NEC Article 250 — grounding and bonding

Reference notes

NEC Article 250 covers grounding and bonding — the most-tested topic on the PE Electrical / Power exam blueprint, and the foundation of every safe electrical installation. Use Next → to walk through the five general requirements (250.4), which AC systems must be grounded (250.20), the grounding electrode system (250.50–66), equipment grounding conductors (250.118–122), and bonding (250.28, 250.96, 250.102).

The five general requirements (250.4(A))

1. Electrical System Grounding — grounded systems are connected to earth to limit voltage from lightning, line surges, or unintentional contact with higher-voltage lines, and to stabilize voltage to earth during normal operation.
2. Grounding of Electrical Equipment — normally non-current-carrying conductive materials (enclosures, raceways) shall be connected to earth to limit voltage to ground.
3. Bonding of Electrical Equipment — those same materials shall be bonded together and to the supply source so a ground fault has an effective return path.
4. Bonding of Electrically Conductive Materials — non-current-carrying materials likely to become energized (water pipes, structural steel, ductwork) shall be bonded back to the supply source.
5. Effective Ground-Fault Current Path — the path must be a low-impedance circuit that facilitates overcurrent device operation. Earth shall not be considered an effective ground-fault current path. This is the single most misunderstood rule in the NEC.

Which AC systems must be grounded (250.20)

AC systems 50 V – 1000 V must be grounded if any of these apply (250.20(B)):

• Phase-to-ground voltage ≤ 150 V — e.g. 120/240 V single-phase, 120/208 V wye, 277/480 V wye
• 3-phase 4-wire wye system where the neutral is used as a circuit conductor
• 3-phase 4-wire delta where the midpoint of one phase is grounded for circuit use

Other systems may be grounded (e.g. corner-grounded delta). Three-phase 3-wire ungrounded delta is a common exception — used historically in industrial plants where a single ground fault should not trip the system.

The grounding electrode system (250.50, 250.52)

At each building or structure, all grounding electrodes that are present must be bonded together to form the grounding electrode system (250.50). Per 250.52(A), the recognized electrode types are:

1. Metal underground water pipe — must contact earth for ≥ 10 ft (3.0 m)
2. Metal in-ground support structure (piling, casing) — ≥ 10 ft
3. Concrete-encased electrode ("Ufer ground") — ≥ 20 ft of bare rebar (≥ ½″ dia) or 4 AWG bare Cu, in concrete with ≥ 2″ cover, in direct contact with earth
4. Ground ring — ≥ 20 ft of bare Cu ≥ 2 AWG, encircling the building
5. Rod and pipe electrodes — ≥ 8 ft (2.44 m); rods ≥ ⅝″ dia if not listed
6. Other listed electrodes
7. Plate electrodes — expose ≥ 2 ft² to soil; ≥ ¼″ thick iron/steel, ≥ 0.06″ thick non-ferrous

Sizing the GEC — Table 250.66

The grounding electrode conductor is sized by the largest ungrounded service conductor:

Important reductions (250.66(A)–(C)):

• GEC to a rod / pipe / plate electrode — need not be larger than 6 AWG Cu (or 4 AWG Al). Common exam trap.
• GEC to a concrete-encased electrode — need not be larger than 4 AWG Cu.
• GEC to a ground ring — need not be larger than the ground ring conductor itself (2 AWG minimum).

Equipment grounding conductors (250.118, Table 250.122)

The EGC is the fault-return path from equipment back to the service grounded conductor. 250.118 lists 14 acceptable types — the most common:

• Insulated/covered/bare copper or aluminum wire (green identification per 250.119)
• Rigid metal conduit (RMC)
• Intermediate metal conduit (IMC)
• Electrical metallic tubing (EMT)
• Listed flexible metal conduit (FMC) — only if < 6 ft total, ≤ 20 A circuit, conduit ≤ 1¼″
• Listed liquidtight FMC — similar size/length restrictions per 250.118(6)
• Cable trays under specific conditions (250.118(13))

Wire-type EGC sizing — Table 250.122, indexed by overcurrent device rating:

Bonding (250.28, 250.96, 250.102)

• Main bonding jumper (MBJ) — at the service equipment, connects the EGC bus to the grounded service conductor (neutral). The single critical conductor that makes the fault path back to the source possible. Sized per Table 250.102(C)(1).
• System bonding jumper (SBJ) — same function at a separately derived system (e.g. transformer secondary). Sized per 250.30.
• Supply-side bonding jumpers — bond raceways/enclosures between the source and the first system disconnect. Sized per 250.102(C)(1).
• Intersystem bonding — bond metal water piping (250.104(A)), gas piping (250.104(B)), and exposed structural metal (250.104(C)) to the grounding electrode system to prevent dangerous voltage differences.

Source

All values, table extracts, and section references in this lesson are verified against NFPA 70 — National Electrical Code, 2020 Edition, Article 250 (Grounding and Bonding). Where exam questions reference newer editions (2023 / 2026), check Article 250 for amendments — the structure remains largely the same but specific table values can shift edition-to-edition.