Electrical Wire Types: The Five Used in American Homes

Almost every US home is wired with one of a handful of methods, and which one is appropriate is decided by where the wiring runs — dry interior space, damp location, outdoors, buried, or exposed to physical damage.

What is NM cable used for?

Non-metallic sheathed cable — usually called NM cable, or by the trade name Romex — is the standard branch-circuit wiring method in most modern US homes.

It's a flat cable containing insulated conductors plus a bare equipment grounding conductor, all inside a plastic outer jacket.

Where it's used: concealed in walls, floors and ceilings in dry interior locations.

Where it isn't: outdoors, in wet or damp locations, buried, or exposed where it could be physically damaged. Some jurisdictions and building types restrict it further.

The jacket is color-coded by size in modern production, which is a manufacturing convention rather than a code requirement — but it's a useful field indicator. See what is Romex / NM cable.

What is UF cable used for?

Underground feeder cable looks similar but has a solid molded jacket rather than a loose sheath, and it's rated for wet locations and direct burial.

Where it's used: outdoors, buried runs to detached garages, sheds, yard lighting and outbuildings.

Burial depth requirements come from the NEC and depend on the wiring method and what's above — a lawn and a driveway have different requirements. Your adopted edition and AHJ govern, and your electrician sizes and buries it.

What is armored cable used for?

AC (armored cable) and MC (metal-clad cable) wrap the conductors in a flexible metal sheath.

Where they're used: exposed runs needing physical protection, some commercial and multi-family applications, and jurisdictions that require metallic wiring methods. Common in some older homes where it may be called BX.

The advantage is mechanical protection. The tradeoffs are cost and stiffness.

Note that in older armored cable, the metal sheath itself may have served as the equipment grounding path. That arrangement can degrade over decades, which is one reason old armored cable installations get scrutinized during inspections. See home electrical inspection before buying.

When is conduit used instead of cable?

Rather than a pre-assembled cable, individual insulated conductors — commonly THHN/THWN — are pulled through a raceway: EMT, rigid metal conduit, or PVC.

Where it's used: exposed interior runs (garages, basements, workshops), outdoor runs, underground, commercial work, and anywhere physical protection or future flexibility is wanted.

Two practical advantages worth knowing as a homeowner:

• Physical protection in garages and shops where wiring could be hit
• You can pull additional conductors later without opening walls, if the conduit has spare capacity

That second point is why oversized conduit is such good value during any electrical project — see EV charger conduit and wire runs, which applies the same logic.

Conduit fill rules limit how many conductors of a given size fit in a raceway, because packed conductors can't shed heat and can't be pulled without damage.

Service entrance cable

SE cable connects the utility service to the panel, and SER is often used for feeders to subpanels. Heavier than branch-circuit wiring and specific to that role.

This is licensed-electrician territory in every practical sense.

Low-voltage wiring

Thermostat wire, doorbell wire, network and coaxial cable, and speaker wire operate at low voltage and are governed by different rules.

Two safety points that matter:

• Never run low-voltage cable in the same box or raceway as line-voltage wiring unless the method is specifically permitted. Mixing them is a real hazard.
• Low voltage doesn't mean no hazard — a shorted low-voltage circuit can still start a fire.

Quick reference

How do you identify electrical wire types?

Useful for assessing a house:

A plastic-jacketed flat cable in walls — modern NM. Normal.

A round, solid-jacketed cable outdoors or entering the ground — likely UF. Appropriate.

Flexible metal sheathing — armored or metal-clad. Fine, but if it's very old, the grounding arrangement is worth checking.

Individual conductors on ceramic knobs and tubes — knob-and-tube. This is old, ungrounded wiring with specific hazards. See knob and tube wiring.

Cloth-jacketed cable — mid-century wiring whose insulation may be deteriorating. See cloth and rubber insulated wiring.

Interior cable running outside or into the ground — a defect. Get it corrected.

Why the method matters for safety

Choosing the wrong method isn't a technicality:

• Indoor cable outdoors degrades in sunlight and moisture, and moisture reaching conductors is a shock and fire path
• Unprotected cable where it can be damaged — a garage wall, a basement stairwell — invites a nail, a shelf bracket or a stored ladder to breach it
• Cable buried without the right rating or depth fails and is expensive to find

The size of the conductor is a separate question with its own rules — see wire gauge explained and the wire and breaker size reference.

Buying NM-B

For ordinary interior branch circuits, the product to know is Southwire's Romex SIMpull NM-B — UL 719 listed, 90°C-insulated conductors applied at 60°C ampacity per the NEC. Match the gauge to the breaker (14 AWG on 15 A, 12 AWG on 20 A — see wire gauge explained):

The bottom line

NM cable handles concealed interior branch circuits, UF handles outdoors and burial, armored and metal-clad handle exposed runs needing protection, and conduit with individual conductors handles shops, outdoor runs and anywhere you might want to pull more wire later. Location dictates the method, and using an interior cable outdoors or unprotected where it can be damaged is a genuine hazard rather than a paperwork issue.

Check circuit capacity with the home electrical load calculator, look up sizes in the wire and breaker size reference, or read wire gauge explained.

Code reference

The requirements behind this guide, for looking up in your adopted edition:

• NEC Article 334 — nonmetallic-sheathed cable (NM-B)
• NEC Article 340 — underground feeder cable (UF)
• NEC Article 330 — metal-clad cable (MC)

Code editions and local amendments vary by jurisdiction. Confirm the adopted edition with your AHJ before relying on any specific article.