How Thick Does Busbar Epoxy Really Need to Be?

Short answer: For engineers writing a busbar coating spec, .012"–.020" (0.3–0.5 mm) of electrical-grade epoxy is typically enough for most busbars, and many specifications call for far more than the voltage requires. What matters more than thickness is the edge: on a sharp 90° edge, even .020" (0.5 mm) on the flats can leave only about .001" (0.025 mm) on the edge, which will fail a hipot test, so Fusion always recommends specifying a radiused edge wherever coating is applied. Electrical-grade epoxies are rated in volts per thousandth of an inch, and typical manufacturer values run 800 to 1,200 V per .001 in. Set thickness from your voltage, your governing standard and a sensible safety factor, not from the idea that thicker is safer.

Fusion coats to your print and spec. Fusion Coatings, Inc. is a job shop that has applied coatings to metal parts in Robesonia, Pennsylvania, since 1969. Customers send prints, the coating spec and quantities; the product and thickness are set by the design engineer, and Fusion applies them as written. Its standard busbar process is electrical-grade thick-film epoxy dip, hipot tested after coating to the customer's specified voltage when required. The guidance on this page is for engineers while the spec is being written. See Epoxy Dip Coating for Busbars.

Edges matter more than thickness

• Coating pulls thin at sharp edges. On a sharp 90° edge, even .020" (0.5 mm) of epoxy applied on the flats can leave only about .001" (0.025 mm) on the edge.
• Thin edges fail hipot. About .001" (0.025 mm) on an edge will fail a hipot test, no matter how thick the flats are.
• Even edge-coverage grades lose film at the edge. Red 3332 (AkzoNobel Resicoat® EL HGD47QF), a grade made for electrical components, is rated for more than 50% edge coverage (ASTM D2967, Method B) at .012"–.016" (0.30–0.41 mm). In that test the edge keeps only about half of the film on the flats, and a sharp edge on a real part can keep far less.
• Specify a radiused edge on the print. Fusion Coatings always recommends a radiused edge wherever coating is applied. A radiused edge lets the coating build evenly, so a .012"–.020" (0.3–0.5 mm) film protects the whole bar instead of just the flats.
• Don't fix an edge problem with more thickness. Adding film to the flats does little for a sharp edge and brings all the drawbacks of over-specifying (below).

What "volts per thousandth" means

Dielectric strength is the voltage a coating withstands, per .001 in. of film, before it breaks down. The data sheet Fusion uses as a primer is the Corvel® 17000 Series electrical-grade epoxy sheet; Corvel® is now manufactured by AkzoNobel. It lists these typical short-time values, measured on a .010" (0.25 mm) film by ASTM D149:

The same sheet describes these epoxies as electrical grade, serviceable to Class B, 266°F (130°C), and used for electric motor cores, low- and high-voltage busbar, hermetic connectors and watt-hour meter coils. It lists cure cycles of 1–20 minutes at 375–400°F (190.6–204.4°C) and film builds from .010" to .100" (0.25–2.54 mm).

Worked example: approximate breakdown voltage by thickness

Multiply thickness in thousandths by the volts-per-thousandth value. For example, .015" is 15 thousandths, and 15 × 800 V = 12,000 V.

Simple multiplication of the .010" (0.25 mm) short-time value. Not a guaranteed breakdown voltage and not a working voltage. See the caveat below.

For comparison, the typical spec range of .012"–.020" (0.3–0.5 mm) works out on paper to roughly 9,600–16,000 V at 800 V per .001 in., and 14,400–24,000 V at 1,200 V per .001 in.

Important caveat: these numbers are not linear, and they are not working voltage

• Not linear. Dielectric strength per thousandth generally falls as film thickness increases, so a film twice as thick does not withstand twice the voltage. The table multiplies a single value measured at .010" (0.25 mm), and real breakdown voltages for thicker films are typically lower than the simple product.
• Short-time lab values are not working voltage. ASTM D149 short-time results show where the coating fails under a rapidly rising test voltage. Continuous operating voltage, transients, temperature, humidity, contamination, aging and coating defects all matter in service.
• Use a safety factor and your governing standard. Set thickness from the requirements of the standard or specification that applies to your equipment (for example UL, IEC or your customer's specification), with an appropriate safety factor. This page is a primer, not a substitute for those requirements.
• Typical, not guaranteed. The values above are typical manufacturer data, not specifications or guarantees. AkzoNobel's current product data sheets still reference these product codes, and the UL recognitions are current. If you'd like the current data sheet for a product, just ask Fusion.

Why over-specifying thickness hurts

Worth weighing while the spec is being written:

• Cost. More film means more powder and more process time on every bar.
• Cure. Heavier builds put more material through the same cure cycle (1–20 minutes at 375–400°F on the Corvel® 17000 Series sheet), so the process has less margin for getting a full, even cure.
• Edges and buildup. Thick films are harder to keep even. Material can build up at holes and slots, and extra film on the flats still doesn't protect a sharp edge (see "Edges matter more than thickness").
• Clearance and fit. Every extra thousandth on each face changes how the bar fits supports, enclosures and neighboring parts.
• Heat dissipation. Epoxy insulates thermally as well as electrically. A thicker layer makes it harder for a current-carrying bar to shed heat.
• Weight. Extra coating adds weight across every bar in an assembly.

Epoxies Fusion stocks for busbars

All four carry UL recognition. Fusion coats with the epoxy your print specifies; if it isn't one of these, Fusion quotes the cost of purchasing that product, so any customer-specified electrical epoxy is fine. Nylon is not the usual choice for busbars; electrical-grade epoxy is the industry standard.

