Thank you for pointing out my error. After applying the Chemistry Code approach to chemical SYMBOLS (C is *not an abbreviation for carbon, it is the chemical SYMBOL for carbon), Section 11.d of the Nemeth Code is what to follow for C-14: no ELI, no NI. Section 9.f is followed for carbon-14 since carbon is a word: yes contract "ar," yes NI. Here is the path of Nemeth Code rules to follow when searching for hyphen rules:
NC §9.b. The numeric indicator must be used after a punctuation mark. However, the hyphen requires special attention (see section f below). [4 examples]
NC §9.f. The numeric indicator must be used after a hyphen when the hyphen follows a word, an abbreviation, or a mark of punctuation. However, also see §11d. [6 examples]
NC §11.d. The numeric indicator must not be used after a hyphen if the hyphen follows a numeral, a letter, or other mathematical expression. [3 examples]
I will delete my original answer so future readers are not misled.