The short answer to the above question is . . . never.
Or at least rarely.
The longer answer is that the ampersand (&) symbol is used in some instances as the legitimate abbreviation for the word and, and is appropriate in notes, bibliographies, and tabular matter. Further, when it appears in the formal name of a company or logo, it is always appropriate. For example, AT&T, Johnson & Johnson, JPMorgan Chase & Co., and PG&E.[1] Occasionally, it is used as a space-saving or stylistic device in the title of a work, such as Nothing About Baseball Is Trivial: Essential Terms, Rules, Stats & History for Fans and Wannabe Fans.[2]
Style guides, such as The Associated Press Stylebook, expressly state that “the ampersand should not otherwise be used in place of and.”[3]
However, when you do use it, here are a few guidelines to remember: (1) If writing a sentence containing serial (or Oxford) commas, you would normally insert that comma before the and; but (2) the comma is omitted when using an ampersand; (3) when the ampersand appears in a company initialism (such as AT&T), there is no space before and after the & symbol.
Let’s again look at the above book title example: Essential Terms, Rules, Stats & History for Fans and Wannabe Fans. (Note, no comma before &). Otherwise, include the serial comma: Essential Terms, Rules, Stats, and History for Fans and Wannabe Fans.
To reiterate, the ampersand should be avoided in almost all instances of formal writing. Instead, spell it out: a-n-d.
I hope this is clear. If so, go forth & conquer! (I mean, go forth and conquer!)
[1] That’s Pacific Gas and Electric for my non-West Coast readers.
[2] How’s that for a sneaky way to slip in the title of my first book. Don’t ask me why I didn’t use an ampersand for the second and. Just . . . don’t ask. Thanks.
[3] AP Stylebook 2017, 17.
The Dean's English 