Elastic guided waves have some potential for non-destructive inspection of civil engineering multi-wire steel cables. However, wave propagation inside such structures is not yet fully understood. This paper investigates multi-wire helical waveguides with special attention to the common seven-wire strand configuration (one straight core surrounded by one layer of six helical wires). A helical coordinate system is first proposed. Though non-orthogonal, this system preserves translational invariance along the helix centreline to explicitly perform a spatial Fourier transform. Then, it is shown that for the analysis of multi-wire helical strands a twisting system - which is a special case of helical systems - is translationally invariant. A semi-analytical finite element method is developed reducing the problem on the cross-section only. A straightforward computation of energy velocity is proposed. Dispersion curves for a single straight wire and a helical wire are first computed to verify the adequacy of the twisting system. Finally the seven-wire strand is analysed using simplified contact conditions. Theoretical dispersion curves are compared to low-frequency magnetostrictive measurements. Good agreement is found for the first compressional-like mode and its associated veering central frequency ("notch frequency").