Three Questions That Pick Your Wire
Choosing the right wire for a new circuit comes down to three things: how many amps the circuit carries, where the wire runs, and how many conductors the load needs. Answer those in order and you will land on the correct cable every time. Here is the decision walk-through.
Step 1: Match the Gauge to the Amps
The breaker amperage sets the wire gauge, because the wire has to safely carry whatever the breaker allows. For common copper NM-B circuits:
| Circuit Amperage | Copper NM-B Gauge |
|---|---|
| 15 amps | 14 AWG |
| 20 amps | 12 AWG |
| 30 amps | 10 AWG |
| 40 amps | 8 AWG |
| 50 to 55 amps | 6 AWG |
These values follow the 60 degrees Celsius column required for NM cable by NEC 334.80. Never put a smaller wire on a larger breaker.
Step 2: Match the Cable Type to the Location
NM-B Romex
Use NM-B Romex for circuits run inside dry, framed walls and ceilings without conduit. This is the standard for almost all interior home wiring.
UF-B
Use UF-B when the run goes underground or outdoors exposed to moisture. It is rated for direct burial and handles soil, water, and sunlight where Romex cannot.
THHN in Conduit
Use THHN single conductors when the circuit runs through conduit, outdoors in raceway, or as a service or feeder in pipe. You pull only the colors and count you need.
SER
Use SER cable for service entrances and feeders to subpanels and equipment such as a standby generator transfer switch.
Step 3: Match the Conductor Count to the Load
Most circuits use two-conductor cable, a hot and a neutral plus ground, written as 14/2, 12/2, or 10/2. Choose three-conductor cable, written as 14/3 or 12/3, when the load needs an extra hot or a shared neutral, such as three-way switches, multi-wire branch circuits, or a dryer that uses both 240-volt and 120-volt power.
Put It Together
A 20-amp indoor kitchen circuit is 12/2 NM-B Romex. A buried 15-amp circuit to a shed is 14/2 UF-B. A 30-amp water heater is 10/2 Romex. A pair of 20-amp circuits sharing a neutral is 12/3. Answer the three questions and the cable picks itself. For the full breakdown of gauges and amperage, see our wire size chart.
Code and Safety Note
This guide is a starting reference, not a load calculation. Follow the National Electrical Code and local amendments, confirm with your authority having jurisdiction, and use a licensed electrician for new circuits, feeders, and panel work.
IB Lighting stocks every common gauge and cable type from Southwire and CerroMax by Cerrowire in our electrical wire and cable collection, with free freight plus free freight insurance on every order. Call (800) 674-9019 and we will help you pick the exact cable for your circuit.