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What size wire to use?

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Posted (edited)
On 12/05/2011 at 15:01, Phil S said:

Eg: a 1 Amp limit and 10V track voltage requires a resistance of LESS THAN 10ohms

a 2 Amp limit and 10V track voltage requires a resistance of less than 5 ohms

a 5 Amp limit and 10V track voltage requires a resistance of LESS THAN 2 ohms

I=V/R a 10Amp limit and 10V track Voltage reqires a resistance of LESS THAN 1 ohm but

a 10 Amp limit and 20V track voltage requires a resistance of less than 2 ohms (eg G Scale)


Phil, I have to disagree with some of your earlier info as posted above.


Those values allow a fault trip, but in reality the actual maximum wiring resistance must be a tiny fraction of those values for proper operation.

You may have intended that but it's misleading at first glance.



For the controller to deliver it's maximum non-trip power to the locomotive(s) on the layout, the resistance must be low enough so the voltage drop is no more than a fraction of the supply voltage; the values as above would allow half or more of the supply voltage to be lost in the wiring with a motor running at stall near the current limit. 


The target resistance should be, say, no more than a tenth of those values at absolute worst case and preferably far lower.


The 32/0.2 advised elsewhere for the supply bus should be OK for most smaller layouts / controllers, that's 1mm^2 and about 20 ohms per kilometer.

With eg. a five metre run of two wires the loop resistance would be 0.2 Ohms; that could be reduced by wiring "ring main" style.



Some actual calculation info:


Typical wire resistances, ohms per metre; for stranded wire not listed, use the cross sectional area equivalent:

7/0.2mm (0.22mm^2) = 0.096

16/0.2mm (0.5mm^2) = 0.039

32/0.2mm (1.0mm^2) = 0.020

30/0.25mm (1.5mm^2) = 0.013

50/0.25mm (2.5mm^2) = 0.08


To work out the voltage drop, multiply the ohms per metre value by the total cable length (round trip total; eg. two wires to a point 2m away = 4m round trip). That's the wiring resistance. (If you use a "ring main" power bus, work out for the total bus length then divide by 4).


Multiply that resistance value by the controller maximum current rating and you get the worst-case voltage drop on the wiring. Ideally that should be less than a volt and personally I'd aim for a tenth of a volt or not much more...



Example; a two metre run of 1mm^2 cable, 32/0.2 or 1mm T&E. Four metres round trip length, 4 x 0.020 = 0.080 Ohms.

A smallish layout, with a 1A controller. At one amp, the voltage drop is (0.080 x 1) = 0.080V which is tiny and will have no effect.


The same cable, a 5m run: 10m round trip so 10 x 0.020 = 0.2 Ohms. 

Say that's on an O gauge layout with a 10A controller; the voltage drop at 10A is (0.2 x 10) = 2V, which is not at all good! Thicker wire or a ring main needed.


Any other gear connected in line, eg. current sensing devices for occupancy detectors, will add to the voltage drops.



As Phil says, it needs working out on a layout-by-layout basis, but ideally with practical numbers rather than guesswork.




Edited by RobjUK
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Phi's words would have  been "just" correct if he had  included the resistance value of the locomotive, and that will be the major part of the resistance.

The wiring as RobJ says, should have as low a resistance value as possible, and his examples look good to me.


I'll just go back to meauring this 1 amp current shunt..

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I seem to have been quoted from PART of a 2011 posting ... 8-)


IN most cases where I have included those figures, I have given 2 sets of information:


FIRST that the aim should be to drop no more than about 2 Volts around the layout under MAXIMUM load  .... so as to avoid the possiblility of 'dropout/brown out' by decoders - especially sound decoders !! at the 'furthest' parts of the layout. - THIS CALCULATION produces a worst-case value for the 'loop' resistance ( EXCLUDING LOCO ) because it is based on

                                     2 Volts acceptable drop  = Max Current x Maximum Loop Resistance.

Some of the values will depend on your track voltage which might be from 12V to 22V but typically 16V on track ( 12V to motor)

For the example I used simple values of 10 and 20 to keep the arithmetic obvious.


