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How To Wire A Long Reverse Loop


Long reverse loops and associated wiring may create inductance that will cause an auto reverser to fail. Larry Maier drafted a wiring solution in response to a user's problems.


I received the PSX-AR reverser that you sent to me and checked it out. It works correctly. The software is Rev D. The latest release is Rev E, but the only difference is the ability to do nested reverse loops (i.e. a crossover in a reverse loop).

I bought two 100 foot lengths of zip cord and wired up a reverser similar to the reverse loop you described to me. It did not work. In fact, it did not work even on the lowest current setting. I measured the short circuit loop current. There was enough inductance to limit the current to well below an amp. Remember, this only happens when the reverser is called on to reverse because the current travels in one wire group and out the other. Normal operation should not see this effect if parallel wires are used.

Larry Maier


  1. Long lengths of wire or track on a layout can have large values of series inductance.
  2. Series inductance acts in a manner to inhibit the flow of the DCC current (similar to too much resistance).
  3. Sufficiently high values of series inductance (long wiring runs) can prevent the proper operation of breakers and reversers because the series inductance limits the DCC current to a value below the trip setting even with a hard short circuit. It can also distort the DCC waveform causing erroneous decoder operation.
  4. The effects of series inductance can be limited on long wiring runs by tightly bundling the feed and return wires together. Lamp cord and speaker wire are two examples of two wires bundled tightly together. Individual wires can also be bundled together by physically grouping them correctly and then binding them with a cable tie every 4 inches. The smaller the spaces between the wires the better.
  5. Reverse loops are a special case since the wires must be tightly bundled together for both polarities of the reverser. See the drawing for the proper method of wiring a long reverse loop.
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