Glass Phenolic Specifications — Sizes, Standards & Tolerances

Glass phenolic laminates — NEMA G3, G5, G7, G9, and G11 — are produced and sold to dimensional and performance standards maintained by NEMA, ASTM, MIL-spec, and IEC. This page consolidates the standards references, standard stock sizes and tolerances for sheet, rod, and tube, and the test methods used to qualify each grade. Use this page alongside the glass phenolic grades guide to write procurement specifications.

At a glance:

• Primary U.S. standard: NEMA LI-1 (defines grades, test methods, and minimum property values)
• Military equivalent: MIL-I-24768 (used in defense and naval procurement)
• International standard: IEC 60893 series (European and global supply chains)
• Sheet stocked from 0.015 in. to 4.0 in. thick; rod from 1/4 in. to 6 in. diameter
• Tube: custom inner and outer diameters; standard wall from 1/8 in. to 2 in.
• Thickness tolerance on sheet: ±5% or ±0.010 in., whichever is greater (NEMA standard)
• Flatness: ≤0.005 in. per foot for standard press-cured sheet

Governing Standards

NEMA LI-1

NEMA LI-1, Industrial Laminated Thermosetting Products, is the primary specification for glass phenolic laminates in U.S. commercial and industrial markets. The standard:

• Defines five glass-fabric grade designations: G3, G5, G7, G9, G11
• Specifies minimum mechanical property values (flatwise and edgewise flexural, tensile, shear, and compressive strength)
• Specifies minimum electrical property values (dielectric strength, arc resistance, volume and surface resistivity)
• Defines conditioning procedures (A = as-received, C = 96h humidity conditioning, D = immersion conditioning)
• Sets dimensional tolerances for sheet, rod, and tube

NEMA LI-1 does not regulate flammability — UL 94 or IEC 60695-11 testing must be separately specified and certified by the laminate manufacturer. Not all glass phenolic formulations automatically achieve UL 94 V-0; confirm with the manufacturer before specifying a UL 94 rating in a purchasing document.

MIL-I-24768

MIL-I-24768, Insulation, Electrical, Laminate, Thermosetting (current revision: MIL-I-24768/12 for glass phenolic types), covers military and naval procurement. The grade designations differ from NEMA:

MIL-I-24768 adds requirements not in NEMA LI-1, including lot traceability, first-article testing, and in some procurement items, qualification approval from the cognizant engineering activity. For commercial projects, NEMA LI-1 is the simpler and faster path.

ASTM D709

ASTM D709, Standard Specification for Laminated Thermosetting Materials, is a broader standard covering all laminated thermosetting plastics (paper, glass, canvas fabric reinforcements). D709 references D709 property tables for each grade, and many purchasing documents cite both NEMA LI-1 and ASTM D709 together.

IEC 60893

IEC 60893, Industrial Rigid Laminated Sheets Based on Thermosetting Resins for Electrical Purposes, is the international standard used in European and global supply chains. The IEC designations use HGW prefix codes:

Note that IEC designations do not map one-to-one to NEMA grades in every case; confirm the specific test values in the product data sheet when substituting IEC-designated material for NEMA-specified material.

Sheet Dimensions and Tolerances

Standard Sheet Sizes

Glass phenolic sheet is pressed in large-format panels and then cut to standard sizes. The most common commercial sheet sizes are:

Sheet Thickness Tolerances

Per NEMA LI-1:

Tighter tolerances (±1–2%) are achievable from some laminate manufacturers by specifying a "close-tolerance" press run, but this adds cost and lead time. For machined parts where the sheet thickness sets a critical dimension, machine the surface after purchase rather than relying on laminate thickness tolerance alone.

Sheet Flatness

Standard press-cured sheet: ≤0.005 in. per foot. Sheet thicker than 1.0 in. is less prone to warpage after cutting; thin sheet (under 1/8 in.) may relax after cutting from a large panel and show bow up to 0.015–0.020 in. per foot. Clamp thin sheet during machining or stress-relieve by storing flat with even support.

Rod Dimensions and Tolerances

Glass phenolic rod is produced by a continuous pulling or batch molding process. Standard stock rod runs from 1/4 in. to 6 in. in diameter.

Rod larger than 3 in. diameter is sometimes described as "billet" and is pressed in cylindrical molds rather than pulled continuously; delivery lead times on large-diameter rod are typically 4–8 weeks.

Tube Dimensions and Tolerances

Glass phenolic tube is wound from pre-preg glass fabric on a mandrel, then oven-cured and extracted. The inside diameter is set by the mandrel; outside diameter is determined by the number of plies wound.

Custom tube dimensions (non-standard ID, OD, or wall) require a minimum order quantity and laminate winding tooling. Most tube suppliers require 2–6 weeks lead time for custom wound configurations. G7 silicone phenolic tube for transformer applications is commonly produced to customer drawings for specific transformer designs; see the G7 silicone phenolic hub for further detail.

Material Certification and Traceability

Certificates of Compliance

Standard commercial glass phenolic is shipped with a Certificate of Compliance (C of C) stating the NEMA grade, lot number, manufacturer, and date of manufacture. The C of C certifies that the lot was produced to the NEMA LI-1 requirements for the stated grade.

Material Test Reports

For applications requiring test data, a Material Test Report (MTR) may be requested from the laminate manufacturer. MTRs include actual measured values (not minimums) for key properties such as flatwise flexural strength, dielectric strength, and water absorption from the specific lot. MTRs are standard practice in power utility and defense procurement.

NEMA Grade Marking

Per NEMA LI-1, sheet panels must be marked with the NEMA grade designation on at least one edge. Rod and tube are marked with a color-coded stripe or labeled end cap. Unmarked material should not be accepted in safety-critical applications — confirm marking with the supplier at order time.

Test Methods Referenced in NEMA LI-1

Conditioning before test is specified in NEMA LI-1 Condition A (as-received), C (96h at 90% RH), or D (24h water immersion). Property values cited in the properties guide are Condition A unless otherwise noted.

Procurement Tips

• Always specify NEMA grade and NEMA LI-1 revision in purchase orders — do not specify by color alone (natural vs. green tint is not grade-differentiating).
• For mil-spec procurement, reference the appropriate MIL-I-24768 type designation.
• Verify UL 94 V-0 flammability independently if the end application requires a UL listing — not all glass phenolic formulations from all manufacturers carry V-0 certification.
• Request MTRs for dielectric strength and arc resistance on safety-critical lots.
• For dimensions outside standard stock, discuss lead times early — large-diameter rod and custom tube may be 4–8 weeks.

See the G10 and FR4 vs. glass phenolic comparison if you are evaluating whether a G10 or G11 specification better fits your requirements.

Standard NEMA G3, G5, G7, G9, and G11 sheet, rod, and tube ship from stock. Certificates of Compliance provided with every order; MTRs available on request.

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