THE  SECOND to show the MAXIMUM RESISTANCE for a loop to be able to have the indicated current flow in a short circuit: YES, a 'limiiting case' to avoid assumptins about how much headroom there would be between 'normal'/continuous output and a short circuit cutout.


(Without the orignal supporting text - assume these currents are the trip current eg 5 Amps for a 3Amp continuus output controller)


(I've decided 'continuous maximum' is a better phrase than 'normal maximum' - as it is to do with the CAPABILITY of thr Controller, and not how many trains you shoose to run.   THERE IS NO NEED FOR LOCO RESISTANCE TO BE IN THESE FIGURES - it is simply to draw attention to the differing current-carrying requirements of differing layouts based on their Controllers Maximum output capability.


(At the time it was written 8 years ago ?? ) there were MANY people claiming, on the one hand, that conversion of their analogue layout needed no changes in wiring  - I can only assume thay have  a 1Amp or similar DCC Controller (and no commercially made Power Feeds with interference Suppresion capacitors inside them !!! - like Hornby, Roco etc.. )  ... or their short circuit protection may not work well...


Conversely there were many advocating the use of 30A Ring Main T+E  (which I used for my Zero-1 layout back in the 80's   (and in one response, someone took great exception to my not specifically mentioning droppers ... )


THE POINT I was trying to make, is that there is NO SINGLE ANSWER - because required information is omitted....  Drawing the parallel to the domestic consumer Unit - Incoming Mains maybe 100A - the house then has 1 or more ring mains of 30A fused/breaker rating, and lighting ring(s) of 5A and cooker and immersion and other specific feeds with appropriate limits.  Size of layout is also not directly relevant.


IN THE SAME WAY ; a model railway layout could be  using a 10A or 5A output controller, but only require a thinner-than 'expected' conductor, by having those areas beyond a circuit breaker provising the protection.   I use PSX's due to their 'intelligence' with coping with initial capacitor charging surges of early sound decoders ... meaning that although my LOFT layout has 4 x 3.2A power districts, it is split into SUB districts with a lower protection  threshold  ... which cut out first - (and auto reset when possible) - whilst leaving the remainder of the layout running as normal.   

Our 3-level transportable H0 layout uses 3 x PSX breakers to separate each level in the same way (from 1 x 3.2A supply)


However, with our transportable G scale (upto 17m long), where thick brass cross section rail provides both the running and the current

(Why have a piece of relatively thin copper wire, when a large section brass rail is in use !!!) - 1 controller of 3.2A at 16V provides all.

The thinnest wire is the initial connection to the small terminals of the Z21 controller 8-)


BUT the GARDEN layout has a slight slope, and may have many engines running  and most with sound and lighting ( including all coaches)

In 125m of track, it uses Massoth rail clamps (screwed fishplates) and multiple parallel power feeds of 2.5mm2 from extra copper/aluminium  wires. Its resistance is less than 0.5 ohms, and can trip the 8A massoth we use for the garden, on a short.

[ I do NOT use Peco 'finescale' G scale track which has a small cross section and relatively  high resistance ]


IN ALL CASES the ABILITY FOR A SHORT CIRCUIT at the track NEEDS to be VERIFIED .. for safety, and adequacy to avoid unnacceptable voltage drop.    Where 2 or more train may be present or operating in a region, the lack of brown-out shuld be observed.


Many also mention 'the need' for dropper wires and problem with fishplates: again this depends on more than is usually mentioned:

Rail Size and Material ... from Brass or Nickel Silver  in N to Gauge 1 code 55, 80, 75, 83,100, 12.   Continuous Rail lengths may be up to 1m, 1.2m or 1.5m.  IN the garden, the brass LGB fisplated may be relied upon (and are okay outside for about a year if not fitted with graphite grease - or Massoth or Hillmam Rail Clamps.   Ballast may have been added with 'wetted' adhesive which get in between fishplate and rail provsing a  nice insulating joint !  Hazardous 'Wire Wool' trees may be by the line, and fall onto it causing sparks and a fire with DCC and inadequate protection.... 



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