Cable + Wire Identification Reference

Why this matters

A walk through any panel or junction box shows cables labeled by code (NM-B 14/2, THHN/THWN-2, MC, BX) + size. Knowing which is which determines safety, ampacity, install location compatibility, and code compliance. The tech who reads cable markings fluently avoids using the wrong cable for the application + flags unsafe situations. This is the field card.

Cable markings on the jacket

Manufacturers print key info every few feet along the cable jacket. Look for:

• Conductor count + size: e.g., "14/2 W/G" = 14 AWG, 2 conductors + ground
• Cable type: NM-B, NMC-B, UF-B, THHN, MC, AC, etc.
• Voltage rating: 600V (common), 300V (some)
• Manufacturer + part number
• UL listing
• Temperature rating: 60°C, 75°C, 90°C
• Material: copper (CU) OR aluminum (AL)
• Year of manufacture: sometimes present

Read the markings before disconnecting / reconnecting anything. The cable's specs determine acceptable use.

NM-B (Romex)

Non-metallic sheathed cable: the most common residential branch wiring. The jacket is marked 90°C, and that refers to the conductor insulation rating only. Its usable ampacity comes off the 60°C column per NEC 334.80. See the ampacity section below before you size anything from the jacket marking.

• White jacket: 14 AWG (15A circuits)
• Yellow jacket: 12 AWG (20A circuits)
• Orange jacket: 10 AWG (30A circuits)
• Black jacket: 8 AWG and 6 AWG (40A, 50A circuits)
• Gray jacket: 4 AWG and larger

Inside the jacket: 2 OR 3 insulated conductors + bare grounding conductor.

Color-coded jacket is post-2001, and it is a manufacturer and NEMA convention rather than an NEC requirement, so do not rely on it alone. Older cable is all-white regardless of size, and a color is not a substitute for reading the AWG marking printed on the jacket.

Use: dry interior locations behind drywall, in framed walls, exposed in unfinished basements (where allowed by code).

Don't use: outdoors, in conduit underground, in damp/wet locations, embedded in concrete.

NMC-B (corrosion-resistant Romex)

Similar to NM-B but rated for damp locations. Less common; specific applications.

UF-B (Underground Feeder)

Direct-burial cable. Solid jacket extruded around the conductors (not just a sheath like NM-B).

Use: outdoor direct burial, wet locations, exterior applications.

Identification: gray jacket typically; thicker than NM-B for same size; "UF-B" marking.

THHN / THWN

Individual conductors, not a multi-conductor cable. Each wire has its own colored insulation, used together in conduit.

• THHN: 90°C, dry locations only
• THWN: 75°C, wet locations OK
• THHN-2: 90°C in dry + wet (modern standard, often dual-rated as THHN/THWN-2)

Use: pulled through conduit (EMT, IMC, PVC, flex). Common for industrial + commercial + residential where conduit is required.

Identification: single conductor in colored insulation (no outer jacket); markings on the insulation itself.

MC (Metal-Clad Cable)

Flexible metal jacket containing color-coded THHN-style conductors.

Use: where NM-B isn't allowed (some commercial, some plenum spaces), where physical protection is needed.

Identification: spiral metal armor on the outside; visible metallic appearance.

AC (Armored Cable, "BX")

Predecessor to MC. Spiral metal armor + paper-wrapped conductors. Older homes have this; rarely installed new.

Identification: similar appearance to MC but older; check markings.

SE (Service Entrance)

Heavy cable from meter to main panel. Multiple variations:

• SEU: service entrance, unarmored
• SER: service entrance, round
• USE: underground service entrance

Use: service entrance + feeder applications.

Identification: large diameter; multiple thick conductors; specific markings.

Low-voltage cable (Class 2 / 3)

Per NEC 725. Examples:

• Bell wire (18 - 22 AWG): doorbell, HVAC thermostat, alarm
• CAT5e / CAT6: ethernet, communications
• Speaker wire: 14 - 18 AWG
• Coaxial (RG-6): TV, antenna, satellite

Identification: smaller diameter; specific application markings (CAT5e, RG-6); typically labeled.

Plenum-rated cable

Used in air-handling spaces (plenums, return-air spaces). Special low-smoke, low-toxin insulation.

