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Author Topic: Wiring a layout for DCC  (Read 9636 times)
USNavyChiefRet

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« on: February 12, 2009, 05:00:12 AM »

I am wiring my layout, adding terminal joiners with soldered wires in various places around the layout. It's not big, yet!  Grin It's only about 12 feet long by 4 foot wide, dogbone style. My questions I guess is this. Don't I have to keep the outside rail separate from the inside rail on my feeder wires? I can't just twist the two feeder wires from the outside rail and inside rail together and attach to one main bus!? I am using Bachmann track and I picked up some presoldered rail joiners at a hobby store here in the Mpls area. I plan to try and add feeder wired around every rail joiner but that's a huge undertaking and trying to solder something that small. WOW! Must take an awfully small tipped soldering iron! Huh?
Do I have to keep those rails separate? I'm not making any cross overs, no figure eights, I will have one passing siding. It's pretty simple right now. I figured start small and work up.  HELP!  Roll Eyes
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Yampa Bob

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« Reply #1 on: February 12, 2009, 06:21:07 AM »

Good Morning Chief
You only need feeders about every 4 to 6 feet around the track if your rail section joiners are tight. Outside and inside rails need to be kept separate to avoid shorts.  Even though you will be using DCC, wire the track as you would for DC, one rail positive (red) ,the other rail negative (black).

Typical bus wires for HO scale, and your size layout, is 14 gauge. Use one red wire and one black wire, regular stranded.  Connect from your main track terminals to the center of the bus wires, not at one end, think "T" connection.

It doesn't matter which rail you choose for red or black, just be consistent. If the prewired joiners have 2 red wires, you should color one wire black with a Sharpie pen so you can maintain polarity.  It helps to put small pieces of red and black tape beside each rail at intervals to keep your wiring correct.  As you connect everything, just keep one rail "red" and the other "black".

Keep the feeders (your joiner wires) short, about 6 to 8 inches.  Alternately you could run the bus wires down the middle underneath the table and run feeders both ways, but for 2 foot feeders you would use 18 gauge wire, up to your shorter and smaller joiner wires.

Before connecting any controller to the track, check with an ohmmeter to make sure the two rails are not shorted somewhere. (crosswired).

The following calculator will help you verify wire sizing, ampacity and voltage drop for length of run.  Set voltage box to 12 volts DC. The next to last box is important, keep "Percent voltage drop" at 2% or less.

http://www.powerstream.com/Wire_Size.htm

Perhaps more than you wanted to know, but this will give you a reference. Don't hesitate to ask if you have more questions.

Regards.
« Last Edit: February 12, 2009, 06:56:21 AM by Yampa Bob » Logged

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USNavyChiefRet

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« Reply #2 on: February 12, 2009, 01:52:15 PM »

O.K.  The wired rail joiners have feeder wires soldered to each and they are quite small, gauge wise, maybe 22 AWG but they are short maybe 6-8 inches, from them I am running 16 AWG to the center where they will tie-in to the main bus which will be 14 AWG. Do you think that will be o.k.?
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SteamGene

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« Reply #3 on: February 12, 2009, 01:57:37 PM »

Hey, Chief,
Sounds good to an old field artilleryman.   Cheesy
Gene
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Yampa Bob

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« Reply #4 on: February 12, 2009, 03:43:21 PM »

Sounds good to go. I verified for 5 amps in case you ever need to add a booster to your DCC controller.

Feeders - 22 gauge at < 12 inches =  5 amps with 1.38% voltage drop.
Secondary bus - 16 gauge at  < 4 feet  = 5 amps with 1.39% voltage drop.
Main bus - 14 gauge at < 9 feet =  5 amps with 1.94% voltage drop.

From main terminal (controller connection), suggest running < 2 feet  14 gauge to the center tap of < 12' bus running centerline of the layout, that gives you about 6 feet of bus each way from the center tap.
« Last Edit: February 14, 2009, 05:34:06 PM by Yampa Bob » Logged

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Jim Banner

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« Reply #5 on: February 13, 2009, 01:04:17 AM »

My philosophy is a little different than Bob's when it comes to wire sizes for DCC.  I believe Bob is basing his on a maximum 2% drop in voltage at a draw of 5 amps.  I base wire size on a maximum of a 1 volt drop.  I suspect that with a 2% drop in voltage, you will not notice any change in speed, even if the change occurs from one block of track to the next.  With a 1 volt difference under the same circumstances, you probably would notice the change if you were looking for it.  However, when you spread that 1 volt drop over 40 or more feet of track, the drop off is so gradual that you don't notice it even if you are looking for it.  At least, no one operating my layout ever has, myself included.

