I am from the impression the code only addresses fuel fired appliances right whether this is the intent or not. My two cents is where by the duct is uncovered it is necessary for being 26 gauge. If it is covered with sheetrock it doesn't matter what it truly is https://garage-attic-lift87542.ampblogs.com/the-single-best-strategy-to-use-for-auxx-lift-attic-71476515