Romex Cable Symbol

The Romex Cable schematic symbol. Standard-specific details are listed below where available.

Definition: The Romex Cable symbol represents a non-metallic sheathed (NM-B) electrical cable used in residential wiring, depicted as a single line or paired conductors enclosed in a sheath annotation, governed by NEC Article 334 (NFPA 70) and manufactured to UL 719 standards, with the designator NM-B indicating 600 V-rated copper conductors in a thermoplastic-insulated, moisture-resistant jacket.

Also known as: NM-B cable, nonmetallic sheathed cable, house wire, Romex wire, NM cable.

What the Romex Cable symbol means

The Romex Cable symbol in a wiring diagram represents a factory-assembled cable containing two or more insulated conductors plus a bare copper grounding conductor, all enclosed in a flame-retardant non-metallic outer jacket. The name 'Romex' is a brand name that has become a generic informal term for NM-B cable in North American residential construction.

In schematic and wiring diagrams, the Romex symbol indicates a pre-jacketed multi-conductor run between an electrical panel, junction box, outlet, or fixture. It communicates cable routing, conductor count, wire gauge, and voltage rating in a single annotation, allowing electricians and inspectors to verify code compliance at a glance.

How to identify the Romex Cable symbol

The Romex Cable symbol is typically drawn as a single bold line or a pair of parallel lines with a surrounding sheath outline, often annotated with conductor count and gauge (e.g., '14/2 w/G' meaning two 14 AWG conductors plus ground). In wiring diagrams the line may include a small label box noting 'NM-B', 'Romex', or the cable specification. Endpoints terminate at device boxes, panels, or fixtures. The symbol differs from a conduit run symbol in that no conduit outline is shown around the conductors.

Function and usage boundary

Romex (NM-B) cable carries current from a distribution panel to branch-circuit loads — outlets, switches, lighting fixtures, and appliances — in dry or damp indoor locations. The bare copper grounding conductor within the cable provides a low-impedance fault-current return path to the panel, enabling overcurrent devices (breakers or fuses) to trip rapidly during a ground fault, protecting against electric shock and fire.

Standards: IEC vs ANSI

IEC 60617	IEC 60617 does not define a specific symbol for Romex/NM-B cable; IEC practice uses a line with conductor count annotation (e.g., a slash with numeral) for multi-conductor cables per IEC 60617-02.
ANSI/IEEE 315	ANSI/NFPA 70 (NEC) Article 334 governs NM-B cable installation. ANSI/UL 719 defines the product standard. In ANSI wiring diagrams, a single line with gauge/count annotation (e.g., '12/2 NM-B') is the accepted representation.
Key difference	IEC uses a generic multi-conductor cable symbol with slashes indicating conductor count; ANSI/NEC wiring diagrams use a labelled single-line notation specific to NM-B. The symbols are functionally equivalent but labelling conventions differ.

The displayed SVG is the live editor implementation, not a licensed reproduction or certification of an IEC or IEEE database glyph. Verify the exact official entry and project edition before issuing work.

Logical terminals

Logical ID	Editor terminal name
a	A
b	B

Where the Romex Cable symbol is used

General-purpose 15 A and 20 A branch circuits in residential homes — lighting and outlet circuits
240 V appliance circuits (dryers, ranges) using 10/3 or 8/3 NM-B
Three-way and four-way switch wiring using 14/3 NM-B for the traveler conductors
Bathroom and kitchen receptacle circuits (GFCI-protected) per NEC Article 210
Garage and basement circuits in dry, protected locations where conduit is not required
Smoke detector interconnected circuits using 14/3 NM-B to daisy-chain alarm units

Key facts

Romex is a brand-name term for NM-B (non-metallic sheathed) cable; NM-B is the correct NEC/UL designation governed by NEC Article 334 and UL 719.
NM-B cable contains two or more 60 °C-rated insulated conductors plus a bare copper equipment grounding conductor, all enclosed in a flame-retardant thermoplastic outer jacket.
Common residential gauges are 14 AWG (15 A), 12 AWG (20 A), and 10 AWG (30 A), with the number before the slash indicating wire gauge and the number after indicating insulated conductor count.
NM-B cable is rated for 600 V and is permitted only in dry or damp locations inside buildings — it is not approved for direct burial, wet locations, or outdoor exposed runs.
The symbol designator in wiring diagrams is typically 'NM-B' followed by gauge/count (e.g., 12/2 NM-B); the IEC equivalent annotation uses conductor-count slashes per IEC 60617-02.
In three-way switch circuits, 14/3 NM-B is used so the red conductor serves as the traveler wire between the two switches.
The bare grounding conductor inside NM-B cable must be connected to the grounding terminal of every device and box it passes through, per NEC 250.148.

Frequently asked questions

What does the Romex cable symbol mean in a wiring diagram?

The Romex cable symbol represents a non-metallic sheathed (NM-B) cable run between two electrical points — panel, switch, outlet, or fixture. The annotation alongside the symbol (e.g., '14/2 NM-B') specifies wire gauge and number of insulated conductors plus a bare ground wire.

What does Romex cable look like in a schematic?

In a wiring diagram, Romex (NM-B) cable is drawn as a single bold line or paired parallel lines with a sheath outline, labelled with the gauge and conductor count such as '12/2 NM-B'. The line runs between device symbols (outlets, switches, panel) to show the cable routing.

What is the difference between NM and NM-B cable?

NM-B is the current NEC designation for non-metallic sheathed cable with 60 °C-rated conductors; 'NM' without the B suffix was an older designation. Modern residential cable sold in the US is NM-B. Both are informally called Romex.

What wire gauge is Romex for a 20 A circuit?

A 20 A branch circuit requires 12 AWG (12/2 or 12/3) NM-B cable per NEC 310.12. Using 14 AWG on a 20 A circuit is a code violation because 14 AWG is rated for only 15 A.

What standard governs Romex cable installation?

NM-B cable installation is governed by NFPA 70 (NEC) Article 334. The product standard is UL 719. IEC installations use a different cable type (H05VV-F or equivalent) governed by IEC 60227.

How do I identify the number of conductors from the Romex symbol annotation?

The annotation format is gauge/count, e.g., '14/2' means 14 AWG conductors, 2 insulated conductors plus 1 bare ground. '14/3' means 3 insulated conductors plus 1 bare ground — four conductors total in the cable.

Can Romex cable be used outdoors or underground?

No. NM-B (Romex) cable is not rated for outdoor exposed runs, wet locations, or direct burial. Outdoor and underground circuits require UF-B (underground feeder) or conductors in conduit per NEC Article 334.10.

Related symbols

Physical breadboard, PCB, and panel mapping

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