Is zakat obligatory on an amount of money in a current account that has reached the nisab, given that there are car loan installments due to the bank, and how is it calculated if it is obligatory?

If the debt encompasses all the money or reduces it below the nisab, and there is nothing to pay off the debt except for that money, then zakat is not obligatory upon you. The evidence for this is what 'Uthman ibn 'Affan, may Allah be pleased with him, said: "This is the month of your zakat, so whoever owes a debt, let him pay his debt, and then pay zakat on the remainder of his wealth."