In practice, this means I can use #14 bus wires up to 40 feet, #12 bus wires up to 60 feet and #10 bus wires up to 100 feet.  If you are checking my calculations, please remember that a 40 bus is 40 feet of wire out and another 40 feet of wire back for a total of 80 feet, and so forth.

Even though I do the calculations based on 5 amps, in practice I rarely if ever have 10 locomotives all drawing power at the far end of any one bus.  When a group of us operate the railroad, we might have half a dozen trains, all double headed.  But normally, no more than half of those would ever be drawing from the same bus.  And usually, one of those trains would be sitting in a siding waiting for the other two.  Very rarely would any bus be carrying more than 2.5 amps, and then only part of that would be out to its far end.  So the voltage drop would rarely if ever exceed half a volt.

I also worry a lot less about voltage drop in feeders than most people do.  I normally keep feeders less than about 3 feet and space them less than 10 feet apart.  That would cause large voltage drops at 5 amps, but unless I ran a solid train of locomotives, I don't know how I could draw 5 amps from a single feeder.  Typically on my layout, a fifty foot bus serves about 100 feet of track so there are at least 10 feeders.  If I could somehow manage to have 4 locomotives all drawing power at one of those feed points, they would be drawing about 2 amps.  But only one half of that would be drawn through the pair of feeders at that point.  Another one quarter would be drawn from the feeders 10 feet away and another one eighth from the next feeders beyond that, and so forth.  No pair of feeders would ever have to carry more than 1 amp.  So my feeders could be 5 times as long as Bob's calculations show or I could use a smaller wire size.  In point of fact, many of my feeders are #24 wire.

Those #10 bus wires have no place on an H0 layout and probably not even on a large scale layout.  Even if you twist your bus wires together you probably should not exceed about 50 feet on any one buss.  Beyond that, you should consider another booster to avoid degraded DCC signals on the rails.  That added booster could be 100' away from the first with 50' buses from each booster extending to a point half way in between.
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Yampa Bob

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« Reply #6 on: February 13, 2009, 03:49:32 AM »

The calculations are not based on a philosophy, rather on  recommendations by the National Electrical Code (NEC) for efficiency and safety.

http://www.mikeholt.com/technical.php?id=technicalvoltagedrop1

Some electricians allow percentage of drop from 3% to 5%, but I have found efficiency of DC motors is reduced significantly above 2%.

In the calculator I linked to, enter 7 feet of 22 gauge wire, 5 amps at 12 volts DC. Actual voltage drop is only 1.162 volts, but the percentage of drop is 9.68%. Now change the voltage to 48VDC.  Voltage drop is still 1.162, but percentage of drop is only 2.42%.  The voltage drop does not depend on the input voltage, just on the resistance of the wire and the load in amps. That's just simple Ohm's law.

Bear with me for one more entry...change the input voltage to 3 volts, again drop is 1.162 volts , but percent of drop is now up to 38.73 %.   At a time when the motor needs 3 volts minimum, it is getting less than 2 volts. Granted, the motors I use commercially have a high starting current, and I have to calculate for locked rotor (stall) current.

True, a single loco will probably never draw the full 5 amps through a feeder, but consider a dead short at one of the feeders when using a booster. Now it is no longer a voltage drop issue, it is a safety issue.  That small wire must now, at least momentarily act as a fuse since it's the weakest link.  22 AWG has an ampacity of 7 amps chassis, but embedded in foam or other roadbed, you might detect smoke.

In the case of a resistance short, there might be 5 amps flowing continously through the wire, and the controller breaker will never trip.  I have seen people use 50 feet of 16 gauge extension cord for a power tool, then they wonder why the plugs melted on the ends, while the tool barely ran.

My Solar Electric designs and installations must comply with NEC and be inspected. When customers pump water from 300 to 500 feet, they can't afford to lose more than 2% of available power, in fact I incorporate LCB (linear current boosters), which allows pumping water with total cloud cover, even in bright moonlight.

