Grounding Electrode System for a Detached Garage Subpanel
A homeowner adds a 100 A subpanel in a detached garage fed by a 100 A feeder from the main service panel in the house. The electrician drives two ground rods at the garage and runs an equipment grounding conductor in the feeder. At the garage subpanel, the electrician installs a grounding electrode conductor from the subpanel's equipment grounding terminal to the two ground rods, and also bonds the neutral bar to the panel enclosure with the factory bonding screw left in place. The inspector observes that the feeder's equipment grounding conductor is a 6 AWG copper, the feeder conductors are 3 AWG copper, and the ground rods are 8 ft apart. The inspector must evaluate the grounding and bonding arrangement and determine what corrections are required.
1. Evaluate the neutral-to-ground bonding at the detached garage subpanel. Is it permitted, and why or why not?
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It is not permitted in this arrangement. The detached garage subpanel is a separate structure fed by a feeder that includes an equipment grounding conductor. Because the feeder includes an EGC, the subpanel is not a service; the neutral must remain isolated from the equipment grounding conductors and the enclosure. Leaving the factory bonding screw in place creates a parallel neutral-to-ground path through the feeder EGC and the grounding electrode system, which can carry neutral current on grounding conductors and metal parts.
The correction: remove the bonding screw so the neutral bar is isolated, and land the feeder EGC on the equipment grounding terminal. The grounding electrode conductor at the garage connects to the equipment grounding terminal (or grounding bus), not to the neutral.
2. Assess the grounding electrode arrangement and the feeder EGC sizing. What, if anything, must change?
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Grounding electrodes: Two ground rods are permitted as the grounding electrode system, and the second rod is required unless a single rod demonstrates 25 ohms or less. The rods 8 ft apart are acceptable; spacing is not the controlling issue. The grounding electrode conductor must be sized per the applicable table based on the largest service/feeder conductor, and it must connect to the grounding electrode system.
Feeder EGC sizing: The feeder ungrounded conductors are 3 AWG copper. The EGC must be sized per the equipment grounding conductor table for the overcurrent device rating and conductor size. A 6 AWG copper EGC may or may not be adequate depending on the 100 A OCPD and the table used; the reviewer must verify against the table rather than assume. If the table requires 8 AWG for 100 A, 6 AWG is adequate; the inspector should confirm the table value and document it.
The key correction is the neutral bonding screw; the EGC and electrode sizing must be verified against the tables, not judged by appearance.