Reference · Electrical · NEMA MG 1 · IEC 60085 · NEC 310.4

Insulation — Rating Chart, Motor Classes & Cable Types

A printable insulation rating chart covering the two questions that drive every electrical specification: which motor insulation class (A, E, B, F, H, N, R, S, C — NEMA MG 1 / IEC 60085) is right for the application, and which cable insulation material (PVC, XLPE, EPR, silicone, fluoropolymer) belongs in the duct, conduit, or motor lead. Includes the 90 °C / 130 °C / 155 °C / 180 °C temperature ladder, NEC Table 310.4 conductor types, and worked examples for sizing copper conductors with the right insulation. Reviewed by a licensed PE.

Figure 1 — Wire cross-section with NEC type letters T, H, W, N and 60/75/90 deg C temperature ratings

Wire size + insulation type calculator

The embedded calculator picks the conductor AWG and matching insulation type from NEC Table 310.4 — including the temperature column (60 °C / 75 °C / 90 °C) that governs ampacity. Enter your load and the wet/dry environment; the tool returns the smallest THWN-2, XHHW-2, or USE-2 conductor that meets ampacity and voltage drop. For motor insulation class selection, see the temperature step in the worked example below.

CALC.003 Wire Size · NEC 310.16 · 6 presets · ampacity + VD

Application[App]

NEC 210.19(A) recommends ≤3% VD on branch, ≤5% combined feeder + branch.

Material[Mat]

Size standard[Std]

Insulation rating[°C]

Ambient temp[Ta]

°C

Load current[I]

or power[P]

System voltage[V]

Power factor[cos φ]

cos φ

Length one-way[L]

VD limit[%]

Conductors in raceway[N]

Parallel sets[Par]

Continuous load (3+ hrs at full current → ×1.25 NEC 210.19)

Recommended size

12 AWG

Both ampacity and voltage drop pass with margin.

0%3%6%

Voltage drop: — V (—%)
Ampacity (derated): —
Required ampacity: —
Recommended OCPD: —
Min EGC (NEC 250.122): —
Power loss in run: — W
V at load: — V

PASS · NEC 310.16 + 210.19(A)

A_min = max( A_vd , A_ampacity ) · NEC 310.16 NEC 240.4(D) · 250.122 · 310.15(B)

Insulation life and dielectric formulas

Eq. 01 — Arrhenius / Montsinger 10 °C rule SI

L_T = L_0 \cdot 2^{-(T - T_0)/10}

·: L = expected insulation life at temperature T; L₀ = rated life at T₀.
·: Empirical: every 10 °C above the rated hot-spot halves the time to thermal breakdown.
·: Used to predict winding life under overload — applies to Class A through H polymer systems.

Eq. 02 — Dielectric strength margin (BIL) SI

BIL \geq 2.5 \cdot V_{LL,max}

·: BIL = Basic Insulation Level (kV peak), per IEEE Std 1.
·: V_LL,max = maximum line-to-line voltage including 10 % over-voltage tolerance.
·: For 480 V system: BIL ≥ 2.5 × 528 ≈ 1.3 kV — easily met by 600 V class wiring.

Eq. 03 — Total motor winding temperature SI

T_{winding} = T_{amb} + \Delta T_{rise} + T_{hot-spot}

·: T_amb = ambient (40 °C is the NEMA MG 1 reference).
·: ΔT_rise = temperature rise above ambient at full load (depends on cooling class).
·: T_hot-spot = additional gradient inside the winding (typically 5–15 °C).
·: Result must not exceed the insulation class limit (130 / 155 / 180 °C).

Standards governing insulation

Two independent ladders apply: the NEMA / IEC thermal class ladder for rotating machines, and the NEC conductor type table for fixed wiring. Both reference the same underlying material chemistry but use different naming conventions.

Standard	Scope	Region
NEMA MG 1-2021	Motors and generators — insulation class definitions, temperature rise limits	North America
IEC 60085 ed. 4	Electrical insulation — thermal evaluation and designation (= NEMA classes)	Worldwide
IEEE Std 1-2019	General principles for temperature limits in rating of electrical equipment	USA
NEC / NFPA 70 — Table 310.4	Allowable conductor insulation types and operating temperatures	USA
UL 83 / UL 4703	Thermoplastic-insulated wires and cables (UL listing for THHN, THWN-2, etc.)	USA
UL 44	Thermoset-insulated wires and cables (XHHW-2, RHH/RHW-2)	USA
IEC 60502	Power cables 1–35 kV — XLPE and EPR insulation systems	Worldwide
IEEE Std 522	Testing of form-wound stator coil insulation for motors	USA
BS 7211 (UK)	Low-smoke halogen-free (LSHF) insulated cables	United Kingdom

Insulation rating chart — motor classes (NEMA MG 1 / IEC 60085)

The insulation class and temperature ladder for rotating machines. NEMA insulation class letters match IEC 60085 designations exactly. The "rise" column is the maximum allowed temperature increase above 40 °C ambient at rated load (per NEMA MG 1-2021).

