NEMA G7
NEMA G7 is woven glass cloth bonded with silicone resin — a thermoset laminate rated to 425°F (218°C) continuous service, making it the go-to insulating material for dry-type transformer coil forms, high-voltage bushings, and any electrical insulation application that runs too hot for phenolic, melamine, or epoxy resin systems.
TL;DR
| Property | Value |
|---|---|
| Parent material | Glass-silicone phenolic |
| Primary use | Transformer coil forms, high-voltage bushings, dry-type transformer insulation |
| Key spec | 425°F (218°C) continuous; dielectric strength ≥ 200 V/mil after heat aging |
| Standards | NEMA LI 1 (Grade G7); MIL-I-24768/17; ASTM D709 |
Chemistry & Reinforcement
G7 = woven glass cloth + silicone resin. This combination is what makes G7 unique among NEMA laminates:
- Reinforcement: Plain-weave woven E-glass cloth (7628 style or similar)
- Resin: Polydimethylsiloxane-based silicone resin (organopolysiloxane)
- Color/finish: Tan to grey; visually similar to G3 but with a slightly more flexible feel in thin sections
- Density: ~1.65–1.80 g/cc
Silicone resin's exceptional heat stability comes from its Si-O-Si backbone. Where phenolic and epoxy resins begin to oxidize and embrittle above 250–285°F, silicone retains its cross-linked network to 425°F continuous. After 20,000 hours at 200°C (392°F), G7 sheet retains measurable flexural strength and dielectric properties — the basis of the "Class H insulation" rating for which G7 is specified.
The trade-off: silicone resin is more expensive than phenolic, melamine, or epoxy, and G7's room-temperature mechanical strength (flexural ~35,000–45,000 psi) is somewhat lower than G10 or G11. G7 is chosen for thermal endurance, not room-temperature structural performance.
Key Properties
The critical G7 test is dielectric strength retention after extended heat aging — NEMA LI 1 requires G7 to meet a minimum dielectric strength value after conditioning at 200°C for 96 hours. No other NEMA laminate meets this requirement. If your application is above 285°F continuous, G7 or G7-equivalent silicone laminates are the only thermoset option in the standard NEMA system.
Typical Applications
G7 is deployed wherever other NEMA grades fail due to high operating temperature:
- Dry-type transformer coil forms — the dominant application. G7 tube wound with copper or aluminum wire forms the coil mandrel in 1-phase and 3-phase dry-type transformers rated Class H (180°C winding temperature). The silicone resin survives the continuous thermal stress of 60–80°C temperature rise above ambient.
- High-voltage bushing cores — G7 tube and rod are machined into bushing cores for medium-voltage (5–69 kV) instrument transformers and power transformers, where the bushing operates continuously at elevated temperature inside the transformer tank.
- Motor stator winding supports — Class H motor construction uses G7 slot liners, phase barriers, and end-turn supports to isolate winding conductors at high temperature.
- Furnace and oven electrical fixtures — insulating brackets, standoffs, and terminal boards inside industrial kilns and drying ovens operating at 300–400°F.
- Heating element fixtures — support rods and clamps for resistance heating elements in industrial furnaces.
- High-temperature test fixtures — electrical test rigs where the test environment itself reaches 400°F.
Standard Sizes
| Form | Common sizes |
|---|---|
| Sheet thickness | 1/16, 1/8, 3/16, 1/4, 3/8, 1/2, 3/4, 1 in. |
| Sheet panel | 24 × 36 in., 48 × 96 in. |
| Rod diameter | 1/4 to 4 in. |
| Tube | Custom OD/ID — winding tube is the primary form; very wide range available |
G7 tube is the most important form for the coil-form market. Wall thicknesses from 1/16 in. up and inner diameters from 1 in. to 24 in. or more can be produced. Standard stock focuses on smaller tubes (ID 1–6 in.); larger coil forms are custom.
Standards Reference
- NEMA LI 1 — Grade G7 is defined with elevated-temperature dielectric strength retention requirements in addition to standard mechanical tests. NEMA LI 1-2020 is current.
- MIL-I-24768/17 — Military glass-silicone specification, Type GSG (Glass-Silicone, General). QPL and lot testing required for defense programs; MIL-I-24768/17 is the direct military equivalent of NEMA G7.
- ASTM D709 — Standard specification for laminated thermosetting materials; heat-aging and dielectric test methods referenced.
- IEC 60893 — International: HGW 2082 designates glass-silicone laminates in the IEC system.
- UL — G7 is recognized as a Class H insulating material (180°C thermal rating) per UL 1446.
Comparison to Neighbor Grades
| Feature | G3 | G7 | G9 | G11 |
|---|---|---|---|---|
| Resin | Phenolic | Silicone | Melamine | Epoxy |
| Max service temp | 250°F | 425°F | 250°F | 285°F |
| Dielectric strength | ≥300 V/mil | ≥200 V/mil | ≥300 V/mil | ≥400 V/mil |
| Arc resistance | Low | Moderate | Very high | Low |
| Moisture resistance | Moderate | Good | Good | Excellent |
| Cost | Lowest | Highest | Moderate | Moderate-high |
| Key market | General insulation | High-temp thermal | Arc-resistant | High-temp structural |
Choose G7 when: operating temperature exceeds 285°F continuously, or your specification calls out Class H insulation or MIL-I-24768/17. Choose G11 for structural load-bearing at 285°F with epoxy machining quality. Choose G9 for arc-resistance priority at 250°F. Choose G3 for the lowest-cost glass-phenolic insulation at 250°F.
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Other high-temperature thermoset grades
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