National Electrical Code 2023 Basics: Grounding and Bonding Part 13
Learn about the rules for auxiliary, common, and lightning protection electrodes and the materials permitted for grounding electrode conductors.
To catch up on Lorenzo Mari’s series on National Electrical Code 2023 Basics: Grounding and Bonding, follow these links:
- National Electrical Code 2023 Basics: Grounding and Bonding Part 1
- National Electrical Code 2023 Basics: Grounding and Bonding Part 2
- National Electrical Code 2023 Basics: Grounding and Bonding Part 3
- National Electrical Code 2023 Basics: Grounding and Bonding Part 4
- National Electrical Code 2023 Basics: Grounding and Bonding Part 5
- National Electrical Code 2023 Basics: Grounding and Bonding Part 6
- National Electrical Code 2023 Basics: Grounding and Bonding Part 7
- National Electrical Code 2023 Basics: Grounding and Bonding Part 8
- National Electrical Code 2023 Basics: Grounding and Bonding Part 9
- National Electrical Code 2023 Basics: Grounding and Bonding Part 10
- National Electrical Code 2023 Basics: Grounding and Bonding Part 11
- National Electrical Code 2023 Basics: Grounding and Bonding Part 12
NEC Sections 250.54, 250.58, and 250.60 deal with auxiliary, common, and lightning protection grounding electrodes. Section 250.62 specifies materials and features for the grounding electrode conductors.
Image used courtesy of Pixabay
National Electrical Code Section 250.54 Auxiliary Grounding Electrodes
The NEC does not require auxiliary grounding electrodes. These grounding electrodes may be employed when establishing a local ground reference in the area of electrically operated equipment – a function not related to clearing faults.
Connect the auxiliary grounding electrodes to the equipment grounding conductors of the types permitted in Section 250.118(A).
Since not related to the electrical safety addressed by the NEC, there is no need to comply with Sections 250.50 or 250.53(C) or the resistance requirement in Section 250.53(A)(2), Exception. Therefore, there is no need of:
- Being part of the grounding electrode system for the service or other supply source.
- Bonding to the building’s grounding electrode system.
- Sizing the grounding electrode conductor per Section 250.66.
- Adding a supplemental electrode if the resistance to earth is higher than 25 Ω.
This section reiterates the rule stated in sections 250.4(A)(5) and (B)(4) regarding the prohibition of using the earth as an effective ground-fault current path. Accordingly, it is not permitted to use grounding electrodes connected to equipment – such as ground rods – in the place of the equipment grounding conductor.
The earth’s return impedance is high enough to limit the ground-fault current to a value that the overcurrent protective device will not “see.” Consequently, the protective device will not operate, energizing the metal parts and exposing persons to electrocution or shock hazard.
The rules in this section are essential in electrical equipment located outdoors, such as metallic lighting poles.
Figure 1 shows an auxiliary grounding electrode connected to the equipment grounding conductor in a metallic lighting pole.
Figure 1. Connection of an auxiliary grounding electrode to a metallic lighting pole. Image used courtesy of Lorenzo Mari
National Electrical Code Section 250.58 Common Grounding Electrode
Use the AC system grounding electrode to ground metallic raceways, boxes, and equipment enclosures in a building or structure. Different services, feeders, or branch circuits supplying a building must use the same grounding electrode.
Figure 2 shows two services in one building supplied by two wye-wye-connected utility transformers. The transformers have primary-to-secondary ties in the neutrals.
Figure 2. Two services in a building connected to a common grounding electrode. Image used courtesy of Lorenzo Mari
Section 250.68(C)(2) permits using the metal structural frame of a building as a conductor to interconnect the electrodes making the grounding electrode system or as a grounding electrode conductor.
Figure 3 shows the flow of an objectionable neutral current through the common grounding electrode when one transformer supplies power to two services. This current may flow through the metal structural frame and is potentially dangerous for anyone touching the structure.
Figure 3. Objectionable neutral current through the common grounding electrode. Image used courtesy of Lorenzo Mari
Note that all the neutral current will flow through the common grounding electrode if the neutral conductor in one service equipment opens.
National Electrical Code Section 250.60 Strike Termination Devices
This section prohibits using the rods, pipes, or plates employed as grounding electrodes for strike termination devices as the grounding electrodes for the electrical supply and equipment – required by Section 250.50. Accordingly, the system grounding and the equipment grounding need a separate grounding electrode system.
This rule does not prohibit or require bonding the grounding electrodes of various systems – including the lightning protection system. The Informational Note N° 1 refers to Section 250.106 for bonding rules regarding lightning protection systems.
Section 250.106 requires bonding the ground terminals of the lightning protection system to the building or structure grounding electrode system. This section does not mention the 1.83 m electrode spacing rule of Section 250.53(B) – which applies when using rod, pipe, or plate electrodes.
Keep a minimum separation of 1.83 m between electrodes of different grounding systems – rod, pipe, and plate types – and bond to the building or structure grounding electrode system to comply with the rules in sections 250.53(B) and 250.106. See Figure 4.
Figure 4. Required bonding and separation between grounding electrode systems. Image used courtesy of Lorenzo Mari
The Informational Note N° 2 in Section 250.60 comments on the advantage of bonding all separate ground electrodes – bonding limits potential differences between electrodes and wiring components.
National Electrical Code Section 250.62 Material for the Grounding Electrode Conductor
The materials permitted for the grounding electrode conductor are copper, aluminum, and copper-clad aluminum. Additionally, the NEC allows the items enumerated in Section 250.68(C) – e.g., interior metal piping, metal structural frame, and rebar-type concrete-encased electrode.
The material must be corrosion-resistant or protected against it. Wire-type conductors shall be solid, stranded, insulated, covered, or bare.
Figure 5 shows two wire-type grounding electrode conductors connecting the bonded neutral and equipment ground bus in the service equipment to a metal underground water pipe – the grounding electrode – and a ground rod supplementing the water pipe.
Figure 5. Wire-type grounding electrode conductors. Image used courtesy of Lorenzo Mari
This section does not require a particular color for the insulated and covered conductors.
Section 200.7(A) prohibits the white and grey colors for other than grounded conductors – like the grounded neutral or phase leg conductors. Section 250.119(A) requires green or green with one or more yellow stripes for the wire-type equipment grounding conductor. This section prohibits using this identification for grounded or ungrounded circuit conductors.
Considering that the grounding electrode conductor is neither grounded nor ungrounded, we may conclude that the green color is acceptable.
Takeaways of Auxiliary, Common, and Lightning Protection Electrodes
- NEC Section 250.54 permits using auxiliary grounding electrodes to boost the equipment grounding conductor.
- Section 250.58 requires bonding the interconnected system of raceways, boxes, and enclosures to the electrode of the AC grounded conductor.
- Section 250.60 prohibits using the grounding electrode of the lightning protection system to ground the grounded conductor and equipment.
- Section 250.62 specifies the materials and qualities of the grounding electrode conductors.