This sentence appears to be a technical electrical engineering standard.
It is saying that only a certain length of high-voltage cable (the exact length is not specified in the sentence) is allowed to be installed in a metal conduit when connecting a high-voltage terminal or lead of a neon transformer (with a voltage over 1000 volts) to the first neon tube. This regulation may be in place to prevent excess voltage drop or other electrical problems that could occur if too much high-voltage cable is used in the conduit.