Glass Phenolic Grades — NEMA G3, G5, G7, G9, G11 Compared

The NEMA glass phenolic family contains five distinct grades, each defined by a different resin system bonded to woven E-glass fabric. All five grades share the same glass reinforcement and general fabrication process, but the resin choice determines operating temperature, arc behavior, moisture uptake, and cost. This guide examines each grade individually, then provides a direct property comparison to support grade selection.

At a glance:

• G3: baseline phenolic resin, general-purpose electrical insulation to 250°F
• G5: melamine resin, improved arc resistance over G3 at the same temperature
• G7: silicone resin, the only grade rated to 425°F continuous
• G9: melamine resin with superior arc and track resistance; highest CTI in the family
• G11: epoxy resin, the high-Tg upgrade path from G10 and FR4; best combined mechanical and electrical properties
• All grades are available in sheet, rod, and tube; consult the specifications page for stocked sizes

How NEMA Grade Designations Work

NEMA Standard LI-1 Industrial Laminated Thermosetting Products assigns two-letter or alphanumeric codes to laminate grades based on reinforcement type and resin system. For glass-fabric laminates, the designation begins with "G":

The number within the designation also carries a historical meaning related to original Army-Navy (AN) and MIL-spec classifications, but for practical purchasing and engineering work, the resin system is the differentiating variable.

NEMA G3 — Phenolic Resin, Glass Fabric

G3 is the foundational glass phenolic grade. It was commercialized in the 1940s and remains in production because it delivers reliable electrical insulation and moderate mechanical performance at the lowest cost in the glass phenolic family.

Resin System

Phenolic (phenol-formaldehyde) resin cures by condensation polymerization, releasing water vapor during the press cycle. The resulting thermoset is hard, brittle compared to epoxy, and begins to oxidize and discolor at sustained temperatures above 250°F. This oxidation does not immediately compromise insulation — phenolic char is a moderate insulator — but it reduces mechanical properties over time.

Key Characteristics

• Continuous-use temperature: 250°F (121°C)
• Flatwise flexural strength: 40,000–50,000 psi
• Dielectric strength: 300–450 V/mil (short-time, 1/16 in.)
• Arc resistance (ASTM D495): 60–90 seconds
• Lowest cost in the glass phenolic family
• Can absorb up to 0.25% water in 24 hours; avoid in high-humidity service without protective finish

When to Specify G3

Use G3 when the service temperature is below 250°F, the application involves simple electrical insulation without arc exposure, and cost is a priority. Typical applications: MCC mounting panels, relay subpanels, distribution panel board backing, and structural spacers in switchgear below 15 kV.

NEMA G5 — Melamine Resin, Glass Fabric

G5 substitutes melamine-formaldehyde resin for phenolic resin while keeping the same woven glass reinforcement. The melamine system changes the arc behavior substantially: when subjected to an arc, melamine resin produces a white, non-conductive char rather than the dark, moderately conductive carbon residue of phenolic.

Resin System

Melamine-formaldehyde is a triazine-based thermosetting resin. The nitrogen content (~67% melamine by weight in the repeat unit) means that arc-induced decomposition releases nitrogen oxides rather than carbon-rich volatiles. The resulting track is less likely to support continued arc propagation along the surface.

Key Characteristics

• Continuous-use temperature: 250°F (121°C) — same as G3
• Flatwise flexural strength: 40,000–50,000 psi
• Dielectric strength: 300–450 V/mil
• Arc resistance (ASTM D495): 100–120 seconds — 30–60 s improvement over G3
• Moderate CTI: 300–400 V
• Slightly lower water absorption than G3 (0.10–0.20%)

When to Specify G5

Choose G5 over G3 whenever the equipment may experience arc events during normal or fault operation — load interrupter switches, open fuse mounts, or disconnect switches where the arc contacts the insulating structure. G5 is also preferred when CTI requirements exceed what G3 can meet.

NEMA G7 — Silicone Resin, Glass Fabric

G7 is the high-temperature outlier in the glass phenolic family. Its silicone resin system is chemically unrelated to phenolic or melamine resins and provides a continuous-use rating of 425°F (218°C) — 175°F above the other organic-resin grades. No other glass-fabric laminate in the NEMA LI-1 standard achieves this temperature without switching to a ceramic or specialty resin.

Resin System

Silicone polymers (polysiloxanes) consist of alternating Si–O backbone bonds with organic side groups. The Si–O bond is thermally stable well above 300°C; organic degradation of the side groups begins above 250°C but proceeds slowly. The result is a resin that maintains flexibility and adhesion to the glass fiber at temperatures where phenolic, melamine, and standard epoxy resins have already carbonized.