Identification: "CMP" (communications, plenum), "OFNP" (optical fiber, plenum), etc.

Use: required in dropped-ceiling returns, attic plenums, mechanical chases used as air ducts. Standard cable in plenum = code violation + fire risk.

Riser-rated cable

Used between floors of buildings (less stringent than plenum). "CMR" "OFNR" markings.

Old + legacy cable types

Knob + tube wiring (pre-1940): separate hot + neutral conductors run on porcelain insulators. Not grounded. Insurance + code issues; rewire recommended.

Cloth-jacketed BX (1940s - 1950s): armored cable with cloth-insulated conductors; brittle.

Cloth-jacketed NM (early Romex predecessors): cloth + rubber insulation; brittle after decades.

Aluminum branch wiring (1965 - 1973): silver-colored aluminum conductors in NM-B-style cable; CO/ALR-rated devices required.

Federal Pacific cables: not specifically the cable but the panel - Federal Pacific Stab-Lok panels have known reliability problems.

Recognizing these alerts you to insurance, code, + safety considerations.

Conductor identification (color inside cable)

NM-B inside the jacket:

• Black: hot (line) - primary hot
• White: neutral
• Red: second hot (3-conductor cable)
• Blue / yellow: in 4-conductor + multi-conductor; per code or designer convention
• Bare or green: equipment grounding conductor

In THHN/THWN through conduit, each conductor is its own color per code conventions. See wiring color codes.

Cable sizing for ampacity

Three different numbers get mashed into one list here constantly, so keep them apart. NEC Table 310.16 has separate 60°C, 75°C, and 90°C columns for the same copper conductor, and NEC 240.4(D) then caps the breaker on small conductors independently of any of them.

Which column applies to NM-B: the 60°C one. This is the part people miss. NM-B conductors carry 90°C insulation, and the jacket says so, but NEC 334.80 requires the ampacity be taken from the 60°C column. The 90°C rating exists only for derating math; you never get to use it as the cable's ampacity. Sizing NM-B off the 75°C column overloads it.

Copper, Table 310.16:

Note the 8 AWG row, because it is the one that bites. 8 AWG NM-B is a 40 A conductor, not 50 A. The 50 A figure belongs to 8 AWG in the 75°C column and does not apply to Romex. 8 AWG NM-B on a 50 A breaker is an overloaded conductor, and it is a common find on DIY range and spa circuits. A 50 A NM-B circuit takes 6 AWG.

Separately, NEC 240.4(D) caps overcurrent protection on small copper conductors at 15 A for 14 AWG, 20 A for 12 AWG, and 30 A for 10 AWG, no matter which temperature column you started from. That is a breaker rule, not an ampacity. Derate from the table ampacity first, then apply the 240.4(D) ceiling last.

Aluminum conductors are sized larger for the same ampacity (rough rule: aluminum needs one to two sizes larger than copper for equivalent current). Derating for ambient temperature and for multiple current-carrying conductors in a raceway applies on top of all of the above. See NEC for the full tables.

Derating applies for ambient temperature + multiple conductors in conduit + cable type. See NEC for full tables.

Identifying mystery cable in older homes

Walk through an older home, see cable you can't identify:

1. Look for jacket markings (may be faded; clean carefully with damp cloth)
2. Note the conductor count + sizes
3. Compare to known cable types
4. Photograph for reference / sharing with senior tech
5. Look up similar cable images online if unsure

When in doubt + cable supports a circuit, treat it as suspect (could be undersized OR wrong-rated for application) + recommend electrician evaluation.

Common service issues + cable

Customer DIY work using NM-B outdoors / in conduit: code violation; replace with proper cable.

References

• NEC Article 310 (Conductors for General Wiring)
• NEC Article 334 (Nonmetallic-Sheathed Cable Type NM)
• NEC Article 338 (Service-Entrance Cable Types SE + USE)
• NEC Tables 310.15(B) and 310.16 (ampacity)
• UL 83, UL 1581 (cable standards)
• Manufacturer technical data (Southwire, Encore, Cerrowire)
• Manuall internal: Wiring Color Codes, Wire Termination Types, Electrical Box Sizing