250.4
General Requirements for Grounding and Bonding
Most apprentices and even some journeymen use the words grounding and bonding interchangeably, but they are two completely different things with two completely different purposes. Section 250.4 makes the distinction crystal clear. Grounding under 250.4(A)(1) means connecting the electrical system to the earth through a grounding electrode. Its purpose is to limit voltage imposed by lightning, line surges, or accidental contact with higher-voltage lines, and to stabilize the voltage to earth during normal operation. Grounding does NOT clear faults — the earth is a terrible conductor with far too much impedance to trip a breaker. Bonding under 250.4(A)(3) and (A)(5) means connecting all non-current-carrying metal parts (conduit, enclosures, equipment frames) together and back to the source via a low-impedance path so that fault current flows fast enough and high enough to trip the overcurrent device. Bonding is what actually protects people from electrocution during a ground fault — it provides the effective ground-fault current path. For grounded systems (250.4(A)), both grounding and bonding are required: the system is grounded to earth, and all equipment is bonded together and back to the source. For ungrounded systems (250.4(B)), there is no system ground to earth (no intentional connection between the circuit conductors and earth), but all equipment must still be bonded together. On an ungrounded system, bonding provides the fault-current path needed to trip the overcurrent device if a second ground fault occurs on a different phase. The key takeaway: grounding stabilizes voltage and dissipates lightning. Bonding clears faults and saves lives. The earth is not a fault-current path.
When You Need This
- Understanding the fundamental WHY behind every grounding and bonding connection you make on the job
- Explaining to a customer, apprentice, or inspector the difference between grounding (to the dirt) and bonding (back to the panel)
- Troubleshooting a ground fault that is not clearing — the effective fault-current path (bonding) may have a high-impedance connection
- Designing or evaluating an ungrounded system (such as a 480V delta) and understanding why bonding is critical even without a system ground
- Studying for the journeyman or master electrician exam — 250.4 concepts underpin every grounding and bonding question on the test
Key Points
Common Mistakes
Believing that a ground rod will clear a fault — the earth has far too much impedance to trip a breaker; only the bonding path (back through the EGC to the source) clears faults
Using the terms 'grounding' and 'bonding' interchangeably — they are different functions with different purposes, and the NEC treats them separately
Thinking an ungrounded system does not need bonding — 250.4(B) still requires all equipment to be bonded together; bonding is what clears faults on the second ground fault
Assuming that connecting equipment to a ground rod provides personnel protection — without a bonding path back to the source, the overcurrent device will never see enough fault current to trip
Ignoring high-impedance connections in the bonding path (loose connectors, corroded conduit joints) that prevent enough fault current from flowing to trip the breaker
Exam Tip
The exam loves to test whether you understand the difference between grounding and bonding. Key phrases: grounding = voltage stabilization and surge/lightning dissipation. Bonding = effective ground-fault current path to trip the OCPD. If the question asks 'what clears the fault,' the answer is always bonding (the EGC path back to the source), never the ground rod. If the question asks 'what stabilizes voltage to earth,' the answer is grounding (the GEC to the electrode). Remember: the earth is NOT an effective ground-fault current path.
Frequently Asked Questions
No. Section 250.4(A)(5) explicitly states that the earth shall not be considered an effective ground-fault current path. The earth has far too much resistance (typically hundreds of ohms) to allow enough current to flow to trip a breaker or fuse. A 120V fault through even 25 ohms of earth resistance produces less than 5 amps — not enough to trip a 15A or 20A breaker. Only the bonding path (EGC back through metal conduit or a green/bare wire to the source) provides enough current to trip the OCPD.
On an ungrounded system (250.4(B)), the first ground fault does not trip anything. The faulted phase simply becomes referenced to ground, and the voltage on the other two phases rises to line-to-line voltage with respect to ground. The system continues to operate. It is the second ground fault — on a different phase — that creates a phase-to-phase fault through the bonded equipment, generating enough current to trip the overcurrent device. This is why ground-fault detection is critical on ungrounded systems.
Ground rods (and other grounding electrodes) serve three purposes: (1) they limit voltage imposed by lightning strikes, (2) they limit voltage from accidental contact with higher-voltage lines, and (3) they stabilize the system voltage to earth during normal operation. Without a grounding electrode, a lightning strike could impose thousands of volts on the system, destroying equipment and creating arc-flash hazards. The ground rod handles surges and stabilization — the bonding system handles faults.
Inline Tools
Grounding Calculator
Size grounding electrode conductors per NEC 250
Related Code Sections
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