When it comes to wiring a Lionel LW transformer, there’s more to consider than just which posts to use. The size of the wires also matters. If you derail a train, 5.5 amps of power can run through the wire for 10-15 seconds before the circuit breaker kicks in. An LW has enough power to melt wire and make it smoke or even catch fire.
Proper wiring for the LW transformer is a bit of a safety issue. It’s not just about preventing voltage drop to keep your train running smoothly. A smooth running train is nice, but safety is a must.
Pinouts for wiring a Lionel LW transformer
The throttle corresponds to the A-U posts. The U post goes to the center rail of your track. The A post goes to the outer rail. The two A posts are interchangeable. This convention confuses people sometimes, but it’s OK. It’s backwards from many of Lionel’s other more popular transformers but it works just fine.
The LW may be the best transformer Lionel ever made for powering accessories. The A-B combination gives a useful 19 volts, excellent for switches. The A-C combination gives 14 volts, ideal for most accessories and for lights. The undocumented B-C combination gives you 5 volts, but bypasses the internal circuit breaker. Use an external circuit breaker if you use the 5 volt connection.
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 an LW, you would make a loop for the A terminal, and a second loop for the U terminal. 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. If you have more than one transformer and you phase them, the other transformer can share the loop that goes to the outer rail. If you use a KW or ZW for running trains, connect the A post from the LW to the U post or bus coming off your larger transformer.
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.
The LW is nice in that the two voltages you’re likely to use for accessories involve the A post. You can run wire loops for the B and/or C posts if you want. Or you can run straight from the transformer post to the accessory if you want. Then you can run a wire from other post on the accessory and connect it to the loop for the A post. Or you can just connect it to the nearest outer rail.
These traits and its relatively high wattage make the LW a fantastic transformer for running lights or accessories.
Adding protective circuit breakers
The LW 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 U post and the track. If you use the B-C posts for an accessory that runs on 5 volts, install a breaker between one of those posts and the accessory. Install the breaker on the post that isn’t going to the track. That way, if a breaker trips, you can tell whether it was the train or the accessory that tripped it.
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.