Key Characteristics

• Continuous-use temperature: 425°F (218°C) — highest in the glass phenolic family
• Flatwise flexural strength: 38,000–48,000 psi (lower than G11 due to silicone's lower modulus)
• Dissipation factor (1 MHz): 0.006–0.015 — the lowest in the family
• Dielectric strength: 250–400 V/mil
• No distinct glass transition temperature
• Cost: highest in the family (50–100% premium over G3)
• Available in silicone phenolic material hub with detailed temperature data

When to Specify G7

Specify G7 wherever the continuous service temperature exceeds 285°F or the application requires retention of dielectric properties through repeated thermal cycling above 200°C. Key applications: dry-type transformer coil forms and winding mandrels (Class H/C), high-voltage condenser bushing tubes, coil support flanges in induction heating equipment.

NEMA G9 — Melamine Resin, Arc/Track-Resistant Specification

G9 uses the same melamine resin system as G5 but is produced to a higher arc and track resistance specification. While G5 achieves 100–120 seconds arc resistance, G9 is consistently tested and certified above 120 seconds. G9 also carries higher CTI (Comparative Tracking Index) values, making it the grade of choice wherever tracking along the insulator surface under contamination (moisture + conductive deposits) is a failure mode.

Resin System

The resin chemistry is melamine-formaldehyde, the same as G5. The higher performance in G9 comes from tighter raw material control, optimized resin-to-glass ratio, and in some formulations, addition of mineral fillers or surface treatments that improve arc extinction. Pressed plate finishing may also differ.

Key Characteristics

• Continuous-use temperature: 250°F (121°C)
• Flatwise flexural strength: 42,000–52,000 psi
• Arc resistance (ASTM D495): >120 seconds (minimum certified value)
• CTI: 400–600 V — the highest in the glass phenolic family
• Dielectric strength: 300–500 V/mil
• Volume resistivity: 10¹⁰–10¹² Ω·cm
• Available in melamine glass phenolic hub

When to Specify G9

G9 is the primary choice for arc chute barriers, de-ionizing plates, exhaust horns, and any surface exposed to repeated arc events in switchgear rated 5–38 kV. It is also specified when IEC 60947 or UL 1066 test standards require CTI values above 400 V, or when contamination (humidity, carbon deposits) on the surface makes tracking resistance critical.

NEMA G11 — Epoxy Resin, Glass Fabric

G11 is the glass phenolic grade that most directly competes with G10 and FR4. Both use woven E-glass reinforcement and epoxy resin, but G11 is manufactured to a higher glass transition temperature (Tg ~165–175°C vs. ~130°C for standard G10) and must pass NEMA LI-1's elevated-temperature mechanical retention tests. G11 does not require flame retardant additives — unlike the "FR" (flame-retardant) designation in G10 and FR4 — but individual formulations may carry UL 94 V-0 ratings.

Resin System

Epoxy resins offer better fiber-matrix adhesion than phenolic or melamine systems, which translates to higher interlaminar shear strength and better retention of flatwise mechanical properties in humid conditions. The higher Tg resin used in G11 is typically a multifunctional epoxy (tetraglycidyl or novolac-based) cured with a high-temperature hardener such as a dicyandiamide or aromatic amine system.

Key Characteristics

• Continuous-use temperature: 285°F (140°C)
• Flatwise flexural strength: 58,000–65,000 psi — the highest in the glass phenolic family
• Dielectric strength: 400–600 V/mil — also the highest
• Water absorption (24h): 0.05–0.10%
• Flexural modulus: 3.0–3.5 × 10⁶ psi
• Arc resistance (ASTM D495): 60–100 seconds (lower than G5/G9 due to carbon-forming epoxy char)
• CTI: 175–250 V (lower than G5/G9 for same reason)

When to Specify G11

Specify G11 when the application needs the best combination of mechanical and electrical properties in a single material, and temperature is below 285°F continuous. G11 is the correct grade for structural insulators, high-voltage bus support brackets, standoff columns, and condenser bushing tubes operating in moderate-temperature environments. Use G7 when temperature exceeds 285°F.

Full Grade Comparison Table

Choosing Between G9 and G11

The most common grade-selection question is G9 versus G11: when does superior arc resistance (G9) outweigh superior mechanical and dielectric strength (G11)?

Use G9 when:

• The part will be exposed to arc events (switching arcs, fault arcs, partial discharge on the surface)
• Tracking resistance is a specified requirement (CTI >400 V)
• Temperature is below 250°F

Use G11 when:

• The part is a structural load-bearing insulator
• Dielectric strength per unit thickness is critical (e.g., thin-wall bushing tube)
• Temperature is between 250°F and 285°F
• The application is wet or humid and moisture uptake must be minimized

For temperatures above 285°F, neither grade applies — specify G7.

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