Ectopic focal excitations due to abnormal calcium (Ca) cycling at the cellular level are believed to underlie cardiac arrhythmias. The mechanisms linking subcellular Ca and focal excitations remain elusive though. This work employs a detailed model of spontaneous Ca release (SCR) to study the formation of Ca-mediated triggered activity in tissue. Due to the stochastic nature of SCR, hundreds of simulations were performed to build statistics regarding frequency, first latency and location of the first ectopic beats. It could be shown that Ca-overload as well as the strength of electrotonic coupling between cells clearly influence the occurrence of ectopic foci. Cable-like structures resembling those in the Purkinje system are more prone to Ca-mediated foci. Further, foci were not evenly distributed, rather they clustered at tissue boundaries.