How to ground the ground wire of a secondary distribution box
Yes, a secondary distribution box (subpanel) still requires a ground wire, which must be separate from the neutral.
Grounding and Neutral Separation
A subpanel is a secondary distribution point that extends electrical service from the main panel. According to the National Electrical Code (NEC), the neutral and ground wires must remain separate in a subpanel. The neutral bus bar should be isolated from the panel enclosure, while a separate grounding bus bar is bonded to the enclosure and connected to the grounding system . This separation prevents neutral current from flowing on metal parts, which could energize the subpanel chassis or connected metal components, creating a safety hazard .
Ground Wire Connection
The subpanel must have a grounding conductor that connects back to the main panel's ground bus. If the subpanel is in the same building as the main panel, a single ground wire is sufficient, sized according to NEC Table 250.122 . This wire ensures that fault currents have a low-impedance path back to the source, allowing overcurrent protection devices to trip quickly .
Grounding Electrodes for Detached Buildings
If the subpanel is located in a detached building, a separate grounding electrode, such as a ground rod, is typically required. This provides an additional path to earth, ensuring safety if the building becomes electrically isolated from the main panel . The grounding electrode conductor connects the subpanel's ground bus to the rod, supplementing the equipment grounding conductor from the main panel.
Summary
- Subpanel in the same building: Requires a separate ground wire to the main panel; neutral and ground must remain isolated.
- Subpanel in a detached building: Requires both a ground wire to the main panel and a local grounding electrode (ground rod).
- Neutral bonding: Only at the main panel; never bond neutral and ground in the subpanel. Proper installation of the ground wire and separation of neutral and ground is essential for electrical safety, code compliance, and effective operation of overcurrent protection devices .
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