What Is the Difference Between Romex, THHN, and UF Wire?

Walk into any GTA hardware store’s electrical aisle and you see rolls of cable labeled Romex, THHN, and UF — plus NMD-90, TECK, MC, and a dozen others. Each has a specific use, a specific set of code requirements, and a specific job it does badly if you pick the wrong one. This guide walks Ontario homeowners through the difference between Romex, THHN, and UF wire, what NMD-90 (the Canadian equivalent) covers, and when to use which.

Short answer: NMD-90 for indoor residential, THHN in conduit, UF direct-buried

Direct answer: in Canada, Romex is called NMD-90 and it is the standard indoor residential cable. THHN is a single-conductor wire designed to be pulled through conduit. UF is direct-burial cable rated for outdoor and underground use. Each is chosen based on the installation location.

NMD-90 (Canadian Romex)

• Type: non-metallic sheathed cable with two or three insulated conductors plus a bare bond wire inside a single outer plastic jacket.
• Use: interior residential wiring — walls, ceilings, and basements. Not permitted in wet locations or buried.
• Sizes: 14/2, 14/3, 12/2, 12/3, 10/2, 10/3 are the common residential sizes.
• Temperature rating: 90°C dry.
• Approval: CSA-approved for Canadian installation.

The “Romex” name in US markets is the brand; the equivalent in Canada is NMD-90 (Non-Metallic Dry rated for 90°C). Ontario Electrical Safety Code requires CSA-certified cable — always look for the CSA mark on the jacket. Our post on cutting and stripping wire covers working with NMD-90.

THHN (individual conductors for conduit)

• Type: single insulated conductor with no outer jacket.
• Use: pulled through conduit or raceways for feeder runs, service entrance, and areas requiring physical protection.
• Sizes: 14 AWG up to 750 kcmil.
• Temperature rating: 90°C dry; some THHN/THWN dual-rated for wet locations.
• Applications: subpanel feeders, EV charger circuits in EMT, industrial installations.

THHN is not permitted in walls without conduit — the individual conductor has no mechanical protection. Use only inside a listed raceway. Our post on running power to a detached garage covers a common use case.

UF (underground feeder)

• Type: non-metallic sheathed cable rated for direct burial and wet locations. Similar construction to NMD-90 but with tougher moisture-resistant jacket.
• Use: direct-buried underground runs (garage, shed, landscape lighting).
• Sizes: 14/2 through 8/3 typical.
• Temperature rating: 90°C wet and dry.
• Burial depth: 900 mm (36 inches) direct-buried per OESC.

In Canada, the equivalent product is NMWU (Non-Metallic Wet Underground). NMWU and UF-B are functionally interchangeable for direct-buried installations. Always check the jacket for CSA certification.

Other cables you might encounter

• MC (metal-clad): flexible metallic outer jacket protecting conductors. Used in commercial and multi-unit residential.
• TECK 90: heavy-duty armored cable for wet locations and industrial use.
• AC (armored cable, BX): older-generation metal-armored cable. Still sometimes seen in existing homes.
• SEU/SER (service entrance): the cable that runs from the utility drop to the meter socket and main panel.
• Low-voltage cable (18/2, 22/2): for thermostat, doorbell, landscape lighting, and communication.

Common mistakes homeowners make

• Using NMD-90 in wet locations. The cable is dry-rated only. Wet exposure degrades the jacket and creates a code violation.
• Burying NMD-90 directly. Not permitted. Use NMWU/UF-B for direct burial or NMD-90 in conduit.
• Using UF/NMWU in dry indoor locations. Not a safety issue but wasteful — the cable costs more than NMD-90.
• Buying US-market Romex without checking for CSA. The Ontario Electrical Safety Code requires CSA certification.
• Wrong gauge for the load. Match wire gauge to breaker rating and load; see our post on breaker sizing.

Cable size and gauge quick reference

• 14/2 NMD-90: 15 A branch circuits, lighting, general receptacles.
• 14/3 NMD-90: 3-way switches, ceiling fan with light kit, split receptacles.
• 12/2 NMD-90: 20 A branch circuits, kitchen counter receptacles.
• 10/2 NMD-90: 30 A dedicated circuits (dryer, water heater, small AC).
• 8/3 NMWU: 40 A EV charger, medium subpanel feeder.
• 6/3 NMWU: 60 A subpanel feeder, large EV charger.

Expert tip from our ESA-licensed electricians

In our experience shopping for cable across GTA hardware stores, the single most common homeowner confusion is between NMD-90 and NMWU. The jackets look almost identical, and both are sold in the same 25-metre and 75-metre lengths. Homeowners planning an outdoor project sometimes buy NMD-90 by mistake because it is cheaper and looks the same. It fails inspection every time. Read the jacket markings before you buy, and if the project is outdoor or underground, insist on NMWU. Same lesson applies to conductor gauge — the 14/2 and 12/2 rolls look identical to the eye. Read the label. Our post on identifying wire gauge covers the visual identification.

Related reading from ezsmart.ca

• Identify wire gauge in wall
• Cut and strip wire
• Breaker vs wire sizing
• Power to detached garage
• Bury indoor wire underground?

Frequently Asked Questions

What cable is standard for indoor Ontario homes?

NMD90 (the Canadian equivalent of Romex-type cable) is the residential dry-location standard.

Where is THHN used?

Individual conductors inside metallic or PVC conduit. Common in commercial and some feeder installations.

What about UF cable?

UF (Underground Feeder) is direct-burial rated. NMWU is the common Canadian equivalent.

Are these interchangeable?

No. Each cable type has specific location and installation rules under the OESC.

Can I use THHN in an indoor wall?

Only inside a proper conduit system. Not in place of NMD90.

Contact us

Unsure which cable type to buy for your GTA project? Book an ESA-certified electrician to spec the right material. Call us at 416-838-9006 or visit our contact page — we will get back to you the same day.

Amir Azimipour

Electrician Since 2008 Journeyman Electrician Designated Master Electrician at EZSMART Corp