Grounding & Bonding Systems
Grounding electrodes, GEC sizing, ground rod requirements, conductor color codes, equipment grounding purpose, and separately derived systems.
- Identify acceptable grounding electrodes per NEC 250.52
- Size the grounding electrode conductor using NEC Table 250.66
- State the 25-ohm rule for supplemental ground rod electrodes
- Apply NEC conductor color code requirements for neutral and ground wires
- Explain the purpose of equipment grounding and size EGCs from Table 250.122
- Describe grounding requirements for separately derived systems
Lección 1
Grounding Electrodes, Ground Rods & the 25-Ohm Rule
Types of Grounding Electrodes
Per NEC 250.52, the following are recognized grounding electrodes:
- Metal underground water pipe - must be in direct contact with earth for 10 feet or more
- Metal in-ground support structure - building steel in direct contact with earth
- Concrete-encased electrode (Ufer ground) - 20 feet of #4 AWG bare copper or 1/2-inch rebar encased in concrete foundation in direct contact with earth
- Ground ring - #2 AWG bare copper encircling the building at least 2.5 feet deep
- Ground rods - minimum 8 feet long, 5/8-inch diameter for iron/steel or 1/2-inch for copper/stainless
- Plate electrodes - minimum 2 square feet of surface area, buried at least 2.5 feet
The 25-Ohm Rule
Per NEC 250.53(A)(2), if a single ground rod has a resistance to earth greater than 25 ohms, a supplemental electrode must be installed. The supplemental electrode must be:
- A second ground rod
- Spaced at least 6 feet from the first rod (though 2x the rod length is recommended)
Exam Tip - Two Rods
When two ground rods are installed, the NEC does not require that the combined resistance meet 25 ohms. The requirement is only that a single rod exceeding 25 ohms must have a supplemental electrode. Once the second rod is installed, no further resistance testing is required.
Water Pipe Electrode Requirements
A metal underground water pipe electrode is one of the best grounding electrodes because it typically has excellent earth contact. However, it must be supplemented by at least one other electrode from the list above. The water pipe alone is never sufficient as the sole electrode because plastic pipe repairs could interrupt the earth contact in the future.
Per NEC 250.52, a metal underground water pipe must be in contact with earth for 10 feet or more to qualify as a grounding electrode. Ground rods must be at least 8 feet long. A single rod exceeding 25 ohms requires a supplemental electrode.