Quick Answer
Insulating washers are non-conductive washers CNC-machined from NEMA-grade epoxy fiberglass laminates (G10, FR4, G11, FR5). They electrically isolate bolts from frames, busbars, and panels, and they block galvanic corrosion between dissimilar metals. IncomePultrusion machines custom insulating washers, spacers, and sleeves to your drawing — standard bolt sizes M3–M24 (#4–1 in) or any custom ID × OD × thickness, quoted within 24 hours.
Custom Insulating Washers, Machined to Your Drawing
Metal washers are the default in most assemblies — until the assembly carries current, mixes metals, or sits outdoors. At that point a metal washer becomes a short-circuit path or a corrosion cell, and an insulating washer becomes a design requirement rather than an option.
IncomePultrusion supplies custom insulating washers machined from certified NEMA-grade laminate sheets sourced from qualified mills. We do not sell off-the-shelf catalog washers; every batch is produced to the buyer’s drawing or sample, which means any ID, OD, thickness, and quantity can be matched without tooling investment. Prototypes are available for validation before production quantities.
Because the base material is a flat laminate sheet, both flat washers and profiled parts — shoulder washers, spacers, sleeves, and standoffs — can be produced in the same batch and from the same certified material lot.
Material Grades: G10, FR4, G11, and FR5
All four grades are woven glass fabric bonded with epoxy resin. The differences are flame retardancy and temperature class. Choose by the assembly’s electrical code and operating temperature, not by habit:
| Grade | Flame Retardant | Typical Continuous-Use Temperature | Choose It When |
|---|---|---|---|
| G10 | No | ~130 °C (266 °F), NEMA Class B | General electrical isolation; no flame rating specified |
| FR4 | Yes (UL94 V-0 class) | ~130 °C (266 °F), NEMA Class B | Flame retardancy is specified by code or customer standard |
| G11 | No | ~155–180 °C (307–356 °F), high-Tg epoxy | High temperature insulating washers for hot assemblies |
| FR5 | Yes | ~155 °C (311 °F), high-Tg epoxy | High temperature and flame retardancy are both required |
Specific dielectric strength, glass transition temperature, and flame rating vary by manufacturer and laminate batch. For applications governed by standards or safety codes, request the material datasheet with your quote. For a deeper grade-by-grade breakdown, see the G10 & FR4 washers page and the insulating washer materials guide.
Standard and Custom Sizes
Washers are machined to drawing, but most orders fall into standard bolt-clearance dimensions. Use this table as a starting point; any dimension can be adjusted:
| Bolt Size | Typical ID | Typical OD | Common Thickness |
|---|---|---|---|
| M3 / #4 | 3.2–3.5 mm | 7–9 mm | 0.5–1.5 mm |
| M4 / #8 | 4.3–4.5 mm | 9–12 mm | 0.5–2.0 mm |
| M5 / #10 | 5.3–5.5 mm | 10–14 mm | 0.8–2.0 mm |
| M6 / 1/4 in | 6.4–6.7 mm | 12–18 mm | 1.0–3.0 mm |
| M8 / 5/16 in | 8.4–8.7 mm | 16–24 mm | 1.0–3.0 mm |
| M10 / 3/8 in | 10.5–10.8 mm | 20–30 mm | 1.5–4.0 mm |
| M12 / 1/2 in | 13.0–13.5 mm | 24–37 mm | 2.0–5.0 mm |
| M16–M24 / 5/8–1 in | 17–25 mm | 30–56 mm | 2.0–6.0 mm |
- Tolerances: ±0.1–0.3 mm depending on diameter and thickness; tighter tolerances quoted on request.
- Thicker parts: spacers, sleeves, and standoffs above 6 mm are machined from thicker laminate or built-up stock.
- Finish: natural laminate color, clean-cut edges, free of delamination; edges lightly deburred.
Insulating Washers for Galvanic Corrosion Isolation
When two dissimilar metals touch in the presence of moisture — an aluminum frame with stainless-steel bolts, copper busbars on galvanized steel hardware — the less noble metal corrodes at the contact point. An insulating washer breaks the metal-to-metal path and stops the galvanic cell from forming.
Typical galvanic-isolation assemblies we supply:
- Aluminum enclosures and frames fastened with stainless or galvanized hardware
- Outdoor electrical cabinets, solar racking, and marine equipment exposed to condensation or salt air
- Copper or aluminum busbars mounted on steel supports, where the washer isolates both electrically and galvanically
For long-term outdoor or chemical exposure where galvanic isolation must be combined with corrosion resistance, see our pultruded options in the FRP washers guide and the corrosion-resistant FRP washer product page.
Electrical Insulating Washers: Busbars, Switchgear, Transformers
Electrical insulating washers are the highest-volume use of this product family. In switchgear, panel boards, battery racks, and motor or transformer terminals, the washer keeps live hardware from finding a ground path through the frame — and preserves creepage and clearance distances around mounting holes.
