Track feeder wire gauge

[The_Ghan]

Gentlemen,

 

With my draft track layout I'm about to start soldering feeder wires.  My question is this: What gauge wire should I use for the short feeder wires? :dontknow:

 

I'm going digital with Digitrax.  I've got 10 AWG red and black trunk wiring in a star layout through power supply - booster - power management and block detection.  The star layout in 10 AWG will continue to just beneath the position that I want to place the track feeders, at roughly 6' intervals.  I bought some "Light Hook-Up Wire" on-line.  It appears to be 20 AWG gauge.  I don't plan to use lengths longer than 3".  Will this do the job? 

 

Cheers

 

The_Ghan

[KenS]

There are two questions to ask when sizing wire for power (including DCC): will it deliver the power, and will it catch fire? The power-delivery problem basically depends on resistance (small wires have more resistance) and manifests as voltage drop. The "catch fire" bit is also due to resistance, because that lost power goes to heat the wire, and if it heats faster than it can cool, it will melt (if you're lucky) or set your table/scenery on fire when it reaches their ignition temperature.

 

But the power requirements of DCC really aren't that large, and the wire doesn't have to be massive, although the really thin wire used on DC layouts has length limits at DCC amperages that could be a problem, and this is why larger wire is recommended for bus wiring. But you don't need to take that too far.

 

For wire size and limits, check an ampacity table like this one.

 

To compute voltage drop use Ohm's Law (Volts = amps x resistance), so 20 feet (use "20x2" since you need to count length out and back) at 4 amps using 14 AWG (2.525 ohms per 1000 feet) is:

 

V = 4 x (20 x 2 x (2.525/1000)) = 0.404

 

10 AWG bus wiring is overkill, unless you plan to use the layout to jump-start your car also. And it's a real pain to work with.  If you had a really large layout, and were using an 8 or 10 Amp supply, 10 gauge might be justified, although 12 should work fine. Otherwise 14 gauge is a good size for 5-Amp systems. At 4 amps, 20 feet of 14 AWG wire will cause a 0.4 volt drop; which shouldn't be a problem. Even N-scale DCC, which uses 12 volts on some systems, could probably operate just fine on a 4 volt drop (that includes loss in the track, which can be much higher than bus loss per foot). For a small layout, even a 16 AWG bus would likely be fine (0.6 volt drop at 20 feet).

 

For feeders, as long as you keep them short, the standard Kato size (24 AWG) is sufficient, and is rated for up to 3.5 amps if the wires aren't inside a constrained space (like a metal tube) where they won't be air-cooled (a short bit through the roadbed isn't enough constrained space to count since conductive cooling to the parts that are exposed will keep it cool). Voltage drop at even 3 amps is just 0.15 volts per foot of feeder.  If you're going to run the feeder entirely within the layout material (i.e., between a plywood baseboard and foam scenery) it would probably be a good idea to use 20 AWG (rated for 1.5 amps in a constrained space), but with real-world train sustained amperages well under an Amp, 24 AWG would probably work fine even then.

[The_Ghan]

Thanks Ken,

 

I'll be sticking with my 10 AWG mains.  I've got the wire and it fits snuggly into screw connectors, etc.  It also gives plenty of surface area to solder to.  Your explanation is well founded and clear.  I suspect that my feeder wire is 24 AWG.  I'm going to see if I can pick up some 18-20 AWG cheap.  Silly to ruin the effort of using 10 AWG mains with 24 AWG feeders, I think.

 

Another thought: I have some track sections that are less than 6' long and would normally be fed by a single pair of wires.  I'm thinking to double up in case one of the connections fails in the future.

 

Cheers

 

The_Ghan

[KenS]

Feeding one section of track twice won't be a problem if the feeders are connected close to each other on the bus and track. As you move either part further apart, you introduce the potential for the two versions of the DCC waveform to interfere due to different propagation times and differing voltages when under load (due to resistance differences for the two paths). Given the low rate of DCC signaling (16 kHz) it's unlikely that interference due to delay would be significant at any reasonable distance, but I haven't done the math. Generally, loops of any kind are a bad idea in a DCC layout, and dual feeders is a form of loop.

