Assorted wiring methods: EMT, PVC, MC, tray

Chapter 3 — Wiring Methods & Materials

Pick the right wiring method, support it, bond it, and mind the details—this is most of what you do on real jobs.

10
Major Articles
15
Quiz Questions
20
Visual Examples
🧭

300 — General Requirements for Wiring Methods

  • 300.4/5Protection from physical damage; burial depths vary by method and occupancy.
  • 300.10Metal raceways must be electrically continuous and effectively bonded.
  • 300.15All splices/terminations in boxes or fittings—no loose splices in raceways.
  • DamageProtect wiring methods from physical damage; add sleeves/guards where exposed to abuse.
  • FittingsUse listed fittings for the wiring method; don't mix EMT and RMC fittings.
EXAM TRAP
Location governs method
Dry vs damp vs wet changes everything: allowed methods, fittings, and conductor ratings. Identify the space first; choose the method second.
Raceway run through mixed-use areas
Method changes when the environment changes.
Bonding bushing on raceway
Maintain a continuous fault-return path.
📎

358 — EMT: The Everyday Raceway

  • 358.12Not permitted where subject to severe physical damage or in some corrosive environments unless protected.
  • 358.30Support within 3 ft of each box and at required intervals (check local adoption).
  • 358.26Total bends ≤ 360° between pull points.
  • EndsReam cut ends; no sharp edges that can damage insulation.
  • FittingsChoose compression vs set-screw fittings by location (wet/damp) and spec.
RULE OF THUMB
Strap the ends
Strap within the first few feet of boxes and keep runs straight—inspectors look for support near terminations every time.
EMT neatly strapped and aligned
Straight, supported, and bonded.
Close-up EMT set-screw fitting
Use fittings listed for EMT—wet-location where required.
🧱

342 & 344 — IMC / RMC: Heavy-Duty Metal Conduit

  • 342/344.10Permitted in most locations, including outdoor and where subject to physical damage.
  • BondingUse bonding bushings/fittings where raceway continuity could be interrupted.
  • CorrosionField cuts need ream and corrosion protection; use proper thread compound.
  • ThreadsUse thread sealant approved for electrical; protect threads against corrosion.
  • Earth/ConcreteSuitable for direct earth and encasement in concrete where identified.
NEC REFERENCE
Bonding fittings
Where concentric/eccentric knockouts or nonmetallic raceway interrupt continuity, use bonding bushings/jumpers to preserve the fault path.
RMC service with bonding bushings
Bonding jumpers across concentric KOs.
IMC riser outdoors
Right method for exposed, tough locations.
🧰

352 — PVC: Nonmetallic Workhorse

  • 352.10Permitted in wet locations; use sunlight-resistant types where exposed to sun.
  • 352.44Provide expansion fittings where thermal movement is expected (roofs/long runs).
  • 352.30Support per manufacturer and Code—intervals depend on trade size and schedule.
  • EGCRun an equipment grounding conductor—PVC does not provide a ground path.
  • ExpansionProvide expansion joints for temperature movement; set the correct take-up.
TABLE
NEC REFERENCE — Expansion & fittings
Calculate thermal movement and add expansion fittings with proper gaps and guides. Use wet-location fittings where required.
PVC with expansion fitting on a wall
Expansion fitting bridging temperature swings.
PVC underground stub-ups
Protect emerging PVC where subject to damage.
🧵

330 — MC Cable: Types & Uses

  • 330.10MC is allowed in many occupancies; check listing for wet locations, health care (HCF), or fire-rated assemblies.
  • SupportSecure near terminations and at required intervals; avoid crushing or over-bending.
  • TerminationsUse listed connectors; anti-shorts as required by the connector/listing.
  • ArmorEGCSome MC types allow armor as EGC when marked; otherwise include an EGC conductor.
  • FittingsUse listed MC connectors; add anti-short bushings if required by the connector/listing.
NOTE
MC Types at a Glance
Always read the product label and local adoption. Don’t assume any MC works everywhere.
MC cable neatly strapped
Strap near terminations; no kinks.
MC connector with anti-short
Use the connector the listing calls for.
🌀

348/350 — FMC & LFMC: Flexible Done Right

  • LengthShort flexible sections for vibration/isolation; don't run long flex in place of raceway.
  • EGCArmor continuity + listed fittings for a reliable fault path; LFMC in wet areas needs wet-rated fittings.
  • Bend radiusRespect minimum bend radius—crushed whips overheat conductors.
  • LengthKeep flex lengths short to relieve vibration and facilitate connections (watch specific length rules).
  • EGCEnsure equipment grounding continuity through armor/fittings or run a separate EGC.
JOBSITE HORROR STORY
Crushed Whip Cooked a Motor
An LFMC whip kinked behind a curb overheated conductors and tripped a rooftop unit. Keep whips short, supported, and with proper radius.
LFMC whip to rooftop unit
Short, supported, wet-rated fittings.
Crushed flexible conduit
Tight bends crush the spiral and damage insulation.
📦

