How important is wire gauge?

Classic Toy Trains Forum
1

I am in the preliminary stages of planning a 12’x24’ O-Gauge Layout. I will (probably) be using traditional engine control with 1 or 2 Lionel ZW Transformers (I haven’t decided yet) and somewhere in the neighborhood of 200 ~ 300 feet of track with the ability to run 2-3 engines and an average assortment of accessories . I keep seeing articles and discussion threads about wire gauge and I’m puzzled.

If the Transformer puts out up to 24Volts AC and under 300Watts why would you need wire fatter than 18AWG? (unless of course it was a longer run than 25 or so feet). Is there some marketing scheme by the big wire manufacturers to sell fat wires or are there any REAL reason(s) for the larger wire gauge recommendations? My hairdryer runs 1875Watts at 120Volts on 16AWG why cant trains run on 18~20 Gauge wire?

(and yes, I do have 900’+ of 18AWG dual conductor wire on a spool and 500’+ of 24AWG 4-conductor on another spool waiting for this layout)

Thanks!
Tom W.

2

The problem is called “voltage drop”. As the length of the wire increases, the reasistance increases. The way to compensate for that effect is to increase the size of the wire (lower number guage).

By thinking of electricity like water, it may be easier to understand. Voltage is the “pressure” with which electricity is forced to flow. Wire guage is the size of pipe. For safety reasons we lower the pressure (voltage). Over short distances, the drop is minimal. But the further you try to push, the more difficult it becomes, and the need for larger wire becomes noticable.

Is the cord on your hair dryer 25’ long?[:D]

3

In the wide wooly world of electricity, the heavy the wire, the better the flow of electricity. So a general rule, you want to use heavier wire on your main track feeds and on accessories that have electromagnets, solenoids or vibrator motors. Accessories with lights only or smaller DC motors can operate with smaller wire.
It also depends on the size of your layout and the types of engines you’re running. On a large layout, 16 gauge wire will save you problems in the future with poor electricity flow. The older open frame AC motors do require more current. Many of the older Lionel engines start up at around 6 volts. The newer engines with speed control and command capability also need more current. I was surprised when I ran a Railking engine with speed control electronics on my layout how much more power it needed to work.
I run mostly the newer pre-command units with DC can motors which require far less current to operate. Though I have a spattering of MPC and postwar Lionel units with AC open frame motors… and they do need a little more juice than the others. I also have set up some throw switches where I can alternate between AC and DC current to the track. I’ve found that the DC motored engines (minus the circuit boards) run much much smoother with DC current to the track.
BUT even with all that, I still use 16 gauge wire from the transformer to the main leads and to the main terminal blocks. Then I use 18 gauge to sidings and shorter insulated blocks. I make my own low-current street lights and I can use 24 gauge for them as well as for some of my own accessories where I used DC motors.
SO I’d stick with a heavier wire for your main track lines and feeds. But you’ll be surprised I think how quickly you’ll use all that wire you already have for other things as your layout grows! Seriously! I can’t believe all the wire I’ve used.
IMPORTANT TIP: Use color coded wire. If you want to save money by using one color, be sure to label everything as you hook it up: you can use self-sti

4

I’m pretty sure that your hairdrier would use 1875 at 125 volts (not 120) which corresponds to 15 amperes, the most you are allowed to draw from a NEMA 1-15R or 5-15R receptacle. AWG 16 seems a little light for a 15-ampere appliance. I would have used AWG 14.

Anyway, at 15 amperes, a 6-foot AWG-16 cord will drop about 720 millivolts, which is negligible compared to 125 volts. On the other hand, a train drawing 2 amperes 25 feet away through AWG 20 will drop 1 volt, which is much more significant compared to, say, 16 volts. The ZW is supposed to put out a maximum of 20 volts; so you could be okay even with a drop of a couple of volts, as might occur running a couple of locomotive motors and lighted cars. However, since it is rated to supply 14 amperes continuously (and probably much more for a while), one should really use at least AWG 14 with a ZW, just for the sake of fire safety.

