The protective hole (knockout) for a distribution box is typically larger than the conduit trade size, with common sizes ranging from 1/2" to 4" depending on the application.

Standard Knockout Sizes

Electrical boxes and distribution panels feature pre-scored knockouts designed to accommodate standard conduit sizes. The actual hole diameter is always slightly larger than the conduit's nominal trade size to allow for fittings, connectors, and bushings . Common examples include:

• Residential wiring: 1/2" to 3/4" knockouts
• Commercial installations: 1" to 2" knockouts
• Industrial or high-voltage systems: up to 4" knockouts For smaller conduits (1/2" to 1"), a practical rule is: hole diameter ≈ trade size + 3/8". For example, a 1/2" conduit typically requires a 7/8" hole, while a 2" conduit may need a 2 1/2" hole . Larger conduits require consulting a detailed chart to ensure proper clearance.

Clearance and Safety Considerations

• The knockout must allow the connector body to pass through, not just the conduit itself .
• Proper sizing ensures secure fittings, prevents wire damage, and maintains mechanical and electrical safety .
• For screw or bolt mounting, clearance holes are usually slightly larger than the fastener diameter. For example, an M3 bolt may require a 3.4 mm clearance hole .

Practical Tips

• Always check the National Electrical Code (NEC) or local regulations for minimum box volume and knockout requirements .
• Use knockout punches or hole saws to create clean, precise openings .
• Ensure that the knockout edges are smooth or fitted with bushings to protect wires from abrasion . By selecting the correct knockout size, you ensure that the distribution box installation is safe, code-compliant, and allows for future maintenance or expansion.