HTS code 8547.10.40.00 covers "Insulating fittings for electrical machines, appliances or equipment, being fittings wholly of insulating material apart from any minor components of metal (for example, threaded sockets) incorporated during molding solely for the purposes of assembly, other than insulators of heading 8546; electrical conduit tubing and joints therefor, of base metal lined with insulating material › Insulating fittings of ceramics › Ceramic insulators to be used in the production of spark plugs for natural gas-fueled, stationary, internal combustion engines" under the US Harmonized Tariff Schedule. This is the official duty rate charged by US Customs when this product is imported into the United States, sourced directly from the U.S. International Trade Commission and refreshed daily.