Does a HELOC use the current equity you have against your mortgage or the value of your home?

Let’s say I bought a house in 2016 for 150k. I’ve paid down to 120k. I currently have 30k “in” the house. Since the housing market explosion, my house is now worth 250k.

Does the HELOC formula use the value of my mortgage or the value of the house to determine the credit line?