Grounding & Bonding Basics
NEC Article 250
Two different jobs often lumped under one word. Know which is which — your breaker will thank you.
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
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.
References
IEEE Std 142, Recommended Practice for Grounding of Industrial and Commercial Power Systems.
UL 467, Grounding and Bonding Equipment.
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Discussion (2)
The most common grounding violation I see in the field is using the equipment grounding conductor as a neutral. Article 250.24(A)(5) prohibits any connection between the grounded conductor and the equipment grounding conductor on the load side of the service disconnect. Once past the main bonding jumper, they must stay separate.
Bonding of fire sprinkler piping is often overlooked. NFPA 13 Section 24.4 requires bonding across mechanical joints where the piping could become energized. In a lightning strike or ground fault scenario, ungrounded metallic piping can carry dangerous voltages. The bonding jumper provides the fault current path back to the source.
Excellent cross-discipline point. NEC Article 250.104 also requires bonding of metal piping systems that may become energized, including fire sprinkler piping. The sprinkler contractor and the electrician both have responsibilities here, and the gap between trades is where violations happen.