DCC BUS WIRE CONFUSION

Model Railroader General Discussion (Model Railroader)
1

I know this has likely been covered sometime in the past, but I can’t find enough answers, so I apologize in advance for asking for help. I’m new to DCC, no system yet and am ready to lay the bus wires. Yes, I have read wiring for DCC and numerous other sites wonderful information and either the following wasn’t there or I just didn’t comprehend it.

In all of the following, I am using 12 gauge solid wire.

I have seen various recommendations in numerous user group e-mails to limit bus line voltage drops to 5%. That’s where my questions lie.

  1. Most formulas available deal with twisted pairs (which is what a bus uses), but all the recommedations I’ve seen don’t say twisted pair. There IS a difference in results. Is the 5% for a single length of distance of 1 wire or or is it for a twisted pair for the same length?
    .

  2. What’s magical about 5%? After all, on a 100 foot run of 12 gauge wire, the difference between a 5% (.7125V) and 6% (.855) drop is < .2 volts! Seems quite negligible to me. Is 10% (1.425) all that bad?

  3. Don’t your rail and track feeds matter? If I’m running track feeds every 3 foot section of track, don’t they and the rail also carry that potential 5 amp current and share in increasing/decreasing voltage drops?

  4. Do I really need 3 boosters (command +2) for a 120 foot run? Formulas say the run can only be 45 feet each to get a .7155 voltage, just a bit above the .7125 allowed.

  5. So, if correct in #4, does this mean a rule of thumb is to purchase and use a booster for every 45 feet of bus run? If so, that can get prohibitively expensive…

  6. Am I correct in assuming each booster needs its own block and you can’t just connect a booster to an existing cable run without isolating it?

  7. If I have a 5 amp system but never plan on using more than 3 1/2 or 4 amps max, can I use those currents for using the formulas? Or does it have to be based on the max capability of the

2

Ken:

First off, stranded and solid wire of the same guage are by definition electrically identical. So no worries there.

I find it helps to get more specific on my amperage needs by figuring out how many trains and locos per train I will be running.

For HO, I use these figures to compute amperage needs.

Non-sound loco: 0.2 amps each
Soundtraxx loco: 0.4 amps each
QSI sound loco: 0.6 amps each

A typical train on my Siskiyou Line will have 3 units on the point (2 non sound, one Soundtraxx) and 2 unit helpers (1 non-sound, 1 sound).

This means the total amp draw per typical train is: .2 + .4 + .2 + .2 + .4 = 1.4 amps.

I expect to have, at most, two such trains in a given power district, for a total of 2.8 amps.

I use 3.5 amp boosters and have more power districts, rather than higher amp boosters and fewer power districts.

I just make sure I can get 3.5 amps to the other end of the layout with only 5% voltage drop. If you get more than 5%, depending on your voltage at your source, it may or may not be a big deal. For example, if you’re putting 16V to the track at the source, even at 10% voltage drop, you will still have 14.4 volts at the far end. As long as you’ve got a solid 12 volts at the other end, you should be fine.

The more voltage drop you get the more speed difference you may see in your loco top speed at the far end of the your layout.

I don’t use twisted power feeders, although I can see if you have inteference issues, this will help make the DCC signal less suseptible to outside interference. In most cases though, it’s probably not necessary.

3

A good place to start

www.wiringfordcc.com

4

I have mixed feelings about Allan’s site. He’s getting pretty anal these days, recommending twisting power bus wires and all kinds of extra hassle that seems to me to be overkill for most people.

He’s strayed a lot from the KISS approach (keep it simple stupid). Allan’s certainly comprehensive in his information, but it might overwhelm newbies.

Just let beginners be forewarned that you probably won’t need to do much of what Allan recommends unless you have issues with your setup.

5

12 Gauge wire is really overkill for the low voltages of DCC. I wired a 20 x 40 foot HO scale club layout with 18 gauge stranded speaker wire from Wal-mart and everything has functioned perfectly for the past six years. We have had as many as 20 locomotives running simultaneously with no voltage drop problems.

