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DCC newbie - wiring question


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Everywhere I look it recommends using a wire size of at least 16/.02 for the track bus, some places twice that size. However, does that wire size need to be that big right back to the DCC power panel - I'm using an NCE starter kit with the Power Panel which has RJ45 plugs for the controller and a small PCB connector on the back for the track connections.

 

 

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Voltage drop proportional to the product of current draw and the resistance of the wire, so the higher your current draw, the larger the voltage drop will be for any given wire size and length.  The way to reduce the voltage drop is to reduce the resistance of the wire, which means using more strands - ie a thicker wire.  Since lots of DCC systems are 5 Amps, compared to typically 1 Amp for a DC controller, this is the reason why DCC requires larger wire sizes.

 

If your layout is to be a small inglenook type layout, with a single locomotive with a DCC system with a lower power output (I think the NCE starter kit is only 2 Amp) then the wire size will be much less critical than it will be if you are planning a larger layout with multiple locomotives and longer wire runs.

 

In general, I'm planning to use 16/0.2 wire as a minimum (ie droppers) and then 24/0.2 for longer runs to track detection units and finally 2.5 mm2 wire for the power bus back to the command station (which can put out 5 Amps).

 

If you are going to be mixing wire sizes, use the larger wires where you'll have the highest current, which will be the output from your command station.  However, as above, I don't think of 16/0.2 wire as being 'big'.

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Its all about minimising voltage drop for the feeds to the track - the bigger the wire size, the lower the resistance and the lower the voltage drop.

 

16/0.2 is roughly 0.5mm2, which is a bit on the small side, particularly if you have a long cable run (say 3 metres or longer).  I use 2.5mm2 wire derived from mains flex for my power bus and 1.0mm2 wire for my accessory bus.

 

Yours,  Mike.

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I dare to suggest that 16/0.2 is fine for the track droppers but is much too light for the Track Bus and you should consider stripping some mains cable  with the minimum being 1mm2 as suggested?

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1 hour ago, KingEdwardII said:

Its all about minimising voltage drop for the feeds to the track - the bigger the wire size, the lower the resistance and the lower the voltage drop.

 

16/0.2 is roughly 0.5mm2, which is a bit on the small side, particularly if you have a long cable run (say 3 metres or longer).  I use 2.5mm2 wire derived from mains flex for my power bus and 1.0mm2 wire for my accessory bus.

 

Yours,  Mike.


not sure your size of layout, but it seems your accessory bus wire is to small. If it’s a large layout I would increase this size, just to give an indication my DCC accessory bus is currently drawing around 4 amps continuously which is supplied by the command station and powers signals only. There is a separate DC bus for those items.

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This is track bus only, no accessories. Two 4' X 2' baseboards, 009 simple layout. I'm using MERG Ezybus for accessories using Cat5e cables, can I use a Cat5e pair for the track connection?

 

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1 hour ago, rynd2it said:

This is track bus only, no accessories. Two 4' X 2' baseboards, 009 simple layout. I'm using MERG Ezybus for accessories using Cat5e cables, can I use a Cat5e pair for the track connection?

 

Hi,

 

I use Cat5e twisted pairs for the droppers and track bus on my N Gauge layout, it's a bit larger than your layout and has around 100ft pf continous run.

I have run some mains cable to the outer extremes and the used Cat5e from there onwards.

 

Everything runs fine with no appreciable voltage drop. Also remember that unless every single piece of track that you use is isolated then the track itself will act as a large DCC bus in parallel with your wiring.

 

For larger scales this may not work quite as well, but your DCC system will only provide enough current to run your trains, my system is rated at 3A and but it only draws 0.8A with nothing moving and that's with 50 or so DCC loco's and some coaches with lighting. My non sound locos typically draw 200mA when running so I would need to have 10 loco's running at the same time before the system would need to supply 3A.

 

Regards,

 

John P

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3 hours ago, Andymsa said:

but it seems your accessory bus wire is to small

Andy,

 

The 1.0mm2 wire that I use for my accessory bus is rated to 10A, which is way more than my power supply is capable of providing.

My accessory bus is drawing a lot less than 4A: I use point motors that have zero draw when not moving and ~150mA when moving. I have a relatively small number of LED lights that take only a small current. My future plans for semaphore signals involve using low power servos involving no more than 150mA when moving. I don't see the need for anything larger than 1.0mm2 for the forseeable future. 

 

Yours,  Mike.

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4 hours ago, WIMorrison said:

you should consider stripping some mains cable  

I'd be careful over the use of the term "mains cable", since it can mean two very different things. There is the solid core cable typically used to provide the supply to mains sockets in a ring main (etc). Then there is the very different stranded core flex typically used to go from a socket to some appliance. 

 

I would not recommend using the solid core cable for wiring a layout - it is not very flexible and it is generally not so easy to use. The stranded core flex is ideal - nice and flexible and easy to work with (hence the name!). Flex cable is generally available with wires having a cross section from 0.5mm2 up to 2.5mm2 - 2 and 3 core cables are the common ones, but it is possible to get greater numbers of cores if required - 5 core is quite common. It is easy to obtain flex and it is relatively cheap. The main drawback is having to strip the flex when you want the individual wires - but there are cases where having the 2 or 3 wires combined is useful, such as when running connections from accessory decoders to point motors. In my case, this involves a 3-wire connection for which I use 0.5mm2 flex.

