The plural morpheme in English is regularly represented by the allomorphs [s], [z] and [ɨz]. The rule is as follows:
(i) if the noun stem ends in a voiceless consonant, the allomorph used is [s], as in kicks [kɪks];
(ii) if the noun stem ends in a voiced phoneme (including a vowel), the allomorphed used is [z] (as in cats [kæts]);
(iii) if the noun stem ends with a sibilant ([s], [z] etc.), the allomorph used is [ɨz] (as in sizes [saizɨz]).
Lyons, John. 1968. Introduction to Theoretical Linguistics. Cambridge: Cambridge University Press.