NEMA G3

NEMA G3 is the foundational glass-fabric phenolic laminate — woven E-glass cloth impregnated with phenolic resin, pressing multiple plies to produce a rigid thermoset with substantially higher flexural strength and dielectric strength than paper or fabric phenolics, at a lower cost than the epoxy-based G10 or G11 grades.

TL;DR

Chemistry & Reinforcement

G3 is the oldest of the NEMA glass-fabric laminates, introduced alongside the NEMA LI 1 standard itself. Its construction:

• Reinforcement: Plain-weave woven E-glass fabric (7628 or equivalent style)
• Resin: Phenol-formaldehyde (phenolic)
• Color/finish: Natural tan to light green; smooth outer faces from press platens
• Density: ~1.70–1.80 g/cc

The glass reinforcement gives G3 the characteristic strength advantage over paper and fabric phenolics — E-glass has a tensile modulus of ~10 million psi, roughly 5–10× higher than cellulose. This means that under the same load, a G3 beam deflects far less than an XXX beam of equal cross-section. The phenolic resin, while not as high-performance as epoxy at elevated temperature, delivers adequate electrical insulation at up to 250°F continuous service.

Key Properties

Typical Applications

G3 glass phenolic covers the broadest application range in the glass-fabric phenolic family:

• Electrical insulation panels — mounting panels, terminal boards, and bus bar supports in switchgear and distribution panels where glass reinforcement provides structural rigidity beyond paper phenolic capability.
• Transformer structural components — core clamping bars, coil support cleats, and end frames in dry-type distribution transformers rated to 250°F winding temperature.
• Standoff insulators and bus supports — machined rod standoffs supporting bare bus bars in medium-voltage substations and motor control centers.
• Switchgear barriers and barriers — low-arc-exposure dividers between compartments where G9's premium arc resistance is not required.
• Printed circuit board substrates (legacy) — G3 was used as a PCB substrate before G10 and FR4 became dominant; some industrial control boards still specify G3 for cost reasons.
• General industrial insulation stock — the standard "glass phenolic sheet" stocked by most distributors when a customer says "glass phenolic."

Standard Sizes

G3 sheet is the most widely stocked glass phenolic form. Large-diameter rod (4–6 in.) and thick sheet (1-1/2–2 in.) may require laminator lead time.

Standards Reference

1. NEMA LI 1 — Grade G3 is defined in NEMA LI 1 with minimum mechanical and electrical requirements. NEMA LI 1-2020 is current.
2. MIL-I-24768/1 — Military glass-phenolic specification. Type GEE (Glass-fabric, Epoxy… note: MIL-I-24768/1 is actually GPO-3 adjacent; check current table; G3 maps to GPC). Verify QPL requirements with your contracting officer.
3. ASTM D709 — Standard specification for laminated thermosetting materials; test methods for all NEMA laminate grades.

Comparison to Neighbor Grades

Choose G3 when: you need the structural and electrical advantages of glass reinforcement at the lowest cost and 250°F service is sufficient. Choose G5 when arc resistance matters but temperature range remains at 250°F. Choose G7 for 425°F continuous service (silicone resin required). Choose G10 when moisture resistance and epoxy-resin machining quality are priorities.

More related guides

Other glass phenolic grades

• NEMA G5 Glass-Melamine
• NEMA G7 Silicone Resin
• NEMA G9 Arc-Resistant
• NEMA G11 High-Temp Epoxy

Glass epoxy grades

• NEMA G10 Spec Page
• NEMA FR4 Spec Page

All grades index

• NEMA Grades Directory