Quick Hits

Article 394 (1 of 1)
Concealed Knob-and-Tube Wiring — Full Article Breakdown!

SLIDE 1 — Scope & Uses Permitted (394.1, 394.10)
🏗️ WHAT IS KNOB-AND-TUBE WIRING?
An early wiring method used in homes built before the 1930s — not installed in new construction today, but still shows up in remodels or as extensions of existing installations. Designed for hollow wall/ceiling/attic spaces, relying on free air for heat dissipation.
✅ WHERE YOU CAN USE IT (394.10)
ONLY for:
1️⃣ Extensions of EXISTING installations
2️⃣ Elsewhere, by SPECIAL PERMISSION (AHJ)
📌 Unlike open wiring on insulators (Article 398, which must stay exposed), knob-and-tube IS allowed to be concealed.

SLIDE 2 — Uses Not Permitted (394.12)
⛔ WHERE YOU CAN'T USE IT
1️⃣ Commercial garages
2️⃣ Theaters and similar locations
3️⃣ Motion picture studios
4️⃣ Hazardous (classified) locations
5️⃣ Hollow spaces insulated with loose, rolled, or foamed-in-place insulation that envelops the conductors
⚠️ WHY #5 MATTERS: that kind of insulation blocks heat dissipation into free air, raising conductor temp and risking insulation breakdown or ignition — this is why K&T is banned in weatherized spaces.

SLIDE 3 — Framing, Clearances & Attics (394.17, 394.19, 394.23)
🪵 THROUGH FRAMING MEMBERS (394.17)
Passing through wood cross members in plastered partitions → protect conductors with noncombustible, nonabsorbent tubes extending ≥75mm (3 in.) beyond the wood member.
📏 CLEARANCES (394.19)
🔹 ≥75mm (3 in.) between conductors; ≥25mm (1 in.) from the surface they cross
🔹 Tight spaces (meters, panelboards, outlets, switches) → enclose conductors in continuous flexible nonmetallic tubing
🏚️ UNFINISHED ATTICS/ROOF SPACES (394.23)
🔹 Accessible by stairway/ladder → run along or through bored holes in joists/studs/rafters; runs within 7 ft of the floor need running boards ≥1 in. on each side (unless run along the SIDES of the framing, which needs no running board)
🔹 NOT accessible by stairway/ladder → same rules, with an exception for pre-existing buildings with <3 ft headroom (wiring permitted on rafter/joist edges)

SLIDE 4 — Support, Devices & Splices + Final Exam (394.30, 394.42, 394.56)
📏 SECURING & SUPPORTING (394.30)
🔹 Rigidly supported on noncombustible, nonabsorbent insulators — no contact with other objects
🔹 Support within 150mm (6 in.) of each tap/splice, and at intervals ≤1.4m (4½ ft)
🔹 Where supports aren't practical → conductors can be fished through dry hollow spaces if individually enclosed in continuous flexible nonmetallic tubing
🔹 Solid knobs → conductors tied on with insulated tie wires
🔌 WIRING DEVICES (394.42)
Switches must comply with 406.9 and 406.42(B).
🔗 SPLICES & TAPS (394.56)
Must be SOLDERED unless approved splicing devices are used — NO in-line or strain splices allowed.

🎉 That's a wrap on Article 394 — Concealed Knob-and-Tube Wiring!

🧠 FINAL EXAM QUESTION FOR ARTICLE 394
Q: Per 394.10, concealed knob-and-tube wiring is permitted ONLY for extensions of existing installations, or where else?
A) New residential construction only
B) By special permission from the AHJ
C) In commercial buildings only
D) It's never permitted anywhere
(Drop your final answer 👇)

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