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Wire Gauge Table (AWG)

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Wire Gauge Table (AWG)
Posted by Anonymous on Tuesday, November 7, 2006 11:57 AM

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

Circuit Watts

Wire gauge (for length in feet)

6V

12V

6V

12V

3'

5'

7'

10'

15'

20'

25'

0 to 2.5

0 to 5

15

30

18

18

18

18

18

18

18

3.0

6

18

36

18

18

18

18

18

18

16

3.5

7

21

42

18

18

18

18

18

18

16

4.0

8

24

48

18

18

18

18

18

16

16

5.0

10

30

60

18

18

18

18

16

16

16

5.5

11

33

66

18

18

18

18

16

16

14

6.0

12

36

72

18

18

18

18

16

16

14

7.5

15

45

90

18

18

18

18

14

14

12

9.0

18

54

108

18

18

16

16

14

14

12

10

20

60

120

18

18

16

16

14

12

10

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Posted by howmus on Tuesday, November 7, 2006 12:36 PM
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.

Ray Seneca Lake, Ontario, and Western R.R. (S.L.O.&W.) in HO

We'll get there sooner or later! 

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Posted by Anonymous on Tuesday, November 7, 2006 12:46 PM
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.
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Posted by Vail and Southwestern RR on Tuesday, November 7, 2006 1:11 PM

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.

 

Jeff But it's a dry heat!

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Posted by nfmisso on Tuesday, November 7, 2006 1:17 PM
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. 
Nigel N&W in HO scale, 1950 - 1955 (..and some a bit newer too) Now in San Jose, California
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Posted by Vail and Southwestern RR on Tuesday, November 7, 2006 1:22 PM
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.

Jeff But it's a dry heat!

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Posted by Adelie on Tuesday, November 7, 2006 1:30 PM
What Nigel said!

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

- Mark

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Posted by Vail and Southwestern RR on Tuesday, November 7, 2006 1:38 PM

 Adelie wrote:
What Nigel said!

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

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.

Jeff But it's a dry heat!

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Posted by Anonymous on Tuesday, November 7, 2006 1:50 PM

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.

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Posted by Soo Line fan on Tuesday, November 7, 2006 1:54 PM

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

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Posted by Vail and Southwestern RR on Tuesday, November 7, 2006 2:04 PM

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.

Jeff But it's a dry heat!

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Posted by Stevert on Tuesday, November 7, 2006 2:07 PM
 Surfstud31 wrote:

 My understanding is that most DCC systems use 12V 0-5 amps - is this correct?



  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
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Posted by Soo Line fan on Tuesday, November 7, 2006 2:17 PM

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

Jim

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Posted by TheK4Kid on Tuesday, November 7, 2006 2:21 PM

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
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Posted by Vail and Southwestern RR on Tuesday, November 7, 2006 2:22 PM
 Soo Line fan wrote:

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

Jim

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.

Jeff But it's a dry heat!

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Posted by Anonymous on Tuesday, November 7, 2006 2:44 PM

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?

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Posted by Vail and Southwestern RR on Tuesday, November 7, 2006 2:50 PM

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.

Jeff But it's a dry heat!

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Posted by Soo Line fan on Tuesday, November 7, 2006 2:55 PM

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

Jim

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Posted by Anonymous on Tuesday, November 7, 2006 3:17 PM
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.
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Posted by Vail and Southwestern RR on Tuesday, November 7, 2006 3:34 PM
You can post progress pics along the way, you know!

Jeff But it's a dry heat!

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Posted by Anonymous on Tuesday, November 7, 2006 3:54 PM
OK gang - I will start a new thread for progress on the NY&P with pics!  Look for it tonite...
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Posted by jim22 on Tuesday, November 7, 2006 4:20 PM
Ok, I'm an EE too.  I don't think you can get hurt with larger guage wire.  I think the behavior of the system under a short condition needs to be considered.  Under a short condition, the maximum current is being drawn all in one spot.  The worst case is that that spot is 25 feet out at the end of the line, and the 12 volts needs to be dropped somewhere.  It should be dropped inside the booster where the short circuit detection can occurr and shut down the supply.  If too much voltage is dropped in the wiring, it may not look like a hard short to the booster, and it may not protect itself and your layout.

All that said, I think I'm going to use slightly lighter gauge wiring in a star network with shorter lengths out from a central location.  We'll see.  I will have several 8' modules.  Maybe each module will be a power district with a 14 gauge feeder to a central location on the module and feeders of lighter wire up to 4 feet long from there.

Jim

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Posted by Vail and Southwestern RR on Tuesday, November 7, 2006 4:37 PM

Another argument for the larger wire.  I have read of the quarter test, if the booster doesn't shut down when  you put a quarter across the track you need a larger bus wire or another feeder.

I think the star would be fine, a four foot 'sub-bus' shouldn't cause trouble as long as it is 18 gauge or so, I'd think.

Jeff But it's a dry heat!

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Posted by grayfox1119 on Tuesday, November 7, 2006 5:11 PM
When you consider the price difference between #12 and #14 wire, it really isn't that much. And when you compare the cost of all your MRR items in total now, and what it may look like in 5 years, the cost factor gets less and less. Wire is a very cheap investment, one that we should never skimp on. I have plenty of #12 wire in various lengths that I have saved from house wiring projects over the years, so there is no doubt in my mind that I will be using #12.
Also, I want the voltage drop under full load to be as low as reasonable.

As for feeders, if you consider what a schematic diagram, or, an equivalent diagram, would look like with feeders every "x" feet along the track/s, you would see a large parallel resistive circuit. The total resistive load that this would look like to the Power Supply would be the sum of:
1/RT= 1/r1+ 1/R2+ 1/R3+ 1/R4..............

