第 2 章，共 5 章23% 占考试比重
Services, Grounding and Bonding, Panelboards and Transformers
Grounding and bonding is the subject electricians most often get wrong in the field and on the exam, largely because two different conductors with similar names do completely different jobs. This chapter separates the grounding electrode system, which stabilizes voltage to earth, from the equipment grounding system, which clears faults, and then adds the service, panelboard and transformer rules that surround them.
Service Conductors, Clearances and Disconnects
Service conductors are the only conductors in a building that are not protected by an overcurrent device ahead of them, so the code compensates with strict rules on where they may run, how far they may extend inside a building and how quickly they must reach a disconnect. Overhead clearances are graded by what passes underneath, from pedestrian areas up to public roads. The service disconnect must be readily accessible and located nearest the point where the conductors enter the building.
Overhead clearances
For conductors not over 150 volts to ground: 10 feet above grade and pedestrian areas, 12 feet over residential driveways, and 18 feet over public streets, roads and alleys.
2023 NEC §230.24(B)Clearance from openings
Service conductors must maintain 3 feet of clearance from windows designed to open, doors, porches and similar locations, except where run above the top of the window.
2023 NEC §230.9(A)Disconnect location
The service disconnecting means must be readily accessible, either outside or inside nearest the point of entrance of the service conductors.
2023 NEC §230.70(A)(1)Number of disconnects
Not more than six service disconnects are permitted for each service, and current rules require each to be in a separate enclosure or in a listed assembly of individual disconnects.
2023 NEC §230.71Minimum dwelling rating
The service disconnecting means for a one-family dwelling must be rated at least 100 amperes, 3-wire.
2023 NEC §230.79(C)Ground-fault protection
Equipment ground-fault protection is required on solidly grounded wye services over 150 volts to ground with a disconnect rated 1,000 amperes or more, with a maximum setting of 1,200 amperes.
2023 NEC §230.95The Grounding Electrode System
All grounding electrodes present at a building must be bonded together to form one system, which is why a house with a metal water service, a footing electrode and a driven rod ends up with all three connected. Each electrode type has its own qualifying condition, and the metal water pipe is unique in that it always requires a supplemental electrode. The grounding electrode conductor is sized from the service conductors, then capped by the electrode it serves.
Water pipe electrode
A metal underground water pipe qualifies only if at least 10 feet is in direct contact with the earth, and it must always be supplemented by an additional electrode.
2023 NEC §250.52(A)(1)Concrete-encased electrode
At least 20 feet of one-half inch or larger reinforcing bar, or 20 feet of bare copper not smaller than 4 AWG, encased in concrete in contact with earth.
2023 NEC §250.52(A)(3)Rod electrodes
Rod and pipe electrodes must be at least 8 feet long with 8 feet in contact with soil, and rods used together must be at least 6 feet apart.
2023 NEC §250.53(A)(3)The 25-ohm test
A single rod, pipe or plate electrode must be supplemented unless it is shown to have a resistance to earth of 25 ohms or less.
2023 NEC §250.53(A)(2)Grounding electrode conductor sizing
Size from the largest ungrounded service conductor: 8 AWG copper for 2 AWG and smaller, 6 AWG for 1 or 1/0, 4 AWG for 2/0 or 3/0, 2 AWG through 350 kcmil, and 1/0 through 600 kcmil.
2023 NEC Table 250.66Electrode-specific caps
The conductor to a rod, pipe or plate electrode need not exceed 6 AWG copper, and to a concrete-encased electrode need not exceed 4 AWG copper.
2023 NEC §250.66Equipment Grounding Conductors and Bonding
The equipment grounding conductor is the low-impedance path that carries fault current back to the source so the overcurrent device opens, and the earth is explicitly not permitted to serve that purpose. It is sized from the rating of the overcurrent device ahead of the circuit, not from the size of the phase conductors, except when the phase conductors are upsized. The main bonding jumper is what ties the grounded conductor to the enclosure and equipment grounding system, and it exists at exactly one place in a premises wiring system.
Earth is not a fault path
The earth must not be used as the sole equipment grounding conductor or effective ground-fault current path.
2023 NEC §250.4(A)(5)Equipment grounding conductor sizing
Size by overcurrent device rating: 14 AWG copper at 15 amperes, 12 AWG at 20, 10 AWG at 60, 8 AWG at 100, 6 AWG at 200, 4 AWG at 300 and 3 AWG at 400 amperes.
2023 NEC Table 250.122Proportional increase
Where ungrounded conductors are increased in size, the equipment grounding conductor must be increased proportionally by circular-mil area.
2023 NEC §250.122(B)Main bonding jumper
A main bonding jumper must connect the grounded conductor to the equipment grounding conductor and enclosure at the service disconnecting means.
2023 NEC §250.24(B)Main bonding jumper sizing
Size from the table used for grounded conductors and supply-side bonding jumpers; where service conductors exceed 1,100 kcmil copper, the jumper must be at least 12.5 percent of that area.
2023 NEC §250.28(D)No neutral bonding downstream
On the load side of the service disconnect, the grounded conductor must not be used to ground equipment enclosures; keep neutrals and grounds separate at subpanels.
2023 NEC §250.142(B)Panelboards and Overcurrent Device Placement
A panelboard is protected as a unit by a device rated no higher than the panelboard itself, whether that device sits inside as a main breaker or ahead of it in the feeder. Beyond that, the code is concerned with clear identification, so that a future electrician can find the right circuit without guessing. Circuit directories must describe the specific load and its location rather than using vague or obsolete descriptions.
Panelboard protection
Each panelboard must be protected by an overcurrent device with a rating not greater than that of the panelboard.
2023 NEC §408.36Circuit directory
Every circuit must be legibly identified as to its clear, evident and specific purpose, with no reliance on transient occupancy for identification.
2023 NEC §408.4(A)Working space
Panelboards require the working clearances of Article 110, including dedicated space above the equipment free of foreign piping and ducts.
2023 NEC §110.26Wire-bending space
Panelboards must provide the required wire-bending space at terminals and gutters for the size of conductors being landed.
2023 NEC §408.55Transformers
Transformer questions usually combine a current calculation with a protection percentage. Start by finding primary or secondary full-load current from the kVA rating and the voltage, remembering the 1.732 factor on three-phase, then apply the protection percentage from the transformer article. A transformer secondary is also a separately derived system, which means it gets its own system bonding jumper and grounding electrode connection.
Three-phase current
Full-load current equals volt-amperes divided by 1.732 times the line voltage; on single-phase it is volt-amperes divided by voltage.
Primary-only protection
Where only primary protection is provided on a transformer of 1,000 volts or less with primary current of 9 amperes or more, the device may be set at 125 percent of primary full-load current, rounded to the next standard size.
2023 NEC §450.3(B)Separately derived system bonding
A transformer secondary that is a separately derived system requires a system bonding jumper and a grounding electrode conductor at the derived system.
2023 NEC §250.30(A)Clearance from combustibles
Indoor dry-type transformers of 112.5 kVA or less must be at least 12 inches from combustible material unless separated by a fire-resistant heat-insulating barrier or completely enclosed.
2023 NEC §450.21(A)Accessibility and ventilation
Transformers must be readily accessible unless they meet specific exceptions, and ventilation openings must not be blocked.
2023 NEC §450.13测试你的知识
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