Materials & Components
Learning Objectives
By the end of this chapter, you should be able to:
Identify the permitted uses, support methods, and bending radius requirements for common conduit types (EMT, IMC, RMC, PVC, ENT, LFMC).
Distinguish between box fill calculations, pull box sizing, and conduit body rules.
Select the correct conductor insulation type and temperature rating for a given termination and ambient condition.
Apply the rules for NM, MC, AC, and SE/SEU cable installation, including support intervals and protection from physical damage.
Recognize when GFCI, AFCI, or both are required based on location and circuit characteristics.
Navigate the 2026 NEC, 527 CMR 12.00, and NFPA 72 for materials-related provisions.
1.1 Metallic Conduit Systems
EMT (Electrical Metallic Tubing) — Article 358
EMT is a thin-wall steel or aluminum raceway. It is not threaded; use compression or set-screw fittings.
Minimum size: 1/2 inch trade size.
Support: Within 3 ft of every box, cabinet, or fitting, and at intervals not exceeding 10 ft.
Bending: Total bends between pull points shall not exceed 360°. Each bend, including offsets and saddles, counts toward this total.
Use: Indoors, dry locations, and in concrete or where protected from physical damage. EMT is not permitted in corrosive or wet locations unless listed for that environment.
IMC (Intermediate Metal Conduit) — Article 342
IMC is lighter than RMC but can be threaded.
Support: Within 3 ft of termination, then every 10 ft.
Corrosion protection: IMC must be protected where subject to severe corrosive influences. Galvanized IMC is suitable for most outdoor and wet locations.
Bending: Same 360° rule between pull points.
RMC (Rigid Metal Conduit) — Article 344
RMC is the heaviest steel conduit, threaded for connections.
Support: Within 3 ft of each box or fitting, then every 10 ft horizontally or vertically.
Bending: 360° maximum between pull points.
Field threading: Threads must be made with standard dies, and at least five full threads must be engaged when making up joints.
Common Exam Trap
Do not confuse the support interval for vertical runs of RMC/IMC with horizontal runs. The 10 ft rule applies to both, but the 3 ft rule from a box is absolute. Also, EMT supports are not required to be within 3 ft if the conduit is fished or concealed, but that exception is narrow.
1.2 Nonmetallic Conduit Systems
PVC (Schedule 40 or 80) — Article 352
PVC is corrosion-resistant and suitable for wet, corrosive, and underground locations.
Support: Within 3 ft of each box or fitting, and at intervals not exceeding 3 ft for trade sizes 1/2 through 1 inch; 5 ft for 1-1/4 through 2 inch; 6 ft for larger sizes.
Expansion fittings: Required where the conduit crosses a building expansion joint or where temperature changes exceed 30°F (about 17°C) for straight runs. Use the manufacturer’s expansion data for calculation.
Bending: PVC can be field-bent with heat; total bends between pull points shall not exceed 360°.
Underground: PVC must be buried at a depth per Table 300.5, typically 18 inches for residential and 24 inches for commercial under parking areas, unless protected by concrete.
ENT (Electrical Nonmetallic Tubing) — Article 362
ENT is a flexible, corrugated plastic raceway.
Use: Indoors only, in walls, floors, and ceilings, and in concealed or exposed locations where not subject to physical damage.
Support: Within 3 ft of each box, and at intervals not exceeding 3 ft.
Bending: Radius shall not be less than the values in Table 362.24.
Not permitted: In hazardous (classified) locations, or where ambient temperature exceeds 122°F (50°C).
LFMC (Liquidtight Flexible Metal Conduit) — Article 350
LFMC is a flexible metal conduit with a liquidtight jacket.
Use: Where flexibility is required, and in wet locations or outdoors.
Support: Within 12 inches of each box or fitting, and at intervals not exceeding 4.5 ft.
Bonding: The metal core must be bonded; use listed fittings that maintain continuity.
Length: No maximum length for LFMC, but if used as an equipment grounding conductor path, the fittings must be listed for grounding.
1.3 Boxes and Fittings — Article 314
Box Fill Calculations
Box fill is based on the volume of the box and the number of conductors and devices.
Each conductor that terminates, passes through, or is spliced counts as one.
Grounding conductors: All equipment grounding conductors count as one, based on the largest one present.
Devices (receptacles, switches): Count as two conductors each, based on the largest conductor connected.
Internal cable clamps: Count as one conductor (based on the largest conductor).
