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Grounding of the casing of the surface-mounted electrical box

A surface-mounted metal electrical box can be grounded by ensuring direct metal-to-metal contact between the device yoke and the box, or by using an equipment bonding jumper when required.General Grounding Requirements

According to the NEC, all metal boxes must be bonded to the equipment grounding conductor to provide an effective ground-fault current path. This ensures that any fault current is safely conducted to the ground, reducing the risk of electric shock or fire (NEC 250.148) . The grounding connection can be achieved in several ways depending on the installation type.

Surface-Mounted Box Grounding

For surface-mounted boxes, NEC 250.146(A) allows grounding without a separate equipment bonding jumper if direct metal-to-metal contact exists between the device yoke (or strap) and the metal box. To achieve this:

  • Remove at least one insulating washer from the receptacle if it prevents metal-to-metal contact .
  • Ensure the screws securing the device to the box provide firm metal-to-metal contact .
  • Cover-mounted receptacles may be used if the box and cover combination is listed to provide continuity, with at least two permanent fasteners securing the device to the cover and mounting holes located on a flat, non-raised portion of the cover .
Equipment Bonding Jumper

If direct contact is not possible or the device is not listed for grounding through the cover, an equipment bonding jumper must be installed. The jumper connects the grounding terminal of the receptacle to the metal box and must be sized according to Table 250.122 based on the overcurrent protection rating . This ensures that removing the device does not interrupt the grounding path .

Key Considerations
  • The grounding method must maintain continuity even if the device is removed or replaced .
  • Multiple grounding conductors within the box should be bonded together and connected to the box to comply with NEC 250.148 .
  • Using listed grounding clips or bonding screws is an acceptable method to achieve compliance . By following these NEC guidelines, a surface-mounted metal box can be safely grounded, either through direct metal-to-metal contact or with an appropriately sized equipment bonding jumper, ensuring a reliable ground-fault path.
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