Watch your amps

Since my December 2017 DCC Corner article on power supplies I have been getting followup questions so let’s take a closer look. First, let me remind you that I said you need to keep track of the total amperage of all the toys you plug in to your layout room electrical circuit. Given that most household circuits have breakers rated at 15 amps that gives you something to stay under. However you also need to remember that 15 amp value is a non-continuous rating, the continuous load should not exceed 12 amps for more than 3 hours. And that’s where the questions started.

Most loads in our layouts are non-continuous. For example, even if you have an 8 amp power supply it doesn’t pull that much current all the time. Matter of fact, the new high efficiency switching power supplies draw practically none. However things like lights, transformers powering Tortoises, and other electric accessories are on all the time. So adding up your amperage load is a balancing game, that is you have to add up the continuous load and then assume some kind of on-time values for your non-continuous devices.

Things brings up the topic of how to find out how much each device draws. In most cases each device will have a label on it somewhere that will tell you its amperage at a given input voltage. If you have a small refrigerator it may draw 5 amps but that is only when the compressor is running. Otherwise its draw may be minimal. In the case of power supplies it will usually tell you the maximum AC input voltage/amperage and the DC output voltage/amperage. The switching power supplies for my 8 amp boosters pull about 2.5 amps at 110/120 VAC and put out 8 amps at 15 VDC. So even if my booster is running at max capacity it only uses 2.5 amps of house current.

That was another question I got, how can you put in only 2.5 amps and get out 8 amps? There are a couple answers to that. One basic principle of transformers is the power in has to equal the power out ( minus a little loss for various reasons). So if you are putting in 110 V at 2.5 amps (2.5 x 110=275) then you can easily get 15V at 8 amps (8 x 15=120).

Transformers are not as efficient as switching power supplies so you lose some in the process. Most switching power supplies now are in the 85-95% efficiency range and have a lot more going on than just dropping the voltage. Even old fashioned AC to DC transformers have a rectifier, capacitor and in many cases a voltage regulator to suck up some juice. The best example is that big transformer on the utility pole outside your house. These take thousands of volts AC at low amperage and convert it to 110/120 VAC at 100-200 amps for your household circuits. At any rate this is not magic and you can get a higher amperage while putting in a lower amount. The important thing is give some thought to what you are plugging in to your outlets and be safe.


  • After reading this column, I am curious about what power supply I can use with my NCE throttle. I am currently using a CVP throttle with their 5 amp.power supply. I purchased an NCE throttle and plan to replace my CVP setup with NCE. Can I use the 5 amp. CVP power supply with the NCE setup? Also, I need more than a 5 amp supply…will a switching power supply work with my new NCE setup? switching supplies are a lot less expensive than buying a supply from NCE.
    I read your column every month and find it very informative.


  • I am using an old (around 12 or 13 years) original Digitrax, I think 5 amp. Power supply. Never a problem. Would it be beneficial to replace it with one of the new “switching” power supplies? I do not need more capacity to run trains, just thinking of saving a few dinero in the long run, and maybe a cooler running P.S.
    Also, I have quite a few wall transformers running street and building lights, many led lights etc. would replace all these wall transformers with a “switching” power supply be beneficial?

  • in electrical circuit applications a continuous load has it’s determined overload protection device ( fuse or circuit breaker protection).located at the beginning of the electrical circuit . A dedicated branch circuit has its branch circuit rating amperage requirements determined by the wire gauge amperage ratting to individual branch conductors . This means a continuous electrical circuit having unbroken conductors from the overload protection to the required ending device( receptacle or switch to the load) The eliminations of splices makes this individual circuit reliable. When you purchase a transformer the better transformer should have fuses or circuit breaker overload protection on the supply side of the electrical feed and the transformer load output. If you don’t have fuse protection on either side of the input voltage and output voltage and a short circuit occurs sorry to say your hard earned money goes up in smoke! To install overload protection is not hard as it looks. Just remember that the transformer has a 120 volt fuse rating and the output overload device has a fuse rating to the output supply voltage .output. input voltage household supply a fuse rating of 125 volts and output fuse rating to transformer supply output.There are many capable devices you can use to install a overload protection safely and dependable at hardware supply’s,electrical supply’s and electronic supply’s and hobby shops. Don’t forget the web.

    • Mike—I should have defined my terms. By a continuous load I mean one that is drawing the stated amperage all the time such as the overhead lights in the layout room. By non-continuous I mean one that is cycling on and off or going from a low to high then low draw at irregular intervals like a small refrigerator or a power supply for a command station or booster.

  • Just a caution,that grey transformer can supply a lot more than 200 amps. They feed many houses,not just yours.

  • Thank-you Larry for this post. It is very helpful.