Pigtails do not count.
Volume allowance per conductor: Table 314.16(B) gives cubic inches: 14 AWG = 2.0 in³, 12 AWG = 2.25 in³, 10 AWG = 2.5 in³, 8 AWG = 3.0 in³.
Example: A 4-inch square box (21 in³) with two 12-2 NM cables (four conductors), one ground, and one receptacle:
Conductors: 4 × 2.25 = 9.0
Ground: 1 × 2.25 = 2.25
Device: 2 × 2.25 = 4.5
Total: 15.75 in³ — acceptable.
Pull Box Sizing — Section 314.28
For straight pulls, the length of the box must be at least 8 times the trade diameter of the largest raceway. For angle pulls, the distance from the raceway entry to the opposite wall must be at least 6 times the trade diameter of the largest raceway, plus the sum of the diameters of all other raceways on the same wall.
Common Exam Trap
Do not count a conductor that passes unbroken through the box as two. It counts as one. Also, a conductor that is spliced and continues counts as one, not two.
1.4 Conductors and Insulation Types — Article 310
Common Types
THHN/THWN-2: Thermoplastic insulation, heat and moisture resistant. Rated 90°C dry, 75°C wet. THWN-2 is rated 90°C wet.
XHHW-2: Cross-linked polyethylene, rated 90°C dry and wet.
Use: THHN is not permitted in wet locations unless dual-rated as THWN or THWN-2. XHHW-2 is suitable for wet locations.
Temperature Ratings and Terminations
Conductor ampacity is based on the insulation rating, but termination temperature limits may reduce it.
Rule (110.14(C)): Terminations are rated 60°C for circuits ≤ 100 A, and 75°C for larger circuits, unless marked otherwise.
Practical effect: If you use 90°C rated wire on a 60°C terminal, you must use the 60°C ampacity column.
Derating: When more than three current-carrying conductors are in a raceway, apply Table 310.15(C)(1) adjustment factors. Neutrals that carry only unbalanced current are not counted.
Ampacity Tables
Use Table 310.16 for standard ampacities. For example, a 12 AWG THHN is rated 30 A at 90°C, but on a 20 A breaker with 60°C terminals, the conductor is fine because the breaker protects it. The limiting factor is the termination.
1.5 Cable Types and Installation
NM Cable (Nonmetallic Sheathed) — Article 334
Use: In one- and two-family dwellings and their attached garages. Not permitted in commercial buildings unless specifically allowed.
Support: Secured within 12 inches of every box or fitting, and at intervals not exceeding 4.5 ft.
Protection: Must be protected from physical damage where passing through studs or joists; use nail plates where the cable is within 1-1/4 inch of the edge.
Bending: Radius shall not be less than 5 times the cable diameter.
MC Cable (Metal Clad) — Article 330
Use: Indoors and outdoors, in wet or dry locations, and in commercial buildings.
Support: Secured within 12 inches of every box, and at intervals not exceeding 6 ft.
Bonding: The metal sheath can serve as the equipment grounding conductor if listed.
Bending: Radius shall not be less than 7 times the cable diameter for cables ≤ 1 inch, and 10 times for larger.
AC Cable (Armored) — Article 320
Use: Dry locations only, typically in residential.
Support: Within 12 inches of boxes, at intervals not exceeding 4.5 ft.
Grounding: The armor and internal bonding strip serve as the grounding path. Do not use a separate ground wire.
SER/SEU (Service Entrance) — Article 338
Use: As service conductors, and as feeder conductors for residential and commercial.
Support: Within 12 inches of terminations, at intervals not exceeding 4.5 ft.
Not permitted: SEU (unarmored) is not permitted for interior wiring unless it passes through a wall and is protected.
1.6 Devices: Receptacles, Switches, GFCI, AFCI
Receptacle Ratings
15 A and 20 A receptacles on 15 A and 20 A branch circuits, respectively. A 20 A circuit may feed 15 A receptacles only if there are multiple receptacles on the circuit.
Tamper-resistant receptacles are required in dwelling units per 406.12.
GFCI Requirements (Dwelling Units) — Section 210.8
GFCI protection is required for receptacles serving countertops in kitchens, bathrooms, garages, outdoors, crawl spaces, unfinished basements, and within 6 ft of a wet bar sink. In 2026 NEC, GFCI is also required for many 240 V receptacles in these areas.
