Wire Gauge Table (AWG)

Model Railroader General Discussion (Model Railroader)
1

I was researching the gauge of wire for my DCC bus and I came across this handy table. If I read it correctly, I should use 18 gauge wire for a DCC bus up to 25’ for a system using 12V 0-5 amps. My understanding is that most DCC systems use 12V 0-5 amps - is this correct? (I know you can get 8 amps too but that’s overkill for me). I am curious because I recently posted a thread about this and most modelers suggested 14-16 gauge wire for a DCC bus. If the chart is correct then 14-16 gauge wire is probably too much and 18 gauge is just right. Any thoughts on this? Am i misreading the table?

Here is the chart:



WIRE GAUGE SELECTION TABLE


Circuit Amperes
2

This is one of those times when more may be better. While you could “get by” with #18, I would use #14 for the main bus. On my Layout I have a Main Bus, several sub-busses, and then feeds to trackage. I use #12 and #14 (stripped form Romex I had laying around), #16 for the sub-bus portions, and #22 as short track feeders (1’ or less). Alan Gartner has a great page devoted to the reasoning for this on his website. http://www.wiringfordcc.com/trakwire.htm

My suggestion would be to read what Alan has to say before you make the decision to use smaller wiring. I have had zero problems with the way I did it.

3

You hit the nail on the head Ray. I have been told by an electrical engineer that using wire that is too big can be just as bad as using wire that is too small. That is exactly what I am trying to avoid. Why use a larger gauge when it is not necessary? I am quite familiar with Allan Gartner’s DCC wiring website - I have been there many times. But this time I decided to do my own homework and when I found this table - I was quite surprised to be honest. I also thought from my previous research that I would need at least 16 gauge wire but the table doesn’t lie. What I am really trying to confirm is that I am reading the table correctly and that most DCC systems are 12V 0-5 amps.

4

In general terms I (also an EE) disagree that using too large a wire can be as bad as too small (in this case), unless you go absurd. Also, it is important to know what the purpose of that table was, since the voltage drop through the wire is what is important, and the table doesn’t tell you what the criteria were for determining the max length.

Anyway, here is a little calculation:

12V, 5A, 18 gauge wire 1.3 Ohms per 100 feet, 16 gauge .818 Ohms/100ft, 14 gauge .516 Ohms/100ft.

A 25 foot bus has 50 feet of wire (it also comes back, and the voltage drop is the same). We are ignoring the help the track MAY give here, but gaps or bad connections reduce that, so we’ll be safe.

So for 18 gauge wire you lose 5A x 1.3Ohms / 2 = 3.25 Volts in the wire, leaving less than 9 at the loco. 16 gauge: loses about 2 Volts, 14 guage loses about 1.25.

These are worst case, five Amps draw at the extreme end, etc., so it probably is never that bad. But it shows that 18 is almost certainly a bad idea, 16 might be ok, 14 is a good idea, and if you are going longer you might want even more.

There is no question that this is overkill, to an expent, and I’m not looking to argue that, but the extra expense and difficulty in going to 14 gauge isn’t much to ensure you don’t have a problem later, when there are more sound locos, etc., and things we haven’t even though of yet hanging on decoders in the tracks.

5

In addition to the points Jeff makes above, you have to remember that DCC is signal plus power in one waveform. The power loss in the wiring also degrades the control signal.

6

Which reminds me, the first thing in the decoder is a rectifier of some sort, there goes another 1.5 V or so. It is conservative, but I don’t think 14 gauge is too big, in almost any situation.

7

What Nigel said!

The concern for our application is not as much power loss as a loss in the integrity of the DCC signal.

8

In essence they are the same thing. The DCC signal isn’t added onto the power ‘signal’, it IS the power signal. The reason you might lose the DCC signal is because of the power loss. Because of the nature of the DCC signal, I expect that the decoder might still be decoding long after the level has dropped to the point that it can’t drive anything.

9

Good stuff guys! Let me play devil’s advocate a little. Jeff points out that his example is probably a worst case scenario. He also states that the track MAY give a lttle help. Here is my question, if you are running your longest length at about 25’ (which is really 50’ according to Jeff) and have feeders all throughout your layout (say about 6’-10’ apart) won’t the track help ALOT? (in this case I am using Kato Unitrack which I have found to have excellent conductivity).

I must admit that I have a very limited understanding of electrical engineering. Jeff’s resistance formula is an excellent resource. But as he pointed out, there is no way to really calculate this problem because of all the variables involved (track, etc.)