So for the feeder load, the resistance would be considerably smaller, which alllows us to use smaller gauge wire, ie #18.
Dick If you do what you always did, you'll get what you always got!! Learn from the mistakes of others, trust me........you can't live long enough to make all the mistakes yourself, I tried !! Picture album at :http://www.railimages.com/gallery/dickjubinville Picture album at:http://community.webshots.com/user/dickj19 local weather www.weatherlink.com/user/grayfox1119
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Posted by jbinkley60 on Tuesday, November 7, 2006 7:02 PM

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.

Ok, so one EE to another, just to confuse folks here, let's talk about the concept of losing voltage in a real world situation.  While I will agree with your calculations, when we are dealing with DCC the actual differential component of the bipolar pulses don't dictate the actual voltage driven to the motor at any given time.  That is determined by the throttle setting.  What a voltage loss would do is limit the maximum voltage being able to be delivered under load, which would ultimately impact the top speed of the locomotive, not a speed less than maximum.  I don't know about you but I don't run mine at top speed.  So as a locomotive moves further away from the DCC source (as measured in the feet of wire) you won't see a sudden slowdown because the wire resistance goes from .05 ohms to say .1 ohms.  Also since a given locomotive won't draw 5A, the actual draw will be spread along the bus and determined by the amperage draw of the various locomotives based upon their distance from the source.  Lastly, since we are dealing with differential voltage pulses, the DC component of this is not a constant as with standard DC and so the actual voltage drop as a function of the DC component and not a straight linear function.  Sound locomotives have capacitors in them and smooth some of this out. 

So what am I trying to say here ?  I agree that a larger wire won't cause electrical issues but can cause mechanical issues.  I agree that in general bigger is better but I don't believe it is as critical as some indicate.  Also there are ways to use smaller wire by using multiple buses.  I will often put a barrier strip in the middle of an area and run 3' 18 ga.  feeders back to it.  This is actually as effective as a 14 ga bus run along the track with smaller gauge feeders.  So while I feel that proper wiring is important, it isn't catastrophic and things that are more important are blocks, which provide fault isolation and an easy way to add additional boosters in the future without cutting rail. Let's don't get too hung up on wire sizes and miss out on other important wiring items. 

This is almost akin to the power routing vs not for switches.  To me that is another of the less filling / tastes great debates...

      

 

Engineer Jeff NS Nut
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Posted by Anonymous on Tuesday, November 7, 2006 7:37 PM
jbinkley, what you said about the mechanical problems resulting from larger wire is what another EE was trying to tell me.  Basically, from what I could tell he was saying, you can actually cause damage to an engine (motor) by using wire that is too big.  I am not an EE and I have no idea why that is but I was concerned about it.  I don't mind using 14 gauge wire as long as I know it will not cause the mechanical problems you and this other EE alluded to.
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Posted by jbinkley60 on Tuesday, November 7, 2006 8:57 PM

 Surfstud31 wrote:
jbinkley, what you said about the mechanical problems resulting from larger wire is what another EE was trying to tell me.  Basically, from what I could tell he was saying, you can actually cause damage to an engine (motor) by using wire that is too big.  I am not an EE and I have no idea why that is but I was concerned about it.  I don't mind using 14 gauge wire as long as I know it will not cause the mechanical problems you and this other EE alluded to.

I wasn't referring to that type of mechanical problem.  I don't see the point he is making.  What I was referring to was having wire that is too big to work with effectively.  At some point you need bigger terminal strips, the wire is harder to bend, bigger holes, etc.etc.  The other point I was making was that if you break your layout up into blocks then your busses get shorter and thus the wire size requirements may be different.  I even put blocks on my main line.  Right now I use two boosters to power it but I could use as many boosters as I have blocks.  This allows me to easily scale it as the current requirements increase.  So, as an example, I could use 4 - 5A boosters and spread 20 amps over the 4 blocks.  The wire size requirements for a 5A block would be much less than a single 20A block that stretched over the entire layout. 

I hope this helps explain my point.

 

Engineer Jeff NS Nut
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Posted by Anonymous on Tuesday, November 7, 2006 9:37 PM

I understand the block concept and and was considering two blocks for my layout.  I am a sole operator right now so I am not sure if it's necessary.  Regardless, I will need at least 10' wire runs.

This EE was trying to explain an example of a motor overheating (or running hot) because of not enough power going over wires that were too large.  When the amps were increased to the motor, problem solved.  I don't get it but then I'm not an EE.

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Posted by grayfox1119 on Tuesday, November 7, 2006 10:34 PM
Surfstud31: Somehow, I think you might have misunderstood what your EE friend was saying regarding the larger wire size. There is no way that a motor can overheat because it's power source has wire size #8 rather than #12 or #14. The larger the wire size, the lower the resistance, and also, the more current it can safely carry without overheating.
The inverse is true. Just look at a toaster. It has very small size wire for the current that it carries, intentionally, so that it glows red hot to toast your bread.

Using a wire size larger than what is required to suppply current to a device, will not cause the device to overheat.
Dick If you do what you always did, you'll get what you always got!! Learn from the mistakes of others, trust me........you can't live long enough to make all the mistakes yourself, I tried !! Picture album at :http://www.railimages.com/gallery/dickjubinville Picture album at:http://community.webshots.com/user/dickj19 local weather www.weatherlink.com/user/grayfox1119
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Posted by Anonymous on Tuesday, November 7, 2006 10:48 PM

I must have misunderstood him Dick because it didn't make sense to me and although I'm not an EE, I thought I understood enough about electricity to know that! LOL!

Oh, Jeff, here's a link to the thread for my layout progress pics:

http://www.trains.com/trccs/forums/946203/ShowPost.aspx

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