 

That said, in 15 years I never had a soldered feeder fail on my old layout, and if one did re-soldering it wouldn't have been too much work (if you're soldering to the bottom of the track, that's a different story; I soldered to the sides).

 

I'm a strong believer in overdesign, but you've got me beat. 

[The_Ghan]

Hmmm ... do you really think there is a chance of interference from double feeds?  I note the Digitrax website recommends feeders every 6'.  What if you have a loop that is 30' in circumference?  Wouldn't you have 5 feeds to the one loop?  What if the engine is traversing sections at the time the DCC command is sent?  Could it not receive the same command twice as it crosses?

 

I'm sure the guys at Digitrax are handling all of this and I'm betting it isn't a problem.  I'll let you know if it is.

 

Yes, I'm over engineering ... and I think it is a bit of overkill as well ... but I only want to do things once.

 

Cheers,

 

The_Ghan

[Bernard]

Ghan - Here is a nice link to wiring for DCC:

http://www.wiringfordcc.com/track_2.htm

 

And there is a great book by Model RR that was a big help to me when I was wiring:

http://www.amazon.com/Dcc-Made-Easy-Railroad-Railroader/dp/0890246165/ref=sr_1_3?s=books&ie=UTF8&qid=1285590996&sr=1-3

 

On my layout I used 16 AWG for the Buses and 20 AWG for the feeder wires. One of the reason for the 20 AWG was that I could get it in a large spool at Home Depot whereas with other lighter gauges, I couldn't get it. (It is used in wiring Door bells in homes)

 

I went Ken's route and did "overkill" and wired every piece of flex track.

There is one section where I have a trolley running and did an experiment with code 55 track. There I had to use 24 gauge wire in order to solder it to the rails.....it was a pain and took twice as long.

For me, wiring a layout is a lot of fun....much more than scenery, go figure.

[The_Ghan]

Bernard,

 

I agree, wiring is a lot of fun ... it is kind of like performing brain surgery in the toolshed, don't you think?

 

Wiringfordcc is a great website.  Thanks for that tip.  These days I find books gather dust on the shelf.  I'm more likely to have my notebook nearby.  Your little sojourn into the world of Code 55 was interesting.  Soldering 24 AWG would drive me crazy ... like trying to get an eyelash out of your eye !!!

 

Looking forward to some brain surgery.

 

Cheers

 

The_Ghan

[Guest Closed Account 1]

Can Kato Double Track Feeders (20-043) be modified safely in parallel to run off one controller?

 

I was told by the LHS that adding Kato Double Crossover (WX310) will send power to the inner tracks.  Wrong answer.

[KenS]

No modification needed.  Kato sells a 3-Way Extension Cord (24-827).  Plug the splitter box on the extension cord into the power pack, and each of the feed wires to one of the outputs of the box. And you'll have guaranteed correct polarity (I have both of these in my supply bin from the old table-top layout, and just tried it; worked fine).

 

You could do the same thing by splicing the two wires together if you didn't want a little blue box tucked away somewhere. The wire should be good for a total of 3.5 Amps, which is more than the track is rated for, or any reasonable set of multi-unit trains would draw.

 

And yeah, the crossover is insulated in the middle, not power-routing like their turnouts. 

[Guest Closed Account 1]

Thanks for the tip.

 

10 gauge wiring is the standard right?  I can permanently improve the feeder piece with my solder station and slow hand. 

[KenS]

I presume you mean for the bus.

 

10 gauge is really heavy bus wire, as noted earlier in this thread. Even 12 would be heavy unless you're running a 10 Amp command station.  14 ga is more typical for N scale with a 5 amp supply.  Which is not to say you can't use 10 ga if you find it easy to work with. Heavier than necessary wire does no harm.

