Conductor Types Decoded
The two-to-five letters stamped on a wire's jacket tell you everything: temperature rating, wet vs dry, oil resistance, where it's allowed. Decode the letters once and you'll never specify the wrong wire again.
The Insulation Letter Code
Every conductor type code is built from a small alphabet of letters. Each letter encodes one property. Stack them in order and you've described the wire.
| Letter | Meaning | Example in code |
|---|---|---|
| T | Thermoplastic insulation (typically PVC) | THHN, THWN |
| H | Heat-resistant — 75°C rating | THWN |
| HH | Higher heat resistance — 90°C rating | THHN |
| W | Wet-location rated | THWN, XHHW |
| N | Nylon outer jacket (oil + abrasion resistance) | THN, THWN |
| X | Cross-linked polyethylene (XLPE) insulation | XHHW, XHHW-2 |
| -2 | 90°C wet AND dry (the "-2" denotes wet-location 90°C, vs. -W which is wet-only 75°C) | THWN-2, XHHW-2 |
| R | Rubber insulation (older types) | RHH, RHW |
| U | Underground service entrance rated | USE-2, UF |
| SE | Service entrance cable | SEU (round), SER (round) |
| MV- | Medium voltage cable (followed by temp rating: 90 or 105) | MV-105 (105°C) |
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How to read THWN-2
- T = thermoplastic (PVC) insulation
- H+W = heat-resistant 75°C, wet-location rated
- N = nylon outer jacket
- -2 = upgraded to 90°C in both wet and dry locations
→ THWN-2 = PVC + nylon, 90°C wet/dry, the workhorse of modern commercial & industrial wiring.
The Conductor Types You'll Actually See
There are dozens of NEC-recognized types. In real practice you specify maybe ten of them, ever. These are the ones.
| Type | Insulation | Temp dry / wet | Where used | Where NOT to use |
|---|---|---|---|---|
| THWN-2 | PVC + nylon | 90°C / 90°C | Most common. Conduit-wired branches and feeders, indoors and out, wet or dry. Default for new construction. | Not for direct burial; not for cable tray (use TC); not for free-air without conduit |
| THHN | PVC + nylon | 90°C / — | Dry locations only. Often replaced by THWN-2 (better rating, similar cost). | Wet, damp, exterior, underground |
| XHHW-2 | XLPE (cross-linked) | 90°C / 90°C | Premium feeder/branch wire. Better insulation toughness than PVC. Often used for service entrance and large feeders. | Slightly more expensive than THWN-2; usually no functional difference for indoor work |
| USE-2 | XLPE | 90°C / 90°C wet | Direct-burial service entrance. Underground feeders. | Some types not labeled for indoor wiring methods — check label for dual rating |
| NM-B (Romex) | PVC | 90°C / — | Residential interior wiring. Dwellings, multi-family ≤ 3 stories. | Commercial buildings (most jurisdictions); wet locations |
| MC (metal-clad) | Conductors in aluminum/steel armor | 90°C | Commercial & industrial in cable tray, exposed, or raceway-free runs. Replaces conduit-wired systems for labor savings. | Direct burial without specific MC-HL types |
| AC (BX) | Conductors in flexible armor (no separate ground) | 90°C | Older commercial wiring. Largely replaced by MC. | Wet locations; new construction generally prefers MC |
| TC-ER | Tray cable, exposed-run rated | 90°C / 90°C | Cable tray with exposed runs (NEC 392). Industrial and DC distribution. | Direct burial; check NEC 336 for limitations |
| SO / SOOW (cord) | Rubber | 90°C / 90°C | Portable equipment, drop cords, temporary connections, generators | Permanent wiring (NEC 400.8 prohibits cord as substitute for fixed wiring) |
| MV-105 | EPR or XLPE, shielded | 105°C / — | Medium voltage cable (5kV, 15kV, 35kV). Substations, MV feeders, utility-side primary. | LV applications (overkill); requires special terminations |
| SE / SER | Multiple conductors in one cable | 90°C / 90°C | Service entrance, residential. Sub-feeders inside dwellings. | Commercial service in most jurisdictions |
| UF | PVC, direct burial rated | 60°C / 60°C | Direct burial, residential outdoor branch circuits | Aerial; conduit (use THWN-2 instead); commercial direct burial (use USE-2) |
Temperature Ratings — and the Termination Trap
A conductor with 90°C insulation can carry more current than the same wire size at 75°C. But you can't always use the 90°C ampacity column — because the device the wire terminates on has its own temperature limit.
| Termination type | Max temp rating | Use which NEC 310.16 column | Common scenarios |
|---|---|---|---|
| Equipment ≤ 100A circuits | 60°C | 60°C column | Most residential breakers; small device terminals |
| Equipment > 100A circuits | 75°C | 75°C column | Commercial breakers, panelboard mains, motor terminations |
| Equipment marked 90°C | 90°C | 90°C column (rare) | Some specialty equipment; check the label, don't assume |
| NEC 110.14(C) exception | — | You may use 90°C ampacity for the derating calculation, but final allowable can't exceed the termination column | This is how 90°C wire derates more gracefully in conduit fill / high ambient cases |
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Why 90°C wire doesn't help most of the time
If you use THWN-2 (90°C rated) on a 200A breaker (75°C terminations), you must size by the 75°C column. The 90°C rating only helps when you're applying derating factors — the higher 90°C ampacity acts as a buffer before the derating drops you below the 75°C column value. For straight ampacity sizing without derating, the 75°C column governs.
