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 attic heat, bundling 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 far can you run 12/2 Romex on a 20 amp circuit?

It depends on the load and your voltage-drop target. Using the NEC Chapter 9 Table 8 resistance of solid 12 AWG copper (1.93 Ω per 1,000 ft), a 16 A load on 120 V stays within 3% drop up to about 58 ft one way (0.03 × 120 × 1,000 ÷ (2 × 16 × 1.93)). At the same load 10 AWG reaches about 92 ft and 8 AWG about 144 ft. The 3% figure is an efficiency recommendation from the NEC informational notes, not a mandatory limit.

When does a 20 amp circuit need 10 AWG Romex?

When the run is long enough that 12 AWG exceeds your voltage-drop target. A 16 A load on 120 V over a 75 ft one-way run drops 2 × 16 × 1.93 × 0.075 = 4.63 V (3.86%) on 12 AWG but 2 × 16 × 1.21 × 0.075 = 2.90 V (2.42%) on 10 AWG. With a 3% target the tool picks 10 AWG and keeps the breaker at 20 A.

Does Romex in a hot attic need a bigger wire?

Sometimes. NM-B conductors are rated 90 °C, so NEC 334.80 lets you start corrections from the 90 °C ampacity, but the result can never exceed the 60 °C value. 12 AWG at 120 °F gets 30 A × 0.82 = 24.6 A, still above 20 A, so it stays at 20 A. Add three 2-wire cables bundled together (6 current-carrying conductors, 80%) and it drops to 30 × 0.82 × 0.80 = 19.7 A, which no longer covers a 20 A breaker, so the tool moves to 10 AWG.

When do bundled Romex cables need derating?

NEC 334.80 requires the Table 310.15(C)(1) adjustment when more than two NM cables with two or more current-carrying conductors pass, without spacing, through the same opening in wood framing that will be sealed with insulation, caulk or foam, or run in contact with thermal insulation without spacing. Count current-carrying conductors: a 120 V 12/2 has two, a straight 240 V 10/2 has two, and the bare ground never counts. 4–6 conductors take 80%, 7–9 take 70%.

How does the continuous-load calculation work?

NEC 210.19(A)(1) requires the conductor to carry 125% of the continuous load before any correction, and 100% of the load after ambient and bundling correction. The tool checks both and uses whichever needs the larger wire.

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, such as a range or most dryers, generally needs three insulated conductors. Follow the equipment instructions and code.

What is the amp rating of 8/2 Romex?

Copper 8 AWG NM-B is rated 40 A, its 60 °C ampacity from NEC Table 310.16 as required by NEC 334.80. The /2 means two insulated conductors plus a bare ground. For comparison, 6/2 NM-B is rated 55 A.

Can #10 Romex go on a 40 amp breaker?

Not on a general-purpose circuit. NEC 240.4(D) limits 10 AWG copper to 30 A overcurrent protection, so a general 40 A circuit needs 8 AWG NM-B. Motor and air-conditioning circuits are exceptions under 240.4(G) and Article 440, where the nameplate minimum circuit ampacity and maximum breaker govern, which is how #12 can appear on a 25 A AC breaker. This calculator does not model those equipment rules.

How is voltage drop calculated here?

Single-phase drop = 2 × current × one-way length × conductor resistance. The tool uses NEC Chapter 9 Table 8 DC resistance at 75 °C for uncoated copper: solid 14, 12 and 10 AWG and stranded 8 AWG and larger, matching how NM-B is built. It does not model reactance, power factor, distributed loads or connection resistance.

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, attic heat and bundling correction, and voltage drop, then lists the longest run each gauge can handle. Final design still requires the adopted local code and installation conditions.

Copper NM-B (Romex) is limited to its 60 °C ampacity, so 14 AWG serves 15 A circuits, 12 AWG serves 20 A and 10 AWG serves 30 A.

Circuit Details

Common circuits

Breaker Amperage (A)

Calculated Load Current (A)

Enter actual calculated current, not the breaker label. Amps = watts ÷ volts.

Load Type

Non-continuous Continuous (3+ hrs)

Continuous loads require 125% sizing per NEC 210.19(A)(1).

