GND-4 — Equipment Grounding Conductors and Bonding Methods
Key Takeaways
- An effective ground-fault current path is intentionally low impedance and returns fault current to the source so the overcurrent device can open; earth is not a substitute.
- Table 250.122 sizes an equipment grounding conductor primarily from the rating or setting of the upstream overcurrent device, while the 2017 proportional-increase rule applies when ungrounded conductors are enlarged.
- Metal raceways and cable constructions can serve as equipment grounding conductors only when they satisfy 250.118 and remain electrically continuous through listed fittings and bonded enclosures.
- A load-side neutral-to-ground connection creates a parallel path for normal neutral current and can place objectionable current on raceways, enclosures, piping, and communications bonding systems.
Exam checkpoints
| Checkpoint | What to verify |
|---|---|
| 1 | An effective ground-fault current path is intentionally low impedance and returns fault current to the source so the overcurrent device can open; earth is not a substitute. |
| 2 | Table 250.122 sizes an equipment grounding conductor primarily from the rating or setting of the upstream overcurrent device, while the 2017 proportional-increase rule applies when ungrounded conductors are enlarged. |
| 3 | Metal raceways and cable constructions can serve as equipment grounding conductors only when they satisfy 250.118 and remain electrically continuous through listed fittings and bonded enclosures. |
Start with the fault-current path
Grounding and bonding do related but different work. Grounding connects a system or equipment to earth. Bonding joins conductive parts to establish electrical continuity and conductivity. Section 250.4(A)(5) requires an effective ground-fault current path that is permanent, electrically continuous, and able to carry the maximum likely fault current safely. Its impedance must be low enough to facilitate operation of the fuse or breaker.
For a line-to-metal-enclosure fault, the intended path is enclosure to equipment grounding conductor, through bonding connections to the source bonding point, and through the grounded source winding back to the faulted phase. The grounding electrode and soil are not substituted for this metallic path. A 120 V fault through 25 ohms of earth would produce only 4.8 A before other impedance; that will not promptly trip a 20 A breaker.
Section 250.118 recognizes wire-type equipment grounding conductors and qualifying metal raceways, cable armor, cable trays, and other wiring methods. An EMT raceway can be the EGC when its fittings and joints provide continuity and it is properly bonded. Corrosion, loose set-screw fittings, an unbonded concentric knockout, or a nonqualifying flexible section can interrupt the path. Installing a wire EGC is not permission to ignore bonding of metal raceways and enclosures.
Size from Table 250.122
Table 250.122 normally uses the rating or setting of the automatic overcurrent device ahead of the circuit. Examples from the copper column are 12 AWG for a 20 A device, 10 AWG for 60 A, 8 AWG for 100 A, and 3 AWG for 400 A. The circuit's calculated load is not the direct table input. Equipment-specific provisions can modify application, and the path may need to be larger where necessary to meet 250.4.
Where one wire-type EGC serves multiple circuits in the same raceway or cable, 250.122(C) sizes it from the largest overcurrent device protecting conductors in that raceway or cable. If 20 A, 30 A, and 60 A circuits share a raceway, the common copper EGC is at least 10 AWG because 60 A is the controlling row. For a motor circuit, 250.122(D)(1) normally uses the rating of the branch-circuit short-circuit and ground-fault protective device. Where an instantaneous-trip breaker or motor short-circuit protector is used, 250.122(D)(2) instead keys the table to the maximum permitted dual-element time-delay fuse selected under 430.52(C)(1), Exception No. 1. Neither method sizes the EGC from the motor overload setting or normal running current.
The 2017 rule in 250.122(B) matters when ungrounded conductors are increased from the minimum size having sufficient ampacity for the intended installation. A wire-type EGC, where installed, is increased proportionately by circular-mil area.
Example: a 20 A copper branch circuit would ordinarily use 12 AWG ungrounded conductors and a 12 AWG copper EGC. Suppose the ungrounded conductors are enlarged to 8 AWG. Chapter 9, Table 8 gives 12 AWG as 6,530 circular mils and 8 AWG as 16,510 circular mils.
Increase ratio = 16,510 ÷ 6,530 = 2.53
Apply 2.53 to the original 6,530-cmil EGC: the result is approximately 16,510 circular mils, so the EGC also becomes 8 AWG copper. Do not import a later-edition replacement for this 2017 proportional method.
For circuit conductors installed in parallel in multiple raceways, 2017 NEC 250.122(F) requires a wire-type EGC in each raceway, sized from Table 250.122 based on the overcurrent device. A 400 A feeder in two raceways therefore uses a 3 AWG copper EGC in each raceway, before any proportional increase—not two smaller conductors whose areas merely total one 3 AWG conductor.
Bond boxes, enclosures, and devices
Section 250.148 requires equipment grounding conductors associated with splices or terminations in a box to be connected together and, for a metal box, bonded to the box. Removing a receptacle or luminaire cannot be allowed to interrupt grounding continuity to downstream equipment. A listed grounding screw or clip, grounding bar, listed pressure connector, exothermic weld, or another method permitted by 250.8 can make a bonding connection. A sheet-metal screw is not an acceptable grounding and bonding connection.
A receptacle grounding terminal is connected to a grounded metal box by a permitted method under 250.146. Listed self-grounding receptacles and qualifying direct metal-to-metal arrangements have conditions; a plastic yoke washer, loose mounting screw, or assumption that contact is adequate is not a method. Bond around impaired knockouts and use bonding bushings or jumpers where the applicable service or higher-voltage rules require them.
Flexible metal raceways deserve a separate check. Some qualify as an EGC only within the size, length, overcurrent, fitting, and use limits in 250.118. Equipment requiring movement or a connection outside those conditions needs a wire-type EGC routed with the circuit conductors and connected at both ends.
Eliminate objectionable current
The grounded conductor normally carries return current; the EGC normally does not. At the permitted service or separately derived system bonding point, they are intentionally connected. Connecting them again on the load side places neutral current on every parallel conductive path: feeder EGC, panel enclosure, raceway, water piping, and intersystem bonds. Section 250.6 addresses this objectionable current.
The repair is to remove the improper neutral-to-case bond and preserve required grounding and bonding—not to disconnect an EGC or defeat a conductive path. In a downstream panel, isolate the neutral bar from the enclosure, bond the equipment grounding bar to the enclosure, and keep grounded and grounding conductors separated. Test continuity and fault-loop integrity after installation rather than relying on conductor color or appearance.
What is the minimum copper equipment grounding conductor shown in Table 250.122 for a circuit protected by a 100 A overcurrent device?
Under the 2017 proportional-increase rule, a 20 A copper branch circuit is enlarged from 12 AWG to 8 AWG. What copper wire-type EGC results from the circular-mil calculation?
A 400 A feeder is installed in parallel through two raceways under the 2017 rule. What wire-type copper EGC is placed in each raceway before any proportional increase?
What is the likely result of bonding the neutral bar to the enclosure in a feeder-supplied downstream panel?