Grounding & Bonding Basics
NEC Article 250

Grounding ≠ Bonding

These two words are used interchangeably in conversation, but the NEC draws a sharp line between them. Both involve green or bare copper, but they do different jobs.

• Grounding — intentionally connecting the electrical system (and equipment) to earth. Gives lightning and stray voltage a low-impedance path to dirt.
• Bonding — electrically connecting all non-current-carrying metal parts together so they sit at the same potential. Gives a ground fault a low-impedance path back to the source so the breaker trips.

The earth is a terrible fault-clearing path. Soil resistance is measured in ohms; a 120V circuit fault through a 25-ohm ground rod delivers only 4.8 amps — not nearly enough to trip a 20-amp breaker. That's why bonding, not grounding, is what protects you from shock. The ground rod is for lightning and system reference.NEC 250.4(A)(5)

The key components

Grounding Electrode (GE) & Grounding Electrode Conductor (GEC)

The GE is the physical connection to earth — a ground rod, concrete-encased electrode (Ufer ground), metallic water pipe within 10 ft of entry, or building steel. The GEC runs from the GE to the neutral bar at the service disconnect. Section 250.66 sizes it (often #4 or #6 copper for common services). NEC 250.52, 250.66

Main Bonding Jumper (MBJ)

This is the single screw or strap inside your service-disconnect enclosure that ties the neutral bar to the equipment grounding bar (and the metal enclosure itself). It is the one and only place the grounded conductor (neutral) and equipment grounding conductor (EGC) are allowed to connect downstream of the utility. Multiple neutral-ground bonds cause current to flow on metal enclosures — a shock and arcing hazard.NEC 250.24(B), 250.28

Equipment Grounding Conductor (EGC)

The green wire (or bare copper, or metallic raceway listed for the purpose) that runs with every branch circuit back to the service. If a hot conductor shorts to a metal enclosure, the EGC carries the fault current straight back to the neutral at the MBJ, the impedance is low, and the breaker trips in milliseconds. Sized by NEC Table 250.122.

Separately Derived Systems (SDS)

Transformers and on-site generators create separately derived systems. Each SDS needs its own system bonding jumper and grounding electrode connection — you cannot rely on the service ground for a downstream transformer. A common mistake on generator installs is omitting the SDS ground (or double-bonding neutral-to-ground at both the ATS and the generator), which creates circulating current and nuisance GFCI trips.NEC 250.30

The Fault-Current Math Every Electrician Should Know

The single most important reason to understand bonding vs grounding is the arithmetic of how a breaker actually trips. Consider a 120V branch circuit where a damaged hot conductor touches a metal enclosure. The fault current path must be low-impedance enough for the breaker to see an overcurrent.

Scenario A — relying on the earth: Current flows from the hot conductor, through the metal enclosure, through a dedicated ground rod at 25 ohms, through the soil, back to the utility transformer's grounded pole. By Ohm's Law, I = 120V ÷ 25Ω = 4.8 amps. A 20-amp breaker sees 4.8 amps and considers that normal running current. The enclosure remains energized at a lethal voltage indefinitely until someone touches it and their body completes a parallel path.

Scenario B — relying on bonding: Current flows from the hot conductor, through the metal enclosure, through the equipment grounding conductor (a #12 copper wire at 0.02 ohms for a short run) back to the neutral bar at the service, then back to the transformer through the service neutral. Total impedance perhaps 0.1 ohm. I = 120V ÷ 0.1Ω = 1,200 amps. The breaker sees six times its rating and trips in approximately one cycle (16 milliseconds). The enclosure is de-energized before anyone is hurt.

Testing the Grounding Electrode System

NFPA 70B §29.2.1 and most insurance-carrier survey checklists recommend periodic ground resistance testing, typically every 5 years or after major electrical work. Three methods are in common use:

• Fall-of-potential (3-point) test — the IEEE Std 81 canonical method. Requires taking the ground electrode out of service (disconnect from service neutral), driving two auxiliary probes (current + potential) in line with the electrode under test, and sweeping the potential probe along the baseline. Produces an accurate standalone resistance reading. Typical acceptance: 25 ohms or less for a single rod (NEC 250.53(A)(2)), 5 ohms or less for critical facilities.
• Clamp-on (induced-current) test — works only on bonded grounding systems with multiple parallel ground paths (a service with building steel, Ufer, and rod all connected). The clamp induces a signal and measures the loop impedance. Cannot measure a single electrode in isolation.
• Two-point (dead-earth) test — measures resistance between two grounding electrodes. Useful for verifying the bond between building steel and the grounding electrode system, not for absolute resistance of either electrode.

Record the test result, the method used, the weather (dry vs saturated soil produces wildly different readings), and the electrode location. Retest after any service upgrade or after ground-level excavation near the electrode path.

Special-Occupancy Bonding Rules

Data centers + IT rooms (Article 250.146)

Isolated ground (IG) receptacles are permitted for sensitive electronics to reduce noise on the EGC, but they must still be bonded to the building's grounding electrode system through an insulated EGC that terminates at the service or separately derived system neutral — NOT at local building steel. Mixing up the IG return with the signal reference grid creates ground loops that destroy equipment.

Swimming pools + wet locations (Article 680)

Pool shells require equipotential bonding per NEC 680.26. A #8 solid copper conductor ties the reinforcing steel, deck perimeter, metal ladders, and water feature metalwork together and to the equipment grounding system. The goal is not to carry fault current but to hold everything a wet swimmer could touch at the same potential so no voltage gradient develops across their body.

Solar PV + battery storage (Article 690)

NEC 690.47 requires a dedicated grounding electrode for the PV array grounding, bonded to the premises grounding electrode system. DC-side grounding is particularly tricky because photovoltaic sources can maintain dangerous voltage even when a breaker is open — the ground-fault detection and interruption (GFDI) requirement in 690.41 exists because of this.

What facility managers should check

• Open-neutral indicators on your panel. Tingling voltage on metal enclosures usually means a bond is loose or missing, not a broken ground rod.
• Double neutral-ground bonds in subpanels. The bonding screw in a subpanel must be removed. Only the main panel (service disconnect) bonds neutral to ground.
• Ground rod integrity. Inspect at least every 5 years; corrosion breaks the lightning path even if it doesn't affect fault clearing.
• Generator and UPS bonding. After any service on an ATS or generator, verify the SDS jumper is installed in exactly one place.
• IT room / data center equipotential bonding. Isolated ground (IG) receptacles and signal reference grids are governed by Article 250.146(D) — common mistake is tying IG terminals to building steel instead of the service ground.

Frequently Asked Questions

References