"30 amp Romex" is the common jobsite shorthand for 10 AWG NM-B (Non-Metallic Sheathed) cable, sized to safely carry up to 30 amps of current based on the 60°C temperature column of the National Electrical Code (NEC). When an apprentice asks for "30 amp Romex," they need 10-gauge wire. But slapping a 10 AWG cable on a 30-amp breaker doesn't automatically make the installation code-compliant or safe. The physical jacket, the termination temperatures, and the continuous load rules all dictate how this cable actually behaves in a real circuit.

What "30 Amp Romex" Actually Means (and What It Isn't)

Romex is a trademarked brand name owned by Southwire, but it has become the genericized term for all NM-B cable. For a 30-amp circuit, the correct specification is 10 AWG copper NM-B. Depending on your needs, this comes in 10/2 (two insulated conductors plus a ground) or 10/3 (three insulated conductors plus a ground).

Under NEC Table 310.16, 10 AWG copper wire in the 60°C column is rated for exactly 30 amps. Here is the critical part that trips up many DIYers: the individual THHN conductors inside the NM-B jacket are actually rated for 90°C. However, NEC Article 334.80 explicitly mandates that the ampacity of NM cable must be determined using the 60°C column, regardless of the inner wire's higher temperature rating. This is because the outer PVC jacket and the paper filler inside NM-B cannot safely dissipate heat at the 90°C threshold without degrading.

What this changes in your installation: Because you are locked into the 60°C ampacity limit, you cannot up-breaker 10 AWG NM-B to 35A or 40A, even if the load calculations suggest you need a slight buffer. Furthermore, you must treat 10 AWG NM-B differently than standard 14 or 12 AWG lighting wire: it requires larger knockout holes, larger wire connectors (like Ideal WingNut 454 or 3M Performance Plus), and specific torque settings on the breaker lugs.

What people commonly confuse it with: Many builders confuse 10 AWG NM-B with 10 AWG THHN pulled in conduit. Individual THHN wires in conduit can use the 90°C column for derating purposes (like adjusting for multiple wires in a pipe), though the final overcurrent protection is still usually limited by the 60°C or 75°C termination ratings of the breaker and receptacle. NM-B does not get this derating grace.

The Math: Ampacity, Derating, and the 80% Rule

Let's look at a worked numeric example to see how "30 amp Romex" performs over distance. Suppose you are running a 120V 30A RV outlet (a NEMA TT-30R) located 80 feet from your main panel.

10 AWG Copper Circular Mils (CM): 10,380

First, we apply the continuous load rule. Under NEC 210.20, if an RV is expected to draw power for three hours or more, it is considered a continuous load. The maximum continuous load on a 30-amp breaker is 80%, which equals 24 amps.

Next, we calculate the voltage drop using the standard single-phase formula: VD = (2 × K × I × D) / CM.

  • K (Copper resistivity constant) = 12.9
  • I (Current) = 24A (continuous load)
  • D (One-way distance) = 80 feet
  • CM (Circular mils for 10 AWG) = 10,380

VD = (2 × 12.9 × 24 × 80) / 10,380
VD = 49,536 / 10,380 = 4.77 Volts

To find the percentage drop: 4.77V / 120V = 3.97%.

The NEC recommends a maximum voltage drop of 3% for branch circuits. At 3.97%, your RV's air conditioners might struggle to start, and the compressor could overheat. While a 3.97% drop isn't a strict NEC violation unless your local Authority Having Jurisdiction (AHJ) enforces the recommendation as mandatory, it is poor practice. The fix: You would bump the wire up to 8 AWG NM-B for the 80-foot run to reduce the voltage drop, even though the breaker and receptacle remain rated for 30 amps.

Where You Meet This in Practice

You won't use 10 AWG NM-B for standard bedroom receptacles or lighting. You will encounter it in specific high-draw appliance and specialty circuits:

  1. RV Receptacles (NEMA TT-30R): A 120V, 30-amp outdoor outlet for travel trailers. This requires 10/2 NM-B (black, white, bare ground).
  2. Electric Dryers (NEMA 14-30R): Modern 240V electric dryers require a 4-prong outlet. This requires 10/3 NM-B (black, red, white, bare ground) to provide both 240V for the heating element and 120V for the control board.
  3. Electric Water Heaters: Many standard 4500W 240V water heaters are installed on a 30-amp double-pole breaker using 10/2 NM-B. (The white wire is re-identified with black tape to serve as the second hot leg).
  4. Large Window AC Units / Mini-Splits: Some 24,000 BTU+ units require a 30A disconnect and 10 AWG wire.
Safety Warning: Never use 10/2 NM-B for a 4-prong dryer outlet by attempting to use the bare ground as a neutral. Modern code strictly requires an insulated neutral (the white wire in 10/3) for dryer circuits to prevent the chassis from carrying return current.