Copper vs Aluminum — When Each Wins
Aluminum is roughly half the cost of copper for the same ampacity but requires larger conductor sizes (lower conductivity), specific terminations, and antioxidant compound. For large feeders, aluminum saves significant money. For branch circuits, copper is universal.
| Property | Copper | Aluminum |
|---|---|---|
| Conductivity | 1.0× (reference) | 0.61× — needs larger size for same ampacity |
| Cost (commodity) | Higher | ~50% of copper for equivalent ampacity |
| Weight | Heavy | ~1/3 of copper — easier installation on long runs |
| Termination | Direct connection acceptable | Requires AL-rated lug, anti-oxidation compound (Penetrox/Noalox), torque per spec |
| Cold-flow (creep) | Stable | Connections loosen over time if not properly torqued — reason for residential aluminum failures in 1970s |
| NEC small wire restriction | OK at #14, #12 | NEC 310.106(B) — minimum #12 for AL conductors generally; for branch circuits, #6 is the practical floor due to terminations |
| Common application | Branch circuits, all sizes; sensitive equipment | Service entrances, feeders ≥ #6, MV cable, utility distribution |
| Atlas DC1 examples | All branches, panelboards, UPS internal | Service entrance from utility (12.47 kV); some 480V feeder runs > 200ft |
Aluminum Sizing Comparison (Same Ampacity)
| Ampacity (75°C) | Copper size | Aluminum size (one step bigger) | Cost savings (rough) |
|---|---|---|---|
| 100 A | #3 AWG | #1 AWG | ~30% |
| 200 A | 3/0 AWG | 4/0 AWG | ~35% |
| 400 A | 500 kcmil | 700 kcmil | ~40% |
| 600 A | 750 kcmil (or 2×4/0) | 2×500 kcmil parallel | ~45% |
| 1000 A | 2×500 kcmil parallel | 2×800 kcmil parallel | ~45% |
Worked Example 1 — Atlas DC1 Conductor Selection Across the System
One reference facility, six different conductor types — each appropriate for its place in the system.
Example 01 · Atlas DC1 spine
Specifying conductor type for each system zone
| Atlas DC1 location | Application | Conductor specification | Why this type |
|---|---|---|---|
| Utility 12.47kV → MV switchgear | MV primary feeder | 15kV MV-105 EPR shielded, 3/c with concentric neutral, AL conductor | MV requires shielding. Aluminum economical at this size. EPR insulation for thermal toughness. |
| TX-A → 480V SWGR-A (mech room) | Transformer secondary feeder, 4000A | Multiple parallel sets of 750 kcmil Cu THWN-2 in cable tray | High ampacity, indoor, dry. THWN-2 is the workhorse. Could substitute XHHW-2. |
| SWGR-A → MCC-MR1 (chillers) | Motor MCC feeder | 3 sets of 350 kcmil Cu XHHW-2 in 4" EMT | Tougher insulation handles repeated mechanical stress; better resistance to oil/coolant in mech rooms. |
| SWGR-A → UPS-A1 | UPS feeder, 1500A | 5 sets of 750 kcmil Cu THWN-2, separate raceways | Critical load — copper for terminating quality; separate raceways prevent magnetic imbalance. |
| UPS-A1 → PDU-A1 (IT hall) | UPS output to PDU, 1500A | 5 sets of 750 kcmil Cu THWN-2 in cable tray (or TC-ER cable) | Cable tray reduces install labor 30-50% vs conduit. TC-ER cable rated for tray. |
| PDU-A1 → RPP-A1-1 (row level) | Sub-feeder, 400A | 1 set of 500 kcmil Cu THWN-2 in 3" EMT | Single set fine at 400A. EMT for indoor finished space. |
| RPP → rack PDU strip | Branch circuit, 30A | #10 AWG Cu THWN-2 in 1/2" EMT, or MC cable in tray | Standard branch wiring. MC cable for faster row turn-up. |
| Site exterior → outdoor lighting | Underground branch, 20A | #12 Cu USE-2 direct buried, or THWN-2 in PVC conduit | USE-2 direct-burial rated and saves the conduit. THWN-2 in PVC is the conduit alternative. |
| Generator paralleling cabinet | Control wiring | #14 Cu MTW or TFFN, color-coded for control circuits | MTW (Machine Tool Wire) or TFFN for tight bends inside control cabinets. |
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Why one facility uses six different conductor types
Insulation choice follows environment + ampacity + termination + install method. A facility-wide standard ("we use THWN-2 everywhere") is unrealistic — MV cables, direct-burial outdoors, control circuits, and tray installations each have specific needs. Specifying the right type per location is part of professional design.