 

Yours,  Mike.

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23 minutes ago, KingEdwardII said:

I'd be careful over the use of the term "mains cable", since it can mean two very different things. There is the solid core cable typically used to provide the supply to mains sockets in a ring main (etc). Then there is the very different stranded core flex typically used to go from a socket to some appliance. 

 

I would not recommend using the solid core cable for wiring a layout - it is not very flexible and it is generally not so easy to use. The stranded core flex is ideal - nice and flexible and easy to work with (hence the name!). Flex cable is generally available with wires having a cross section from 0.5mm2 up to 2.5mm2 - 2 and 3 core cables are the common ones, but it is possible to get greater numbers of cores if required - 5 core is quite common. It is easy to obtain flex and it is relatively cheap. The main drawback is having to strip the flex when you want the individual wires - but there are cases where having the 2 or 3 wires combined is useful, such as when running connections from accessory decoders to point motors. In my case, this involves a 3-wire connection for which I use 0.5mm2 flex.

 

Yours,  Mike.


you can also get multistranded mains cable, this type is used in conduit generally. I use two sizes of this 1.5 and 2.5mm sq. I don’t bother with striping cable I just buy 100 meter drums at £10.00 a drum approx the time and effort to strip cable is just not worth it. But as you point out avoid solid type.

 

as to the accessory bus wire I assume this is Dcc, it’s not about its current carrying abilities but how much volt drop and resistance that’s introduced using smaller sized wire.

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8 hours ago, Andymsa said:


you can also get multistranded mains cable, this type is used in conduit generally. I use two sizes of this 1.5 and 2.5mm sq. I don’t bother with striping cable I just buy 100 meter drums at £10.00 a drum approx the time and effort to strip cable is just not worth it. But as you point out avoid solid type.

 

as to the accessory bus wire I assume this is Dcc, it’s not about its current carrying abilities but how much volt drop and resistance that’s introduced using smaller sized wire.

As I said, accessory bus is NOT DCC, it's Ezybus by MERG and uses Cat5e cables.

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2 hours ago, WIMorrison said:

Single core mains cable is universally used as the accessory or track bus by most modellers and works perfectly.

Ok but what would you run from the control panel to the mains cable?

 

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40 minutes ago, rynd2it said:

Ok but what would you run from the control panel to the mains cable?

 

 

By " Mains Cable" I hope you are talking about the cable used for the track bus?

As Andymsa says above, similar sized cable.

 

Here's something that's being missed.

With the PowerCab, the Power Bus (Track Bus) doesn't originate from the Power Cab Panel (PCP).

The system's Power Bus output comes from within the PowerCab handset itself, as it contains both the Command Station and the system's Booster.

As such, the Power Bus output is carried down 2 of the 6 wires in the flat handset cable, to the PCP.

That cable has thicker gauge wire cores than a normal 6-core RJ cable, hence the warnings on the NCE website, not to use substitute, non-proprietary 6-core alternatives.

The PCP is nothing but a glorified junction box or terminal block. It doesn't do anything else.

 

So, in answer to the question  "what would you run from the control panel to the mains cable track bus?

Bear in mind if the track bus wires are not connected directly to the terminals on the back of the PCP, there will be 3 sections of wiring between the output from the PowerCab, to the track feeds.

The 6-core RJ cable from the PowerCab to the PCP,

the connection from PCP to the Track Bus and

the Track Bus.

All of these sections together form the PowerCab's Power Bus.

There's a 4th element, or section, as the rails themselves are also part of the Power Bus.

 

 

.

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26 minutes ago, Ron Ron Ron said:

 

By " Mains Cable" I hope you are talking about the cable used for the track bus?

As Andymsa says above, similar sized cable.

 

Here's something that's being missed.

With the PowerCab, the Power Bus (Track Bus) doesn't originate from the Power Cab Panel (PCP).

The system's Power Bus output comes from within the PowerCab handset itself, as it contains both the Command Station and the system's Booster.

As such, the Power Bus output is carried down 2 of the 6 wires in the flat handset cable, to the PCP.

That cable has thicker gauge wire cores than a normal 6-core RJ cable, hence the warnings on the NCE website, not to use substitute, non-proprietary 6-core alternatives.

The PCP is nothing but a glorified junction box or terminal block. It doesn't do anything else.

 

So, in answer to the question  "what would you run from the control panel to the mains cable track bus?

Bear in mind if the track bus wires are not connected directly to the terminals on the back of the PCP, there will be 3 sections of wiring between the output from the PowerCab, to the track feeds.

The 6-core RJ cable from the PowerCab to the PCP,

the connection from PCP to the Track Bus and

the Track Bus.

All of these sections together form the PowerCab's Power Bus.

