Is 12B a transferrable skill into energy and construction jobs?

Corps of Engineers is one of the most blue collar regiments of the Army. 12B focuses more on the combat aspects of being an engineer, but you still learn valuable skills such as mechanics for example. Jobs like 12K (plumber), 12N (heavy equipment operator), and 12R (electrician) are some of the more technical skilled trade jobs. They also offer certifications for welding and carpentry. 12B isn't a bad gig, but I've sat in equipment digging fighting positions while they had to dig them by hand with E-tools. Just some perspective.

You have some oh, what I like to call, conflicting illusions.
You want a transferable skill, you currently have a transferable skill. Your current MOS is a job which will translate to a comparable civilian job.
The job that you want to go into requires apprenticeship, and outside of doing those actual jobs as an MOS, the skill set will not transfer into the job that you want.
If your goal is to attend an apprenticeship then why would you waste time to reclass and spend another three years in the army?
Start researching trade schools, save some money, and have your new career ready to go when you transition.

12B is not that kind of engineer. 12B is landmines, wire, explosives, and (in the mechanized world) some special vehicular equipment. It's infantry with a picket pounder and a demo kit.

BUT, there ARE some engineer MOSes that are closer to what you're thinking of. Go here: https://www.hrc.army.mil/content/CMF%2012%20Professional%20Career%20Map
Then click on the link for Construction Engineer and look through that MOS listing.