“The New Covenant is taken either broadly or strictly.
V The New covenant is also taken in a twofold manner either broadly, inasmuch as it stands for the covenant of grace in general made with sinners , which existed under the Old Testament as well before Christ appeared as under the New after he had been manifested; or strictly, for the covenant of grace promulgated after the manifestation of Christ in the flesh, which should continue to the end of the World”
Francis Turretin, Institutes of Elenctic Theology Vol 2, pg 234
In essence, the C of G and NC are interchangeable
More here: http://www.semperreformanda.com/2013/10/the-westminster-standards-use-covenant-of-grace-and-new-covenant-interchangeably-2/