In this paper, we propose an efficient space-frequency coded orthogonal frequency-division multiplexing (OFDM) system for high-speed data transmission over frequency selective multi-wire power line channels. By transmitting the same data symbol over two uncoupled pair of wires and over two different carriers, with a frequency separation between carriers greater than the coherence bandwidth of the channel, the proposed scheme extracts both space and frequency diversity in the absence of channel knowledge at the transmitter. Numerical results are provided to demonstrate the significant performance improvement obtained by the proposed scheme over conventional single-wire OFDM systems when the frequency selective power line channel is corrupted by impulsive noise.