第 3 章，共 5 章21% 占考试比重
Raceways, Boxes, Cables and Underground Installations
Wiring method questions split cleanly into two kinds: table lookups that reward memorization and fill calculations that reward practice. Box fill and conduit fill are the two calculations you will see most, and both hinge on counting things correctly before any arithmetic starts.
Box Fill Calculations
Box fill counts volume allowances, not physical objects, and each allowance is worth a fixed number of cubic inches based on conductor size: 2.00 for 14 AWG, 2.25 for 12 AWG, 2.50 for 10 AWG and 3.00 for 8 AWG. Work through the box in a fixed order every time so you do not skip an allowance. Conductors that both originate and terminate inside the box, such as pigtails, and small fittings like wire connectors are not counted at all.
Conductors
Each conductor that enters the box and terminates or passes through counts as one volume allowance based on its own size.
2023 NEC §314.16(B)(1)Clamps
All internal cable clamps together count as one allowance, based on the largest conductor in the box.
2023 NEC §314.16(B)(2)Support fittings
Each fixture stud or hickey counts as one allowance based on the largest conductor in the box.
2023 NEC §314.16(B)(3)Devices
Each yoke or strap holding a device counts as two allowances based on the largest conductor connected to that device.
2023 NEC §314.16(B)(4)Grounding conductors
All equipment grounding conductors together count as one allowance, based on the largest one present.
2023 NEC §314.16(B)(5)Not counted
Pigtails that begin and end inside the box, wire connectors and locknuts or bushings on the outside of the box are not counted.
Conduit Fill
Raceway fill exists to keep conductors from overheating and from being damaged during the pull. The allowable percentage depends only on the number of conductors, not on their size, and the odd dip for exactly two conductors reflects the risk of jamming in a bend. When every conductor is the same size and insulation type, Annex C gives the answer directly and saves several steps of arithmetic.
Fill percentages
One conductor may occupy 53 percent of the raceway area, two conductors 31 percent, and three or more 40 percent.
2023 NEC Chapter 9, Table 1Nipples
A raceway nipple not exceeding 24 inches in length between enclosures may be filled to 60 percent.
2023 NEC Chapter 9, Table 1, Note 4Conductor and raceway areas
Take conductor cross-sectional areas from Chapter 9, Table 5 and raceway areas from Chapter 9, Table 4.
Annex C shortcut
Where all conductors are the same size and insulation type, read the permitted number directly from the Annex C table for that raceway.
Bend limit
No more than 360 degrees of total bend, the equivalent of four quarter bends, is permitted between pull points.
2023 NEC §358.26Support and Securing Requirements by Wiring Method
Every wiring method has a pair of numbers: how close the first support must be to a box or fitting, and how far apart supports may be on the run. Grouping them makes them easier to recall, since rigid raceways are supported at longer intervals than cables, and PVC support spacing grows with trade size because larger PVC is stiffer. Mixing up the cable numbers is the most common error on these questions.
EMT
Secure within 3 feet of each box or termination and support at intervals not exceeding 10 feet.
2023 NEC §358.30(A)Rigid metal conduit
Secure within 3 feet of each box, with support spacing that varies by trade size and may reach 20 feet for larger sizes on straight runs.
2023 NEC §344.30PVC conduit
Secure within 3 feet of each box, and support one-half through 1 inch every 3 feet, 1 and 1/4 through 2 inch every 5 feet, with longer intervals for larger sizes.
2023 NEC §352.30Type NM cable
Secure within 12 inches of each box with clamps, or 8 inches of a single-gang nonmetallic box without clamps, and support every 4 and 1/2 feet.
2023 NEC §334.30Type AC and Type MC cable
Type AC is supported every 4 and 1/2 feet, Type MC every 6 feet, and both are secured within 12 inches of every termination.
2023 NEC §330.30Liquidtight flexible metal conduit
Secure within 12 inches of each box or fitting and support at intervals not exceeding 4 and 1/2 feet, with limited exemptions for short flexible connections.
2023 NEC §350.30(A)Underground Wiring and Burial Depths
Burial depth depends on what is protecting the conductors and what is above them, so the table is read as a grid of wiring method against location. The general pattern is that metal conduit needs the least cover, listed nonmetallic raceway needs more, and unprotected direct-buried cable needs the most. A reduced depth is allowed for low-voltage residential circuits only when GFCI protection is present.
Direct-buried cable
24 inches of cover in locations not otherwise specified.
2023 NEC Table 300.5Rigid and intermediate metal conduit
6 inches of cover in locations not otherwise specified.
2023 NEC Table 300.5Listed nonmetallic raceway
18 inches of cover where buried without concrete encasement in locations not otherwise specified.
2023 NEC Table 300.5GFCI-protected residential circuits
12 inches of cover for residential branch circuits rated 120 volts or less and 20 amperes or less that are GFCI protected.
2023 NEC Table 300.5Emerging from grade
Conductors emerging from grade must be protected by enclosures or raceways from below grade to at least 8 feet above finished grade.
2023 NEC §300.5(D)(1)Wet location conductors
The interior of a raceway underground is a wet location, so conductors must be listed for wet locations, such as THWN-2 or XHHW.
2023 NEC §310.10(C)Physical Protection and Pull Boxes
Where cables pass through framing, the code assumes someone will eventually drive a fastener into the wall, so it requires either setback or a steel plate. Pull and junction boxes for large raceways are dimensioned so conductors are not bent tighter than their insulation can stand. Both rules produce simple calculations that appear regularly on the exam.
Framing member setback
Cables through bored holes must be at least 1 and 1/4 inches from the nearest edge of the framing member, or protected by a steel plate at least 1/16 inch thick.
2023 NEC §300.4(A)(1)Straight pulls
The box length in the direction of the pull must be at least eight times the trade size of the largest raceway.
2023 NEC §314.28(A)(1)Angle and U pulls
The distance from the raceway entry to the opposite wall must be at least six times the trade size of the largest raceway plus the sum of the diameters of the other raceways on the same wall and row.
2023 NEC §314.28(A)(2)Boxes must remain accessible
Junction, pull and outlet boxes must be installed so the wiring inside is accessible without removing any part of the building finish.
2023 NEC §314.29测试你的知识
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