AFCI Requirements — Section 210.12
AFCI protection is required for branch circuits supplying outlets in dwelling unit family rooms, dining rooms, living rooms, parlors, libraries, dens, bedrooms, sunrooms, recreation rooms, closets, hallways, and similar areas. Combination AFCI breakers are typical.
Common Exam Trap
GFCI protects against ground faults; AFCI protects against arc faults. A dual-function breaker provides both. Do not assume a GFCI receptacle protects the entire circuit downstream unless wired for feed-through protection.
1.7 Wire Connectors and Terminations
Splices must be made in listed devices (wire nuts, push-in connectors) and enclosed in a box.
No splice is permitted in a raceway or cable unless it is a listed device accessible for inspection.
Torque: Terminations must be torqued to the manufacturer’s specification. Use a torque screwdriver for aluminum and large copper conductors.
Aluminum conductors: Must use anti-oxidant compound and listed connectors rated for aluminum (CO/ALR).
Pigtails: A pigtail is a short conductor used to connect a device; it does not count for box fill.
1.8 Busways — Article 368
Use: For feeders in commercial and industrial settings.
Support: Busways must be securely supported at intervals not exceeding 5 ft, unless otherwise listed.
Protection: Must be protected from physical damage and not installed in hazardous locations unless listed.
Tap off: Plug-in devices must be listed for the busway.
1.9 Code Navigation — Where to Find It
| Topic | NEC Article/Section | Notes |
|---|
| EMT | 358 | Support 358.30, bends 358.26 |
| IMC | 342 | Support 342.30 |
| RMC | 344 | Support 344.30 |
| PVC | 352 | Support 352.30, expansion 352.44 |
| ENT | 362 | Support 362.30 |
| LFMC | 350 | Support 350.30 |
| Box fill | 314.16 | Tables 314.16(A), (B) |
| Pull boxes | 314.28 | Sizing rules |
| Conductors | 310 | Ampacity tables 310.16 |
| Terminations | 110.14(C) | Temperature limits |
| NM cable | 334 | Support 334.30 |
| MC cable | 330 | Support 330.30 |
| AC cable | 320 | Support 320.30 |
| SER/SEU | 338 | Support 338.10 |
| GFCI | 210.8 | Dwelling and commercial |
| AFCI | 210.12 | Dwelling areas |
| Receptacles | 406 | Tamper-resistant 406.12 |
| Busways | 368 | Support 368.30 |
| Underground burial | 300.5 | Depth table |
| 527 CMR 12.00 | MA amendments | Check for stricter rules |
| NFPA 72 | Fire alarm wiring | Chapter 12 for wiring methods |
1.10 Practical Field Points
Always check the label on the conduit or cable for the UL listing and temperature rating.
Use a torque screwdriver on breaker lugs and aluminum terminations.
When running EMT outdoors, use compression fittings, not set-screw, to prevent moisture ingress.
For PVC expansion, measure the run length and ambient temperature range; install expansion couplings at the midpoint.
For MC cable, use a listed anti-short bushing (red head) at every termination to protect the conductors from the cut sheath.
NM cable in an attic must be protected where within 6 ft of the scuttle hole if the attic is accessible.
GFCI test monthly per manufacturer instructions; record on the label.
1.11 Common Exam Traps
140.Counting grounds: All equipment grounds count as one conductor, not one per cable.
141.Support distances: EMT is 10 ft; MC is 6 ft; NM and AC are 4.5 ft. Do not mix them up.
142.Bending radius: EMT and RMC use the 360° rule; MC and AC use a radius multiplier.
143.Temperature derating: A 90°C wire does not automatically allow a 90°C ampacity if the termination is 60°C or 75°C.
144.Wet locations: THHN alone is not permitted; use THWN or THWN-2.
145.Box fill: A device counts as two conductors, but a pigtail counts as zero.
146.PVC support: 3 ft from box, then 3 ft intervals for small sizes — not 10 ft like EMT.
147.AFCI vs GFCI: AFCI is for arc faults in bedrooms and living areas; GFCI is for ground faults in wet areas. They are not interchangeable.
148.SEU cable cannot be used for interior branch circuits.
149.LFMC support: 12 inches from the box, not 3 ft.
Summary
This chapter covered the core materials and components you will select and install daily. Master the support intervals, bending rules, box fill calculations, and termination temperature limits. In the open-book exam, your first move should be to the Code Navigation table above, then to the specific article. Practice locating tables quickly — Table 310.16, 314.16(B), and 300.5 are your most-used references.
