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There have been a few discussions about the addition of a plug and cord to a boiler (or furnace) to allow for easy connection to emergency power (generator or battery/inverter). There was much worry about safety, and of course, code compliance. This is what I did (see photo):
Single oulet dedicated 15 A circuit.
14/3 SJOW cord about 2 ft long.
Kept service switch and MC cable pigtail to boiler.