Scenario Walkthrough: The Melted RV Receptacle

To understand why wire sizing and device selection matter, let's look at a real-world failure mode.

The Setup: A homeowner installs a TT-30R RV outlet in their driveway using 10/2 NM-B and a standard residential-grade 30A single-pole breaker. They plug in a large 5th-wheel RV for a summer weekend.

The Numbers: The RV's internal power converter, refrigerator, and two roof AC units pull a combined 28 amps continuously. The ambient outdoor temperature is 95°F.

The Outcome: After four hours, the homeowner smells melting plastic. The TT-30R receptacle face is warped, the plug prongs are scorched, but the 30-amp breaker in the panel has not tripped.

What Went Wrong: This failure was caused by three compounding errors:

  • Continuous Load Violation: 28 amps exceeds the 24-amp continuous load limit (80% of 30A) for the breaker and receptacle.
  • Device Grade: The homeowner used a cheap, big-box-store residential-grade 30A plug. Heavy-duty continuous loads require industrial-grade receptacles (like Hubbell or Bryant) with massive internal brass contacts designed to dissipate heat.
  • Thermal Buildup: The receptacle was housed in an enclosed plastic weatherproof "bubble" cover. At 95°F ambient, the heat generated by the 28A load had nowhere to go. The internal temperature of the box easily exceeded the 60°C rating of the NM-B insulation and the receptacle's termination limits.

The breaker didn't trip because a standard inverse-time thermal-magnetic breaker is designed to hold slightly above its rated current for hours before tripping. 28A is simply not high enough to trigger the thermal trip curve of a 30A breaker in a timely manner.

Installation Quirks: Stripping, Torque, and Box Fill

Working with 10 AWG NM-B requires different physical techniques than 14 or 12 AWG wire.

Stripping the Jacket

Do not use a standard wire stripper to slit the outer PVC jacket of 10/2 or 10/3; you will almost certainly nick the 10 AWG copper conductors, creating a hot spot. Use a dedicated NM-B slitting tool (like the Jonard JNM-10) or carefully score the jacket longitudinally with a utility knife, keeping the blade flat against the inner paper separator.

Torque Requirements

Under NEC 110.14(D), any termination rated 100 amps or less must be tightened to the manufacturer's specified torque using a calibrated torque screwdriver. For a typical 30A breaker (such as an Eaton BR or Square D QO), the required torque for 10 AWG wire is usually between 35 and 45 inch-pounds. Always check the label printed on the breaker or the panel door. Hand-tightening 10 AWG often results in loose connections that arc and burn under a 24A continuous load.

Box Fill Calculations

10 AWG wire takes up significantly more space in a junction box. Under NEC 314.16, each 10 AWG conductor counts as 2.5 cubic inches of box fill. If you are splicing three 10/2 cables in a junction box, you have 6 current-carrying conductors (6 × 2.5 = 15 cu in) plus equipment grounds (1 × 2.5 = 2.5 cu in), requiring a minimum 17.5 cubic inch box. A standard single-gang nail-on box will not work; you need a deep 4x4 or 4x11 metal or PVC junction box.

FAQ: Common 10 AWG NM-B Questions

Can I use 10 AWG Romex on a 20-amp breaker?

Yes, it is perfectly legal to use a larger wire on a smaller breaker. However, 10 AWG wire is physically too thick to terminate on the screws of most standard 20-amp receptacles. You will need to pigtail the 10 AWG wire down to a 12 AWG wire inside a junction box using a large wire nut, or purchase specialized heavy-duty receptacles with deep back-wire clamps rated for 10 AWG.

Is there a 30-amp aluminum Romex?

No. NM-B cable is manufactured exclusively with copper conductors. If you need aluminum wire for a 30-amp circuit (which would require 8 AWG aluminum to achieve 30A ampacity), you must use SE (Service Entrance) cable or individual aluminum THHN/THWN-2 wires pulled inside a conduit.

Why is 10 AWG NM-B jacket colored orange?

Following the industry-wide color-coding standard adopted in the early 2000s, NM-B jackets are colored by gauge to help inspectors and electricians identify wire sizes at a glance. White is 14 AWG (15A), Yellow is 12 AWG (20A), Orange is 10 AWG (30A), and Black is 8 or 6 AWG (40A-55A).

Can I run 30 amp Romex through floor joists without conduit?

Yes, NM-B cable is designed to be run through the bored holes of wooden framing members, including floor joists and studs, without the need for conduit. However, if you are running it through metal studs, you must use plastic bushings or grommets in the punched holes to prevent the sharp metal edges from slicing the PVC jacket and causing a ground fault.