Undersized wire is the leading cause of house fires originating in electrical systems — the NFPA logs roughly 45,000 such fires every year in the US. Choosing the right electrical wire for house circuits is not guesswork; it is a matter of matching amperage, length, and installation method to National Electrical Code tables. The good news is that for 90% of residential work, the right answer falls into one of four or five common gauges, and once you understand the system the choices become obvious.
This guide covers what each wire type is for, the amperage each gauge handles, and which mistakes homeowners make most often when adding circuits, running new outlets, or replacing old wiring.
Common House Wire Types
- NM-B (Romex) — standard interior wiring for dry, protected locations
- UF-B — direct burial or wet locations, feeding detached garages or lamp posts
- THHN/THWN — individual conductors used inside conduit, commercial and garage subpanels
- MC (metal-clad) — flexible armored cable, commonly required in multi-family or commercial settings
- Service entrance (SEU/SER) — heavy cable feeding the main panel from the meter
For almost all interior residential work, NM-B is what you will buy. Home Depot, Lowe’s, and electrical supply houses stock it in 25, 50, 100, and 250-foot rolls. Expect to pay $0.40 to $1.20 per foot depending on gauge and conductor count.
Wire Gauge and Amperage
Wire gauge is counterintuitive — smaller numbers mean thicker wire. The NEC sets maximum amperage ratings based on the 60°C column for residential NM-B cable. Going over the rated amperage for a given gauge is a fire risk, period.
- 14 AWG — 15-amp circuits, bedroom and living room lighting and outlets
- 12 AWG — 20-amp circuits, kitchen, bathroom, laundry, garage outlets
- 10 AWG — 30-amp circuits, electric dryers, water heaters, larger AC units
- 8 AWG — 40-amp circuits, electric ranges, some central AC units
- 6 AWG — 50-60 amp circuits, electric stoves, small subpanels
- 2-4/0 AWG — service entrance, 100-200 amp main feeds
When in doubt, size up. Going from 14 AWG to 12 AWG costs about $0.15 more per foot and gives you room to upgrade later without pulling new wire.
Conductor Count
NM-B cable is labeled with the gauge, conductor count, and ground. “12-2 with ground” means 12 AWG wire, two insulated conductors (hot and neutral), plus a bare copper ground. “12-3 with ground” adds a red conductor, used for three-way switches, split outlets, and 240-volt circuits that need a neutral.
For standard outlets and lights, buy 12-2 or 14-2. For three-way switches and shared-neutral multiwire branch circuits, use 12-3 or 14-3. For a 240-volt electric dryer with no neutral, 10-2 is sufficient; modern dryers that require a neutral need 10-3 or 10-4.
Voltage Drop Over Long Runs
Wire resistance causes voltage to drop over distance. The NEC recommends no more than 3% drop on branch circuits. For runs over 50 feet on a 20-amp circuit, upsize from 12 AWG to 10 AWG. Detached garages 100 feet from the main panel often need 8 AWG even for a 30-amp feed.
A simple rule of thumb: double the distance, upsize one gauge. Voltage-drop calculators online give exact numbers if you know circuit length, load, and conductor size.
Installation Code Basics
NM-B runs through studs must be protected by nailing plates when drilled holes are within 1.25 inches of the stud face. Cable must be secured within 8 inches of every box and every 4.5 feet along its run. Running NM-B through conduit outdoors is not allowed — that is what UF-B or THHN in conduit is for.
Every splice must happen inside a covered junction box. Wire nuts or push-in connectors (like Wago lever nuts) are code-compliant; soldering and taping is not. Tamper-resistant outlets are required in all living spaces in every state that has adopted the 2011 or newer NEC.
Circuit Planning for New Additions
The 2020 NEC requires dedicated 20-amp circuits for kitchen countertops (at least two), laundry, bathroom, and the refrigerator. Bedrooms need arc-fault protection. Bathrooms, kitchens, garages, basements, and outdoor outlets need GFCI protection.
For a finished basement, figure one 20-amp general outlet circuit per 600 square feet, plus dedicated circuits for any 240-volt equipment (welder, treadmill, electric baseboard heat). Undersizing circuit count is the most common planning mistake — you end up running extension cords six months later.
Tools for Residential Wiring
Wire strippers (Klein 11055), lineman’s pliers, a voltage tester (non-contact Fluke 1AC), diagonal cutters, a cable staple hammer, and a good flashlight cover 95% of residential work. Add a fish tape for pulling wire through finished walls and a multi-meter for troubleshooting.
Budget around $120 for a quality starter set. Cheap Harbor Freight strippers work for a few jobs but wear out fast and leave ragged copper, which makes bad connections.
When to DIY Versus Call an Electrician
Homeowners in most states can legally wire their own homes with a permit. Adding outlets on existing circuits, replacing light fixtures, and running new circuits in open framing are reasonable DIY projects for someone willing to read the code. Work inside the main panel, service upgrades, and generator transfer switches belong with a licensed electrician — mistakes there cause fires or electrocution.
Always pull permits for new circuits. Inspection catches mistakes before they matter, and unpermitted work often invalidates homeowners insurance if there is ever a fire claim.
