9.3 Equipment and Box Grounding Connections

Why This Matters on the TX Journeyman Exam

These are pure Knowledge items — short, fact-driven, and repeated every cycle. PSI loves the range/dryer 4-wire cord rule, the pigtail rule, and the "sheet-metal screw is not allowed" connection rule. Miss any of the three and you typically miss two items, because they come in paired scenarios.

General Rule — NEC 250.110

NEC 250.110 requires that equipment that is fastened in place or connected by permanent wiring methods (raceway, cable, direct burial) be grounded — that is, connected to the EGC. Exposed non-current-carrying metal parts likely to become energized must be grounded.

The fixed-in-place criterion matters: a cord-and-plug-connected tool is covered by 250.114, not 250.110.

Specific Equipment — NEC 250.112 through 250.126

NEC 250.112 lists specific equipment that must be grounded regardless of position, including:

• Motor frames (NEC 430.242 cross-reference) — every motor frame must be grounded, no voltage threshold.
• Switchboard and panelboard frames.
• Pipe organs (legacy, still in the article).
• Motor controllers, motor generator sets, electric heating equipment, and luminaires.
• Information technology equipment and audio/RF equipment per their respective articles.

Motors — NEC 430.242

For motor circuits, 430.242 requires the motor frame to be connected to the EGC. The EGC for a motor circuit is sized from Table 250.122 using the motor branch-circuit short-circuit and ground-fault protective device rating — not the motor full-load current. Where that protective device is rated above 800 A, the EGC is sized per 250.122(A) "not smaller than shown, and not required to be larger than the largest ungrounded circuit conductor."

Ranges and Dryers — NEC 250.140 and 250.142

This is the exam-favorite worked item.

The 4-wire cord rule

NEC 250.140 permits the frames of ranges, wall-mounted ovens, counter-mounted cooking units, and clothes dryers to be grounded to the grounded circuit conductor (neutral) only on existing branch circuits installed under earlier Codes that allowed it. For new installations, the frame must be grounded to the equipment grounding conductor — a 4-wire cord-and-plug connection is required.

NEC 250.142(B) makes the prohibition explicit: on the load side of the service disconnect, the grounded circuit conductor (neutral) shall not be used for grounding equipment. So a new range or dryer receptacle must be a 4-slot NEMA 14-30R (dryer) or 14-50R (range), with the equipment grounding conductor bonded to the receptacle's grounding terminal and to the frame through the 4-wire cord's green conductor.

Worked example — new electric range

A new 40 A, 240/125 V electric range is installed in a kitchen. The existing 3-wire SE cable (two ungrounded + bare neutral) cannot be reused to bond the frame.

• Compliant path (new branch circuit): Run 6 AWG copper ungrounded conductors, a 10 AWG copper EGC, and a separate 10 AWG neutral (or 8 AWG neutral if required by 220.61) in a 4-wire cable or raceway to a NEMA 14-50R receptacle. Use a 4-wire range cord (NEMA 14-50P). The range's green frame lead bonds to the EGC, not to the neutral.
• Non-compliant path: Reusing a 3-wire cable and bonding the frame to the neutral on a new branch circuit violates 250.140 and 250.142(B).

The 3-wire exception (existing only)

NEC 250.140 Exception 1 still allows frames of ranges and dryers to be grounded to the neutral on branch circuits installed prior to the 1996 NEC, where the grounded conductor is not smaller than 10 AWG copper or 8 AWG aluminum and is insulated (or part of Type SE cable with the neutral bare). Existing 3-wire receptacles may remain; replacements may use a 3-wire cord if the existing branch circuit and receptacle are 3-wire.

Methods of Grounding Equipment — NEC 250.134

NEC 250.134 permits grounding fixed equipment by any of the EGC types in 250.118 or by an equipment grounding terminal bar in the equipment. The connection must be to an equipment grounding conductor, not to a grounded (neutral) conductor on the load side of the main disconnect (250.142(B)).

EGC Splices and Connections in Boxes — NEC 250.148

NEC 250.148 governs how EGCs are handled inside a box. The rule that shows up on every TX exam:

Where more than one EGC enters a box, all EGCs shall be spliced or joined together inside or on the box, with a pigtail for the connection to devices. Removing a receptacle, switch, or other device shall not interrupt the grounding continuity to downstream devices.

The pigtail is the entire exam point. A receptacle wired with the EGC looped around its grounding screw and continuing onward is not compliant — removing the receptacle breaks the EGC to downstream receptacles. Use a wire nut or listed splicing device to join all incoming EGCs plus a pigtail that lands on the receptacle grounding screw.

Exception: where the EGC is part of a listed multiwire branch-circuit assembly and the box is large enough to allow a single EGC to loop through, but the practical rule is: pigtail every box.

Connection Means — NEC 250.8

NEC 250.8 restricts how grounding conductors are connected to electrodes, enclosures, and fittings. Permitted means:

Prohibited: sheet-metal screws, wood screws, and self-tapping screws (250.8(B)). They do not maintain consistent contact pressure and can loosen under fault current. This is a one-line exam item — "A #10 sheet-metal screw may not be used to connect a grounding conductor to an enclosure."

Boxes as Grounding Points — NEC 314.31

NEC 314.31 requires a box used at a receptacle, switch, or other outlet to have an intentional grounding means — a tapped hole to accept a grounding screw, a listed grounding clip, or an equipment grounding terminal bar bonded to the box. A screw used for this purpose must be listed for grounding (green head, 10-32 thread typical). The box itself must be bonded to all entering EGCs (250.148).

For nonmetallic boxes, an equipment grounding terminal bar must be provided and connected to all EGCs entering the box. The terminal bar is not bonded to the box (it cannot be — the box is nonconductive), but it provides the common point for the EGCs and the device pigtail.

Summary of Cross-References

• Article 100 (definitions of grounded, grounding, bonded, effective ground-fault current path)
• 250.4(A)(2)/(3)/(5) — performance requirements
• 250.8 — permitted and prohibited connection means
• 250.110 / 250.112 / 250.126 — equipment grounding scope
• 250.134 — methods of grounding fixed equipment
• 250.140 / 250.142 — range/dryer 4-wire rule
• 250.148 — EGC splicing in boxes (pigtail rule)
• 314.31 — boxes as grounding points
• 430.242 — motor frame grounding