Fines and penalties mostly relating to high numbers of A&E patients is contributing to a multi-million pound deficit at NGH.
The hospital is currently overspent by £12.5m this financial year, but forecasts by finance director Phil Bradley predict that will increase to £18.5m by April.
A complex system of penalties is to blame, with NGH being penalised for huge numbers of patients through the doors of its A&E departments even though it has no control over the influx. If numbers breach a ceratin threshold, the hospital gets less money per patient.
An NGH spokeswoman said: "The number of patients who attend our A&E department has risen and they are more unwell.
"This means we need more staff and facilities available to look after them, leading to the opening of winter escalation beds.
"We are working with colleagues in the Northamptonshire Health and Care Partnership to reduce length of stay and improve discharge because it is better for our patients, enables us to admit patients more quickly and makes the best use of our resources."
Northampton General is far from alone in suffering from a large deficit - most NHS acute hospitals are failing to balance the books because of penalties.
Although the fines are funnelled back into the NHS, and trusts cannot go bust, it means hospitals are unable to spend as they would like on improvements.
However, hospital bosses are hopeful for an improvement. The tariff issue has been recognised nationally and the payment system could well be changing to remove this under-funding in 2019/20.