I find that when transfecting large plasmids, it’s important to keep in mind the molar mass of DNA. For example you would need 3ug of a 12kb plasmid to match the molar mass of 1ug of a 4kb plasmid. This can have big effect on the result, as theoretically you need equal plasmid copy number to match expression potential.
Also, viafect seems to work best for larger plasmids.
Thank you for your input. I actually tried Jet prime with upto a 1:10 ratio with 1ug plasmid for RPE,HEK cells (toxicity was 70%). I did get TE in RPE cells. But nothing in HEK cells.
Generally, jet prime shouldn't be so toxic to HEK cells. What are you trying to transfect? if it is an ion channel try using a specific blocker post-transfection.