Grounding & Bonding

Grounding & Bonding — Master Exam Study Chapter

AR-MST · 2023 NEC (NFPA 70) · Open Book

Learning Objectives

After completing this chapter, you will be able to:

1.1 Foundational Definitions and the “Why” of Grounding

The NEC uses precise language. A grounded conductor is a current-carrying conductor that is intentionally connected to earth (typically the neutral). A grounding conductor connects the non-current-carrying metal parts of equipment to the earth or to the grounded conductor. A bonding jumper provides electrical continuity between metal parts that must be at the same potential but are not intended to carry load current under normal operation.

The three core purposes of grounding and bonding are: (1) limit voltage surges from lightning or line faults, (2) provide a low-impedance path for fault current to clear overcurrent devices, and (3) stabilize voltage to earth during normal operation. A master must understand that bonding is what clears faults; grounding is primarily for lightning and voltage stabilization. Do not confuse the two on site or on the exam.

1.2 Services: Grounding Electrode System and GEC Sizing

For a service, the grounding electrode conductor connects the service disconnecting means to the grounding electrode (ground rod, concrete-encased electrode, water pipe, etc.). Per 250.66, the GEC is sized from the largest ungrounded service conductor, not the sum of parallel conductors. For example, if you have 4 parallel 500 kcmil conductors per phase, you size the GEC from 500 kcmil, not from 2000 kcmil.

Table 250.66 gives the minimum GEC sizes: up to 2 AWG copper (or 1/0 aluminum) for 3/0 and smaller service conductors; 1/0 copper for 250 kcmil through 500 kcmil; 2/0 copper for 600 kcmil through 900 kcmil; and 3/0 copper for 1000 kcmil and larger. Note the aluminum equivalents are one to two sizes larger. A concrete-encased electrode (Ufer) requires a GEC of at least 4 AWG copper, regardless of the service size — this is a common trap.

250.52 lists acceptable electrodes: metal underground water pipe (at least 10 ft in contact with earth), concrete-encased electrode (at least 20 ft of ½-inch rebar or 20 ft of bare 4 AWG copper), ground ring (at least 20 ft of 2 AWG bare copper), and ground rods (at least 8 ft long). If a water pipe is used, it must be supplemented by an additional electrode per 250.53(D).

Inspection point: Verify that the GEC is protected from physical damage (250.64) and that all connections are accessible and listed for direct burial if underground. Check that the ground rod clamp is listed and that the rod is driven to its full 8-ft depth unless rock is encountered — then you may drive at an angle not exceeding 45°.

1.3 Main Bonding Jumper and System Bonding Jumper

The main bonding jumper (MBJ) is located at the service disconnecting means and connects the grounded (neutral) conductor to the equipment grounding terminal. Per 250.28, the MBJ must be sized from Table 250.102(C)(1) — not from Table 250.66. For a service with 500 kcmil copper phase conductors, the MBJ is 1/0 copper. The MBJ can be a wire, bus bar, or screw. In a service panel, the green bonding screw is the MBJ; it is installed only at the first disconnecting means.

Critical rule: The neutral-to-case bond is made only at the service. Downstream, the neutral must be insulated from the equipment grounding conductors. If you install a subpanel and bond the neutral to the enclosure, you create a parallel path for neutral current on the EGC — a violation of 250.6 and a shock hazard. On the exam, look for the phrase “separate neutral and ground bars in subpanel” as the correct answer.

For multiple disconnects (e.g., a split-bus panel or six disconnects for a large service), the MBJ is installed in the service equipment enclosure, and a grounding electrode conductor tap is run to each disconnect per 250.64(D)(1). Each tap must be sized from the phase conductors feeding that disconnect, but no smaller than the GEC required for the service.

1.4 Separately Derived Systems (250.30)

A separately derived system (SDS) has no direct electrical connection to the supply conductors except through a transformer, generator, or other source. The most common SDS is a dry-type transformer (e.g., 480V to 208Y/120V) or a standby generator with a transfer switch.

Grounding requirements: The SDS must have its grounded conductor (neutral) connected to a grounding electrode at the source or at the first disconnecting means per 250.30(A). The GEC for an SDS is sized from Table 250.66 using the largest ungrounded phase conductor of the SDS secondary. For a 208Y/120V system derived from a 75 kVA transformer, the secondary conductors are typically 4/0 or 250 kcmil, so the GEC is 2 AWG copper.

Bonding requirements: The system bonding jumper connects the neutral to the equipment grounding conductor at the SDS source. This jumper can be installed at the transformer, at the first disconnecting means, or anywhere between them — but only at one location. If installed at the transformer, a supply-side bonding jumper must run with the phase conductors to the first disconnect, sized per Table 250.102(C)(1).

The 5-ft rule: If the transformer is located within 5 ft of the first disconnecting means, you may bond the neutral at the disconnect and run a GEC from the transformer to the electrode. If the distance exceeds 5 ft, the bonding jumper must be at the transformer, and the GEC must be run to the electrode. This is a classic exam trap — read the distance carefully.

