Is it ever correct to use dependant instead of dependent?

The simple answer to the question of when dependant should be preferred to dependent is—for the American writer, anyway—maybe never. But that's only the simple answer.

In British English, dependant tends to be used for the noun, as in "a person's spouse and dependants," while dependent is the usual choice for the adjective, as in "a person's spouse and dependent children." In American English, dependent typically does both jobs.

This wasn't always the case: dependant is the older of the pair. The word is derived from French dépendant, which was borrowed into English during the Middle Ages with two different meanings: a literal one, "hanging down" (dépendant is the present participle of the French verb dépendre, meaning "to hang down”) and an extended one, “determined or conditioned by another.”

For two centuries, the English adjective was frequently spelled with final -ant or -aunt. But in the 16th century, the spelling of the word began to shift toward the -ent ending, influenced by the Latin form of the word, dēpendēnt. It is this Latinized spelling of the adjective that is predominately found in American and British English today.

The French-derived dependant, however, still has life across the pond—as a noun. The noun form is newer; it dates to the early 16th century, around the time that the spelling of the word was shifting. Why the -ant spelling for the noun had more staying power in British English is unknown. As the Oxford English Dictionary notes, it was possibly influenced by other nouns, such as defendant and assistant.

In summary, dependant can be used for the noun in either British or American English, but dependent for either noun or adjective is a safe choice in American English.