This paper proposes a new method of determining whether a country is likely to be able to sustain its current account deficits without defaulting on its debt. It is based on the notion of the national intertemporal budget constraint (IBC) which is derived from a forward solution of the balance payments identity. Although the problem is formally similar to that considered in the literature on the government IBC, a more general solution is obtained by allowing the primary deficit to be endogenous instead of exogenous. It is shown that the existence of negative feedback from net indebtedness to the primary deficit - a wealth effect - is sufficient. This method supplants the ad hoc approach previously adopted in the current account literature. Whilst many of the same variables appear, a formal structure for analysing them is provided.