DCC Power Bus cable size

[TonyW]

New to DCC and jumping in with both feet at the deep end.

 

I am building a large (ish) layout 21' x 13' and I have been told to use 2.5mm sq domestic mains cable for the power bus, is this OK. 

 

I have checked a few web sites including DCC Concepts which recommends 12 to 14 AWG for this size layout, so from this 2.5mm sq would seem OK but would like to double check with those who have done this before.

[RFS]

Yes that's a good size and what I use on my layout which is 32' x 8'. 

[corax67]

It's fine - used on a 40'x20' eight track tail chaser with no problems.

[TonyW]

Yes that's a good size and what I use on my layout which is 32' x 8'. 

 

It's fine - used on a 40'x20' eight track tail chaser with no problems.

Thanks, better safe than sorry!

[Phil S]

There are 2 requirements to be met by your cabling, with perhaps a 3rd as an additional 'safety feature'.

There is NO SINGLE ANSWER because these MINIMUM requirements DEPEND on the NORMAL MAXIMUM and CUT_OUT CURRENTS of YOUR CHOSEN Controller/Central Unit/Booster.

 

1/  The wiring - whether a combination of track itself, and or associated bus wiring - which 'forms a loop':

From the controller to eg the OUTER rail, and normally via locos, coaches and accessories, RETURNS via the INNER rail

[ Note these labels purely for 'discussion': the current is AC, and it is up to you which wire goes to which rail ... as long as they don't go to the same one !!!! ]

 

THIS MUST be capable of conducting the MAXIMUM NORMAL CURRENT without 'heating up' and melting any insulation... ...

 

but ALSO it should conduct it with so little resistance to the flow, that the VOLTAGE DROP in the wiring is MINIMAL .... eg no more than perhaps 2 VOLTS at the FURTHEST PART of its run, WHEN the MAXIMUM CURRENT is being taken by one or more locos etc in that area. 

 

[ When there is NO LOAD, there is NO CURRENT, and therefore NO VOLTAGE Drop .. so simply measuring a 'STATIC' layout with nothing moving will NOT show how much the voltage drops under load ]

 

A SOUND DECODER is likely to be the most sensitive device which would be affected by the voltage drop.  In theory decoders should work with the voltage dropping to as low as about 7V.

 

IF the power supply is from an 'old-fashioned'/traditional TRANSFORMER, the output voltage - at the controller - will drop under load, and rise with no load: if this is NOT regulated within the controller, then this can be seen as SIMPLE coach lighting (without on-board regulators) changing in brightness to match: take care with '12V' LED strips for example... many take double the current at 14V - the extra heat is noticeable!

 

2/ THE SAFETY CUT OUT or 'Penny' test:  WHAT HAPPENS? ... when a short circuit occurs on the track due to a derailment?

Given the conditions of requirement 1 above, a LARGE CURRENT could flow ... with ALL the voltage being 'dropped' in the wiring....

... to prevent this going into meltdown, and other possible damage to components, the CENTRAL UNIT or BOOSTER or closer local breaker... MUST CUT the current when a safe value is exceeded .... this varies a lot with different controllers, and therefore, so does the acceptable resisitance of the wiring !!

 

EG A Basic 1A controller (much like a  traditional analogue controller switched to 1 section): 16V @ 1 A  x Rmax ... Maximum resisitance = 16 ohms   Maybe 2A for CUTOUT? implies Rmax = 8 ohms

 

A 3 Amp Controller with 16V dcc needs a loop resistance of LESS than  16/3 or 5.3 ohms  for the normal maxuimum current to be able to flow .... maybe Rmax of 2 to 3 ohms to CUTOUT

A 5 Amp Controller with 16V dcc needs a lower loop resistance (ie thicker wiring)  of  less than 16/5 or 3.2 ohms, and  therefore to ensure the protective cutout operates maybe Rmax of 1-2 ohms

 

A 10 Amp controller - more typically used for G Scale in the Garden, usually benefits slightly from a higher voltage of 22V:   So 22/10 = 2.2 ohms MAXIMUM resistance for NORMAL running, and LESS than 1 ohm to ensure the protective cutout operates in ANY short circuit.

 

So, you can see that a single value for wiring cannot be quoted: it is up to YOU to check your system will CUT OUT SAFELY when ANY PART of the TRACK is SHORTED.

NOTE that EVEN LOWER RESISTANCES ARE TO BE PREFERRED: In the examples above, when the maximum current flows there would be 'zero' volts across the track - the calculations were based on A FAULT_CONDITION in which the Controller would NOT cutout, because it was its designed NORMAL MAXIMUM. (Hence suggesting half that to ensure tripping)

 

3/ My final 'requirement' is to ensure that whatever wiring you use within your layout CANNOT BE CONFUSED with any mains wiring used in the vicinity.

    When choosing wiring by 'rating': 'MAINS' cabling is based on set standards and assumptions about wiring in plaster or conduit etc, and acceptable tempratures, and insulation thickness.

 

For our LOW VOLTAGE wiring - we do not need the heavy insulation .... having more copper and less plastic is better !  

So I recommend 'heavy current loudspeaker flex' rated at '15A' or similar, in clearly identified Red and Black or White and Black figure of eight format - where possible.   

 

HOWEVER, on my indoor layout (5m x 8m OO) I actually also use 'sold for mains' 6A  twin flex in differing colour sleeving for different power districts or purposes.   But in all cases -  check for minimal, acceptable voltage drop when under load, and then the safety cut-out MUST operate on a short!

 

Note that many people 'convert' from analogue to DCC whilst keeping their existing Sectional wiring - which was probably adequate for 1 loco in each section,  and probably no coach lighting or sound.

This is PROBABLY adequate, on safety grounds, where the new DCC controller ONLY has a 1A rating ... but unlikely if it has a 5A normal maximum (and cutout somewhere above that )

[Crosland]

New to DCC and jumping in with both feet at the deep end.

 

I am building a large (ish) layout 21' x 13' and I have been told to use 2.5mm sq domestic mains cable for the power bus, is this OK. 

 

I have checked a few web sites including DCC Concepts which recommends 12 to 14 AWG for this size layout, so from this 2.5mm sq would seem OK but would like to double check with those who have done this before.

It doesn't have to be "domestic mains cable", that's just a convenient and relatively cheap (in quantity) way of buying the correct gauge wire.

 

Andrew

[TonyW]

It doesn't have to be "domestic mains cable", that's just a convenient and relatively cheap (in quantity) way of buying the correct gauge wire.

 

Andrew

Actually it is very cheap as I have a lot left over from our house build.

[Crosland]

You can safely use it, as long as you do the "penny test": at the point furthest from the feeder of that section you put a metal coin across the track. The booster/command station should trip out immediately. If not, decrease the size of the section it feeds or move the feeders closer to the end. Mind that this may mean the other side of the section can subsequently fail the penny test!

Not sure exactly what you mean, but you should really have a feed (or "dropper") from the bus to every piece of track, so max length of approx 3' for a length of flexi-track.

 

You then do the coin test at the farthest point from the booster. If the wiring fails the coin test then it is the bus wire that is inadequate rather than the location of the feeders.

 

Andrew