Loss measurements were made at 60 Hz on 6‐m lengths of 1‐ and 3‐cm‐diam superconducting niobium tubes used as the inner conductor of a coaxial line. These tubes carried transport currents up to 1700 and 5300 A, respectively. The losses were found to fit the relationP=fAEc1(H/Hc1)n,wherefis the frequency,Ais the surface area,Ec1is the energy loss per unit surface area per cycle atHc1, the lower critical magnetic field, andHis the peak magnetic field at the surface of the superconductor. The exponentnequals 3 forH≤Hc1andn≥4 forH≥Hc1. The ac losses showed a 25% linear increase with temperature from 4.2 to 5 K. For the case of an eccentric coaxial line, where the current and magnetic field vary about the circumference of the superconductor, the ac losses were found to agree with those predicted from calculated values of the current and magnetic field distributions. In particular, when the peak surface field is less thanHc1andRis the ratio of the maximum to minimum surface magnetic fields about the circumference of the superconductor, the losses were a factor (3R2+2R+3)/4Rgreater than those found at an equivalent current for a concentric configuration.