 

Feeders are geerally in the 20 - 26 ga range, although which is mostly a matter of personal prefference. If you're going to feed 5 amps through one feeder (i.e., if you have enough trains on the section of track fed by that feeder to draw 5 amps) you'd need 22 ga or heavier, but that wouldn't be a good idea with sectional track (Unitrack, for example is only rated for 3 amps, and that's likely true of anything dependent on friction-fit rail joiners).  Kato's choice of 24 ga (rated for up to 3.5 amps) makes sense for their track.

[The_Ghan]

Webskipper,

 

The wiring on my layout is "supersized" to ensure I get excellent results from DCC.  I'm using Digitrax and transponding and leaving nothing to chance.  I've even shortened all my feeders to 4" and used 18 AWG wire.  For the bus wire I'm using 10 AWG - I like the though of only dropping 1 Ohm per 1000' of wire but hey, who lays 1000' of wire anyway?  14 AWG will do the job.

 

Cheers

 

The_Ghan

[CaptOblivious]

The hing to remeber when using large gauge solid core wire for DCC buses or feeders is the "skin effect": the thicker the wire, the greater the impedance presented to signals at about the DCC frequency. DCC is about 12kHz, so wires smaller than 14AWG do not show this effect. 10AWG will show a significant effect, meaning that, for DCC signals (NOT plain DC) the effective resistance will be much higher than you claim. Don't have the calculations handy just now to say by how much, but you can look it up.

 

In short: for DCC larger wire doesn't always mean better signal integrity. Best vbalance between signal integrity and power transmission is probably at 14 AWG.

 

Update:

Found this nifty tool:

http://ampbooks.com/home/amplifier-calculators/wire-inductance/

 

Suppose that DCC is a 12KHz signal (it's not, because it's variable, but that looks like a good limiting case). Then here is the total effective resistance of various AWG sizes, in ohms per kilometer:

[table]

[/td][td]DC DCC

10 1 28.74

12 1.588 59.26

14 2.525 61.26

16 4.016 63.82

18 6.385 67.25

20 10.150 72.01

22 16.500 79.56 [/table]

 

So…looks like the skin effect doesn't worsen with increased cross-section, but it does mean that the distortion calculations have to be modified a bit. Anyway, 10AWG solid-core is still better, on these calculations, than small wires. So I stand corrected!

[inobu]

This is what I learned, You can have pure gold bus wires, if the connections are loose or make little to no contact onto your track rails you will have voltage drops through out your layout no matter the gauge.

 

I would focus on connection point the most. Take a part the feeder and joiners and you will be surprise/shocked at what you find.

 

Inobu

[inobu]

The hing to remeber when using large gauge solid core wire for DCC buses or feeders is the "skin effect": the thicker the wire, the greater the impedance presented to signals at about the DCC frequency. DCC is about 12kHz, so wires smaller than 14AWG do not show this effect. 10AWG will show a significant effect, meaning that, for DCC signals (NOT plain DC) the effective resistance will be much higher than you claim. Don't have the calculations handy just now to say by how much, but you can look it up.

 

In short: for DCC larger wire doesn't always mean better signal integrity. Best vbalance between signal integrity and power transmission is probably at QR AWG.

 

Funny you bring this aspect to the table. I was going to post my thoughts but felt it might be considered way out there.

 

I'm going to use CAT 5 on my next build because I see DCC as data communication more so than power distribution. Treating the rail as a data link will aid you in achieving optimum operation. When we look at if from the power perspective we just look for continuity. When we look at it from the data perspective it is signal quality which can drive you to creates a better layout medium resulting in a solid power distribution. 

