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, 90°C: the most common residential branch wiring.
- 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 (NEC change); older cable is all-white regardless of size - look for the AWG marking 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
Per NEC Table 310.16 (75°C copper):
- 14 AWG = 15A (NM-B)
- 12 AWG = 20A (NM-B + THHN)
- 10 AWG = 30A
- 8 AWG = 50A
- 6 AWG = 65A
- 4 AWG = 85A
- 2 AWG = 115A
- 1 AWG = 130A
- 1/0 AWG = 150A
- 2/0 AWG = 175A
- 3/0 AWG = 200A
- 4/0 AWG = 230A
Aluminum conductors are sized larger for the same ampacity (rough rule: aluminum needs 2 sizes larger than copper for equivalent current).
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:
- Look for jacket markings (may be faded; clean carefully with damp cloth)
- Note the conductor count + sizes
- Compare to known cable types
- Photograph for reference / sharing with senior tech
- 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