What Wire Do I Need? The Complete Electrical Cable Selection Guide
Choosing the right electrical wire is a two-part decision: what type of cable, and what gauge. Get both right and your installation is safe, code-compliant, and passes inspection. Get either one wrong and you are either creating a safety hazard or pulling the wire out and starting over. This guide walks you through the decision systematically for every common residential and light commercial application.
Step 1: Determine Where the Wire Will Be Installed
Location determines cable type. The environment the wire lives in is the first filter.
Inside Finished Walls, Ceilings, and Floors (Dry Indoor)
This is the most common scenario for residential wiring. Standard NM-B (Romex) is the correct cable type. NM-B is UL-listed for concealed residential use in dry locations. It is the most affordable cable type, the fastest to install, and what your inspector expects to see inside residential walls throughout the US (with some local exceptions).
Exposed Runs in Basements, Garages, or Attics
NM-B is permitted for exposed runs in residential construction in many jurisdictions, but it offers no physical protection. If the cable could be bumped, hit by a stored item, chewed by a rodent, or subject to any physical damage, MC (Metal Clad) cable is the better choice. MC cable carries a metal armor jacket that resists physical damage and is accepted in both residential and commercial exposed locations. It costs more per foot but eliminates the need for conduit in most exposed applications.
Outside Above Ground
NM-B is not rated for outdoor use. For above-ground outdoor installations, use conduit (EMT, PVC, or rigid) with THHN individual conductors inside, or use weatherproof junction boxes and conduit fittings to keep all wiring protected from moisture exposure.
Underground (Direct Burial)
Only UF-B (Underground Feeder cable) is approved for direct burial in earth without conduit. NM-B buried underground will fail from moisture infiltration within months to years. UF-B has a solid waterproof jacket construction that resists moisture indefinitely. Burial depth for UF-B must be at least 24 inches for residential direct burial per the NEC.
Conduit (Any Location)
When pulling individual conductors through EMT, PVC conduit, or rigid conduit, use THHN/THWN-2 individual conductors. THHN is the standard conduit conductor for all circuit types. Pull one conductor for each function needed: black and white for 120-volt circuits, black, white, and red for 240-volt circuits with neutral, plus a green or bare ground wire for every circuit.
Commercial Buildings
Most commercial building codes require MC cable or conduit wiring methods throughout. NM-B (Romex) is generally not permitted in offices, retail, restaurants, medical, or institutional buildings. Use 12/2 or 14/2 MC cable for branch circuits, or run THHN through conduit.
Step 2: Determine the Required Gauge
Gauge is determined by the breaker size, which is determined by the electrical load. Match these:
15-amp breaker: 14 AWG wire (14/2 NM-B, 14/2 MC, or 14 AWG THHN). Used for: lighting circuits, bedroom outlets, living area outlets.
20-amp breaker: 12 AWG wire (12/2 NM-B, 12/2 MC, or 12 AWG THHN). Used for: kitchen outlets, bathroom outlets, garage outlets, outdoor outlets, laundry, dishwasher, garbage disposal.
30-amp breaker: 10 AWG wire (10/2 NM-B or 10 AWG THHN). Used for: electric dryer, electric water heater, 30-amp EV charger, AC disconnect.
40-amp breaker: 8 AWG wire (8 AWG THHN or SER cable). Used for: electric range sub-feed, 40-amp EV charger, large HVAC disconnect.
50-amp breaker: 6 AWG wire. Used for: electric range full circuit, 50-amp RV outlet, large subpanel feeds.
Step 3: Determine the Number of Conductors
Most circuits need 2-conductor cable (one hot, one neutral, plus ground): 120-volt outlets, lighting, dedicated appliance circuits. Use /2 cable: 12/2 NM-B, 14/2 UF-B, etc. Three-conductor cable (two hots, one neutral, plus ground) is needed for three-way switch wiring, split receptacles, multi-wire branch circuits, and 240-volt appliances requiring a neutral. Use /3 cable: 12/3 NM-B, 14/3 NM-B, 10/3 NM-B for ranges.
Quick Decision Reference
New bedroom or living area outlet: 14/2 NM-B on 15-amp breaker. Kitchen countertop outlet: 12/2 NM-B on 20-amp breaker. Bathroom outlet: 12/2 NM-B on 20-amp breaker. Electric dryer: 10/3 NM-B on 30-amp breaker. Underground to detached garage: 12/2 UF-B (24 inch burial depth). Exposed basement ceiling run: 12/2 MC cable. Commercial office outlet: 12/2 MC cable or 12 AWG THHN in conduit. Three-way switch wiring: 14/3 NM-B on same circuit gauge as the lighting circuit.
IB Lighting stocks every cable type in this guide: NM-B (Romex) from Southwire and Cerrowire, UF-B, MC cable, and THHN in all standard gauges with free freight insurance on every order. Browse our complete wire and cable selection or call (800) 674-9019 for expert help selecting the right cable for your specific project. Our team includes a licensed master electrician available to answer technical questions.