Hello,

Are there any rules for running #10 to Junction boxes in basement right below heaters and tapping off with 12/2 to them less than 6'. I'm running 30 amp circuit for 6000 watts of baseboard heat in parallel

The rule that would apply is in the NEC NFPA 70 Section 210.19 (A) (4) Exception #1 (c), limiting this tap to 18 inches.Careful placement of the junction boxes should get you close enough.