Bonding is extremely important anywhere in an electrical system, but there are some areas where regular bonding methods might not be sufficient. In areas like services, 277V or 480V circuits, and some hazardous (classified) locations, extra measures must be taken to ensure that the bonding connections will withstand the ground-fault conditions and prevent arcing from a ground fault. The term “assured bonding” is intended to help clarify the special care required to ensure a safe installation.

Assured bonding of service enclosures and raceways
A ground fault at service equipment, enclosures, or raceways that contain service conductors can cause tremendous destruction if not cleared quickly. Because of this danger, the NEC requires you to bond the following metal parts containing service conductors to an effective ground-fault current path in accordance with the assured bonding requirements contained in 250.92(B):

• Metal raceways that contain service conductors
• Enclosures, such as disconnects, meter enclosures, or wireways that contain service conductors
• Metal raceway that contains the grounding electrode conductor [250.64(E) and 250.102(C)]

Assured bonding doesn't apply to raceways or enclosures that contain feeder and
branch-circuit conductors.

Enclosures and raceways that contain service conductors must be assured bonded to an effective ground-fault current path by one of the following methods:

• Bond enclosures and raceways to the grounded (neutral) service conductor by exothermic welding, listed pressure connectors, listed clamps, or other listed fittings (250.8). An equipment-bonding conductor isn't required within rigid nonmetallic conduit that contains service conductors [250.142(A)(1) and 352.60 Ex. 2] because fault current uses the grounded (neutral) service conductor as the effective ground-fault current path to the power supply [250.24(B)].
• Raceways are considered suitably bonded when threaded couplings or threaded bosses are wrenched tight on enclosures.
• Raceways are considered suitably bonded that are connected with threadless raceway couplings and connectors that are made wrench tight.
• When a metal service raceway terminates to an enclosure with a ringed knockout, one end of the service raceway must be bonded with a bonding jumper sized in accordance with Table 250.66 [250.102(C)].
• When a metal service raceway terminates to an enclosure without a ringed knockout one end of the raceway must be bonded by a bonding jumper or a bonding-type locknut. Standard locknuts aren't suitable for this purpose. A bonding-type locknut differs from a standard-type locknut in that it has a bonding screw with a sharp point that drives into the metal enclosure. Assured bonding one end of a service raceway provides the low-impedance path for fault current to return to the utility power supply.