yeah, I know. That is my quick and dirty that I give to people to make sure they always end up on the safe side. Kinda like the "add up all you fuse ratings method...) Nothing like having your power cable turn into a pile of copper slag from running too small of a cable. Also I assume that since he will probably be running bridged 4-ohm mono, the real calc would endup being somewhere between 1400w and 1600w depending on how efficeint the amp is etc. This would put him somewhere just below 20 amps. I would say bump it up one power notch too to be safe since we all know that what we get out put is really only 65-85% of the power drawn since some is wasted and turned into heat due to amp inefficentcy (sp?) Since we don't have all the details of the amp speaker combo and run length, it is hard to really say, but at least the calc and chart are available. Good luck with the system. With that much power, it should pound.
