Wiring methods
Wiring methods are the cables and raceways the NEC recognizes for running conductors, covered in Chapter 3 of the 2023 NEC. Article 300 sets rules that apply to all of them, such as protection from physical damage (300.4) and minimum burial cover (Table 300.5(A)). Each method then has its own article with uses permitted, uses not permitted, and securing and support rules: Article 334 for NM cable, Article 358 for EMT, and Article 352 for PVC conduit.
What wiring methods are and why they matter on the exam
A wiring method carries and protects conductors from the panel to the load. Cable assemblies such as NM, AC, MC, and UF have the conductors built in. Raceways such as EMT, RMC, and PVC are installed complete first, and the conductors are pulled in afterward (300.18(A)).
Exams test Chapter 3 with short lookups: how far from a box, how often to strap, how deep to bury. Most Chapter 3 articles share a numbering pattern:
- .10 is uses permitted.
- .12 is uses not permitted.
- .24 and .26 cover bends.
- .30 is securing and supporting.
For EMT support, go straight to 358.30. For PVC, go to 352.30. For more open-book strategy, see how to pass the journeyman electrician exam.
The key rules (with NEC sections)
General rules in Article 300
- Bored holes in wood, 300.4(A)(1). Keep the edge of the hole at least 1-1/4 in from the nearest edge of the framing member, or protect the wiring with a steel plate or sleeve at least 1/16 in thick. RMC, IMC, PVC, and EMT do not need the plate.
- Metal framing, 300.4(B)(1). NM cable through punched holes in metal studs needs listed bushings or grommets covering all metal edges.
- Parallel to framing, 300.4(D). Cables and raceways run along studs, joists, or rafters must be at least 1-1/4 in from the nearest edge, or be protected by a plate or sleeve.
- Under metal roof decking, 300.4(E). Keep cables and raceways at least 1-1/2 in below the decking.
- Burial cover, 300.5(A). Table 300.5(A) sets minimum cover by wiring method and location. Cover is measured from the top of the cable or raceway to the top of finished grade.
- Emerging from grade, 300.5(D)(1). Direct-buried conductors coming out of the ground need protection from the required cover depth (no more than 18 in below grade is required) to at least 8 ft above grade.
- Underground is wet, 300.5(B). The inside of an underground raceway is a wet location, so conductors must be listed for wet locations.
Type NM cable, Article 334
- Where it is allowed, 334.10 and 334.12. NM is permitted in one- and two-family dwellings and in other buildings of Types III, IV, and V construction, with limits. It is not permitted in wet or damp locations or embedded in concrete.
- Securing, 334.30. Secure NM within 12 in of every box, cabinet, or fitting and at intervals not exceeding 4-1/2 ft. Horizontal runs through bored holes at those intervals count as supported.
- Bends, 334.24. The inside bend radius must be at least five times the cable diameter.
- Physical damage, 334.15(B). Where exposed to damage, protect NM with RMC, IMC, EMT, Schedule 80 PVC, or another approved means.
- Basements, 334.15(C). Cables run at angles to joists in unfinished basements may be fastened to the joist bottoms only if they have at least two 6 AWG or three 8 AWG conductors. Smaller cables go through bored holes or on running boards.
EMT, Article 358
- Securing, 358.30(A). Fasten EMT within 3 ft of each box, cabinet, conduit body, or other termination, and at intervals not exceeding 10 ft. Where framing does not permit fastening within 3 ft, unbroken lengths may be fastened within 5 ft.
- Through framing, 358.30(B). Horizontal runs through framing members spaced 10 ft or less count as supported, but still need fastening within 3 ft of terminations.
- Bends, 358.26. No more than 360° of bends between pull points.
- Cut ends, 358.28. Ream cut ends to remove burrs. EMT may not be threaded.
PVC conduit, Article 352
- Securing, 352.30. Fasten PVC within 3 ft of each termination and support it at the spacing in Table 352.30, which grows with trade size. Trade sizes 1/2 through 1 need support every 3 ft.
- Physical damage, 352.10. Where subject to physical damage, use PVC identified for that use, such as Schedule 80.
- Expansion, 352.44. Install expansion fittings where the expected length change between secured points is 1/4 in or more.
Worked examples
Example 1: strapping an EMT run
A straight 38 ft EMT run connects two junction boxes. What is the minimum number of straps under 358.30(A)?
- One strap within 3 ft of each box: 2 straps, at 3 ft and 35 ft.
- Span between them: 38 − 3 − 3 = 32 ft.
- 32 ÷ 10 = 3.2, so round up to 4 spans of 10 ft or less.
- Four spans need 5 straps in total, 8 ft apart.
Example 2: stapling NM cable
A 20 ft run of NM cable goes along studs between two boxes. How many staples at minimum?
- One staple within 12 in of each box leaves 20 − 1 − 1 = 18 ft between them.
- 18 ÷ 4.5 = 4 spans exactly, so 5 staples, 4-1/2 ft apart.
Example 3: PVC expansion
A 150 ft straight run of PVC is installed outdoors where the temperature swings 90°F between winter and summer. PVC expands about 3.38 × 10⁻⁵ in per inch of length per °F.
Length change = Coefficient × Length (in) × Temperature change- Length in inches: 150 × 12 = 1,800 in.
- Change: 0.0000338 × 1,800 × 90 = 5.48 in.
That is far more than 1/4 in, so expansion fittings are required under 352.44.
Example 4: reading Table 300.5(A)
UF cable is direct buried across a yard to a detached shop. How deep must it be?
- Find the row first: a general location, not under a building, slab, driveway, or street.
- Find the column: direct-buried cables or conductors.
- For a 240 V circuit, Table 300.5(A) requires 24 in of cover.
- If the run is instead a residential 120 V branch circuit with GFCI protection and overcurrent protection of 20 A or less, that column applies and the requirement drops to 12 in.
Common exam mistakes
- Mixing up NM and EMT distances. NM is 12 in from the box and every 4-1/2 ft. EMT is 3 ft from the box and every 10 ft.
- Using Schedule 40 PVC where it is exposed to damage. The answer is usually Schedule 80 or a metal raceway.
- Measuring cover to the bottom of the trench. Cover is measured to the top of the cable or raceway.
- Requiring steel plates for EMT, RMC, IMC, or PVC in bored holes near the stud edge.
Practice questions
Frequently asked questions
How often does NM cable need to be stapled?
Under 334.30, within 12 in of every box, cabinet, or fitting and at intervals no greater than 4-1/2 ft. Horizontal runs through bored holes at those intervals count as supported.
How deep does PVC conduit need to be buried?
In a general location, Table 300.5(A) requires 18 in of cover for nonmetallic raceway listed for direct burial without concrete encasement. Driveways, streets, buildings, and concrete cover change the requirement.
What is the support spacing for EMT?
Section 358.30(A) requires fastening within 3 ft of each termination and at least every 10 ft.
When is a steel nail plate required?
Under 300.4(A) and 300.4(D), a steel plate at least 1/16 in thick is required when a cable is closer than 1-1/4 in to the edge of a wood framing member.
How many bends are allowed in a conduit run?
Articles such as 358.26 and 352.26 allow 360° total between pull points, counting offsets and kicks. See conduit bending, conduit fill, and conductor ampacity for the conductors inside, or try the practice test.