Wiring a Lionel KW transformer

Last Updated on August 1, 2017 by Dave Farquhar

When it comes to wiring a Lionel KW transformer, there’s more to consider than just which posts to use. The size of the wires also matters. If you derail a train 8 amps of power can run through the wire for 10-15 seconds before the circuit breaker kicks in. You don’t want the insulation to melt and catch fire.

Proper wiring for the KW transformer is a bit of a safety issue.

Pinouts for wiring a Lionel KW transformer

First things first: The two big throttles correspond with the A-U and B-U posts. The U posts go to the outer rails of your track. The A or B post go to the center rail. Each throttle controls its own loop of track.

If you’re going to power accessories off the KW as well, you probably want the C-D posts for 14 volts, or D-U posts for 19-20 volts. The 20 volts provided by early KWs is too much for many accessories, so the C-D posts are usually a better choice. If you have an early KW and want an 18- or 19-volt output from it, use a 5-ohm, 25-watt resistor on the D-U posts. You should be able to get one for under $5.

Proper wire sizing

If your wire runs are no more than 10 feet long, you can get by with #16 gauge wire for your track. This will keep voltage drop at an acceptable level and sufficiently carry the current. If you have a larger layout requiring runs of 10-15 feet, step up to #14 gauge wire. For longer than 15 feet, use #12 gauge wire.

For your accessories, you can generally use thinner wire. The draw will be lower and there is much lower risk of short circuits. For lights, #18 gauge wire is usually sufficient. For switches, some people drop even lower, using phone cable.

In this application, there’s no difference electrically between solid and stranded wire. Stranded wire is more flexible and less likely to break. Breaks in wire are hard to troubleshoot, so it’s better to go with stranded wire if you can.

Be sure to get more than one color. It’s much easier when you come back years later to splice in another wire if it’s color-coded.

Bus wiring or star wiring

There are two ways to run wires throughout your layout. Both have advantages.

The old-fashioned way is to use bus wiring. With bus wiring, you make loops of wire all the way around the underside of your layout. For a KW, you would make a loop for the A and B terminals, and a third loop for the U terminals to share. Also, when you figure your wire lengths for your wire gauge, only half of it counts, since it’s a loop. The electricity will take the shortest route.

Then, at every point where you need feeder wires to your track, drill holes in the table, run wires up to the track, and splice those wires into the loop. I like to just cut the wire, strip back a bit of insulation, then twist all three wires together and secure them with a wire nut. There are special connectors for splicing without stripping wires, but I don’t find them as reliable as wire nuts. You’ll need yellow or red wire nuts due to the size and quantity of wires involved. You can use thinner 18-gauge wire for the short distance between the track and your bus, since the bus does the heavy lifting.

For star wiring, you run pairs of wires all the way from the transformer posts to the points on the track where you need a connection. Some modern electronics require this method to work properly. The downside to this method is it uses more wire.

Regardless of the style of wiring you use, you need feeder wires every 3-4 track joints to minimize voltage drop. Start over with your counting at each switch or crossing, as switches and crossings cause more voltage drop than regular track sections.

I prefer bus wiring, but if you run newer trains with sound and command control, you may have to use star wiring.

Adding protective circuit breakers

The KW has an internal circuit breaker, but it can take several seconds to trip. To protect your wiring and your layout, it’s best to add additional circuit breakers. Lionel’s 91 and 92 circuit breakers were each rated at 3 amps. Alternatively, you can use an American Flyer 1275 or Marx 420 circuit breaker. I like the American Flyer 1275 because it has an adjustment knob.

Normally a single train will not draw 3 amps, and a 3 amp breaker trips instantly in the case of a derailment.

To protect your wiring, install a breaker between your transformer’s A post and the track, and between your transformer’s B post and the track. This setup is also convenient, since a train derailing on one track won’t cut off power to the other loop. You’ll know which one derailed because it’s the one that stopped.

Some people use modern-production resetable miniature circuit breakers rated at 3 amps, but I haven’t been able to find one that even a ZW can trip. Until I can identify one that works reliably, I’ll continue to recommend the old Lionel ones.

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