Inspection point: For a generator SDS, verify that the neutral is bonded to the generator frame only if the transfer switch does not switch the neutral. If the transfer switch is a 3-pole switch (neutral not switched), the generator is not an SDS, and the neutral must be bonded at the service only.

1.5 Equipment Grounding Conductors (EGC) and Parallel Runs

The EGC (250.118) can be a wire, conduit, cable tray, or metal raceway. When using wire-type EGCs, size per Table 250.122 based on the overcurrent device rating ahead of the circuit — not the load. For a 100A feeder, the EGC is 8 AWG copper, even if the ungrounded conductors are 2 AWG for voltage drop. If you increase the ungrounded conductors for voltage drop, you must also increase the EGC proportionally per 250.122(B).

Parallel runs: When conductors are run in parallel (multiple conductors per phase), the EGC must be run in parallel as well, sized per Table 250.122 based on the rating of the overcurrent device. Each parallel EGC must be full-size — you cannot split a 1/0 EGC into two 6 AWG conductors. The EGC must be in the same raceway or cable as the phase conductors to ensure low impedance during a fault.

Trap: For a 400A feeder with 2 parallel runs of 3/0 copper per phase, the EGC in each raceway must be sized for the 400A OCPD — that is 3 AWG copper per Table 250.122. Do not size it for 200A per raceway.

1.6 Feeder and Service Sizing Calculations

Service conductors (250.24 and 310.12): For a dwelling unit service, you may use Table 310.12 which allows the 83% derating of the neutral and phase conductors for services rated 100A through 400A. For example, a 200A residential service can use 2/0 copper (200A × 0.83 = 166A, and 2/0 copper is rated 175A at 75°C). This table applies only to dwelling units, not to commercial or industrial services.

Feeder conductors per 215.2(A)(1): The feeder must have an ampacity of at least the sum of the non-continuous loads plus 125% of the continuous loads. For a 100A continuous load, the feeder must be rated at least 125A. The overcurrent device must also be rated at least 125% of the continuous load, unless it is listed for 100% continuous operation (rare, and requires a specific breaker and panelboard).

Neutral sizing: The grounded conductor must be sized to carry the maximum unbalanced load per 220.61. For a 3-phase, 4-wire wye system, the neutral carries the unbalanced current of the line-to-neutral loads. You may not reduce the neutral below the minimum required for the load, but you can often use a smaller neutral than the phase conductors for unbalanced lighting loads. However, for a 3-phase, 4-wire delta with a high-leg, the neutral must be full-size because the high-leg loads are typically line-to-line.

Inspection point: Verify that the neutral is not switched or fused (230.75). The neutral must be solidly connected to the service disconnect. Check that the neutral bus is properly bonded to the enclosure only at the service.

1.7 Overcurrent Protection Coordination and Tap Rules

Service overcurrent protection per 230.90: The service disconnecting means must have a rating not less than the computed load. The OCPD must be located at or near the point of entrance. 240.21 allows taps — conductors that connect to a feeder or service without an OCPD at the tap point. The most common is the 10-ft tap rule (240.21(B)(1)): the tap must have an ampacity not less than the load served, must not exceed 10 ft in length, must be enclosed in a raceway, and must terminate in an OCPD rated not more than the tap conductor ampacity.

The 25-ft tap rule (240.21(B)(2)) allows up to 25 ft if the tap conductors have an ampacity of at least one-third of the rating of the OCPD protecting the feeder. For example, a 400A feeder can have a 25-ft tap of conductors rated at least 133A (one-third of 400A). This is common for feeding a small panel from a large busway.

Coordination for elevators, fire pumps, and health care requires selective coordination (240.12) — the upstream device must open only after the downstream device has cleared the fault. For a master, this means you must verify that the time-current curves of breakers do not overlap. On the exam, remember that fire pump feeders must have OCPDs that are sized to allow the locked-rotor current of the pump motor to pass without opening (695.4).

1.8 Motors and Generators

Motor circuits per 430.22: The branch-circuit conductors must have an ampacity of at least 125% of the motor full-load current (FLC) from Tables 430.247 through 430.250. The motor overload protection (430.32) is separate from the branch-circuit short-circuit and ground-fault protection (430.52). The latter is sized from Table 430.52 — typically 250% of FLC for a standard squirrel-cage motor with an inverse-time breaker. The EGC for a motor branch circuit is sized from the rating of the branch-circuit OCPD per Table 250.122.

Generators as separately derived systems: When a generator is used as a backup source with a transfer switch that switches the neutral, the generator is an SDS and must be grounded per 250.30. The neutral of the generator must be bonded to the generator frame, and a GEC must be run to a grounding electrode at the generator location. If the transfer switch does not switch the neutral, the generator is not an SDS, and the neutral is bonded at the service only.

Inspection point: For a generator with a 4-pole transfer switch (switching the neutral), verify that the neutral-to-case bond is removed at the generator if the bond is made at the transfer switch. Double bonding creates a parallel neutral path.

1.9 Code Navigation — Where to Find It

1.10 Inspection and Supervision Points

As a master, you are responsible for the final sign-off. On every grounding and bonding inspection, verify:

1.11 Common Exam Traps

End of Chapter — Grounding & Bonding.