Red 2036: AkzoNobel Interpon 100 AG111QF, current data sheet summary

Interpon 100 AG111QF ("17-4001 ECB1363A RED 203") is an electrical-grade functional epoxy for fluid-bed application. It is made by AkzoNobel Powder Coatings at 3 Commerce Drive, Reading, Pennsylvania, a short drive from Fusion in Robesonia. Typical values from the current data sheet (rev. 2, July 28, 2020):

Red 3332: AkzoNobel Resicoat® EL HGD47QF, current data sheet summary

Resicoat® EL HGD47QF ("Epoxy Powder Coating for Electrical Components", product codes 17-4005 / ECA-1283) is applied by fluid bed for high builds and can also be sprayed. It is made by AkzoNobel. Typical values from the current data sheet (rev. 2, June 23, 2023):

Gray 2770: AkzoNobel Resicoat® EL HLD17QF, current data sheet summary

Resicoat® EL HLD17QF (product codes 17-7006 / ECB-1363A) is an electrical-grade functional epoxy with a hard finish and high edge coverage, applied by electrostatic spray. It is made by AkzoNobel. The data sheet lists it for electric motor cores, low- and high-voltage busbars, hermetic connectors and watt-hour meter coils. Its recommended film of .004"–.006" (0.10–0.15 mm) is the manufacturer's range for electrostatic spray; on busbars, Fusion builds the film to the thickness on the customer's print, which for busbars is typically .012"–.020" (0.3–0.5 mm). Typical values from the current data sheet (rev. 1, Oct 22, 2020):

Typical values from the manufacturers' data sheets, not guaranteed specifications. Current data sheets for other products are available from Fusion on request.

How Fusion tests busbar coating

Customers usually specify the test voltage they need, and Fusion Coatings hipot tests busbars after coating when required. A typical example is an AC 3,700 V test at 50/60 Hz for 1 minute. Fusion Coatings, Inc. is ISO 9001:2015 certified.

Current work, for scale: Fusion is currently coating a copper busbar 4 ft × 6 in × 1/2 in thick (about 1.22 m × 152 mm × 12.7 mm).

Frequently asked questions

How thick should epoxy be on a busbar?

For engineers writing a busbar coating spec, .012"–.020" (0.3–0.5 mm) of electrical-grade epoxy, applied by thick-film dip, is typically enough for most busbars. The right figure depends on the voltage, the standard you design to and the safety margin you apply, not on making the coating as thick as possible, and edge design usually matters more than extra thickness. Fusion Coatings is a job shop: it coats to the product and thickness on your print and spec.

Why do sharp busbar edges fail hipot tests?

Coating pulls thin over a sharp 90° edge. Even when .020" (0.5 mm) is applied on the flats, a sharp edge can be left with only about .001" (0.025 mm) of coating, which will fail a hipot test. Fusion Coatings always recommends specifying a radiused edge on the print wherever coating is applied.

What does "volts per thousandth" mean?

It is dielectric strength: the voltage a coating withstands per .001 in. of thickness before it breaks down in a lab test. Typical manufacturer values for Corvel® electrical-grade epoxies range from 800 to 1,200 V per .001 in., measured at .010" (0.25 mm) by ASTM D149 short-time test. These are laboratory breakdown values, not working voltages, so engineers should apply a safety factor and follow the governing standard or customer specification, such as UL or IEC requirements.

Is a thicker epoxy coating always better on a busbar?

No. Above what the application needs, extra thickness adds cost, cure time, edge buildup, fit and clearance problems, weight, and thermal resistance that makes it harder for the bar to shed heat. Dielectric strength per thousandth also tends to drop as film thickness increases, so doubling the thickness does not double the protection.

How does Fusion test busbar epoxy coating?

Customers usually specify the test voltage they need, and Fusion Coatings hipot tests busbars after coating when required. A typical example is an AC 3,700 V test at 50/60 Hz for 1 minute.

Which electrical-grade epoxies does Fusion stock?

Fusion stocks Red 2036, AkzoNobel Interpon 100 AG111QF (product codes 17-4001 / ECB-1363A); Red 3332, AkzoNobel Resicoat® EL HGD47QF (product codes 17-4005 / ECA-1283); and Gray 2770, AkzoNobel Resicoat® EL HLD17QF (product codes 17-7006 / ECB-1363A). It also stocks 3M Scotchcast 260, applied when a print specifies it. Nylon is not the usual choice for busbars; electrical-grade epoxy is the industry standard.

What if my print calls for an epoxy you don't stock?

That's fine. Fusion Coatings coats with the electrical-grade epoxy your print specifies. If it isn't one of the products Fusion stocks, the quote includes the cost of purchasing that product, so any customer-specified electrical epoxy can be applied.

Request a quote: send your prints and coating spec

Send your prints, the coating spec (product and thickness) and quantities, plus any hipot test voltage, and Fusion will quote coating to your spec. If the specified epoxy isn't one Fusion stocks, the quote includes purchasing it. Call 610.693.5886, email pjones@fusioncoatingsinc.com, or request a quote. More on the process: Epoxy Dip Coating for Busbars.

Source note: dielectric and process values on this page come from the Corvel® 17000 Series data sheet (Corvel® is now manufactured by AkzoNobel) and the current AkzoNobel data sheets for Interpon 100 AG111QF, Resicoat® EL HGD47QF and Resicoat® EL HLD17QF, plus 3M figures for Scotchcast 260, and are typical values, not guaranteed specifications for current products. Current data sheets are available from Fusion on request.