Guide to Safe Grounding With Flexible Metal Conduit

Electrical safety remains paramount in construction and maintenance work, with proper grounding being one of the most critical aspects. Among the various components involved, metal flexible conduit (MFC) presents unique challenges when used as grounding path. Recent discussions in the electrical community have highlighted widespread confusion about when and how these conduits can safely serve as equipment grounding conductors.

Contrary to common assumptions, not all metal flexible conduits qualify as acceptable grounding paths. The National Electrical Code (NEC) establishes strict requirements for Liquid-Tight Flexible Metal Conduit (LFMC) when used as an equipment grounding conductor. These regulations exist because improper grounding through flexible conduits can create significant safety hazards during electrical faults.

Electrical professionals must adhere to these essential requirements when considering metal flexible conduit for grounding purposes:

All LFMC installations must use listed fittings specifically designed for the purpose. These components undergo rigorous testing to ensure proper conductivity and mechanical strength. Substandard fittings may compromise the entire grounding system's effectiveness.

For conduits sized between 3/8-inch and 1/2-inch (12-16mm metric), the circuit's overcurrent protection must not exceed 20 amps. This restriction applies to lighting circuits and similar low-current applications.

Conduits from 3/4-inch to 1 1/4-inch (21-35mm) have more complex requirements. The circuit protection cannot exceed 60 amps, and the grounding path cannot include any smaller flexible metal conduits, which could fail under high fault currents.

The total length of flexible metal conduit in any grounding path must not exceed 6 feet (1.8 meters). This limitation controls impedance in the grounding circuit, ensuring it can properly conduct fault currents.

Conduits with stainless steel cores require supplemental grounding conductors due to stainless steel's relatively poor conductivity compared to other metals.

Even when meeting all standard requirements, installations involving vibration or movement demand additional precautions. The NEC requires separate equipment grounding conductors in these situations because mechanical stress can compromise conduit connections over time.

The extensive regulations stem from fundamental electrical principles. Flexible metal conduits inherently have higher resistance than solid conductors, and their connections may degrade over time. These factors could prevent proper fault current conduction, potentially leaving energized surfaces during electrical faults.

• Select appropriate conduit sizes based on circuit requirements
• Use only listed fittings from reputable manufacturers
• Minimize conduit lengths in grounding paths
• Avoid mixing conduit sizes in the same grounding circuit
• Install supplemental grounding conductors when required
• Conduct regular inspections of flexible conduit connections

Understanding these requirements helps ensure electrical systems meet safety standards while providing reliable performance. As with all electrical work, adherence to code requirements and manufacturer specifications remains essential for preventing hazardous situations.