Ground Wire Sizing Guide: NEC Requirements Explained

Proper grounding is one of the most critical aspects of electrical safety. Ground wires provide a low-resistance path for fault current, enabling protective devices to operate quickly and protecting people from electric shock. The National Electrical Code (NEC) specifies minimum ground wire sizes based on the circuit being protected, and understanding these requirements is essential for safe, code-compliant installations.

Understanding Grounding Terminology

The NEC distinguishes between several types of grounding conductors, each with different sizing requirements and functions. Confusing these terms is a common source of errors in electrical design.

Equipment Grounding Conductor (EGC)

The equipment grounding conductor connects non-current-carrying metal parts of equipment to the system ground. This is the green or bare wire that runs with the circuit conductors in cables and conduit. Its purpose is to provide a path for fault current if a hot wire contacts the equipment enclosure, allowing the circuit breaker or fuse to trip quickly.

Grounding Electrode Conductor (GEC)

The grounding electrode conductor connects the electrical system to the grounding electrode, typically at the main service panel. It connects to grounding electrodes such as ground rods, concrete-encased electrodes, or metal water pipes. This conductor establishes the earth reference for the electrical system.

Main Bonding Jumper

The main bonding jumper connects the equipment grounding system to the grounded (neutral) conductor at the service. This connection is made only at the main service equipment and creates the path that allows fault current to return to the source and trip protective devices.

Equipment Grounding Conductor Sizing

NEC Table 250.122 specifies the minimum size of equipment grounding conductors based on the rating of the overcurrent device protecting the circuit. This table ensures the ground wire can safely carry fault current until the protective device operates.

When to Upsize the EGC

While Table 250.122 provides minimum sizes, there are situations where larger equipment grounding conductors are required or recommended. When circuit conductors are increased in size for voltage drop, the EGC must be proportionally increased.

Grounding Electrode Conductor Sizing

NEC Table 250.66 specifies grounding electrode conductor sizes based on the size of the largest ungrounded service-entrance conductor. The GEC connects the system to the grounding electrode and must be sized to carry the current necessary to operate protective devices during ground faults.

Sole Connection to Ground Rod

When the grounding electrode conductor connects only to a ground rod or pipe electrode, the conductor is not required to be larger than 6 AWG copper or 4 AWG aluminum. This exception recognizes that the resistance of the ground rod itself limits the current that can flow, making larger conductors unnecessary.

Concrete-Encased Electrode

For grounding electrode conductors connected to concrete-encased electrodes (Ufer grounds), the conductor is not required to be larger than 4 AWG copper. The low resistance of concrete-encased electrodes provides excellent grounding, but the NEC limits the required conductor size due to practical installation considerations.

Common Grounding Configurations

Residential Service Grounding

A typical 200-amp residential service with 2/0 AWG copper or 4/0 AWG aluminum service conductors requires a 4 AWG copper or 2 AWG aluminum grounding electrode conductor to water pipe or concrete-encased electrodes. When connecting to supplemental ground rods, a 6 AWG copper conductor is typically sufficient.

Subpanel Grounding

Subpanels require an equipment grounding conductor sized according to Table 250.122 based on the feeder overcurrent protection. The neutral and ground must remain separate in subpanels only the main service panel has the neutral-to-ground bond. A 100-amp subpanel feeder requires at least an 8 AWG copper EGC.

Detached Building Grounding

Detached buildings supplied from the main building require careful grounding consideration. An equipment grounding conductor must be run with the feeder conductors. In many cases, a grounding electrode system must also be installed at the detached building and bonded to the equipment grounding conductor.

Ground Wire Installation Best Practices

• Keep grounding electrode conductors as short and straight as possible to minimize impedance
• Protect exposed grounding conductors smaller than 6 AWG from physical damage
• Use listed connectors rated for the application when connecting to grounding electrodes
• Never splice grounding electrode conductors unless using irreversible compression connectors or exothermic welding
• Bond all grounding electrodes together with a bonding jumper when multiple electrodes are present
• Install equipment grounding conductors in the same raceway or cable as the circuit conductors
• Use green or green with yellow stripe insulation for insulated grounding conductors

Ground Wire Material Considerations

Both copper and aluminum are permitted for grounding conductors, but copper is preferred for most applications due to its superior conductivity, resistance to corrosion, and reliable termination characteristics.

Copper Ground Wires

Copper is the standard material for grounding conductors and is required in some applications. Copper provides excellent conductivity, resists corrosion in most environments, and makes reliable connections at terminations. For outdoor and direct burial applications, bare copper develops a protective patina that resists further corrosion.

Aluminum Ground Wires

Aluminum grounding conductors cost less but must be one or two sizes larger than copper equivalents. Aluminum is prohibited from direct contact with earth or concrete and requires anti-oxidant compound at terminations. Due to these limitations, aluminum is less commonly used for grounding despite its cost advantage.

Testing and Verification

After installation, grounding systems should be tested to verify proper connections and adequate fault current capacity. Ground resistance testing measures the electrode connection to earth, while continuity testing verifies that all equipment grounding connections are intact and low-resistance.

A properly installed grounding system will have ground electrode resistance under 25 ohms per NEC requirements, and equipment grounding paths should show very low resistance (typically under 1 ohm) from any point back to the service. Higher readings indicate loose connections, undersized conductors, or other problems requiring correction.