314 — Boxes & Conduit Bodies: Fill and Fit

  • 314.16Box-fill math: each conductor counts; all EGCs together as one; internal clamps count; devices can count as two.
  • Conduit bodiesLBs marked 'for pulling only' don't permit splices regardless of volume.
  • CoversBoxes and bodies require covers and must stay accessible.
  • VolumeUse box-fill rules: conductor volume by AWG; devices may count as two conductors on the yoke.
  • EGC CountAll equipment grounding conductors together count as one of the largest size present.
EXAM TRAP
Box-Fill & LB Splice
Two big misses: devices counting as two conductors (largest on the yoke) and pull-only LBs (no splices).
Device box crowded with conductors
Do the math—heat and nuisance trips come from overfill.
Conduit body marking
Check 'splicing permitted' and volume.
⛏️

300.5 — Underground Wiring: Depth & Details

  • CoverCover depth varies by method/occupancy—PVC vs RMC vs direct-bury are different.
  • EmergenceProtect where raceway emerges from grade; rigid elbows often required for damage protection.
  • Locate/markCall before you dig; use warning tape/tracer where required.
  • CoverCover depth depends on method/occupancy—verify before trenching.
  • BackfillProvide proper backfill/bedding to prevent raceway damage.
RULE OF THUMB
Depths aren’t one-size
Direct-bury cable, PVC, RMC, and low-voltage lighting all have different cover depths. Confirm the method, then check the table.
Trench with conduit at correct depth
Depth verified before backfill.
Warning tape in trench
Mark the run to avoid future surprises.
📐

300 — Bends, Fill, and Pull Points

  • 360° ruleTotal bends between pull points ≤ 360°.
  • Bend radiusFollow method-specific minimum bend radius for conductors/raceway.
  • FillUse Chapter 9 tables for fill—changes by conductor count and raceway type.
  • 360°Max 360° of bends between pull points; add a box if more turns are needed.
  • RadiusRespect minimum bend radius per method/conductor; tight bends overheat insulation.
NEC REFERENCE
Add a pull point
If you’ll exceed 360° or struggle to pull, add a box or body. It saves insulation and labor.
Conduit run with pull point
Pull points make long runs practical.
Tight bend radius warning
Tight bends = damaged insulation.
🪜

392 — Cable Tray: Systems, Supports, Separation

  • TypesLadder vs ventilated vs solid-bottom; support per manufacturer spacing.
  • FillTray fill and ampacity depend on type, spacing, and cable rating.
  • SeparationSignal/communication often require separation or barriers—check the spec.
  • FillRespect tray fill limits and side-rail height (per listing and NEMA/NEC guidance).
  • SupportKeep support spans within manufacturer's max; add mid-span supports for heavy runs.
TABLE
Tray Snapshot
Tray typeStrength / VentilationCommon use
LadderHigh / ExcellentPower feeders, long spans
Ventilated troughMedium / GoodBranch circuits, short spans
Solid-bottomLow / PoorControl/IT (with heat concerns)
Values are illustrative—always follow the manufacturer’s charts for span spacing, loading, and fill.
Ladder tray run
Long spans with strong rungs.
Ventilated tray with branch circuits
Mind fill and support spacing.

Wiring Methods — What to Take to the Job

Location First

Dry/damp/wet decides the method, fittings, and conductor ratings.

Support & Secure

Strap near enclosures and at required intervals. Sag and drift fail inspections.

Fault Path Matters

Raceways/fittings must maintain grounding/bonding continuity.

Bends & Fill

≤360° between pull points; respect bend radius and fill tables.

Boxes & Bodies

Do the box-fill math; check LB markings before you splice.

Environment & Corrosion

Use materials/finishes listed for the space you’re in.

Knowledge Check

Answer all questions, then click Submit Answers. You’ll see your score after submitting. Nothing is graded until then.

1

EMT in a dry location is secured with straps. What is the maximum distance from each box a strap must be located (trade size 1 in. and smaller)?

2

Nonmetallic raceways (PVC/RNC) expand and contract with temperature. When is an expansion fitting required?

3

What’s the minimum cover for Schedule 40 PVC feeder (120/240 V) direct-buried under a one‑/two‑family residential driveway?

4

Maximum cumulative degrees of bend permitted between pull points in a run of raceway?

5

EMT support spacing (general) is required at intervals not exceeding:

6

Flexible metal conduit (FMC): maximum interval between supports for trade sizes 1‑1/4 in. and smaller?

7

Direct‑buried UF cable for a 120 V, 20 A GFCI‑protected branch circuit in residential lawn: minimum cover?

8

Allowed maximum conductor fill in a single raceway (more than two conductors)?

9

Rigid metal conduit (RMC) under a residential driveway: minimum cover?

10

At what number of current‑carrying conductors in a raceway do you begin ampacity adjustment (derating)?

11

RNC (PVC) support spacing for sizes 1 in. and smaller (horizontal runs)?

12

MC cable must be secured within ____ of every box and supported at intervals not exceeding ____.

13

LFMC (liquidtight flexible metal conduit) in wet locations requires:

14

Maximum cumulative degrees of bend permitted between pull points in FMC?

15

When a raceway crosses a building expansion/settlement joint, use: