The 25 ohms language thus applies here also, meaning if a rod is employed the exception cited in italics above also will apply.
The 25 ohms language in NEC applies solely to rods, pipes or plates. It does not apply to other type electrodes such as water pipes, building metal frames, concrete-encased electrodes (Ufer), ring (counterpoise) or other electrodes described in NEC, part 250.52.
Really good information on this subject is in NFPA 780 (2011 version):
- From section B.4.3 "... this does not necessarily mean the resistance of the ground connection should be low, but rather that the distribution of metal in the earth ... should be such to permit the dissipation of a stroke of lightning without damage."
- From section B.4.4 "... low resistance is desirable, but not essential ..."
- From section B.4.5 "... the following rule: the more extensive the underground metal available, the more effective the protection.”