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Wire Sizes to Use in DCC

Posted by on Aug 7, 2015 in Miscellaneous, Tony's Tips |

Since we all use wire, you would think that wire size would be common knowledge with modelers. That may not be so! I got a phone call from a club that had just smoked some of the wiring on a DC layout. Their question to me was what size wire should they be using? It seems that the wire they used was scrap telephone wire, the price was right!

When DCC was first announced, it sounded like all you had to do was to connect the DCC system in place of your DC power pack and you were finished. On the other hand, there have been recent articles that say to remove all the old wiring and start over for DCC? The best and least expensive approach is somewhere in between.

Wire Basics

All wire has some resistance. When current flows through a wire, the resistance causes a voltage loss. The combination of voltage loss and the current flow produces some heat. In model railroading we are not as interested in the heat, but the amount of voltage loss in the wire. This voltage loss may causes a train to slow down in some blocks, especially those that are a distance from the power source. This loss can also affect DCC System shut down when a short circuit occurs.

There are two types of wire used for wiring a layout, solid wire and stranded wire. Solid wire should only be used in permanent locations where it will not be flexed. Stranded wire is made up of many strands of smaller wire. The smaller the wire used, the more flexible the wire. For model railroading there is no difference in the current capacity between the solid and stranded wire at the same gauge. Stranded wire can be used in both permanent locations or where it will be flexed. (In your home the wire in the walls is solid and the cords you plug in are stranded wire.)

Although rail is not wire, we do use it like wire to conduct power to our locomotives. The old brass track was a good conductor of electricity, but when it oxidizes the oxides were non-conductive. Nickel-Silver rail is not a good conductor of electricity, but when it oxidizes the oxides are conductive. Long runs of Nickel-Silver rail can cause voltage losses like under size wiring. The cure is to frequently run a “feeder” wire down from the rails to main buss wire, small bare solid wire is used. Some modelers recommend a feeder wire for each piece of rail. Rail joiners can also be a source of voltage loss.

Is Your Layout Wiring OK?

Wiring used for DC may work for DCC. The difference is in DCC you can have more than one engine operating in a block. This means you can have more current flowing to a block. DCC does provide more voltage on the rail than you normally have with DC. This helps with the voltage loss, but does not fix it.You may have to increase wire size. Many clubs have started to convert to DCC by replacing one of their DC cab’s power packs with a DCC system. This allows time to slowly modify the wiring to better utilize the advantages of DCC.

Is the old wiring OK? There is a simple way to test wiring with DCC called “The Quarter Test”. Simply take a US quarter (coin), and short the rails. The DCC System should trip and shut down. If the DCC System does not shut down, it may be caused by undersized wiring, poor connections, or too much resistance in the rails.

A better method is to measure the voltage drop.

Use a constant load on the rails like an automotive tail light with a couple of clip leads attached to act as a load on the track (About 1-2 Amps). Then you can hunt down any excessive voltage losses. A meter is necessary. Most meters can not accurately measure DCC voltage but you can use the AC readings as a reference. Better yet use the “RRampMeter”, specifically designed for model railroaders.

What Size Wire to Use?

The voltage drop or loss should be the determining factor in the choice of wire size. To determining the size of wire three things are needed, the length of the run, the amount of current that will flow and the amount of voltage loss. The chart shown is for a 1/2 volt drop running one way. If you have wire running out to the rails and then back again, the loss would be one volt. DCC Systems are rated at about 2 Amps to 10 Amps. The most popular is the 5 Amps.

Length in Feet for a 1/2 Volt Drop

Wire Size
1 A
2 A
3 A
5 A
10 A
8
796
398
265
159
80
10
501
250
167
100
50
12
314
157
105
63
31
14
198
99
66
40
20
16
124
62
41
25
12
18
78
39
26
16
8
20
50
25
17
10
5