Romex is a brand name for Type NM-B (non-metallic sheathed) cable, the most common residential wiring product in the US. It contains two or three insulated conductors plus a bare ground wire wrapped in a plastic jacket. NEC Article 334 governs its installation.

What wire gauge do I need for a 20-amp breaker?

12 AWG NM-B is the standard choice for a 20 A circuit (rated 20 A at 60 °C). 10 AWG is required if the circuit runs at 75 °C or carries a continuous load at the breaker's full rating.

Can I use 14 AWG wire on a 20-amp breaker?

No. NEC 240.4(D) prohibits protecting 14 AWG conductors with anything larger than a 15 A overcurrent device. Using 14 AWG on a 20 A breaker is a code violation and a fire hazard.

What is the NEC 80% continuous-load rule?

NEC 210.19(A)(1) requires that branch-circuit conductors be sized at 125% of the continuous load (loads that run for 3 or more hours). In practice this means the wire and breaker must handle the load at 80% of their rated ampacity.

Does Romex come in different configurations?

Yes. Common types include 14-2, 14-3, 12-2, 12-3, 10-2, and 10-3 (gauge-conductors). The "-2" variant has one hot + neutral + ground; "-3" adds a second hot wire for 240 V or multi-wire branch circuits.

What is the difference between 60 °C and 75 °C rated wire?

NM-B cable is rated for use at 60 °C terminations per NEC 334.80, even if the insulation itself is rated 90 °C. Most residential breakers and outlets are rated for 60 °C or 75 °C. Always use the lower temperature column when sizing conductors unless all terminations are confirmed 75 °C rated.

Romex Wire Size Calculator

Name: Romex Wire Size Calculator
Availability: InStock
Author: Nham Vu

Enter your breaker amperage and load type to find the correct Romex (NM-B) wire gauge per NEC 310 guidelines.

Circuit Details

Breaker Amperage (A)

Load Type

Non-continuous Continuous (3+ hrs)

Continuous loads require 125% sizing per NEC 210.19(A).

Termination Temperature Rating

NM-B cable must be derated to the lowest-rated termination (NEC 334.80).

Circuit Voltage

Fill in the circuit details and click Find Romex Size.

NM-B Ampacity Reference

AWG	60 °C (A)	75 °C (A)	Max Breaker

Summary

Enter your breaker amperage and load type to find the correct Romex (NM-B) wire gauge per NEC 310 guidelines.

How it works

Select your circuit breaker amperage from the dropdown.
Choose whether the load is continuous (runs 3+ hours) or non-continuous.
Pick the installation temperature rating (60 °C or 75 °C).
The calculator applies NEC 310 ampacity tables and the 80% continuous-load rule.
Your recommended Romex gauge, wire color code, and ampacity are displayed instantly.

Use cases

Choose the right Romex cable when wiring a new bedroom or living room circuit.
Verify that existing wiring is rated for an added appliance or upgraded breaker.
Size NM-B cable for a kitchen, laundry, or workshop sub-circuit.
Confirm wire gauge for a 240 V outlet (dryer, EV charger, range).
Match Romex AWG to a replacement circuit breaker size.
Plan rough-in wiring for a home addition or garage.

Frequently Asked Questions

Last updated: 2026-07-01 · Reviewed by Nham Vu