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Cast Resin Busway (IP68) vs Sandwich Busway: Outdoor and Harsh Environment Guide

By DHYN Engineering Team  ·  August 3, 2026

Quick answer: Choose a cast resin busway for outdoor runs, wet or corrosive sites, tunnels and anywhere the duct might see standing water, because the conductors are fully encapsulated in epoxy resin and the standard rating is IP68. Choose a sandwich busway for indoor, dry locations where you want a smaller, cheaper duct with plug-in tap-off points. Most projects use sandwich indoors and cast resin for the outdoor sections.

These two products solve the same problem, moving large currents from A to B, but they are built on opposite philosophies. A sandwich busway keeps the weather out with an enclosure. A cast resin busway assumes the weather will get in and makes sure it cannot touch anything that matters. Knowing which philosophy your site needs is the whole decision.

What a cast resin busway actually is

In a cast resin busway, the copper or aluminum conductors are cast solid inside a block of epoxy resin mixed with mineral filler. There is no air gap, no ventilation path, nothing for moisture or dust to travel along. The result reaches IP68 as standard, meaning continuous immersion under conditions stated by the manufacturer, not just rain and spray. We covered what the IP digits mean in our IP54 vs IP65 vs IP68 guide; the short version is that IP68 is the only rating that takes submersion seriously. Like sandwich ducts, cast resin types are type-tested under IEC 61439-6.

A sandwich bus duct, by contrast, stacks insulated bars flat inside a metal housing. Compact, cool-running, easy to fit with tap-off outlets. But the enclosure is a raincoat, not a submarine hull, and the joints are only as good as the gaskets the installer torqued that day.

Head to head comparison

	Sandwich busway	Cast resin busway
Protection rating	IP54 standard, IP65 with sealed joints	IP68 standard, immersion rated
Environment	Indoor, dry, clean	Outdoor, wet, dusty, corrosive, coastal
Size for same rating	Smaller	Larger and noticeably heavier
Heat dissipation	Good; housing acts as heatsink	Slower through resin; check derating
Tap-off outlets	Easy, plug-in boxes along the run	Rare; mostly point-to-point feeder duty
Site joints	Bolted joint packs, torque checked	Factory sealed or site cast with resin kits
Typical ratings	250A to 6300A	Commonly 400A to 5000A
Price	Baseline	Often 1.5 to 2 times higher per meter

When cast resin is the right call

Outdoor links between buildings are the classic case. A duct on a trestle or bracket sees rain, sun, condensation cycling and the odd bird nest, year after year. Resin does not care. Tunnels and basements with groundwater risk are the second case; an IP68 duct shrugs off the flood that ends a sandwich run. Third, corrosive atmospheres: wastewater treatment plants, fertilizer and chemical works, coastal sites with salt air. Chlorides and acid vapors eat plated copper through any gasket eventually. They do not eat through solid epoxy. Fourth, direct burial and duct-bank routes, where the run literally sits in wet soil. No metal-enclosed product belongs there.

There is a fire angle too. Good cast resin systems use halogen-free, flame-retardant epoxy, and because there is no internal air, fire has nothing to feed on inside the duct. It is not a substitute for a true fire-rated circuit, but it removes one failure mode.

When sandwich is the better answer

Indoors, in dry conditions, sandwich wins almost every time. It is smaller, so it fits the shaft that was sized too small anyway. It is cheaper per meter at every rating we quote. It takes plug-in tap-off boxes every half meter or so, which cast resin mostly cannot do, so for high-rise risers and workshop distribution there is no contest. And if a section is ever damaged, a sandwich duct can be opened, inspected and repaired; a resin block cannot. For the indoor 90 percent of a typical project, sandwich is the sensible default, and you only pay the resin premium on the outdoor meters that need it.

Installation notes specific to cast resin

Plan the lifting before the truck arrives. Resin sections run heavy, often double the weight of the sandwich equivalent, and long runs may need a crane rather than a scissor lift. Respect the thermal expansion fittings the manufacturer specifies; a solid resin block still expands, and a rigid 60-meter run with no expansion provision will push its end flanges around. Site joints must be made exactly per the manual, with the resin or sealing kit fully cured before backfilling or flooding is even possible. Rushing a joint cure is how an IP68 product becomes an IP40 story.

What to ask the manufacturer for

Five documents separate a real cast resin product from marketing talk: the IEC 61439-6 type test reports, the IP68 test report stating the immersion depth and duration actually tested, the resin material datasheet showing halogen-free and flame-retardant classification, the short-circuit withstand rating, and the derating table for ambient temperature, because resin sheds heat slower and a 45°C Middle East rooftop is not a 25°C test lab. A supplier who can send all five within a day has done this before. Tell us the route, the environment and the rating you need, and we will tell you honestly which parts of it should be cast resin and which parts should stay sandwich.