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?

For the basic copper NM-B 60 °C ampacity screen, 12 AWG corresponds to 20 A. A long run may need a larger conductor for voltage drop, and correction factors or local amendments can require a different design.

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.

How does the continuous-load calculation work?

This screen multiplies the entered continuous current by 125%. It does not assume that every circuit may simply be loaded to a universal 80% value; equipment listing and the applicable branch-circuit rules still control.

Does Romex come in different configurations?

Yes. In common cable naming, “-2” and “-3” count insulated conductors and do not count the equipment grounding conductor. A straight 240 V load may use two hots without a neutral, while a 120/240 V load that requires a neutral generally needs three insulated conductors. Follow the equipment instructions and code.

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

Under NEC 334.80, the allowable ampacity of Type NM-B cable must not exceed the 60 °C rating from the applicable ampacity table, even though modern NM-B has 90 °C conductor insulation. The 90 °C rating may be used for adjustment and correction calculations, but the final result still cannot exceed the 60 °C ampacity.

Does this calculator include voltage drop?

Yes. It uses the entered one-way length, load current, source voltage and maximum drop to perform a simplified single-phase resistive screen. It does not model reactance, power factor, multiple distributed loads, connection resistance or operating temperature, so verify the final design with an appropriate professional tool.

Romex Wire Size Calculator

Enter breaker rating, actual load current and one-way run length. The tool checks the breaker, 125% continuous-load sizing, copper NM-B 60 °C ampacity and a simplified voltage-drop screen; final design still requires the adopted local code and installation conditions.

Circuit Details

Breaker Amperage (A)

Calculated Load Current (A)

Enter actual calculated current, not the breaker label.

Load Type

Non-continuous Continuous (3+ hrs)

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

NM-B Ampacity Rating

NM-B allowable ampacity cannot exceed the 60 °C conductor rating (NEC 334.80).

Circuit Voltage

One-way Run Length (ft)

Maximum Voltage Drop (%)

Length is panel to load. The screen uses the round-trip conductor path for single-phase circuits.

Fill in the circuit details and click Find Romex Size.

NM-B Ampacity Reference

AWG	60 °C (A)	Ampacity Basis / OCPD Limit

Safety and code scope

This is a preliminary copper NM-B screen, not an electrical design or permit approval. The voltage-drop result is a simplified resistive estimate using 11.2 ohm-cmil/ft and a round-trip single-phase path. It does not model reactance, power factor, distributed loads, connection resistance, ambient/bundling correction, terminal limitations, motor or appliance rules, dwelling-service allowances, fault current, grounding, wet-location suitability or local amendments. Verify the currently adopted code and obtain AHJ approval where required. Review the NFPA Article 334 record, Southwire NM-B specification and Southwire voltage-drop calculator.

Summary

How it works

Select the circuit breaker rating and enter the calculated load current.
Choose whether that entered current is continuous or non-continuous.
Enter the one-way cable run and the maximum voltage-drop target.
Compare both the NM-B 60 °C ampacity screen and simplified resistive voltage-drop requirement.
Review the preliminary AWG result, calculation breakdown and installation limitations.

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: 2026-09-26 · Reviewed by Nham Vu