Types of Wire: Complete Guide for Homeowners

Walk into the electrical aisle at Home Depot and you’ll find a dozen spools labeled with codes like NM-B, THHN, UF, and SEU — all different types of wire, each built for a specific job. Picking the wrong one isn’t just inefficient, it’s a code violation and a fire risk. This guide breaks down every common residential wire type, what the letters mean, what gauge to use for what amperage, and where each belongs — indoors, outdoors, buried, or inside conduit.

Reading a Wire Label: What the Letters Mean

Every wire is stamped with a code string like “12-2 NM-B WITH GROUND 600V.” Here’s what it tells you:

• First number (12): conductor gauge — AWG (American Wire Gauge)
• Second number (2): number of insulated conductors, ground wire not counted
• Letters (NM-B): insulation and jacket type
• Voltage rating (600V): maximum rated voltage

Letter decoder: N = nonmetallic, M = sheathed, B = 90°C-rated insulation, T = thermoplastic, H = heat-resistant, HH = higher heat, W = water-resistant, N = nylon jacket, U = underground, F = feeder.

Common Gauge Sizes and Their Amp Ratings

AWG gets smaller as the number gets larger. Standard residential gauges:

• 14 AWG: 15 amps, lighting circuits
• 12 AWG: 20 amps, outlet circuits, small appliances
• 10 AWG: 30 amps, water heaters, electric dryers, small AC units
• 8 AWG: 40-50 amps, electric ranges, large AC
• 6 AWG: 55-65 amps, subpanel feeders
• 4 AWG: 85 amps, large subpanels
• 2 AWG: 115 amps, service entrance for smaller homes
• 2/0 AWG: 175 amps, service entrance for average homes

Aluminum wire carries about 60% of copper’s amperage at the same gauge, so a 2/0 copper service becomes 4/0 aluminum for the same 175-amp capacity.

NM-B (Non-Metallic Sheathed Cable, aka Romex)

The workhorse of US residential wiring. A flat plastic jacket contains two or three insulated conductors plus a bare copper ground. Colored by gauge to make identification easy:

• White jacket: 14 AWG (15-amp lighting)
• Yellow jacket: 12 AWG (20-amp outlets)
• Orange jacket: 10 AWG (30-amp appliances)
• Black jacket: 8 AWG or 6 AWG (40-amp and up)

NM-B is approved for interior dry locations only. You cannot bury it, run it outdoors, or install it in damp areas. A 250-foot roll of 12-2 NM-B runs $90 to $130 depending on copper prices.

UF-B (Underground Feeder)

Looks similar to NM-B but the conductors are embedded in a solid plastic jacket rather than wrapped in paper filler. UF-B is rated for direct burial (at least 24 inches deep, 12 inches under concrete) and wet locations. Use it for detached garage feeders, shed lighting, landscape outlets, and pool equipment (with GFCI). Same gauge-to-color coding as NM-B in some brands; grey jacket in others.

THHN and THWN (Thermoplastic Nylon-Coated)

Individual insulated conductors that must be pulled through conduit. Used for commercial buildings, exposed runs, and any wet or damp install. Letter breakdown: T = thermoplastic, H = heat resistant 75°C, HH = 90°C, W = wet location, N = nylon outer jacket. THHN/THWN-2 is the most common version at Home Depot.

You’ll see it in 500-foot spools in black, white, red, green, and blue. Pull three or four colors through the same conduit to make up a circuit. Great for running out to a shop building inside PVC conduit, rewiring a panel area cleanly, or replacing ancient knob-and-tube in an accessible attic.

SE, SEU, and USE (Service Entrance Cable)

The heavy gauge cable running from the utility drop or transformer into your meter and panel. Three types:

• SEU: service entrance, unarmored, for aerial drops
• SER: service entrance, round, with ground — used for subpanel feeders
• USE: underground service entrance, direct-burial rated

Service-entrance cable is usually 4/0 aluminum in residential 200-amp services. Only a licensed electrician should work on service conductors, and they require utility coordination to disconnect.

Low-Voltage and Data Cables

Not all wire in a home carries 120V. Common low-voltage cables:

• Cat5e/Cat6: Ethernet data, 4 twisted pairs, plenum or riser rated
• RG6 coax: TV and cable internet
• 18/2 thermostat wire: 18 gauge, 2 conductors, for standard T-stats
• 16/2 speaker wire: in-wall rated for home audio
• 12/2 or 10/2 low-voltage landscape wire: direct-burial for 12V lighting

Never run low-voltage in the same stud cavity as 120V wiring unless both are in separate conduits — crosstalk and induced noise are real issues, and code requires minimum separation.

Stranded vs. Solid Conductors

NM-B in residential gauges is solid copper — a single strand you can bend and terminate under a screw. Larger gauges (6 AWG and up) and any THHN pulled through conduit are usually stranded, meaning dozens of small strands twisted together. Stranded wire flexes easier and pulls better, but needs a crimp connector or a lug-style termination; a standard outlet screw isn’t rated for fine-stranded wire without a ferrule.

Aluminum vs. Copper

Aluminum costs less per pound but corrodes at connections and has lower conductivity. It’s still used for service entrance and large subpanel feeders, where the cost savings at high gauge is significant. Avoid aluminum branch wiring — homes wired in the 1970s with aluminum branch circuits have elevated fire risk, and insurance premiums reflect that. If your home has aluminum 15-amp or 20-amp circuits, consider rewiring or using COPALUM or AlumiConn connectors to pigtail copper at every device.

Matching Wire to the Job

Use this quick reference before your next trip to the hardware store:

• Bedroom lights and outlets: 14-2 NM-B on 15A, or 12-2 on 20A
• Kitchen small-appliance circuits: 12-2 NM-B (code requires 20A)
• Dedicated bathroom circuit: 12-2 NM-B, 20A GFCI
• Electric range: 8-3 NM-B, 40A
• Electric dryer: 10-3 NM-B, 30A
• Central AC 3-ton: 10-2 NM-B, 30A
• Detached shed: 12-2 UF-B buried 24 inches, 20A with GFCI
• Panel-to-subpanel: 6-3 NM-B or 6/3 SER, 60A

Code and Safety Notes

Every US jurisdiction enforces some version of the NEC (National Electrical Code), updated every three years. Your permit office will tell you which code cycle applies and whether AFCI/GFCI protection is required on specific circuits. Never splice wires outside of an accessible junction box, and don’t mix aluminum and copper at a termination without the right connector. When in doubt, have a licensed electrician verify the design — a $150 consult is cheaper than an insurance denial.