  A network of cosmic strings would lead to gravitational waves which may be
detected by pulsar timing or future interferometers. The details of the
gravitational wave signal depend on the distribution of cosmic string loops,
which are produced by intercommutations from the scaling network of long
strings. We analyze the limits imposed by energy conservation, i.e., by the
fact that the total amount of string flowing into loops cannot exceed the
amount leaving the long strings. We show that some recent suggestions for the
cosmic string loop production rate and distribution are ruled out by these
limits. As a result, gravitational waves based on such suggestions, in
particular "model 3" used in LIGO data analysis, are not to be expected.