- Busbar and battery connections: isolate copper or aluminum conductors from steel bolts and racks
- Switchgear and panel boards: prevent unintended grounding paths and short circuits at mounting points
- Transformer and motor terminals: thin washers maintain spacing in compact terminal stacks
- Chassis and instrument mounting: shoulder washers isolate connector shells and panels where the design calls for a floating, non-grounded part
Where the assembly intentionally needs a ground path, an insulating washer is the wrong part — tell us which mounting points must stay isolated and which must stay bonded, and we will machine the kit accordingly.
High Temperature and Thermal Insulating Washers
Standard G10/FR4 is rated for continuous electrical service around 130 °C. Above that, the correct answer is the high-Tg grades: G11 (155–180 °C class) or, when flame retardancy is also required, FR5. High temperature insulating washers from these grades are used near motors, transformers, heating equipment, and power electronics where a standard laminate would soften or lose dielectric margin.
A note on naming: buyers occasionally ask for “FR10” or “FR11” washers. These are not NEMA laminate grades — in nearly all cases the requirement is actually FR4/G10 (standard) or G11/FR5 (high temperature). Send the operating temperature and any flame-rating requirement, and we will confirm the correct grade with a datasheet before production. Details: G11 & FR5 high-temperature laminate.
Shoulder Washers, Spacers, and Sleeves
Beyond flat washers, the same laminate can be machined into spacers, sleeves, and custom-machined G10 & FR4 parts such as:
- Shoulder (step) washers: the shoulder centers the washer in the panel hole and isolates the bolt shank from the hole wall — required when a flat washer alone leaves the bolt touching the panel
- Tubular spacers and standoffs: maintain fixed spacing between boards, panels, or busbars
- Sleeved bushings: full-length bolt isolation through thick panels
If your assembly has a through-bolt in a clearance hole and both the bolt head and the shank must be isolated, specify a shoulder washer with the hole diameter and panel thickness on the drawing. More: G10 & FR4 spacers, bushings & shoulder washers.
How to Get a Quote (24-Hour Response)
Insulating washers price is driven by material grade, sheet thickness, dimensions, tolerance, and batch quantity — there is no catalog price list because every batch is machined to order. To receive a quote within 24 hours, send:
- Material grade: G10, FR4, G11, or FR5 (or just the operating temperature and flame requirement)
- ID × OD × thickness, or a drawing / sample
- Quantity per batch and annual volume
- Tolerance requirements
- Application environment (voltage, temperature, outdoor/chemical exposure)
- Whether flat washers, shoulder washers, spacers, or a mixed kit are required
Submit these details through the quote request page. Prototypes can be machined for validation before moving to production quantities.
Frequently Asked Questions
Are insulating washers enough to provide insulation at a mounting hole?
A flat insulating washer isolates the bolt head or nut from the surface, but the bolt shank can still touch the wall of a clearance hole. For full isolation at a mounting hole, use a shoulder washer (or a shoulder washer plus flat washer stack) so the shank is sleeved as well. Creepage and clearance distances should be checked against the assembly’s voltage class.
Can insulating washers prevent galvanic corrosion?
Yes. Galvanic corrosion needs metal-to-metal contact plus moisture. A non-conductive laminate washer between the dissimilar metals breaks the contact and stops the galvanic cell. For outdoor or marine assemblies, combine the washer with isolated bolt shanks (shoulder washers or sleeves) so no incidental metal path remains.
What is the difference between G10, FR4, G11, and FR5?
All four are epoxy fiberglass laminates. FR4 is the flame-retardant version of G10; FR5 is the flame-retardant version of the high-temperature G11. Standard grades serve to about 130 °C continuous; G11/FR5 serve in the 155–180 °C class. See the grade table above and the FR4 vs G10 comparison.
Are “FR10” or “FR11” real washer materials?
No. NEMA does not define FR10 or FR11 laminate grades. Requests for “FR10” almost always mean FR4 or G10, and “FR11” usually points to the high-temperature grades G11 or FR5. If your drawing lists one of these names, confirm the actual temperature and flame requirements before ordering.
Do insulating washers affect grounding?
Yes — that is their purpose. An insulating washer removes the electrical connection between the fastener and the panel. In assemblies where a part must remain grounded, do not use insulating washers at that location; where the design calls for a floating or isolated part, use them at every metal-to-metal contact point, including the bolt shank.
Should I use flat washers or shoulder washers?
Use flat washers when only the bearing face needs isolation. Use shoulder washers when the bolt passes through a clearance hole and the shank must also be isolated from the hole wall — typical for panel jacks, instrument mounting, and galvanic isolation of through-bolts.
What size insulating washer do I need for an M8 or 1/4-inch bolt?
As a starting point, ID slightly above the bolt diameter (for example 8.4–8.7 mm for M8) and OD at 2–3 times the bolt diameter. See the size table above for standard bolt sizes from M3 to M24, or send a drawing for custom dimensions.
Can you supply both insulating washers and corrosion-resistant FRP washers?
Yes. Laminate G10/FR4/G11 washers serve electrical isolation; pultruded FRP washers serve chemical and marine corrosion resistance. Both are available from IncomePultrusion — see the FRP washer product page for the pultruded line.
Request a Quote for Insulating Washers
Send your drawing, sample, or bolt size and material requirement through the quote request page — a quotation with material datasheet support is returned within 24 hours. For material selection background, read the insulating washer materials guide; for grade details, see G10 & FR4 washers.