On any structure devoted in whole or in part to any nonconforming use, work may be done in any period of 12 consecutive months on ordinary repairs, or on repair or replacement of nonbearing walls, fixtures, wiring or plumbing, to an extent not exceeding 10% of the current replacement value of the structure; provided that the cubical content of the structure, as it existed at the time of passage of this chapter, shall not be increased.