In this paper, a methodology to calculate the shielding effectiveness (SE) of enclosures with slots at low frequency in the near magnetic field is presented. This modeling technique is based on the magnetic moments and the transmission line (TL) approach and makes specific assumptions. It is applied to a slot of small size relative to the dimensions of the enclosure. The slot is electrically short. This technique is validated in the case of a real enclosure with numerical simulations and experimental data. This method is dedicated to the case of mechatronics boxes using in electric and hybrid vehicles (EHVs), it allows obtaining the results with sufficient accuracy precision for dimensioning a magnetic shielding in a short time.