AWG 14 is of course readily available in the form of NMC non-metallic-sheathed cable used for house wiring at very reasonable prices.

As I have said before, I solder my rail joints; and I have had no indication of voltage drop on my 11x18-foot layout with a one-point feed. I am convinced that, until you get to much larger layouts, the main problem is with the joints, not with the resistance of the rails.

5

I have some solid core (copper) old bell wire @ 18 ga…is there a law that says you have to used stranded wire? The bell wire takes a little more heat to solder and it isn’t as flexible, but I have quite a bit of it and hope I can use it. My table is only 4’ X 8’ and I run only one train at a time…

Jim Duda

6

No Jim, the only law here is Ohm’s law!

voltage = current x resistance

The key to this formula is to know the resistance pre foot of the material that you are using. That number is a function of the material, such as copper or aluminum, and the size of the wire. I’m sure that there is a table somewhere showing those values, but I don’t have one handy.

The rest is a matter of personal preference, beside, with your layout size, this issue is almost moot.

7

Quote:

“The rest is a matter of personal preference”

I have to mildly disagree with this statement: there’s also the non-trivial matter of fire safety.

Thinner wires heat up more when they carry a significant amount of current. I tend to thing that anything above 1 amp is pretty significant. If the wire isn’t sized right, the wire will get too hot & can then start a fire.

If you’re referring to stranded versus solid wire, I agree, that’s a matter of personal preference. However, stranded wire is more flexible & is less susceptible to breakage.

Tony

8

Absolutly correct Tony, I should have mentioned that part. As a matter of fact in my younger days I actually experienced a few meltdowns.

In a controlled reaction this effect can be useful for things like smoke units, toasters, and hair dryers. Its the unintentional and unexpected wire heating that can be very dangerous.

Modern electrical codes, and UL testing, help to protect us from most of these hazards every day, but nobody is watching over us when we wire our layouts.

9

so then (only for the sake of argument) if a 12 AWG stranded carry 20 Amps at 120Volts why couldn’t a 18AWG handle 6 to 8 Amps at 24 Volts, a fraction of that? any electricians reading this with the formula? i know there is a way to figure this out and prove it’s safety and… I would really like to use the wire in my basement that i didn’t have to pay for. [:D]

thanks again,
Tom W.

10

That is about the limit for that size wire, just make sure that you don’t try to squeeze more amps through it. The way you do that is by installing a fuse or circuit breaker. That way the fuse or breaker becomes the weak link, and you won’t fry the wire.

Even though I’m not a licensed electrician, I have a lot of experience working with high and low voltage wiring. I have completely wired 2 homes under permit, and have done quite a few train layouts, including a large public display. I have also worked along side a couple of electrical engineers, and a couple of electricians.

To sum up here, there are 2 seperate issues here, that both have wire size in common.

The first and most important is CURRENT. Too many AMPS through too small a wire can be dangerous!

The second issue is VOLTAGE drop. The longer the wire run, the more of an issue this becomes. As lionelsoni explained earlier, because the starting voltage is low to begin with, a loss of a volt or two can make a difference in the way your trains run.

Have fun and stay safe.

11

thanks again for all your help and input. i also found some links that provide estimates for voltage drop and resistance.

http://www.pelco.com/support/tools/wiregacalc.aspx

http://www.tnt-audio.com/clinica/wirecalculator.xls

http://www.mogami.com/e/cad/wire-gauge.html

http://www.electrician.com/vd_calculator.htm

I am handy with wires and electric stuff and am comfortable doing eletric work in my home but have forgotten all the formulas I’d learned in trade school 20 years ago. OY!

I’m becoming a born again model/toy railroader and am excited to find that this forum seems to work and respond so well. Thanks all.
Tom W.

12

For the inquiring minds who want to know, the diameter of a wire as a function of AWG number is

diameter = .005 inch * 92^((36-AWG)/39)

where * denotes multiplication and ^ exponentiation.

