Grounding and Bonding at a Detached Garage with Subpanel
A detached garage 60 ft from an Alaska home is fed by a 60 A feeder from the main panel: 6 AWG copper conductors (two hots, neutral, equipment ground) in Schedule 80 PVC buried 18 in. The subpanel has a separate ground bar and neutral bar, and the installer bonded the neutral bar to the panel enclosure with a green screw. A metal water pipe enters the garage and is bonded to the subpanel ground bar. The main panel has a grounding electrode conductor to a ground rod and to the metal water service. The inspector red-tags the job, citing the neutral bonding and the grounding electrode system.
1. Identify the grounding and bonding errors and explain the correct configuration for a detached-building subpanel.
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Error 1 - neutral bonded at subpanel: In a feeder to a separate structure, the neutral and equipment grounding conductors must be separated at the subpanel; the neutral bar must be isolated (floating) and the equipment ground bar bonded to the enclosure. The green bonding screw must be removed. Bonding neutral to ground at the subpanel creates a parallel return path on the grounding conductor and energizes the enclosure under fault - a serious shock/fire hazard.
Error 2 - grounding electrode system: A detached structure with a feeder requires a grounding electrode system at the building (ground rod(s), and bonding to any metal water piping and structural steel). The metal water pipe entering the garage must be bonded, and a ground rod (or two, 6 ft apart) must be installed and connected to the subpanel ground bar with a grounding electrode conductor sized per the table (for 6 AWG feeder, typically 8 AWG Cu GEC). The installer bonded the water pipe to the ground bar, which is correct, but there is no grounding electrode conductor to a rod - the inspector's citation is valid.
Error 3 - burial depth: Schedule 80 PVC requires 18 in burial - that is met. Verify the 6 AWG feeder ampacity: 6 AWG Cu at 75°C = 65 A, so a 60 A feeder is fine.
Correct configuration: Remove green screw; isolate neutral; install ground rods and GEC; bond water pipe and any metal enclosures; ensure equipment ground is continuous from main panel.
2. Explain why the 6 AWG equipment grounding conductor is or is not adequate, and what would change if the feeder were 100 A.
Read the worked answer
Equipment grounding conductor sizing is based on the overcurrent device protecting the feeder, not the phase conductors. For a 60 A feeder, the EGC must be at least 10 AWG copper (per the EGC table). The installer used 6 AWG, which is more than adequate - no violation there. The inspector's red tag is about the neutral bonding and missing electrode, not the EGC size.
If the feeder were 100 A: Phase conductors would need 3 AWG Cu (100 A at 75°C) or 1 AWG Al. The EGC would need to be at least 8 AWG copper (per the table for 100 A). The 6 AWG EGC would still be adequate for 100 A, but the phase conductors would be undersized at 6 AWG (65 A) - so the feeder itself would have to be upsized. Burial depth for Schedule 80 PVC remains 18 in.
Also note: if the garage has a metal water pipe, the bonding jumper to it must be sized per the bonding table (for 6 AWG, 8 AWG Cu is typical), and the grounding electrode conductor to the rod is sized per the GEC table. The key exam point: EGC size follows the OCPD, not the phase conductor size, and the subpanel neutral must float.