Whether you're working in construction, maintenance, or electrical inspection, one of the most fundamental skills is identifying electrical cables. In residential, commercial, and industrial settings, proper cable identification ensures safety, compliance with codes, and efficient maintenance. Misidentifying a cable can lead to faulty connections, overloads, or hazardous conditions.
This guide explains how to identify electrical cables in the U.S. by color, markings, insulation types, and application. From household NM-B cables to industrial THHN wires, you’ll learn how to recognize and differentiate them with confidence.
Why Cable Identification Matters
Correctly identifying electrical cables isn't just about efficiency—it’s a matter of safety. Mislabeling or misinterpreting a cable can lead to:
- Electric shock or electrocution
- Equipment damage from incorrect voltage or amperage
- Non-compliance with the National Electrical Code (NEC)
- Costly downtime during repairs or inspections
- Fire risks due to overloaded or mismatched wires
Knowing how to read the markings, understand color coding, and differentiate insulation types can prevent these issues.
Cable Types Commonly Found in U.S. Installations
Electrical cables in the U.S. fall into several categories based on their usage and insulation. These are the most common types you're likely to encounter:
1. Non-Metallic Sheathed Cable (NM or NM-B)
NM-B cable is used primarily in residential wiring. It consists of multiple conductors (usually copper), insulated and bundled within a plastic sheath.
- Common brands: Romex, Southwire
- Applications: Light fixtures, outlets, switches
- Rated for: Indoor use only
- Markings: “NM-B” along with gauge (e.g., “12/2 NM-B”), voltage (e.g., “600V”), and temperature rating
2. Armored Cable (AC) and Metal-Clad Cable (MC)
These cables are used in commercial and industrial buildings where additional mechanical protection is needed.
- AC cable includes paper-wrapped conductors inside a flexible metal jacket
- MC cable features insulated conductors in a metallic sheath, often with a ground wire
- Markings: Gauge, type (e.g., “MC”), voltage rating, temperature, and sometimes date of manufacture
- Applications: Commercial lighting, branch circuits, feeder wires
3. THHN / THWN-2 Wire
THHN (Thermoplastic High Heat-resistant Nylon-coated) and THWN (with added Water-resistant properties) wires are commonly used for conduit installations in both residential and commercial settings.
- Sold as single conductors
- Rated for dry and wet locations
- Markings: “THHN” or “THWN-2,” wire gauge, voltage rating (600V), and manufacturer
- Color-coded insulation
4. UF-B Cable (Underground Feeder)
UF-B cable is designed for underground use and damp environments. The conductors are embedded in solid plastic, providing extra protection from moisture and damage.
- Applications: Outdoor lighting, underground feeds to outbuildings
- Markings: “UF-B,” gauge, voltage, and temperature rating
- Color: Gray outer sheath
5. Service Entrance (SE) Cable
SE cable is used to bring power from a utility source into a building.
- Variants: SER (round), SEU (flat)
- Common in residential panel installations
- Markings: “SE” or “SER,” gauge, voltage, and temperature
- Color: Usually gray, with aluminum or copper conductors
How to Identify Electrical Cables by Markings
Cable manufacturers include essential information directly on the insulation or sheath. This information is required by
NEC standards and includes:
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Wire type (e.g., NM-B, THHN, MC)
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Gauge size (e.g., 12 AWG, 14 AWG)
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Voltage rating (usually 600V)
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Temperature rating (e.g., 90°C dry or wet)
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Certification marks (UL, CSA)
- Manufacturer name
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Date or batch codes (important for compliance and recall tracing)
Example:
12/2 NM-B 600V 90°C (UL) E123456 SOUTHWIRE 2023
This means a 12-gauge, 2-conductor non-metallic cable rated for 600 volts and 90°C.
Understanding Color Codes for Wires
Wire insulation colors help identify the function of each conductor in a cable. The
NEC recommends specific color codes, especially in residential and commercial buildings:
For Residential Wiring (NM-B, UF-B)
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Black or Red: Hot (line) wires
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White or Gray: Neutral
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Green or Bare Copper: Ground
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Blue or Yellow (less common): Switch legs or travelers in 3-way/4-way circuits
For Commercial or Industrial Wiring (THHN/THWN)
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Black: Hot (line 1)
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Red: Hot (line 2)
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Blue: Hot (line 3) in 3-phase
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White: Neutral
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Green: Ground
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Orange: High-leg delta (usually in 120/240V 3-phase systems)
Always check local amendments, as some states or cities may have variations or additional requirements.
Identifying Cable Gauge (AWG)
The
American Wire Gauge (AWG) system assigns a number to indicate the diameter of a wire. The smaller the number, the thicker the wire. Common sizes include:
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14 AWG: Lights and outlets (15-amp circuits)
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12 AWG: General-purpose outlets (20-amp circuits)
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10 AWG: Water heaters, air conditioners
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8 AWG and lower: Large appliances, subpanels, feeders
The gauge is printed directly on the wire or sheath. When in doubt, use a wire gauge tool to confirm.
Learn more: Understanding American Wire Gauge (AWG).
Tools for Cable Identification
A few essential tools can make identifying cables safer and more accurate:
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Cable markers: Pre-printed or write-on labels for organizing wires
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Continuity testers: Confirm connections in bundled wires
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Multimeters: Check voltage, current, or continuity for specific conductors
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Cable tie labels: Combine bundling with visual identification
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Thermal transfer labels: Permanent identification for industrial wires
Accurate labeling is especially important in environments with dense or complicated wiring systems, like control panels, switchboards, or IT infrastructure.
Tips for Field Identification
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Always de-energize the circuit before inspection or testing
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Use headlamps or flashlights in panels or crawl spaces to see markings clearly
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Clean dirty cables with a dry rag before reading the sheath
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Check junction boxes and enclosures—the cable type and gauge may be visible at entry points
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Document your findings—use a wire map or schematic to track circuits
When working with older buildings, note that color codes and cable types may not follow modern standards. Cloth-insulated or aluminum wires may require special care.
Labeling and Long-Term Identification
To reduce guesswork in the future, always label cables during installation or renovation. Use:
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Self-laminating thermal labels for durable, indoor/outdoor applications
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Write-on tags for quick field labeling
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Shrink tubing labels for permanent, tamper-resistant ID
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Color-coded cable ties to mark phases or functions
A well-labeled wiring system reduces troubleshooting time, speeds up repairs, and helps meet NEC record-keeping standards.
Learn more: A Guide to Self-Laminating Thermal Labels: Durable, Flexible, and Built to Last.
Final Thoughts
Identifying electrical cables accurately is one of the most important skills in the electrical trade. Whether you’re a licensed electrician, a maintenance technician, or a DIY enthusiast, knowing how to read cable markings, understand color codes, and select the right type of wire for the job keeps everyone safe.
Understanding the difference between NM-B, MC, THHN, UF, and SE cables—and knowing how to identify their functions, ratings, and gauge—can mean the difference between a safe, code-compliant job and a potential hazard.
Always follow
NEC standards, use proper labeling practices, and treat every unidentified cable with caution. The more confidently you identify cables, the more efficiently and safely you’ll complete any electrical project.
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