Class	Hot-spot max (°C)	Rise above 40 °C amb (K)	Typical materials	Common application
A	105	60	Cotton, cellulose, paper, varnish	Obsolete in modern industrial; small toys, vintage motors
E	120	75	Polyester enamel, polyvinyl-formal varnish	Small Euro fractional-HP motors (often replaced by B)
B	130	80	Mica, fiberglass, polyester-imide enamel	1960s–1990s industrial; small modern HVAC fan motors
F	155	105	Polyimide enamel, Nomex®, epoxy-glass laminate, VPI epoxy	Today's NEMA Premium / IE3 / IE4 industrial standard
H	180	125	Aramid (Nomex®), mica-glass, silicone resin	Oilfield, traction, foundry, submersible, VFD-fed
N	200	145	Polyimide (Pyre-ML), mica-Nomex, silicone	High-temperature traction, aerospace
R	220	165	Polyimide film (Kapton®), mica-glass, silicone	High-temp aerospace, military
S	240	185	Mica-glass, ceramic-bonded, polyimide	Specialty aerospace, downhole drilling
C	> 240	> 185	Mica, glass, ceramic, PTFE (Teflon®)	Furnaces, jet engines, nuclear

NEC conductor insulation types — Table 310.4 abridged

The cable insulation material reference for NEC fixed wiring. "T" = thermoplastic, "X" = thermoset (cross-linked), "H" = high-heat, "W" = wet location, "N" = nylon outer jacket. A c copper line insulation for HVAC condenser feeders is typically THWN-2 or XHHW-2 in conduit.

Type	Insulation	Max op temp	Wet location	Thermoplastic / Thermoset	Common use
TW	PVC	60 °C	Yes	Thermoplastic	Branch wiring (largely superseded by THHN)
THW	PVC + nylon	75 °C wet / 90 °C dry	Yes	Thermoplastic	Service entrance, feeder
THHN	PVC + nylon	90 °C dry only	No	Thermoplastic	Most common branch and feeder wire (in conduit)
THWN-2	PVC + nylon	90 °C wet/dry	Yes	Thermoplastic	Modern default — replaces THHN + THWN
XHHW-2	XLPE (cross-linked PE)	90 °C wet/dry	Yes	Thermoset	Industrial feeder, marine, harsh environment
USE-2	XLPE	90 °C wet	Yes (direct burial)	Thermoset	Underground service entrance, direct buried
RHH / RHW-2	EPR (ethylene-propylene rubber)	90 °C wet/dry	Yes	Thermoset	Heavy industrial, mining, oil & gas
MTW	PVC + nylon	90 °C wet	Yes	Thermoplastic	Machine-tool wiring (NFPA 79)
SF-2 / SFF-2	Silicone rubber	200 °C	Dry only	Thermoset	Fixture wire, high-temp appliance
FEPB	FEP / FEP-Glass	200 °C	Dry	Thermoplastic	Plenum-rated, high-temp appliance
MV-105	EPR or XLPE	105 °C	Yes	Thermoset	Medium-voltage power cable 5–35 kV