6

I have agree with Joe. I just got done wiring a 27 by 27 club layout. It has a 8 amp base unit(Digitrax) and 2 8 amp boosters. We have 16 power districts(4 PM42 units) and have used 12 gauge wire(the price was right) for the bus wiring. Every section of rail has a feeder dropped from it - I have not measured, but I doubt we have any voltage drop(bus length is about 30’ in some cases).
At home I have a 5 amp cmd station and a 5 amp booster, 4 power districts and I used #18 wire, for a 25 by 20 layout. The feeders from the rail to the bus are #22. I have had no problems as well.
Remember, you can go 45’ each way from the booster based on your formula. Allan’s site has good basic info, but is really over-kill. Just good basic wiring is the key to sucessful DCC operation. Some of my bus wires ae 32’ long with #18 wire!

Jim

7

Well, the link on the above posts gets me a prompt that says “this page cannot be displayed” so I can’t check it out (as of 3:00PM CDT 7/28/05). When I worked around radio station equipment the reason we twisted pairs around audio gear was to keep RF interference out of the audio. A strong field (like from a transmitter) will get into wiring and if the pair carrying audio is twisted the interfering signal tends to cancel itself out. Those sorts of conditions usually don’t exist in a home layout situation unless you live next door to a powerhouse radio station’s transmitter or some other unlikely circumstance. When I ran the bus on my layout I used heavy wire (don’t remeber the gage), and since it’s a shelf type layout I ran the one bus wire about six inches away from the other one. No need to twist anything and it’s obvious which bus wire a drop from the rails goes to. I believe kenkal, in his zeal to do it right, is making this harder than it really is.

Ed

8

Thanks to all who responded. As I indicated, I had read Alan’s Wiring for DCC and that’s primarily why I had all my questions.

I can’t say I’m any less confused, but what I take away from this, by actual users, is that I shouldn’t have a problem with longer than 45 foot bus runs as long as I get 12 volts DCC at the other end, using 12 gauge wire for the track bus is OK but maybe overkill, 20 gauge track feeds every flex length is OK, could be smaller and NOT twisting the track bus wires is OK. So, that’s what I’ll try following. Does any of this change if the layout is in my concrete walled basement (no power lines or broadcasting antennas around except my upstairs wireless network for my 2PC/TIVO/VONAGE) and I go radio?

BTW, the reason I’m not feeling particularly less confused is because some users recommend 1 thing for bus gauge, lengths, and twisting pairs, other users another, Alan recommends another, Digitrax another and NCE yet another. I used to think, incorrectly I guess, the reason for twisted pairs was to avoid interference caused by induced currents from one wire’s magnetic field to the next. Sigh… All I want is a well wired layout. Hopefully I’m able to learn and retain something from all your help. [:(]

Thanks again. Ken

[quote]
QUOTE: Originally posted by ebriley

Well, the link on the above posts gets me a prompt that says “this page cannot be displayed” so I can’t check it out (as of 3:00PM CDT 7/28/05). When I worked around radio station equipment the reason we twisted pairs around audio gear was to keep RF interference out of the audio. A strong field (like from a transmitter) will get into wiring and if the pair carrying audio is twisted the interfering signal tends to cancel itself out. Those sorts of conditions usually don’t exist in a home layout situation unless you live next door to a powerhouse radio station’s transmitter or some other unlikely circumstance. When I ran the bus on my layout I us

9

I agree with the KISS approach. I’ve been involved with a couple of DCC layouts, one small one large and there is nothing special done there at all. THe large one was wired using household mains cable for the main buss with light gauge feeders off it. We see little interference issues from it.

Tim

10

Here are a couple of opinions from an engineer, although not a DCC expert…

The actual frequency of the DCC signals is pretty slow, and the signal is huge compared to the noise you might be fighting. I think twisting would be a waste of time. Not to mention that the rails are there, and twisting them is probably a bad idea.

If you use a 12 gauge bus wires:
Resistance per foot- I found several numbers, all less than .2 Ohms per 100 feet
Assuming 12 Volt signal ,and 3 Amps current ,then there is a drop of .6 Volts, or 5% per 100 feet. BUT, the track is there too, so the “real” resistance is going to be less, so the bus can be longer. Also, you probably have more that 12 Volts to start with, so some more cushion is built in. And the load is probably not all at 100 feet. Also, if you have a 100 foot loop, I’d go 50 feet each direction, with the break in the middle, giving a 100 foot bus with no more than 50 foot runs, cutting the max loss in half.