Bottom line, I don't size wire for performance alone, safety comes first. I agree this is overkill for a layout, but I like my wiring to stay cool.  Cool
« Last Edit: February 14, 2009, 05:35:45 PM by Yampa Bob » Logged

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pdlethbridge
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« Reply #7 on: February 13, 2009, 10:01:00 AM »

Then there is the practical side. I never intend to run more than 2 engines, and even that would only be when someone else brought his power cab. I, along, only run one loco at a time with a max of 3 decoders running at once briefly and then 2 decoders when a train is running. The decoders are , 1, for the engine, 2, for the auto reverser, and 3, for the turntable that can be shut off. Even though the switch machines are decoder controlled, each decoder, a digitrax ds 64, has its own power supply. So I feel very comfortable with the 18 gauge buss with 18 gauge feeder wires spliced in. The buss is about 14 feet in length and the feeder wires range from a few inches up to about 2 1/2 feet. The buss runs on the front edge of the layout with the feeder wires extending to it. I'm disabled so this makes it very convenient. The ds 64s are at the front edge too and everything is covered by a curtain for neatness. Thus the question. Am I safe?
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Nathan

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« Reply #8 on: February 13, 2009, 10:31:08 AM »

As for the National Electrical Code,  it is for high voltage house  and commercial wiring, and even says that other users are covered by other NFPA codes.   I spent a number of year as a volunteer fire man and was the one that got our department to join NFPA and get the copies of the codes.  I spent my time reading through them so I could understand what to look for at a fire so I could pass the informtion alng to the Fire Marshel.

In pritical use, like Jim says, when you have multiple feeders on a block the voltage drop is lower then the 'caluclated drop'.  On our large scale club layout the main lines are about 280 feet long.  When we only had 2 feeders per loop and depended on the track from that point, with draws of 10 amps we were seeing 4 volts of drop.  Going to 6 blocks with feeders for each block going to both ends of each block we have brought this down to 1 volt drop with a multi unit PA-PB-PA with 11 lighted passenger cars drawing about 10 amps.  Most trains we run draw between 2 and 4 amps and see about 1/2 volt drop at most.
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Yampa Bob

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« Reply #9 on: February 13, 2009, 05:56:11 PM »

As a practicing Electrician for over 40 years, and former Building Inspector for 10 years, I assure you the current NEC has provisions in place for regulating low voltage DC installations.  Enforcement and interpretation varies by state and individual inspector, more rigid in western and remote areas where renewable energy applications (solar, wind and hydro) are more prevalent.

Anyway, I won't debate that issue any further as it serves no purpose other than argument. Everyone is free to believe what they want, but I'm the one that has to deal with inspectors, and their interpretation and decision is always final. 

Of course no one is going to inspect your layout, wire it any way you want. Electricity doesn't discriminate.  When you have a short, the electrons don't hold a meeting and say: "Well, gee guys, this is just a model railroad layout, let's not start a fire this time".

As with all things in life, there is a practical side and safe side to any issue. You take the practical side, I'll stick to playing it safe, and we'll both be happy railroaders.  Cool
« Last Edit: February 13, 2009, 06:15:58 PM by Yampa Bob » Logged

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pdlethbridge
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« Reply #10 on: February 14, 2009, 10:56:45 AM »

I'd like to be save too so should I replace the buss to 14 gauge and make it longer to shorten the track leads or what?
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Yampa Bob

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« Reply #11 on: February 14, 2009, 05:31:55 PM »

Paul
As I mentioned before, many electricians allow for 3% to 5% depending on load requirements.

Your system is rated at less than 2 amps. Using the calculator I plugged in 2 amps for an 18 ga. bus 14 feet long. Voltage drop is only .37 volts, percent of drop 3%.

For the feeders, 18 ga. for 3 feet, voltage drop is only .08 volts, or .66%.

I suggest you are within safe design limits, and you shouldn't have any concerns. As mentioned by others, the bus is probably not that critical. The problem arises when modelers run those 22 ga. extension wires all around the layout for an EZ Command, then later add a 5 amp booster.  To me, planning a layout from the start for 5 amps makes good sense.

Keep in mind, when I design any DC system, I have to factor in a worst case scenario, called a "catastrophic event", just like an Engineer who must over design a building or bridge. 

All the practical solutions offered in the posts will probably never create problems, but again I can't risk my reputation on "probably".  I've never had a system failure for any reason, and some have been in operation for more than 15 years.
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pdlethbridge
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« Reply #12 on: February 14, 2009, 05:42:31 PM »

Now what if I changed the buss to 25 feet of 14 gauge copper stranded speaker wire and keep the 18 gauge feeders but keep them under 2 feet. What would it take to crash something like that?
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Yampa Bob

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« Reply #13 on: February 14, 2009, 06:12:31 PM »

Use the calculator.  .26 volt drop or 2%. ,probably not worth the effort or expense.

Personally, I wouldn't use speaker wire as the insulation may not have adequate temperature rating or abrasion resistance. 

I use only THHN/THWN wire rated at 90 deg C.
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I know what I wrote, I don't need a quote
Rule Number One: It's Our Railroad.  Rule Number Two: Refer to Rule Number One.
pdlethbridge
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« Reply #14 on: February 14, 2009, 06:13:46 PM »

how about house wire, like 14/2?
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