The reason I am wondering all this is because I do have a readily available supply of 18 gauge wire. I’m thinking I might give it a try and if I have problems, then just replace the 18 gauge with 16 gauge or 14 gauge. Does this seem to be a stupid idea and a waste of time? Be honest, you won’t hurt my feelings and may save me a lot of trouble.

10

The voltage drop for a 2-wire, 25’ run using 18ga will be 1.646 volts. Now if your power supply is for example, a 12 volts you would only have 10.354v available to the trains. A loss of over 13%! Take into consideration other factors such as rail joiners and connections and it is conceivable that you will loose or “drop” 2 volts. If that is acceptable to you then that is fine. To me I would want all the power I paid for.

Lets use the same example, this time with 14ga wire. The drop will still be .648 volts. What I am saying is that even thought the circuit has the ability to provide enough current, your power supply will not have the ability to overcome the loss using small wire. Wire is so inexpensive in the scheme of building a model railroad it is the last place you want to economize.

Here is a link with voltage drop calculator. http://www.powerstream.com Select “Technical Resources” then select “Wire and Current Capability Chart.

Jim

11

Here is what I’d say. IF you use the 18 gauge it will probably work, probably fine, probably forever. BUT. When it doesn’t three years from now when you add a nice noisy sound consist, or extend the layout, or a gap develops in the rail that isn’t there now, and things get squirrely, but just now and then, and you really can’t figure it out, are you going to remember that you ‘cheated’ and used potentially inadequate wire? I’m guessing not. You can do it if you want, and you’ll probably be ok, but…I wouldn’t. It is up to you, of course.

Soo Line Fan, did you remember that you have 25 feet out and another 25 feet back? I think it is actually twice what you said (unless I slipped up in my head, which is possible), and still your point is valid. Why cheat in the basics? Just like you said.

12

Well, it depends on the DCC system in use and any selections/adjustments you may have made, but for HO scale the track voltage is usually closer to 15 volts.

Keep in mind that the run of the mill decoder doesn’t boost the voltage, it just rectifies/filters/controls it, and in doing so it usually introduces a voltage drop. Therefore, to get the full 12V at the decoder outputs, you need a couple volts more than that available on the tracks.

The bottom line is that you should go with the 16 or better yet 14 AWG wire. I know there’s a lot of relatively cheap 18 gauge wire out there (zip cord for wiring lamps and speakers, etc), but 14 or 16 stranded isn’t that expensive in bulk, especially when you consider it as a percentage of what the entire layout is costing you.

Steve

13

I meant a total run of 25’ (12.5 out and back) so both of our calculations are correct. Sorry for any confusion.

Jim

14

Being an ASEE and HAM radio operator myself I have to totally concur with Jeff.
Who knows what new toys will develop down the line,that we will add to our layout, and absorb more power from our power source?
I’d rather spend a little more now, and avoid that old saying “PAY ME NOW OR PAY ME LATER”.
Plus the hassle and frustration of rewiring a layout.
Just my two cents worth!

Ed

15

It’s ok, I just wanted to make sure I hadn’t messed up, I think there is enough misinformation floating around that it is good to be accurate when we can, and I wanted to fix it if I’d goofed. The best part is that your number pointed out that 18 gauge isn’t even really enough at 12.5 feet! I’m sure there are lots or cases where it works, because of all of the things that are usually not worst case, and all of the helpful unknowns, but that’s luck, not engineering.

16

OK guys - you’ve convinced me. I will be running alot of sound locos (in fact so far ALL my DCC locos are sound), looking forward to DCC uncoupling someday, and those new MTH locos which just eat the power. These future power needs and your much heeded advice have convinced me to upgrade to 16 or 14 gauge for the main DCC bus. Now, is 14 or 16 gauge better for the main DCC bus with a max length of 25’ (one way)?

Also, I assume I can use all that 18 gauge wire I have for AC accessories like internal building lights?

17

I’d use 14, there’s no compelling reason not to.

You can use the 18 for anything else, as long as it isn’t you bus wire.

18

I agree, the other part of this is that a one way run of 10, 25 or 50’ is actually doubled in regards to voltage drop. If using DC one side will be greater due to the block switches.

Jim

19

Thanks gang! The decision is made. 14 gauge wire for the main bus and 18 gauge for AC accessories. Thanks so much everyone!! Will post pics when layout is complete.

20

You can post progress pics along the way, you know!