Identify the worst-case operating temperature. Add the ambient (typically 30–40 °C for indoor industrial), the temperature rise of the conductor at full load, and any hot-spot allowance. For a 480 V motor running in a 40 °C foundry: ambient 40 °C + winding rise 80 °C + hot-spot 10 °C = 130 °C → minimum Class B insulation (130 °C). For PVC-insulated branch wiring at 30 °C ambient with 50 °C rise: 80 °C → THW (75 °C) is marginal — pick THWN-2 (90 °C).
Pick the insulation class — NEMA / IEC. NEMA MG 1 (= IEC 60085) defines six standard classes by maximum hot-spot temperature: A 105 °C, E 120 °C, B 130 °C, F 155 °C, H 180 °C, N 200 °C, R 220 °C, S 240 °C, C above 240 °C. Most modern industrial motors are Class F insulation with Class B temperature rise (a 25 °C safety margin).
Choose between thermoplastic and thermoset for cables. Thermoplastic insulations (PVC, polyethylene PE, polypropylene) soften and re-form on heating — easier to extrude, lower cost, lower temperature ceiling (typically ≤ 90 °C). Thermoset insulations (XLPE, EPR, silicone rubber) cross-link permanently during cure and resist softening — higher temperature limit (90–250 °C), better short-circuit withstand. NEC THHN/THWN-2 = thermoplastic; XHHW-2 = thermoset (XLPE).
Verify dielectric strength margin. Insulation must withstand line-to-ground voltage plus surge transients (typically 2.5 × Vline-line per IEEE Std 1). For a 480 V system: 480 × 2.5 = 1 200 V minimum BIL (Basic Insulation Level). Standard 600 V class building wire (THHN/XHHW-2) easily covers this with 1 500 V tested dielectric.
Pick a NEC conductor insulation type that matches. NEC Table 310.4 lists every approved conductor insulation: TW (60 °C, wet/dry), THW (75 °C wet), THWN-2 (90 °C wet/dry), XHHW-2 (90 °C wet/dry, thermoset), USE-2 (90 °C wet, direct burial), MTW (90 °C wet, machine-tool wiring), SF-1/SF-2 (200 °C silicone fixture wire), MV (medium-voltage XLPE up to 35 kV).

Worked example — 75 HP motor in a 40 °C foundry

Specify the insulation system for a 480 V, 3-phase, 75 HP TEFC induction motor with a 1.15 service factor running continuously in a 40 °C ambient foundry:

Step 1 — temperature budget: NEMA reference ambient is 40 °C. Service-factor 1.15 motors are allowed an additional 10 K rise per NEMA MG 1 § 12.43, so design winding rise = 105 K + 10 K = 115 K. Hot-spot allowance 10 K.

Step 2 — total winding temperature: 40 + 115 + 10 = 165 °C. Class F (155 °C) is insufficient. Need Class H insulation (180 °C), leaving 15 °C margin.

Step 3 — material specification: order a Class H system — Nomex® / mica-glass slot liner, Kapton® polyimide enamel on magnet wire, silicone or epoxy-H VPI ground-wall impregnation. Spec line: "Class H insulation system per NEMA MG 1 § 1.66.4 with Class F (105 K) rise per nameplate; Inverter Duty per NEMA MG 1 Part 31".

Step 4 — motor leads: from motor terminal box to VFD — 600 V class XHHW-2 (XLPE thermoset, 90 °C wet/dry) in rigid metal conduit. For VFD applications: use shielded VFD cable (XLPE inner with copper-tape shield + outer jacket) to control bearing currents.

Step 5 — feeder cable: 75 HP @ 480 V draws ~ 96 A FLA × 1.25 (continuous) = 120 A. Per NEC 310.16 75 °C Cu column → #1 AWG THWN-2 (130 A rated). For the foundry environment with rooftop conduit reaching 60 °C: derate to 90 °C column with adjustment factor 0.71 → #1/0 THWN-2 (170 × 0.71 = 121 A).

Thermoplastic vs thermoset — when to use which

The fundamental cable-insulation decision: thermoplastic insulation (PVC, PE, PP, FEP) softens reversibly when heated; thermoset insulation (XLPE, EPR, silicone) cross-links permanently during cure and resists softening up to its decomposition temperature. The trade-off:

Property	Thermoplastic (PVC / PE)	Thermoset (XLPE / EPR)
Max continuous temp	60–105 °C	90–250 °C
Short-circuit temp limit	160 °C (PVC) / 250 °C (PE)	250 °C (XLPE/EPR)
Behavior on heating	Softens, can be remelted	Stays solid until decomposition
Cost	Lower (simpler manufacturing)	Higher (cure step required)
Ozone / UV resistance	Moderate (PVC poor unjacketed)	Excellent (EPR especially)
Recyclability	Easier (re-extrudable)	Harder (chemical recycling only)
Examples	THHN, THWN-2, T, TW, MTW, FEP	XHHW-2, USE-2, RHH/RHW-2, MV-90

Modern industrial practice favors thermoset (XHHW-2, EPR, silicone) for harsh environments, VFD applications, and any installation where short-circuit temperature withstand or service life under thermal cycling matters. Thermoplastic (THHN, THWN-2) remains dominant in commercial / residential because of cost.

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