I'm putting a new down flow air handler in my house and am trying to get the wiring done asap. Originally had a 10kw heat kit which had a set of two breakers and electrician said would require a 6/2 run. Now I'm thinking of bumping up to 15kw or 20kw. If I go 20kw would I likely just do a second 6/2 run from the panel and the heat kit would have 4 breakers? Also wondering if standard procedure is to run NM-B through crawl space to junction box on joist under unit then come up through floor with THHN in conduit and into furnace?