I'm wiring a 240v 1500w heater and I ran a #12 wire because the circuit is also going to double as a A/C circuit for the summer. Anyhow I followed the wiring diagram on heater and wired to a 2pole 15amp and I turned on breaker and it popped so I pulled apart the heater and looked it all over again and the same happened. If anyone has a suggestion I would truly appreciate it!

Sounds like you have a short somewhere. With the breaker off, disconnect the heater. Measure the resistance between the 2 terminals of the heater. Should be well above zero. You can calculate it. Check each hot wire to ground. Should be open. Also check one terminal of the heater to ground, again, should be open. Also check one terminal of the heater to ground, again, should be open.