You can borrow up to 80% of the home's value minus the dollar amount of your first mortgage. For example: If your home is valued at $200,000, then 80% of your home's value is $160,000. If you still owe $90,000 on the mortgage than you would have $70,000 available to borrow for your HELOC. ($160K - $90K = $70K available to borrow.)
The minimum HELOC amount is $5,000, but the borrower can allow the HELOC to have a $0 balance.