So, I don’t think there is any worry with a bus of 100 feet, probably quite a bit longer. If you are picking up noise, it is probably in the loco, like next to the motor, and not in the bus. With DCC signals, if you are getting noise enough to matter in the bus, I don’t think I want to be in the room. But you might be able to cook hot dogs. :slight_smile:

I don’t see how you arrived at #4, but I think the key is having enough current for the load, the distance isn’t going to be a bit issue, unless you are talking about a really big layout. Someone correct me if I am thinking wrong, somehow.

Joe- I saw somewhere that you said the EasyDCC Booster3’s short detection was too quick for the taillight scheme, but that was several years back. Is that still true, or do you know?

Jeff

11

Jeff:

EasyDCC’s Booster3 short detection is definitely too fast to use them with the auto taillights. That’s why I use Lenz LV101’s … 3.5 amps, and they are nice and slow to respond to a short … just right!

Tony’s Trains Exchange sells refurbished LV101’s for $75 each, which is a great price. However, they are used units so it can take a while to get some in stock, usually 3-4 months. But if you’re not in a hurry, the price/performance of the LV101s is hard to beat if you believe in more power districts and lower amp boosters like I do.

12

Thanks, Joe.
I’m in the planning, figuring it out phase, so time isn’t a problem. That might be a great way to go, I’ll look into them. I am aiming for a middled sized N-scale layout, but I want operation to be easy, so that my kids, their friends, and others can operate without getting frustrated. I figure if they are in the garage, I know what they are up to!

Jeff

13

I have a question about the 45 foot run limit. Is this the total run limit or can you feed a 90 foot long district if the booster is physically placed in the center of the district and essentially feeds two 45 foot districts?

14

IF 45 feet is the limit (and I am inclined to think it is significantly more than that), you could feed 90 feet, 45 each way.

Jeff

15

This is helpful, but I’m a just a “tad” confused.

I’m downloading this topic into a folder so that I can read my DCC info and then make sense of this. Sometimes it’s rough being a linear thinker!

Thanks!

16

Jeff , thank you for your response. I like your explanations. I am a little more comfortable with my concerns now.

The way I arrived at #4, is:
a) no more than 5%voltage drop seems to be the recommended thing in DCC, but no one ever says why, except Joe’s explanation is great – just keep 12 volts on the track and it’s fine. Since the NMRA track voltage for HO out of the booster should be 14.250Volts, then 5% is a voltage drop of .7125 volts
b) I used one of the many formulas on the web, such as http://www.securityideas.com/securitycam/howtocalvold.html
which gives a formula for 12 gauge wire with 5 amps of current as:
length of run/100, times the voltage drop factor for 100 feet of cable (which is 1.59 in the table for the 1amp + 4amp columns)

c) so, using the formula, a 100 foot run comes up with a 1.59 voltage drop, which is over 11%, far exceeding that 5% recommendation.

d) to get to the 5% we supposedly need, a run must be around 45 feet (.7155 drop) to get the recommended maximum 5% voltage drop.

e) I erred in not remembering that you could split the runs by putting the booster in the middle. So, depending on how the layout is configured, you “could” need only the one booster with a “star” wiring of 45 feet or less each for 3 runs or more.

In my case of a basically L shaped layout of 45x90, 3 would be required (left leg run of 45 feet, right leg run of 45 feet, another booster for the rest. Again, just to satisfy that supposed 5% max drop restriction.

Of course, being the novice I am, I apologize if I made any stupid errors in the above.

What’s very interesting guys-- I visited a guy’s 2500 square foot basement and under garage floor layout (yes, 2500 square feet of RR, not the size of his basement& garage). What a fantastic layout he has!

However, he used 14 gauge wire, NOT twisted and has only 4 10 amp boosters for the whole thing. He never has any problems wi

17

Ken:

Just curious, who is the guy with the 2500 sq foot basement/under garage layout? Anyone we know? Sounds an awful lot like Bruce Chubb’s layout, which is why I’m asking …

18

I figured it out later, the 5 A was what I was missing. I think a star makes a lot of sense.