There's a 4th element, or section, as the rails themselves are also part of the Power Bus.

 

 

.

The track bus wires cannot be directly connected to the back of the PCP unless I mount the PCP directly on one of the baseboards, even then the wire size would probably be too large for the connector supplied with the PCP. I was planning on a detachable control panel (it's a portable layout) which is no problem with the EzyBus which only needs two pairs in a CAT5e cable.

 

However, as you point out above, the PowerCab is connected through an RJ cable which I have just measured at 6.5 mm wide. There are 6 wires in it and therefore they cannot be bigger than 1.0 mm so why would I need anything heavier to connect the PCP to the track bus?

 

My track bus will only ever carry the load for one or two 009 locos, no sound or lights, so I was hoping to get away with a smaller wire size.

 

 

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25 minutes ago, rynd2it said:

.....However, as you point out above, the PowerCab is connected through an RJ cable which I have just measured at 6.5 mm wide. There are 6 wires in it and therefore they cannot be bigger than 1.0 mm so why would I need anything heavier to connect the PCP to the track bus?

 

My track bus will only ever carry the load for one or two 009 locos, no sound or lights, so I was hoping to get away with a smaller wire size.

 

 

 

Nail.

Head.

Well almost.

The PowerCab is low powered system, therefore 1.0 to 1.5mm Power Bus wires would be sufficient.

The only caveat, is if a higher powered Booster is added later on, or if another more powerful DCC system is connected to the layout, then there may be questions about the suitability of the wiring.

 

Don't over analyse it.

Approach the issue from the end of just installing the thickest gauge wire that's practical, or sensible, rather than from the other end of how thin can the wire be.

 

 

.

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13 hours ago, jpendle said:

Everything runs fine with no appreciable voltage drop. Also remember that unless every single piece of track that you use is isolated then the track itself will act as a large DCC bus in parallel with your wiring.

 

For larger scales this may not work quite as well, but your DCC system will only provide enough current to run your trains, my system is rated at 3A and but it only draws 0.8A with nothing moving and that's with 50 or so DCC loco's and some coaches with lighting.

 

Whilst you don't want any appreciable voltage drop during operation the crucial aspect is what happens when you get a short? If the wiring is not good enough (too high resistance) it may not be detected.

 

11 hours ago, KingEdwardII said:

The 1.0mm2 wire that I use for my accessory bus is rated to 10A,

 

A current rating is basically meaningless, unless you know how it was measured. The only useful rating is the resistance per unit length. Use that with the maximum length wiring run (remember to count out and back from the booster) to calculate the total resistance. Only then, knowing your nominal track voltage, acceptable voltage drop and maximum current, can you determine if that wire is suitable for your application. For a typical UK layout, however, 1mm^2 is probably adequate :)

 

Even with "mains" cable you need to know what you are doing. Cooker cable is much thicker, not just because of the higher current but to guarantee only a small voltage drop (yes, I know current, voltage and resistance are linked). A few volts drop in a much thinner 6A lighting circuit is neither here nor there. Put 6A through the same cable in a 12V DCC system and the voltage drop becomes significant.

 

To keep things simple it's better to over specify and not have to worry about the maths. Use  the thickest cable you reasonably can for the bus. The droppers can be much thinner. They are much shorter and will drop very little voltage even when pushed beyond their supposed current rating.

 

There was an amusing post once (not here) where someone tried to claim thin droppers would cause your layout to catch fire :banghead:

 

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Andrew,

 

I think we all agree that it's the voltage drop across the wire that counts. And I also agree with you that it is always better to over specify.

I probably don't need a 2.5mm2 track bus on my layout, with its maximum cable run of the order of 5m and a max of 4 or 5 locos running at the same time, but the hassle of having to rip out and replace underspecified cable just isn't worth considering.

 

Perhaps to assist the OP in deciding what cable to use, I'll provide a link to a useful Voltage Drop Calculator tool, which can give hard numbers to the use of any given size of wire, taking into account its length:

 

https://www.omnicalculator.com/physics/voltage-drop

 

Yes, short lengths of thinner wire are typically used - like the droppers from the track bus to the track which have to be soldered to the track and be as unobtrusive as possible. Their short length and the lower amount of current they have to carry mean that they are OK.

 

Mike.

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4 hours ago, KingEdwardII said:

Their short length and the lower amount of current they have to carry mean that they are OK.

 

Sorry, and I know I can be a bit of a pedant :) but droppers have to be able to carry the full overload current in the case of a short circuit, especially if a single dropper feeds an isolated section of track (such as a frog). It's only the short length that allows thinner wire to be used.

 

 

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12 minutes ago, Crosland said:

 

Sorry, and I know I can be a bit of a pedant :) but droppers have to be able to carry the full overload current in the case of a short circuit, especially if a single dropper feeds an isolated section of track (such as a frog). It's only the short length that allows thinner wire to be used.

 

 

How do you calculate the full overload current? Knowing, as you do, the equipment I'm using (NCE PowerCab, 009 locos) how to determine this?  

 

And isn't it possible to build in an overload protection?

 

 

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