DC Wiring


Active Member
Jul 9, 2005
The point of this thread is to consolidate much of the information regarding DC wiring into a single thread. Andrew (moderator) has proposed moving this information into the Academy.

Wiring for DC control has become a lost art because of the interest in DCC. The model railroading press used to publish annually instructions on how to wire for DC. It doesn't anymore. Instead, instructions for installing, using, and operating with DCC get all the press space. But DCC is not for everybody!

In particular, DC is still used by the following groups (and likely to remain so for a while to come):
  • families looking to expand in the hobby beyond their first train set to their 1st real layout.
  • model railroaders who normally operate their layouts by themselves, but sometmes may have a second operator
  • layouts where multiple running trains are segregated naturally to areas of the layout with little to no interaction with each other due to the track plan
  • those for whom the cost of DCC is too high compared with the benefits they expect to receive
This thread is dedicated primarily to DC control situations where simultaneous operation of 2 trains is the maximum expected. The components and wiring for 3 or more simultaneous trains over shared track are much more expensive and complex, both to install and operate. For 3 or more trains over shared tracks, serious consideration should be given to DCC control instead.

There are several wiring books that cover most of the mechanics of installing DC wiring. The Atlas wiring book emphasizes use of Atlas electrical components, and shows how to wire Atlas layouts, but has very minimal discussion of theory.

The guiding principal of DC control is a separate throttle for each train, and each train receives power only from the throttle controlling it. To do this, the track is broken into electrically separate sections of track called blocks. Each track block has an electrical switch (usually a toggle switch or Atlas Selector) wired to it that is used to select which throttle will control that particular block.

In operation, as train A advances, the block in front of the train is switched to throttle #1. The block behind train A is switched off or switched to throttle #2 for train B. Similarly, as train B advances, the block in front of train B is switched to throttle #2. And the block being vacated is switched off, or back to throttle #1 for train A. In this way, train A is always connected to throttle #1 and train B to throttle #2, no matter where on the layout either train goes. The key is that the 2 trains cannot occupy the same track block at the same time. Then they are both connected to the same throttle, and cannot be stopped, or direction or speed changed without the same happening to the other train.

One topic that I have seen missing from almost all wiring books is how to figure out where to put the blocks in your layout. How you block your layout will determine how much operational flexibility you do or do not have. Too few blocks, or blocks with the wrong boundaries, and the result is your trains will not be able to move without becoming cross-connected to each others' throttles. Too many blocks mean way too much time wasted on switching blocks, and excess wiring.

Rules for blocking in DC: These are the rules I came up with for selecting block boundaries to gain maximum operational flexibility.
Rule #1: Every continuous route or loop needs at least 2 blocks per train (4 blocks minimum for 2 trains) that will be operating on the circle, oval, or loop simultaneously. This gives one block for the train to occupy, and a second block to advance into. Two trains following each other on a simple loop layout need 4 blocks on the loop alone. If there are less, each train must stop until the block in front of it is vacated.

Rule #2: Both tracks of every passing siding must be separate blocks. This allows one train to be stopped (or moving) on the siding while the other train passes on the main.

Rule #3: Every dead end spur where you might want to park or operate a locomotive while another train goes by on the main branch of the spur turnout must be a separate block. Power routing turnouts can be used instead of a block toggle for blocks on dead end spurs. The power routing turnout cuts power to the spur when the turnout is thrown for the main.

Rule #4: Block boundaries at turnouts shall be at the frog end of the turnout, normally close to the clearance point. The clearance point is where a stopped train will not be hit or side-swiped by a train using the other branch of the turnout.

Rule #5: When possible, blocks shall be at least as long as the longest train. When blocks are all a train length long or longer, you don't have to worry about using more than 2 blocks at any given time for a train. The train length rule is a mandatory requirement for reversing loops and sections.

yours in DC wiring


Feb 5, 2007
Calgary Alberta
Great Timing.
Yup, this one's a keeper for the vault.

I've just laid out my N scale transistion era 32" x 96" double oval with crossover. One loop is the mountain line the other a prairie line. I need to isolate/power the passing tracks, an industry line, a mine spur, a passenger station, engine sheds and maybe even a turntable.

I have a DC pack and some DTDP switches so the DC over DCC savings will be spread out to buy engines and cars.