Snubbers and inductance, again

A friend of mine just saw the new NCE snubbers and asked me how many he should buy for his layout. His main power bus is abut 100’ long with an 8 amp booster so it should benefit from having snubbers. This seems to be one of those topics that keeps on coming back so let me just try and boil it down to some simple guidelines.

  1. Inductance is a property of wires carrying an AC current (DCC is a type of AC) which can lead to various issues with DCC. Symptoms include loss of control, run away trains, reduced track power in areas a long way from the booster, failure of the booster short circuit protection to work all the time, unexplained changes in CV settings, and blown decoders. If you experience any of these symptoms read on, otherwise pat yourself on the back and count yourself lucky.

  2. If your power buses are greater than 30′ long, and/or you are using boosters rated at 5 amps or greater then you might expect to see inductance related issues.

  3. If you built your layout before inductance was identified as a potential problem and you did not twist your wires, and they are longer than 30′, or you use boosters rated at 5 amps or greater, then snubbers may eliminate inductance related issues.

  4. If you build a layout and plan to use buses longer than 30′ and boosters of 5 amps and larger you can prevent inductance related issues by twisting your bus wires 2-3 times per foot, or using zip cord for power buses.

  5. If you are not having any inductance related issues then don’t worry about it.

For more information on this topic search for inductance or snubber in the search box at the top of the website page, read my DCC Corner column in the May 2015 Model Railroader, or read my book “Wiring Your Model Railroad”.


  • I have had some of the symptoms that have been described here and in a lengthy article on inductance in the February (2018) issue of the NMRA Magazine. Mostly a few decoders that lost their locomotive numbers, evidently at random. This problem seems to have improved as I’ve replaced early decoders with newer ones as I’ve upgraded to sound. However, I recently noted two locomotives with damage I suspect was caused by a short circuit that wasn’t blocked by the breaker; one with a front axle warped and “melty” looking and the other with a truck sideframe that may have overheated.
    My bus cable is standard household #12 Romex wire with a ground in the cable – the ground isn’t attached to anything though. I use a 7amp EZDCC boosters for two power districts. Others are protected by the Power Shield breakers. Some of my bus wires may be 50’ long. For each block I take #18 branches from both sides of the bus cable for each detection section, run both wires through a DPDT “kill switch”. The detector (several different designs used) is between this toggle and the track. From the detector #22 wires attach to the rails at multiple points in the block.
    HERE”S WHAT”S CONFUSING: The NMRA Magazine article says to “place the filter between any current sensing detectors and the command station/booster”. This would mean the filter would be very close to the booster since most mainline blocks are signaled. But I always thought the snubber should be at the end of the bus.
    So where should I install the RC filter?

    • First, PSX and other circuit breakers do not act fast enough to capture the voltages spikes associated with inductance. They do work with sustained shorts to prevent booster shutdowns and overheating at the point of the short. That is why somehting like a snubber is needed in addition to the circuit breaker.

      The snubber does use a little current, probably abiut 50 milliamps each. Consequentyl if it is placed no the power bus after the detector it will be seen as a load just like a light bulb, loco, or detection wheelset. So they have to be placed before the detector. In my book I showed a way to use a capacitor to eliminate this problem but I personally have never tried it. You can also find more about this on Allan Fartner’s Wiring for DCC website. On my layout I have the detectors placed on a sub-bus off the main power bus and the so I can isolate the detector and snubbers. Either try the capacitor trick or you may need to modify your wiring.

      Your bus wires are pretty long and combined with the 7 amp rating of your booster sounds like a potential inductance issue. Remember that inductance is a cumulative issue so a 10’ sub-bus dropped off the end of a 50’ power bus is 60’ total. I suggest keeping all bus runs under 30’ by using either a centrally located booter with multiple buses spreading out star-like or but using boosters distributed ariund the layout. Good luck.

  • Hi Laryy,
    I’m laying track on my top deck, so wiring and circuit breakers are no far off. I was planning to go from a centrally located switched outlet to a power distribution panel(12 zip wire). From there I will have 5 blocks with only one being about 45 feet(14 Ga) red/blk zip wire.

    I plan to use NCE EB1 circuit breakers for 4 blocks, and the DCC Specialties PSX-AR for the fifth since it includes a reverse loop for continuous running and for staging. I bought some NCE snubbers for the end of all runs, overkill maybe, but they’re cheap. Does this make sense?

  • Larry,
    In reading your post, two things come to mind. 1.Even considering the 5 guidelines offered above, is there a standard calculation that be used when deciding how many snubbers to install on your layout based on buss wire length, booster amperage, or number of feeders, etc.? 2. Is there a good method to measure the amount inductance present on your layout? Old an oscilloscope be used connected to mesaure the amount of unwanted inductance? And what that wave pattern look like?