Kentony Shops

Enhancing Your model railroading experience.

Calculating Power Supply Requirements in Watts

Power supply requirements for toy train layouts can be calculated in watts or in amperes.  This discussion calculates power supply requirements in watts. 

Classic Toy Trains published an excellent article about toy train transformers in their January 1997 issue.  It was written by Dennis Eichenberg with photos by William Zuback. 

The article traces the history of toy train transformers from their earliest beginnings to the electronic power packs of the modern era.  It is well worth reading.  It contains a lot of good information packed into just four pages.

Other information sources include; The Lionel Corporations Instructions for Assembling and Operating the same thing.  Here is our best shot of as we understand it.

NOTE: The calculations of watts, amps, and volts below will look very precise, but they’re really just rough estimates and should be considered as such. Thats why the calculations in amps and watts come out different and why you should always round up to be safe.

Toy Train Transformers

Toy train transformers reduce household voltage to a safe level for operating toy trains.  Toy train transformers are rated in watts.  This is a measure of the amount of electric power the transformer can take from the household line without overheating.  It is not a measure of the wattage the transformer can deliver to the layout. 

A 15 amp, 110-volt household circuit can safely supply about 1650 watts of power (15amps x 110 volts).  Plugged into that circuit, a 275-watt Lionel ZW* transformer takes 275 watts from it.

NOTE: Material we’ve reviewed indicates that up to 25 percent of a transformers rated wattage is used internally by the transformer to reduce the household voltage to a safe level for toy train use.  The internal usage is reflected in a warming of the transformer.

The above material recommends that atransformer not be loaded to more than 75 percent of its rated wattage to prevent overheating and excessive power loss over an extended period of use.  Thus, a 275 watt transformer should be loaded to not more than 200 watts (275watts x 0.75 = 206watts).

How Much is Enough?

The Classic Toy Train 1997 article and Lionels 1949 instruction booklet provp>

We’ve drawn on both in the table below, but have added estimated wattage requirements for two-motor locomotives. It appears to us that if a one-motor locomotive draws 30 to 40 watts, one with two motors ought to draw twice that amount.

Type of EquipmentRequirements
in Watts
O, O27, S and Standard Gauge Locomotives
Locomotive without whistle15-25 watts
Locomotive with whistle30-40 watts
Locomotive with two motors and whistle60-80 watts
Operating accessories & automatic track signals15 watts
6-volt lamps1.5 watts
12-volt lamp (small)2 watts
12-volt lamp (large)3 watts
18-volt lamp5 watts

Total wattage requirements depend on how many lights, accessories, track signals and locomotives will be running at any one time on your layout.  Also, locomotives and accessories that are not maintained will have higher wattage requirements than shown in the table. 

Here are some guidelines:

  1.   Include the locomotive that draws the most wattage.  If two or more locomotives will be running at the same time, include the combination that draws the most.
  2. Include operating accessories that will most likely be operating when the locomotives are running. 
  3. Include automatic track signals that will most likely be operating when the locomotives are running.
  4.  Include lamps in passenger cars, cabooses and other illuminated cars that will most likely be on the track when the locomotives are running.  Include the combination that draws the most.
  5. Include lamps in switches and switch controllers.
  6. Include lamps in streetlights, floodlight towers, non-operating accessories and buildings.

Lets Do The Math

Sample Layout
In Watts
One Motor and Whistle1 Loco30-40 watts (use 40)40
Two Motors and Whistle1 Loco60-80 watts (use 80)80
Operating Accessories3 accessories15 watts each45
Automatic Track Signals2 signals15 watts each30
Lamps (6 volts)
4 x (2 lamp Floodlights)8 lamps1.5 watts each12
2 x (2 lamp Passenger Cars)4 lamps1.5 watts each6
Lamps (12 volt small)
5 Streetlights5 lamps2 watts each10
6 x (2 lamp Floodlight towers)12 lamps2 watts each24
2 x (4 lamp Floodlight towers)8 lamps2 watts each16
Lamps (12 volt large)
14 x switches14 lamps3 watts each42
7 x switch controllers7 lamps3 watts each21
Lamps (18 volts)
1 x Caboose1 lamp5 watts each5
Estimated Total Watts331
  1. Add 25 percent to the Estimated Total Watts to account for the wattage used up internally by the transformer:
  • 331 watts x 1.25 = 414 watts
  1. Round up to the next 100 watts to cover any miscalculations and to allow for expansion:
  • 414- watts rounds up to 500 watts required for the layout
    • This 500-watt requirement can be spread over a number of transformers.  For example, one for track power to run the locomotives and lights in cars and switches and another for accessories floodlights, streetlights etc.
    • Where more than one transformer is used, make sure they are in phase with each other. 
  1.  But if its still not enough to justify getting the transformer or transformers you REALLY want, just add more.  Like they say:

    A true toy train enthusiast needs at least one ZW!

Leave a Comment