Paper Phenolic Grades — NEMA XX, XXX, XXXP, XXXPC Compared

NEMA paper phenolic comes in four standard grades: XX, XXX, XXXP, and XXXPC. All four share the same paper-reinforced phenolic resin construction; the differences lie in paper ply selection, resin formulation, resin content, and the cure state in which the laminate is supplied. Each grade was developed to optimize a specific processing or performance characteristic — choosing incorrectly adds unnecessary cost or produces field failures. This guide maps each grade to its defining properties, test requirements, and application fit.

At a glance:

• NEMA XX: lowest cost, general-purpose electrical insulation in dry indoor environments
• NEMA XXX: superior moisture resistance; use where humidity cycling or outdoor enclosures are a factor
• NEMA XXXP: B-stage (post-formable) resin; heat to 160–180°C, bend or form, cool to lock shape permanently
• NEMA XXXPC: room-temperature die punching; optimized for terminal strips and high-volume blanked parts
• All four grades: dielectric strength 500–600 V/mil perpendicular, natural tan/brown color, 120°C continuous service
• Governed by NEMA LI-1 (Industrial Laminated Thermosetting Products standard)

How NEMA Grade Designations Work

NEMA LI-1 defines paper-base phenolic grades with an "X"-prefixed alphanumeric code: a single X indicates paper reinforcement; additional letters encode resin system and processing. Glass-base grades (G10, G11, FR4) and cotton-base grades (CE, L, C) use different prefix codes.

The grade designation is a performance specification, not a manufacturing recipe. Two laminators can produce "NEMA XXX" from different paper weights and resin formulations as long as both pass all LI-1 tests. Request actual test data, not just grade certification, for critical applications.

Grade XX — General Purpose

NEMA XX is the baseline grade: standard kraft paper plies at moderate resin content (35–45% by weight), fully cured in the press. NEMA LI-1 minimum requirements include dielectric strength 600 V/mil perpendicular at 1/16", volume resistivity 10¹¹ Ω·cm (dry), arc resistance 120 seconds, and flexural strength 15,000 psi flatwise.

Water absorption is not tightly controlled in Grade XX — typical 0.50–0.80% at 1/16" thickness. After moisture cycling, volume resistivity can drop 3–4 decades from dry values.

When to Specify XX

Use Grade XX when the part operates in a consistently dry, climate-controlled environment and cost is the primary constraint. Do not specify XX in outdoor, condensing, or high-humidity environments — the modest cost difference vs. XXX is not worth the reliability risk.

Grade XXX — Enhanced Electrical and Moisture Performance

NEMA XXX is produced with a denser paper ply — often a higher-grade alpha-cellulose or densified kraft — and a phenolic resin formulation with higher resin content (typically 45–55%) that reduces moisture uptake by filling more of the paper fiber void volume. The result is a laminate that maintains better electrical properties after moisture exposure.

Properties and Performance

NEMA LI-1 Grade XXX requirements for 1/8" specimen (tighter than XX in all electrical categories):

• Dielectric strength perpendicular (short-time): 600 V/mil minimum at 1/16"
• Volume resistivity (dry): 10¹² Ω·cm minimum (vs. 10¹¹ for XX)
• Volume resistivity (after moisture conditioning): 10¹⁰ Ω·cm minimum
• Water absorption (24 hr, 1/16"): 0.35% maximum (vs. no requirement for XX)
• Dissipation factor (60 Hz): lower than XX

The moisture conditioning requirement distinguishes XXX from XX more clearly than any other test. Grade XXX must demonstrate that volume resistivity remains above 10¹⁰ Ω·cm after the NEMA conditioning cycle — Grade XX has no such requirement.

When to Specify XXX

Use Grade XXX when:

• The enclosure or environment involves humidity above 60% RH, seasonal outdoor temperature swings, or condensation events
• The insulating panel will be stored or transported in non-climate-controlled conditions before installation
• The electrical circuit specification requires stable insulation resistance after moisture conditioning
• A reliable baseline for low-voltage switchgear qualification is required

Grade XXX costs approximately 10–20% more than Grade XX. For most terminal board and insulation panel applications, the modest price premium is worth the improved reliability margin.

Grade XXXP — Post-Formable

NEMA XXXP is fundamentally different in processing state from XX and XXX. While XX and XXX are fully cured in the press and shipped as rigid laminates, XXXP is cured to the B-stage — a partially cross-linked intermediate state where the resin is solid and dimensionally stable at room temperature but has not completed its final polymerization.

Processing Behavior

At room temperature, XXXP panels are rigid and behave like a fully cured laminate. When heated uniformly to 160–180°C the resin softens, the laminate becomes pliable, and it can be bent over a mandrel or pressed in matched tooling. Cooling below 100°C while held in the formed shape allows the resin to re-cure; the formed part is dimensionally stable and can be machined normally. Minimum forming radius is 6–10× sheet thickness. Allow 5–15 minutes at temperature depending on thickness before forming.

When to Specify XXXP

XXXP is the correct grade for:

• Formed bus bar support frames with curves or bends that would require expensive machining if made from flat stock
• Curved arc chute insulators in switching equipment
• Tubular coil bobbins formed by rolling flat sheet around a mandrel
• Custom-shaped electrical panels that must fit non-planar equipment contours
• Low-to-medium volume formed insulator production where thermoforming tooling would be uneconomic

Grade XXXPC — Cold-Punchable

NEMA XXXPC is formulated to enable die blanking and punching at room temperature without pre-heating, without delamination, and without excessive die wear. The "C" modifier in the designation indicates cold-punching capability. The paper ply weight and resin flexibility are adjusted — relative to standard XX — to allow the laminate to shear cleanly across the ply stack under die force, rather than fracturing and delaminating.

Processing Behavior

XXXPC is fully cured but uses a resin system with slightly more elasticity than XX or XXX, allowing the die punch to shear cleanly through the ply stack rather than crack the workpiece. Maximum practical punching thickness is 1/4" (6.35 mm); die clearance should be 5–8% of material thickness per side. Edge quality on punched holes is adequate for electrical insulation and assembly; for precision fit-up, drill and ream after punching.

When to Specify XXXPC

• High-volume terminal strip production (>1,000 pieces per run)
• Punched connector wafers, switch insulators, and relay insulation parts
• Any application where progressive-die or transfer-die stamping replaces machining for economic reasons
• Applications where room-temperature punching is a process requirement (no oven access or heated die equipment)

XXXPC costs slightly more than Grade XX due to the controlled formulation, but the savings in conversion cost (punching vs. machining) are typically 5–50× per piece at volume.

Grade Comparison Table

Specifying the Correct Grade in Purchase Orders

When ordering paper phenolic, include the NEMA grade designation in the purchase order line item. Generic orders for "phenolic sheet" without a grade specification risk receiving XX when XXX is needed or vice versa. The correct call-out format is:

Phenolic Laminate Sheet, NEMA LI-1 Grade [XX / XXX / XXXP / XXXPC], [thickness], [sheet size], [quantity]

For rod and tube, specify:

Phenolic Rod, NEMA LI-1 Grade XX (or XXX), [diameter], [length], [quantity]

Note that XXXP and XXXPC are primarily sheet products; rod and tube in these grades is less commonly stocked and may require minimum order quantities from primary laminators.

For full size availability and stocked dimensions, see Paper Phenolic specifications. For the electrical and mechanical property data supporting grade selection, see Paper Phenolic properties.

More related guides

Cross-cluster suggestions to help shoppers and engineers explore adjacent topics:

Applications

• Electrical Insulation

Industries

• Electrical Power

Compare to other materials

• Phenolic Paper Vs. Glass