As a practical matter it can be very useful to know that wire area and resistance very nearly halve or double every 3 wire sizes and decrease or increase by a factor of 10 every 10 wire sizes. If you can remember that AWG 10 wire has a diameter of about 100 mils (.1 inch) and a resistance of 1 ohm per 1000 feet (note that 1, 10, 100, and 1000 occur once each in the rule!), you can calculate in your head most of what you will ever need to know about wire.

Except “ampacity”, or current-carring ability. This is a little fuzzier than resistance, since it depends on the type and thickness of insulation, ambient temperature, and how hot you want to let the wire get. However, a good starting place is 30 amperes for AWG 10 and 20 amperes for AWG 12, with a rule that these numbers are halved every 4 wire sizes (not 3).

Thus, for example, AWG 20 has a resistance of 8 ohms per 1000 feet (a factor of 10 greater than 1 ohm per 1000 feet for being 10 sizes smaller than AWG 10) and can be expected to carry about 5 amperes safely (a factor of 2*2 less than 20 amperes for being 8 sizes smaller than AWG 12).

For the resistance, you may have to do some roundabout calculations: For example, to get the resistance of AWG 18, go 2 factors of 10 up to AWG 30 (100 ohms per 1000 feet), then 4 factors of 2 back down to AWG 18 (6.25 ohms per 1000 feet). Or you could go one factor of 10 down to AWG 0 (.1 ohm per 1000 feet) and 6 factors of 2 back up to AWG 18 (6.4 ohms per 1000 feet). This gives virtually the same answer–remember this is just an approximation–but close enough for our purposes.

The reason that you should use AWG 14 with a ZW is not that you need such low resistance to keep the voltage drop low, nor that you need the ampacity for the current th

13

Hey Tom W. Thanks for those websites. I use small gauge wire (18 awg.) with very negligble voltage drop (less than .1 volts) on a 105 sq. ft. O-gauge layout. I doubt that any locomotive draws more than a couple amps, so short runs with smaller wire shouldn’t be a problem.
Tim R.

14

The issue isn’t only how much current the locomotives are pulling, it’s also how many amps any other cars that have center rail pickups are pulling (e.g., lighted cars). If you have a triple lashup pulling 10 lighted passenger cars, you’re going to be drawing a lot of current. I don’t know the figures, but I wouldn’t be surprised to hear it was something like 10 amps or more.

When you wire your layout, you have to take into account how many lighted cars as well as how many loocomotives you’re going to be running. And if you’re running TMCC locos, you have to figure the maximum number that will be in each power district on your layout.

While you’re at the wiring, and if you’re using older power sources with slow acting circuit breakers, you should consider putting in some fast acting breakers or fuses into the writing. The breaker in the transformer is there to protect the transformer; however, an overloaded wire can start a fire before the transformer’s circuit breaker pops.

This issue can be even worse if there’s a short circuit; in this case, the resistance is close to zero & huge currents (100 amps & more) can be flowing through the wires.

On the OGR forum, there’s a poster that uses the name FMaguire. He’s an EE and has posted a number of very detailed & well written posts on wire sizing & circuit breakers.

Tony

15

does any body know where to find what these trains can truly draw in amps at whatever voltage? and what is a “big” draw for a lionel engine( or any O-gauge makers product)? I can figure the cars with lights by counting bulbs and wattage easy enough, but the engines and locos…? I’m having trouble looking up any figures for them. (i know that a shorted trackor engine can draw something like “1.8 gigawatts” (see the movie “back to the future”) but a properly running, high-draw engine or loco set, what does lionel (or whomever) say thier motor will draw? does anyone test this stuff for posterity or to satisfy thier obsessive/compulsive nature?

primarily, I’m just curious to know… when I learn something new I usually beat it the topicto near death. as I am coming back to this hobby I am seeing loads of articles and postings saying “use bigger wire” but not exactly saying why. I worked in a building with lots of electricians and they’d always find the “load” of the equipment and figure out wire size, type, fusing, etc. before connecting anything.

