It is technically possible to use a 15% cream, but you have to consider that you are actually using way more water in your ganache and that would impact seriously in the shelf life.
What problem are you experiencing? Maybe there is another way to solve it.
Thank you for you answer i did not think about what you said but of course you're right. Some of my ganache have a too high fat content and it breaks or there is a deposit if fat at the bottom though i followed the recipe or the proportions of half the weight of the chocolate . And it is specially annoying with my caramel ganache interieur with a pinch of salt. I will appreciate any insight from more experienced chocolatiers thank you
Well proportions depend on the type of chocolate and how much cocoa butter it has, what other ingredients you use with water content, shelf life you want and how hard you want it to set.
e.g. For molded bonbons I've used up to 1/1.5 heavy cream to chocolate for milk chocolate 45% with success (and even 1.4 for dark 70%) with 3 weeks shelf life, using invert sugar to lower water activity. Of course for dipped confections It would've been impossible to do the same. You just need to get a very good understanding of the theory behind the ganache system.
If you don't want to add more cream you can indeed use cream with less fat content and try to counter water activity with inver sugar instead of cane partially or even totally (provided its not a lot). Also remember that the texture of the ganache will change too with less dairy fat in it.