 

Inobu

[CaptOblivious]

The hing to remeber when using large gauge solid core wire for DCC buses or feeders is the "skin effect": the thicker the wire, the greater the impedance presented to signals at about the DCC frequency. DCC is about 12kHz, so wires smaller than 14AWG do not show this effect. 10AWG will show a significant effect, meaning that, for DCC signals (NOT plain DC) the effective resistance will be much higher than you claim. Don't have the calculations handy just now to say by how much, but you can look it up.

 

In short: for DCC larger wire doesn't always mean better signal integrity. Best vbalance between signal integrity and power transmission is probably at QR AWG.

 

Funny you bring this aspect to the table. I was going to post my thoughts but felt it might be considered way out there.

 

I'm going to use CAT 5 on my next build because I see DCC as data communication more so than power distribution. Treating the rail as a data link will aid you in achieving optimum operation. When we look at if from the power perspective we just look for continuity. When we look at it from the data perspective it is signal quality which can drive you to creates a better layout medium resulting in a solid power distribution.  

 

Inobu

 

Speaking of tables, please see the table in my modified post above.

 

 

Funny, I was just wondering the same thing: Whether using Cat5 was a good way to go. We can be sure that the impedance at DCC frequencies is minimized; it's already twisted for us. There are a lot of other pairs in the wire that wouldn't be needed, though, unless you sent the signal down each pair to maximize power transmission capabilities. What is the maximum power that can be sent down a pair of Cat5 wiring?

[David]

Funny, I was just wondering the same thing: Whether using Cat5 was a good way to go. We can be sure that the impedance at DCC frequencies is minimized; it's already twisted for us. There are a lot of other pairs in the wire that wouldn't be needed, though, unless you sent the signal down each pair to maximize power transmission capabilities. What is the maximum power that can be sent down a pair of Cat5 wiring?

 

I don't think it's a lot, even if you use Cat5e. Power over ethernet specifies something like 300mA in total (at something around 10 or 12V), and the Cat5e cables I think are rated for up to 500mA. With a specific brand of cable, under good conditions, you could probably exceed those numbers, but I don't see a safe way to push 1A or more - they're ultimately 24 gauge wires, twisted or not.

[CaptOblivious]

Funny, I was just wondering the same thing: Whether using Cat5 was a good way to go. We can be sure that the impedance at DCC frequencies is minimized; it's already twisted for us. There are a lot of other pairs in the wire that wouldn't be needed, though, unless you sent the signal down each pair to maximize power transmission capabilities. What is the maximum power that can be sent down a pair of Cat5 wiring?

 

I don't think it's a lot, even if you use Cat5e. Power over ethernet specifies something like 300mA in total (at something around 10 or 12V), and the Cat5e cables I think are rated for up to 500mA. With a specific brand of cable, under good conditions, you could probably exceed those numbers, but I don't see a safe way to push 1A or more - they're ultimately 24 gauge wires, twisted or not.

 

is that 500mA per pair, or in total for all eight pairs? Because if its per pair, and if you duplicated the DCC signal on all eight four pairs, you could conceivably transmit up to about 3A 1.5A down the cable (derating a bit)…which is about right not good enough for my liking.

[ShinCanadaSen]

From the Power over Ethernet Wikipedia page:

 

"Power over Ethernet or PoE technology describes a system to pass electrical power safely, along with data, on Ethernet cabling. PoE requires category 5 cable or higher for high power levels, but can operate with category 3 cable for low power levels.[1] Power can come from a power supply within a PoE-enabled networking device such as an Ethernet switch or can be injected into a cable run with a midspan power supply.

 

The IEEE 802.3af-2003[2] PoE standard provides up to 15.4 W of DC power (minimum 44 V DC and 350 mA[3][4]) to each device.[5] Only 12.95 W is assured to be available at the powered device as some power is dissipated in the cable.[6]

 

The IEEE 802.3at-2009[7] PoE standard, also known as PoE+ or PoE plus, provides up to 25.5 W of power.[8] Some vendors have announced products that claim to comply with the 802.3at standard and offer up to 51 W of power over a single cable by utilizing all four pairs in the Cat.5 cable.[9] Numerous non-standard schemes had been used prior to PoE standardization to provide power over Ethernet cabling. Some are still in active use."