“boil it down for me woodsy” - my wife would say.
i dont wanna buy new wire if i dont have to.

You have all been very helpful and I look forward toreading thru this forum as I start building and wacthing them run again. [:)] [:)]

thanks,
Tom W.

16

Tom, the problem is that the current drawn depends on the locomotive–older types draw more, those with multiple motors draw more, those with faster gearing draw more–, the condition of the locomotive–poor lubrication, bad bearings, rubbing magnetraction magnets draw more–, the track–sharp curves draw more–, speed–faster draws more–, and load–longer trains, older cars, heavier cars, grades, curvy track draw more. Over all, one can assume that the current will vary from about 1 ampere to 5 amperes in normal running. If you run well lubricated new one-motor locomotives with modern cars and short trains on level track, you will be closer to the bottom of that range. If you run heavy two-motor pre-war or post-war locomotives pulling long trains of postwar cars, you will be at the top. It’s not a problem that lends itself very well to much precision.

17

I’ve seen a table published showing the maximum amount of current that different types of locomotives can be expected to draw. I think this table was in Neil Besolough’s TMCC book, but I’m not sure. I’m at work, so I don’t have access to the book to check.

I’ll call these numbers “reasonable approximations of maximum current draw,” or RAMCD. What I mean is that these numbers are kind of like the MPG estimates you see published for cars: “Your milage may vary” depending on driving style, etc. As Bob said, there are lots of factors that affect how much current a locomotive actually draws, and a particular engine that you own may draw more or less than the figures in that table.

So what you need to do is figure out how many of the engine with the highest draw (probably an engine with a double universal or PullMor motor) you’ll be running at one time on any given power district, multiply by the RAMCD for that engine, then add up all the wattages on the bulbs in your average consist, and you’ll have a figure for current that you can use for planning.

Next, find a wire size that can handle that load, plus a safety factor, then find some fast acting circuit breakers that can handle the same load plus the same safety factor, and begin wiring.

Note that it doesn’t matter how much voltage is being applied to the tracks for this exercise; you’re interested in the maximum amount of current that the wire will be able to carry. That will probably be at maximum voltage, anyway. If the voltage is less than maximum, then the current draw will be less, too. Unless you end up running more stuff than you allowed for in your plan at that lower voltage setting…

Good luck!

Tony

18

One more thing comes to mind: There are limits to how much power you can supply to a power district that have nothing to do with the wire.

If you’re running TMCC, the most power you can deliver to any one power district is 400 watts through a TPC 400. At 18 volts, that’s about 22.22 amps. If you have a derailment leading to a short circuit, that’s enough current to weld a wheel to your rails! I’ve even read about people having had this happen!

What I would do is consider how much current what I want to run will likely draw at a worst case, then compare that with the power I have available. I personally would go with 15 amps as a max current draw and wire & breaker accordingly. This seems like adequate power for most applications.

Tony

19

Another good point Tony. I’m not even sure if it means anything to the person who originally posted the question, but it means a lot to me. He may or may not be using TMCC, but I am.

I’m just starting to build my layout, and my plan is to have standard blocks for both signaling and power, even though I have no intention of running any conventional engines. This way there is only one train in any block at any time (hopfully), allowing me to provide overload protection to each block at about 8 amps max. I don’t run a lot of lighted cars, or old “high draw” engines, or even multiple units, so for me 8 amps should be enough.

I have a lot of materials left over from that public display I did 10 years ago, including some “serious” homemade power supplies that could easily be turned into welders.[:D]

20

I am getting back into this with grand kids. you guys are talking about wire gauges & voltage & amps i never thought about. i am going to build them a 027 layout with 53 year old lionel stuff. birkshire’s, gang cars, ect. the old stuff, & some new stuff, intermodal cranes ect. boy do i have to do some homework. thanks for the sites on wire gauge. will be back here a lot now. thanks guys.