For a secondary distribution box, grounding rods should be at least 8 feet long, with multiple rods spaced at least 6 feet apart if needed to achieve a resistance of 25 ohms or less.

Grounding Rods and Electrodes

The most common grounding electrodes for a sub-panel or secondary distribution box are ground rods. Each rod must be at least 8 feet in length and fully driven into the earth. If a single rod does not achieve a resistance to earth of 25 ohms or less, a second rod must be installed, separated from the first by a minimum of 6 feet to prevent overlapping spheres of influence and improve grounding efficiency. Alternative electrodes, such as concrete-encased electrodes (Ufer grounds), can also be used, requiring at least 20 feet of bare copper conductor or steel rebar encased in concrete, providing superior performance in certain soil conditions (NEC Section 250.52(A)(3)), .

Grounding Conductor Sizing

The grounding electrode conductor (GEC) connecting the rods to the distribution box should be sized according to the largest ungrounded feeder conductor supplying the sub-panel, following NEC Table 250.66. For Ufer grounds, a 4 AWG copper conductor is typically sufficient, even for large services . The conductor must be continuous and securely connected to the dedicated grounding bus in the panel, avoiding double-tapping neutrals or using substandard materials .

Secondary Panel Grounding Considerations

• Separation of neutral and ground: In a sub-panel, the neutral bus must not be bonded to the panel enclosure; only the main service panel provides the single neutral-to-ground bond .
• Low-impedance path: Proper grounding ensures fault currents return safely to the source, allowing overcurrent protection devices to trip quickly and stabilizing the system against surges or lightning .
• Material quality: Use copper or copper-bonded steel rods for durability and low resistance. Avoid aluminum for buried runs, as it corrodes over time .

Practical Tips

• Test the resistance of each installation; if it exceeds 25 ohms, add additional rods.
• Ensure all rods are fully driven vertically into the soil for maximum contact.
• Maintain continuous, properly sized conductors from the rods to the panel's grounding bus.
• Avoid shortcuts like chemical treatments or substandard rods, which can compromise safety and longevity . By following these guidelines, a secondary distribution box will have a reliable, low-resistance grounding system that meets safety standards and ensures proper operation of protective devices.