The ergodic magnetic limiter is a device designed to generate a cold boundary layer of chaotic magnetic field lines at the peripheral region of a tokamak, with the main purpose of reducing the deleterious effects of the plasma-wall interaction. In the TCABR tokamak an ergodic limiter was constructed and recently installed inside the vacuum chamber. We developed a theoretical model for the action of an ergodic magnetic limiter in a large aspect-ratio tokamak taking into account the finite width of the limiter. The theoretical results are in good agreement with measurements of the vacuum magnetic field created by the limiter. Poincaré maps of field line flow are computed to reveal the resulting magnetic field line structure due to the ergodic limiter and show that the operation of the ergodic limiter in the TCABR tokamak is feasible and results in a chaotic boundary layer for limiter currents of about 6% of the plasma current. © 2005 IOP Publishing Ltd.