On calculuses of fractions

Can anyone supply a reference to the fact that if you add to the hypotheses of a calculus of right fractions the assumption that if {s_i: X_i --> Y_i} is a family of arrows, all in Sigma, then so is \prod s_i: \prod X_i --> \prod Y_i, then you can conclude that if the original category has all limits, so does the fraction category and the canonical functor to the fraction category preserves them. Michael Barr +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++