The requirement to separate the grounded (neutral) conductor and the equipment grounding conductor in a subpanel has been in the National Electrical Code (NEC) for a significant time, but the exact wording and enforcement have evolved. Here's a breakdown: NEC 1996 - This is typically referenced as the most prevalent year of code change when separating the neutral and ground in a subpanel became a standard electrical practice. It more explicitly enforced that in a subpanel, the neutral and ground must be kept separate, and the neutral bus must be insulated from the subpanel enclosure.

Reasoning for the Change

The primary reason for this separation is to prevent neutral current from flowing through the equipment grounding conductors. When the neutral and ground are bonded in a subpanel, a portion of the neutral current can return to the source (main panel) via the grounding path. This is undesirable for several reasons:1.

Electromagnetic Interference (EMI)

Ground current can induce EMI in nearby electronic equipment.2.

Stray Voltage

It can create stray voltage on grounded metal objects, potentially causing nuisance shocks.3.

Safety Concerns

It can compromise the effectiveness of the grounding system, especially in fault conditions.

Important Considerations



Main Panel Exception

The neutral and ground are always bonded at the main service panel (or the first disconnecting means in a separately derived system). This creates a single point where the neutral and ground are connected, ensuring a low-impedance path for fault current back to the source.

Retrofitting

Retrofitting existing installations to comply with current code can be complex and sometimes not strictly required unless significant changes are being made to the electrical system. However, it's always recommended to consult with a qualified electrician to assess the safety of the existing setup and determine the best course of action.

Local Codes

Local codes may vary, so it's essential to check with the local electrical inspector or authority having jurisdiction (AHJ) for specific requirements in your area.

In summary, while the concept of separating neutral and ground in subpanels existed before, the 1996 NEC is commonly cited as the point where it became a more clearly defined and widely enforced requirement. Flag for review