The plural morpheme in English is regularly represented bz the allomorphs /s/, /z/ and /iz/. The rule is as follows:
(i) kicks /kiks/;
(ii) if the morph ends in one of the voiced phonemes (including the vowels), s is represented by /z/ (cats /kats/);
(iii) if the morph representing the noun morpheme with which s is combined to form the plural ends with a sibilant (/s/, /z/ etc.), s is represented by /iz/ (sazis /saiziz/).
Lyons, John. 1968. Introduction to Theoretical Linguistics. Cambridge: Cambridge University Press.