NEC (National Electrical Code) Article 250 covers grounding and bonding for electrical installations to protect from electrical shock and ensure correct operation of the electrical system.

Key Considerations

• The main goal of grounding is to limit voltages caused by lightning, line surges, or accidental contact with higher-voltage lines and to stabilize the voltage during normal operations.
• Systems that need to be grounded include those operating at 50 volts or more if they are supplied by a utility or connected to outside sources(NEC 250.20).
• Effective grounding is required for systems such as transformers, motors, and other major equipment.
• The grounding electrode system connects the building's electrical system to the earth. Various electrodes can be used, including metal water pipes, concrete-encased electrodes, ground rods, and ground rings (NEC 250.50).
• Bonding ensures electrical continuity and the effective connection of conductive parts to prevent potential differences that can cause shock or fire hazards.
• Article 250 mandates the use of ground fault protection for specific equipment and scenarios, such as swimming pools and hot tubs.
• The size of the GEC (Grounding Electrode Conductor) depends on the size of the service entrance conductors. For example, for a service entrance conductor of 2 AWG or smaller, an 8 AWG copper conductor is typically required.
• Systems like generators or transformers that are not directly connected to the main service must be grounded and bonded according to specific rules to ensure safety. Grounding electrodes and bonding jumpers must be appropriately sized and installed.

Bonding (NEC 250.90–250.102)

Bonding ensures all metal parts of an electrical system (like enclosures, raceways, or service panels) are electrically connected to maintain the same potential, preventing voltage differences that could lead to electrical shock. Key points include:

• Bonding jumpers are conductors used to ensure continuity between metallic parts that are normally non-current-carrying, such as raceways and equipment enclosures.
• The grounded conductor (neutral) must be bonded to the service equipment to ensure that a fault will trip the overcurrent device (breaker or fuse).
• Any separate metallic water pipes, gas lines, or other conductive piping must be bonded to the grounding electrode system.

Ground Fault Protection (NEC 250.32, NEC 250.36)

Ground fault protection must be applied in certain installations to protect personnel and equipment:

• Ground Fault Circuit Interrupters (GFCIs): Required in locations such as bathrooms, kitchens, outdoors, and garages. These devices are designed to protect people from electric shock by detecting imbalances between the hot and neutral wires.
• Ground Fault Protection for Equipment (GFPE): Provides protection for equipment from damaging ground faults. This is required for certain installations, like for services over 1000 amperes and 150 volts to ground.

Grounding and bonding of cable systems

• Service Entrance Cables (SER and SEU Cables): Article 250 requires that service entrance cables are properly grounded and bonded to ensure a safe electrical path for fault currents. Grounding these cables helps protect against lightning, surges, and accidental contact with higher voltages.
• Equipment Grounding Conductors (EGC) in Cables: Many multiconductor cables, such as Type MC (Metal-Clad) and Type TC (Tray Cable), include an equipment grounding conductor as a part of their design. Article 250 provides the guidelines for these conductors' sizing, connection, and installation to ensure they perform correctly during a fault.  For example, if a circuit is protected by a 20-ampere breaker, the minimum EGC size required is 12 AWG copper. When multiconductor cables, like Type MC or Type TC, contain an internal EGC, it must be properly terminated at both ends of the run to ensure the grounding path is continuous and reliable. Article 250 requires that the sheath be bonded to the EGC to prevent any potential voltage differences between the sheath and the ground.

Bonding requirements for cable trays and conduits

• Cable Tray Systems: NEC Article 250 mandates that all metallic cable trays used to support cables be bonded together to create an electrically continuous system. This bonding ensures that the cable tray system does not pose a shock hazard and that fault currents can safely flow to the grounding system.
• Metal Conduits and Cable Armor: Metallic conduits, armoured cables (like Type AC and Type MC), and metallic cable sheaths must be bonded according to Article 250. This prevents dangerous voltage differences between conduits and other metal parts that could lead to shock or fire hazards.

Other Cable Considerations

• Shielded Cables (e.g., Instrumentation and Communication Cables): Cables with shields (such as Type ITC and coaxial cables) need proper grounding according to Article 250 to ensure that electromagnetic interference (EMI) is minimized and that the shield performs its protective function effectively.
• Voltage Separation: Article 250 influences how cables of different voltages are managed in a system. For instance, cables rated above 600 volts must either be separated from lower voltage cables by a solid barrier or be of specific types that provide adequate insulation, such as Type MC cables.
• GEC Routing with Cables: Article 250 specifies that grounding electrode conductors should only be run within the same raceway or conduit as power cables if properly insulated and protected, ensuring that the electrical integrity of both the GEC and the power cables is maintained.