Section 2.4: Grounding and Bonding Systems

Section 2.4: Grounding and Bonding Systems

Grounding and bonding are the cornerstones of electrical safety in residential wiring. Although often confused, they serve distinct purposes. Grounding connects the electrical system's grounded conductor (neutral) and metallic enclosures to the earth. This limits voltages imposed by lightning, line surges, or accidental contact with higher-voltage lines. Bonding joins metallic parts of the electrical system, piping, and structural components together to establish an electrically conductive, low-impedance path. This path ensures that fault currents are carried back to the service panel, triggering the overcurrent protection device (breaker) to clear the fault immediately.

The Grounding Electrode System (IRC E3608)

All grounding electrodes present at a building must be bonded together to form the grounding electrode system. The IRC identifies several acceptable electrodes, which must be interconnected.

1. Metal Underground Water Pipe

A metal underground water pipe in direct contact with the earth for 10 feet or more must be used as a grounding electrode.

• Supplemental Electrode Required: A metal water pipe must always be supplemented by an additional electrode (such as a driven rod or concrete-encased electrode) to ensure a backup ground path if the water pipe is later replaced with non-metallic PEX piping.
• 5-Foot Rule: The grounding electrode conductor (GEC) connection to the water pipe must be made within the first 5 feet of where the pipe enters the building. This minimizes the length of interior piping used as a ground path and ensures the connection remains visible and accessible.

2. Concrete-Encased Electrode (Ufer Ground)

A concrete-encased electrode, commonly known as a Ufer ground, is highly effective due to the large surface area of the concrete foundation in direct contact with the earth.

• Requirements: It must consist of at least 20 feet of bare copper conductor (not smaller than 4 AWG) or one or more steel reinforcing bars (rebar) not less than 1/2 inch in diameter.
• Location: The electrode must be encased in at least 2 inches of concrete and positioned near the bottom of a concrete foundation or footing that is in direct contact with the earth.

3. Rod and Pipe Electrodes

Driven rod electrodes are the most common electrodes used in residential construction.

• Dimensions: Rod electrodes must be at least 8 feet long and have a minimum diameter of 5/8 inch (for steel or iron) or 1/2 inch (for copper-coated steel).
• Installation: The rod must be driven vertically into the earth to a depth of not less than 8 feet. If rock prevents vertical driving, the rod may be driven at an angle not exceeding 45 degrees from the vertical, or buried in a trench that is at least 2.5 feet (30 inches) deep.
• Spacing and Resistance: A single rod electrode must be supplemented by a second electrode (spaced at least 6 feet apart) unless the single rod has a resistance to ground of 25 ohms or less. In practice, inspectors generally require two rods to bypass the need for expensive soil resistance testing.

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Bonding Metallic Piping Systems

To prevent metallic pipes from becoming energized and presenting a shock hazard, the IRC requires them to be bonded to the electrical system.

Metal Water Piping Bonding

The interior metal water piping system must be bonded to the service equipment enclosure, the grounded conductor at the service, or the grounding electrode conductor. The bonding jumper must be sized in accordance with Table E3611.1.

Metal Gas Piping Bonding

Other metal piping systems, including gas piping, must be bonded.

• The EGC as a Bonding Means (E3609.7): The IRC permits the equipment grounding conductor (EGC) of the circuit that is likely to energize the gas piping (such as the circuit supplying a gas furnace, range, or water heater) to serve as the bonding means. No additional external bonding jumper is required, provided the EGC is properly sized and connected.

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Service Bonding Jumpers (E3609)

The electrical continuity of all metal service equipment, meter enclosures, and service raceways must be maintained. Because of the high fault currents available at the service, standard locknuts are not considered sufficient for bonding.

• Methods: Bonding must be assured using threaded hubs, bonding bushings with bonding jumpers, or specialized service-grade bonding locknuts.
• Main Bonding Jumper: Within the service disconnect enclosure, a main bonding jumper (typically a screw or green strap) must connect the grounded service conductor (neutral) to the equipment grounding bus and the metal enclosure.

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Grounding Electrode Conductor (GEC) Sizing (Table E3611.1)

The GEC connects the service grounded conductor to the grounding electrodes. It is sized based on the size of the largest ungrounded service-entrance conductor.

GEC Sizing Table

Maximum Sizing Exceptions

The IRC limits the required size of the GEC for certain types of electrodes, regardless of the size of the service-entrance conductors:

1. To Rod, Pipe, or Plate Electrodes: The portion of the GEC that is the sole connection to these electrodes is not required to be larger than 6 AWG copper or 4 AWG aluminum.
2. To Concrete-Encased Electrodes: The portion of the GEC that is the sole connection to a Ufer ground is not required to be larger than 4 AWG copper.

Step-by-Step Grounding Electrode System Sizing Walkthrough

To design the grounding system for a standard 200-ampere residential service:

1. Identify the Service-Entrance Conductors: A 200A service uses 2/0 AWG copper ungrounded conductors.
2. Sizing the Grounding Electrode Conductor (GEC) to Water Pipe (Table E3611.1):
   • For 2/0 AWG copper service conductors, Table E3611.1 requires a 4 AWG copper GEC.
   • This conductor must connect within the first 5 feet of the metal water pipe entrance.
3. Sizing the GEC to the Driven Rod Electrodes:
   • Even though the service size dictates a 4 AWG copper GEC for the main system, the exception under Section E3611.1 permits the GEC to a driven rod electrode to be capped at a maximum of 6 AWG copper.
   • Therefore, a 6 AWG copper conductor can run from the panelboard to the two driven rods.
4. Sizing the GEC to the Concrete-Encased (Ufer) Electrode:
   • The exception under Section E3611.1 permits the GEC to a Ufer ground to be capped at a maximum of 4 AWG copper.
   • Because 4 AWG is already required by the service size, a 4 AWG copper GEC is run to the foundation rebar connection.