NM Cable vs Conduit: Which Wiring Method for the Job
Romex NM-B cable vs THHN in EMT conduit — ampacity column limits, where each is permitted, protection, derating, and future-proofing compared.
Updated August 20, 2026
NM-B cable and THHN-in-conduit are less two products than two philosophies of wiring. Nonmetallic-sheathed cable arrives as a finished assembly — conductors, jacket, and all — that staples to framing as fast as a crew can walk; conduit arrives as an empty steel raceway that must be bent, strapped, and only then filled with individual conductors pulled through it.
The code treats them differently in a way that surprises people: the copper inside modern NM-B is 90 °C-rated THHN-class wire, yet NEC 334.80 pins the cable’s allowable ampacity to the 60 °C column, while the same conductor run in EMT starts from the 90 °C column for derating math (with terminations usually capping the final answer at the 75 °C value). The column rows below map that gap straight out of NEC Table 310.16 — same copper, different ceiling, purely because of the wiring method around it.
Everything else trades speed against service life: NM-B wins the install race and loses on physical protection, exposure, and the ability to add circuits later; a conduit system costs more labor up front and then behaves like infrastructure — re-pullable, repairable, and legal in places Romex may never go.
Side by Side
| Spec | NM-B cable (Romex) | THHN in EMT conduit |
|---|---|---|
| Governing NEC article | Article 334 — Nonmetallic-Sheathed Cable | Article 358 (EMT) + Chapter 9 fill tables |
| Ampacity column — 12 AWG copper (A) | 20 A (60 °C column, NEC 334.80) | 30 A (90 °C column for derating; terminations usually cap use at 75 °C) |
| Ampacity column — 10 AWG copper (A) | 30 A (60 °C column, NEC 334.80) | 40 A (90 °C column for derating; terminations usually cap use at 75 °C) |
| Ampacity column — 6 AWG copper (A) | 55 A (60 °C column, NEC 334.80) | 75 A (90 °C column for derating; terminations usually cap use at 75 °C) |
| Where permitted | One- and two-family dwellings and (with construction-type limits) other structures; not in wet locations and not where exposed to physical damage — NEC 334.10/334.12 | Broadly permitted — dwellings, commercial, industrial, exposed work, and (with fittings suited to the condition) damp locations |
| Physical protection | Plastic jacket only — needs nail plates near edges and cover in framing; vulnerable exposed | Steel raceway shields conductors along the whole run; the standard answer for exposed walls |
| Fill & derating obligations | None per cable, but bundled cables and thermal insulation contact trigger 310.15 adjustment | Conduit fill per Chapter 9 (40% for 3+ conductors) and ampacity adjustment when more than three current-carrying conductors share the raceway |
| Re-pull & future capacity | None — adding a circuit means fishing a new cable through closed framing | Pull string and spare fill make new or upsized circuits a pulling job, not a demolition job |
| Labor profile | Fastest rough-in there is: drill, pull, staple; one trade, light tooling | Bending, cutting, reaming, strapping, then a separate wire pull — slower and more skilled, priced accordingly |
Which One for the Job
Finished-basement branch circuits
NM-B cable (Romex)
Inside framed, drywalled walls of a dwelling, NM-B is squarely within its Article 334 permission, protected by the finish, and dramatically cheaper to rough in. Conduit buys nothing behind drywall that the framing and nail plates don’t already provide.
Unfinished garage or shop walls
THHN in EMT conduit
Exposed runs below about eight feet live where bikes, lumber, and machinery hit walls — the "subject to physical damage" condition that pushes NM out under 334. EMT takes the impact, looks intentional, and leaves spare fill for the welder or compressor circuit that always comes later.
Commercial tenant fit-out
THHN in EMT conduit
NM cable’s permissions narrow sharply outside residential construction types, while suspended-ceiling and steel-stud commercial space is EMT’s native habitat. Fit-outs also churn — walls move, circuits get reassigned — and a raceway system absorbs that with pulls instead of rewires.
Attic circuit passing through batt-insulated bays
Either
Both methods legally traverse insulation; the governing fact is thermal, not mechanical. NM-B is already priced for it by the 60 °C column, while conduit through hot attic air may owe ambient-temperature correction on the 90 °C column — run the derating math for the actual route before assuming either wins.
Frequently Asked Questions
Why is Romex limited to the 60 °C ampacity column when its wire is rated 90 °C?
NEC 334.80 makes the cable assembly, not the conductor, the limiting element: NM cable is stapled against wood, bundled through bored holes, and buried in insulation where heat cannot escape the jacket. The 90 °C conductor rating still helps — derating calculations may start from it — but the final ampacity may never exceed the 60 °C value, which is why 12 AWG NM lands at 20 A.
Is it legal to run Romex inside EMT?
Generally yes — NM cable may be run through raceway sections for protection (a common move where a cable drops down an unfinished wall), provided the conduit is sized for the cable’s cross-section and the assembly stays out of wet locations, since the inside of conduit outdoors is considered wet. What the conduit does not do is upgrade the cable’s ampacity: 334.80 still applies.
How full is a conduit actually allowed to be?
Chapter 9, Table 1 caps three or more conductors at 40% of the raceway’s internal area, with the familiar one-size-up practice existing precisely so pulls stay feasible and future circuits fit. Fill is a separate check from ampacity adjustment — more than three current-carrying conductors in the pipe also derates every one of them under 310.15(C)(1).
Which method costs less over the life of the building?
NM-B nearly always wins the install invoice, and in a stable dwelling that is usually the end of the story. The equation flips where circuits change: every added or upsized run in a conduit system is a wire pull through existing raceway, while the NM equivalent reopens walls. Shops, garages, and commercial space tend to repay conduit’s premium; sealed residential walls rarely do.
Try the Calculators
Sources
- NEC Article 334 — Nonmetallic-Sheathed Cable: 334.10/334.12 uses permitted and not permitted; 334.80 ampacity at the 60 °C column
- NEC Article 358 — Electrical Metallic Tubing (EMT): uses permitted, securing and supporting
- NEC 2023 Table 310.16 (via the conductor-ampacity calculator’s verified table) and 310.15 adjustment/correction rules
- NEC Chapter 9, Table 1 and Annex C — conduit fill limits for conductor combinations