Worked Example 2 — Residential Service + Branches
Example 02 · Alternate scale
Single-family home · 200A service · interior wiring + outdoor branches
| Location | Specification | Why |
|---|---|---|
| Utility transformer → meter base (overhead) | Triplex (USE-2) AL service drop, sized to NEC 310.12 | USE-2 weather + UV resistant; AL economical for utility-scale distribution. NEC 310.12 = "residential 83% rule". |
| Meter → main panel (interior) | 4/0 AL SER cable | SE/SER cable is the residential service entrance standard. AL for cost savings. |
| Main panel → sub-panel (laundry / garage) | 4-conductor #6 Cu (3 hot + 1 ground) NM-B (or 4-cond MC for garage) | NM-B = "Romex" for residential interior. MC required where physical protection needed. |
| Branch circuits (outlets, lighting) | #14, #12 Cu NM-B | Standard residential branch wire. Cu-only at small sizes per NEC. |
| Range, dryer (240V) | #6 (range) or #10 (dryer) Cu NM-B with separate ground | Modern residential 240V branches require 4-wire (2 hot + neutral + ground). |
| Outdoor receptacles, garage door | #12 Cu UF-B direct burial OR THWN-2 in PVC conduit | UF saves conduit cost. PVC + THWN-2 is more rigorous and easier to repair. |
| Pool / hot tub circuits | #10 or #8 Cu THWN-2 in PVC, GFCI protected | Specialized rules per NEC 680. PVC conduit (no metal in pool area). |
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NEC 310.12 — the residential 83% rule
For 100A through 400A 1φ-3W residential services only, NEC 310.12 lets you use one wire size smaller than NEC 310.16 would require. Example: 200A service with 4/0 AL (NEC 310.12) instead of 250 kcmil AL (NEC 310.16). Saves significant cost. Applies ONLY to residential service entrance.
Drill — Quick Self-Check
Work each problem mentally; reveal to check. Goal: reflex, not deliberation.
Drill 1 · Decode the letters
Decode XHHW-2:
X = XLPE. HH = 90°C. W = wet. -2 = 90° wet AND dry. XLPE insulation, 90°C wet/dry.
Premium feeder/branch wire.
Drill 2 · Termination temp
Equipment marked '75°C' rated. Wire is THWN-2 (90°C). Which ampacity column?
75°C column
NEC 110.14(C) — termination governs.
Drill 3 · Cu vs AL
Need 200 A ampacity. Cu = 3/0 AWG. AL ≈ ?
4/0 AWG (one size larger)
AL conductivity = 61% of Cu.
Drill 4 · Direct burial
Underground conductor without conduit, residential branch — type?
UF or UF-B (NEC 340)
USE-2 for service entrance. THWN-2 not direct-buryable.
Drill 5 · Residential 83% rule
200A 1φ residential service. NEC 310.16 calls for 250 kcmil AL. Per NEC 310.12, what's allowed?
4/0 AL (smaller — the 83% rule)
NEC 310.12 ONLY for residential service entrance, 1φ-3W.
If You See THIS, Think THAT
| If you see… | Think / use… |
|---|---|
| "THWN-2" called out | PVC + nylon, 90°C wet/dry. Default for new commercial/industrial conduit-wired work. |
| "THHN" only | Dry locations only — 90°C dry but no wet rating. Largely obsolete in favor of THWN-2. |
| "XHHW-2" | XLPE insulation (tougher than PVC), 90°C wet/dry. Premium choice for large feeders, MV transitions. |
| "USE-2" in residential | Direct-burial service entrance. NEC 338 — also rated as RHH/RHW-2 for indoor use when so labeled. |
| "MV-105" | Medium voltage cable, 105°C. 5/15/35 kV applications. Requires shielding + special terminations. |
| "NM-B" | Romex. Residential interior only — most jurisdictions ban from commercial buildings. |
| "MC" (metal-clad) | Conductors in metal armor. Tray-rated, cable-managed install. Replaces conduit-wired systems for labor savings. |
| "TC-ER" or "TC" | Tray cable, exposed-run rated. NEC 392 cable tray installations. |
| Termination ≤ 100A | NEC 110.14(C): 60°C ampacity column. Even if wire is 90°C-rated. |
| Termination > 100A | NEC 110.14(C): 75°C column. THWN-2 / XHHW-2 90°C rating used only for derating margin. |
| Aluminum conductor specified | Use AL-rated lugs, antioxidant compound (Penetrox/Noalox), torque per spec. NEC 110.14 enforces. |
| Direct burial without conduit | USE-2 (for service or feeder); UF-B (for residential branches). NOT THWN-2. |
| Residential service ≤ 400A 1φ | NEC 310.12 — smaller residential service conductor allowed (the "83% rule"). |
| "TC-MC" or "MC-HL" | Specialized variants: TC-MC for tray + low temp; MC-HL for hazardous (Class I Div 1) locations. |