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.

Hottest Ambient (°F)

Bundled Current-Carrying Conductors

86 °F (30 °C) is the table baseline; use attic temperature for attic runs. Three 2-wire cables through one sealed hole = 6 conductors (NEC 334.80).

Fill in the circuit details and click Find Romex Size.

Maximum One-Way Run by Gauge

AWG	Corrected A	Max run (ft)

Romex (NM-B) Wire Size Chart — Copper, NEC 2023

60 °C ampacity is the final NM-B limit (NEC 334.80). The 90 °C column is only the starting point for ambient and bundling correction. Resistance is NEC Chapter 9 Table 8 at 75 °C.

AWG	Cable	60 °C (A)	90 °C base (A)	Breaker	Ω / 1,000 ft	Typical use
14 AWG	14/2, 14/3	15	25	15 A (240.4(D))	3.07	Lighting, general 15 A receptacles
12 AWG	12/2, 12/3	20	30	20 A (240.4(D))	1.93	Kitchen, bath, laundry, garage 20 A
10 AWG	10/2, 10/3	30	40	30 A (240.4(D))	1.21	Water heater, dryer, small AC
8 AWG	8/2, 8/3	40	55	40 A	0.778	Range, cooktop, 32 A EV charger
6 AWG	6/2, 6/3	55	75	50 A common	0.491	50 A range, 40 A EV charger, subpanel
4 AWG	4/3	70	95	70 A	0.308	Subpanel feeder
3 AWG	3/3	85	115	80 A common	0.245	Subpanel feeder
2 AWG	2/3	95	130	90 A common	0.194	Subpanel feeder

Ambient temperature correction (90 °C column, NEC Table 310.15(B)(1))

Ambient °F	78–86	87–95	96–104	105–113	114–122	123–131	132–140	141–149
Factor	1.00	0.96	0.91	0.87	0.82	0.76	0.71	0.65

Safety and code scope

This is a preliminary copper NM-B screen, not an electrical design or permit approval. Ampacity follows NEC 2023 Table 310.16 with NEC 334.80 (60 °C final limit, 90 °C correction base), Table 310.15(B)(1) ambient correction and Table 310.15(C)(1) bundling adjustment, and 210.19(A)(1) continuous-load sizing. The breaker check is conservative: it does not apply the 240.4(B) next-size-up allowance. Voltage drop uses NEC Chapter 9 Table 8 DC resistance at 75 °C and a round-trip single-phase path. It does not model reactance, power factor, distributed loads, connection resistance, 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.

Quick answer

Copper NM-B (Romex) is limited to its 60 °C ampacity, so 14 AWG serves 15 A circuits, 12 AWG serves 20 A and 10 AWG serves 30 A. A 20 A circuit takes 12/2 NM-B; past about 58 ft at 16 A (3% drop on 120 V) it needs 10 AWG, and hot attics or bundled cables can also force the next size up.

Examples

NM-B copper size	60 °C ampacity
14 AWG (14/2)	15 A
12 AWG (12/2)	20 A
10 AWG (10/2)	30 A
8 AWG (8/2)	40 A
6 AWG (6/2)	55 A

Enter your breaker, load and run length to see whether voltage drop, attic heat or bundling pushes the wire up a size.

Summary

How it works

Pick a common circuit preset or select the breaker rating and enter the calculated load current.
Choose whether that entered current is continuous or non-continuous.
Enter the one-way cable run, the maximum voltage-drop target, the hottest ambient temperature along the run and how many current-carrying conductors are bundled together.
The tool checks each copper NM-B size against 125% continuous sizing at 60 °C, the 90 °C-based ambient and bundling correction (capped at the 60 °C value, NEC 334.80), the breaker rating and voltage drop using NEC Chapter 9 Table 8 resistance.
Review the smallest size that passes, the calculation breakdown and the maximum run length for every gauge.

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, water heater).
Check how far a 12/2 or 10/2 run can go before voltage drop exceeds 3%.
Check whether attic heat or several cables through one bored hole force a larger gauge.

Frequently Asked Questions

Last updated: 2026-10-09 · Reviewed by Nham Vu