 

and

 

"Category 5 cable uses 24 AWG conductors, which can safely carry 360 mA at 50 V according to the latest TIA ruling."

 

so it would seem to me that we can't transmit enough amperage over Cat5(+).

[CaptOblivious]

The IEEE 802.3at-2009[7] PoE standard, also known as PoE+ or PoE plus, provides up to 25.5 W of power.[8] Some vendors have announced products that claim to comply with the 802.3at standard and offer up to 51 W of power over a single cable by utilizing all four pairs in the Cat.5 cable.[9] Numerous non-standard schemes had been used prior to PoE standardization to provide power over Ethernet cabling. Some are still in active use."

 

and

 

"Category 5 cable uses 24 AWG conductors, which can safely carry 360 mA at 50 V according to the latest TIA ruling."

 

so it would seem to me that we can't transmit enough amperage over Cat5(+).

 

51W of power equals: @ 12v, 4.25A; @14V, 3.65A. That sounds like plenty of current to me!

[inobu]

We do push the limits on CAT 5 but I don't think a block or feeder segment runs 3A at any given time.

 

My main thought is to look at the rail as a data link more so than a power link. I think you can dial in your layout's DCC better with this approach.

 

Inobu

[CaptOblivious]

We do push the limits on CAT 5 but I don't think a block or feeder segment runs 3A at any given time.

 

My main thought is to look at the rail as a data link more so than a power link. I think you can dial in your layout's DCC better with this approach.

 

Inobu

 

 

I certainly take your point. I've spent a lot of time lately staring at DCC traces in my oscilloscope. Anyway, perhaps never 3A intentionally, but during a short-circuit…it would be nice if the DCC bus didn't literally melt down every time a car derailed ;)

[inobu]

Breakers are critical for sure.

 

Inobu

[KenS]

Cat 5 is 24 AWG (cat 6 might be heavier, or not, depends on the manufacturer), which means a limit of 0.577 amps in a constrained space (and I think a jacketed multi-strand cable counts as one).  If you have a 4-pair cable and you use all pairs that's 2.3 A max, which is too low for a bus, even if you're using something like a Zephyr.  One short-circuit (engine run the wrong way into a switch) that fails to trip the breaker because it's not quite short-circuited enough, and you might have a fire under the layout.

 

The 51W number assumes 44 volts (0.58 amps per pair on two pairs).  You can't just divide that by 14 (or whatever) to raise the amperage; your amperage is limited regardless of the voltage, because the limit is based on heating (Joule Heating) and heating depends solely on current and resistance, regardless of voltage. 

 

Breakers also are known to have problems if the wire is too small (I think it has something to do with the resistance in a long bus), which can keep the breaker from tripping in a short, componding the problem.

 

You might never have a fire or other serious failure, particularly with a 3-amp breaker or supply, as there is a safety margin in any of these standards. There may be an additional margin due to a jacketed cable being not quite a fully constrained space (there is some radiative cooling), but there's no easy way to know what it is. But the problem is still there, and it gets worse with longer runs or larger supplies. There are good reasons everyone recommends 16 AWG or larger wire for DCC bus use.

[inobu]

Kens,

 

You bring up a good point of a potential fire. Anything under the layout has to be secure, meaning it must be reliable. It looks like CAT 5 cannot provide the level of support needed in the realm of overloads or short circuits.

 

I wanted to take it one step further in that I wanted to run BERT (Bit Error Rate Test) test over the layout. If you can run clean BERT's then you got a good running layout. The BERT test could also help in isolating weak points.

 

You really need data grade wiring to do that. The only problem is data normally runs at low power levels and we operate at higher levels. oh well. 

 

Inobu