  We explore sufficient conditions for inseparability in mixed states with a
globally conserved charge, such as a particle number. We argue that even
separable states may contain entanglement in fixed charge sectors, as long as
the state can not be separated into charge conserving components. As a witness
of symmetric inseparability we study the number entanglement (NE), $\Delta
S_m$, defined as the entropy change due to a subsystem's charge measurement.
Whenever $\Delta S_m > 0$, there exist inseparable charge sectors, having
finite (logarithmic) negativity, even when the full state is either separable
or has vanishing negativity. We demonstrate that the NE is not only a witness
of symmetric inseparability, but also an entanglement monotone. Finally, we
study the scaling of $\Delta S_m$ in thermal 1D systems combining high
temperature expansion and conformal field theory.
