Your iMac Late 2014 should be the Retina 5K. It supports UEFI, so an EFI installation is possible. If you have a 3TB Fusion drive, it requires special treatment due to FAT and Windows needs. Even though Windows supports GPT, the MSR part (in step 4) must reside in a FAT partition which cannot be after 2TB address space on disk. OS X also creates its EFI as diskNs1, where s1 is the first slice.
1. Backup OSX.
2. Create a partition large enough for the Windows ISO and add 2GB. Burn the ISO to this partition.
3. Use BCA and download the BC drivers. They are stored in /Library/Application\ Support/Bootcamp in a DMG files called WindowsSupport.dmg. Copy this file somewhere other than this directory. You will need it after Windows is installed. You will need a USB for this step even though the saved file is used. The USB can be 1-to-2 GB. It is to allow BCA to write the contents of the DMG to it.
4. Create a partition for Windows and format it as Free Space. This will be split into MSR (128MB) and MSD partition (this will become NTFS).
5. Boot your iMac from the partition in step 2, by holding the Alt key when booting up.
6. Install Windows to the partition created in step 4.
7. Install BC drivers from the DMG saved in step 3.