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DCC newbie - wiring question


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21 minutes ago, rynd2it said:

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

 

Yes.  If you divide your layout into zones or "power districts" and fit district cut-outs, they should be rated for a lower maximum current than the limit the command station (or booster) will apply.

The advantages of these devices, more applicable to a large layout, are that

  • you don't have to examine the whole layout to locate the cause of the short
  • only the relevant part of the layout stops in the event of a short, and the rest keeps going, very useful at exhibitions

It's rather like one of your upstairs lights blowing a fuse (or the modern MCB equivalent) but the rest of the house electrics are still working rather than having the 100A master fuse for the  house blowing.

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37 minutes ago, rynd2it said:

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?  

 

My understanding is that the NCE PowerCab starter set is a 2 Amp system, so under a short circuit scenario your PowerCab will probably be able to supply something like 2.1 or 2.2 Amps to the track, before shutting itself down, so that is the current that you should be designing for.

 

40 minutes ago, rynd2it said:

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

 

The NCE PowerCab will have it's own overload protection, but you can set up a circuit breaker with a lower rating - for example, the DCCSpecialities PSX can be set to cut out at 1.27 Amps

https://www.dccconcepts.com/wp-content/uploads/2017/08/PSX-Quick-Reference-Guide.pdf.

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23 minutes ago, Michael Hodgson said:

 

Yes.  If you divide your layout into zones or "power districts" and fit district cut-outs, they should be rated for a lower maximum current than the limit the command station (or booster) will apply.

The advantages of these devices, more applicable to a large layout, are that

  • you don't have to examine the whole layout to locate the cause of the short
  • only the relevant part of the layout stops in the event of a short, and the rest keeps going, very useful at exhibitions

It's rather like one of your upstairs lights blowing a fuse (or the modern MCB equivalent) but the rest of the house electrics are still working rather than having the 100A master fuse for the  house blowing.

It's a tiny 009 layout, one or two little locos.

 

Power districts is overkill

 

 

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16 minutes ago, Dungrange said:

 

My understanding is that the NCE PowerCab starter set is a 2 Amp system, so under a short circuit scenario your PowerCab will probably be able to supply something like 2.1 or 2.2 Amps to the track, before shutting itself down, so that is the current that you should be designing for.

 

 

The NCE PowerCab will have it's own overload protection, but you can set up a circuit breaker with a lower rating - for example, the DCCSpecialities PSX can be set to cut out at 1.27 Amps

https://www.dccconcepts.com/wp-content/uploads/2017/08/PSX-Quick-Reference-Guide.pdf.

Thanks for that

 

 

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I had a funny surprise last week when I opened up a second hand z21 (white) I had purchased. Whether it was proprietary or home made  I couldn't tell, as it used the appropriate connector, but the power cable from the z21 to the track was no better than 16/0.2 - I havnt replaced it yet as I' m only doing test runs, but whoever had it before me must've had some issues I would've thought.

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Why? 
 

Assuming it was a short cable that shouldn’t cause an issue and as the z21 is only rated at 3A with the constant current capacity of 4A for 16/0.2 you are within the wire capacity. The voltage drop for a couple of feet to the layout or track bus will be negligible.

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There's useful table here https://www.canford.co.uk/TechZone/Article/MetricAWGWireSizeEquivalents

 

I use single strand 24AWG twisted pair wire on my layout, stripped from CAT5E network cable.

One strand of 24AWG is equivalent to 7/0.2 multi strand wire, and their effective resistance is 76.4ohms/km. Which is not a lot of ohms per metre.

 

I think that in practice it's bad solder joints and other connections that add most of the resistance to model railway wiring, not the actual wires themselves.

 

Regards,

 

John P

Edited by jpendle
Typo
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The command station (or booster) will have fault protection to cutout in the event of a short circuit. It takes a finite time during which ohms law determines the current that will flow. The level at which the booster trips will be set by design and component tolerances.

 

The problem with poor wiring is that the resistance is too high which limits the current. The current could be above the maximum rating for the booster but not enough to trip it, allowing a high current to flow for an indeterminate time.

 

Moving away from layout wiring, the worst case is poor wiring in a loco where all of the resistance is concentrated in a small length of wiring (or other conducting path). All of the power is dissipated in a small area leading to the pictures of melted bogies.

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10 hours ago, jpendle said:

I think that in practice it's bad solder joints and other connections that add most of the resistance to model railway wiring, not the actual wires themselves.

 

True. Nickel Silver rails are much poorer conductors than copper wire, add in dodgy rail joiners and you see why it's recommended to connect droppers to every piece of track for all but the smallest layouts.

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"Mains cable" isn't really anything to do with current rating as it is available in different core sizes.

e.g. I have 0.5mm on my table lamp with a 3A fuse in the plug and 1.5mm on my 3kW fan heater with a 13A fuse.

 

"Mains rated" refers to the voltage it can withstand.

Higher voltage rating usually means thicker insulation, not thicker copper cores.

 

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1 hour ago, newbryford said:

"Mains cable" isn't really anything to do with current rating as it is available in different core sizes.

 

"Mains rated" refers to the voltage it can withstand.

Higher voltage rating usually means thicker insulation, not thicker copper cores.

 


this is not correct, there are many factors that will determine a cable size to be used in a given situation. And I’m going to be controversial now, this really does show why people have so many issues with wiring layouts. As Crosland  rightly points out, resistance of the cable is the important thing, so yes a small 7/02 wire may take the voltage and is rated for current but is it fit for purpose, possibly not.

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1 hour ago, newbryford said:

"Mains cable" isn't really anything to do with current rating as it is available in different core sizes.

e.g. I have 0.5mm on my table lamp with a 3A fuse in the plug and 1.5mm on my 3kW fan heater with a 13A fuse.

 

"Mains rated" refers to the voltage it can withstand.

Higher voltage rating usually means thicker insulation, not thicker copper cores.

 

It's everything to do with current. The thicker core sizes can carry heavier currents. It has to be thick enough to have a small voltage drop at those higher currents but, as I said above, what is small voltage drop in a 240V circuit (a few volts) is a large drop in a 12V circuit. Using an under-rated cable means (a) it may get warm (b) the appliance may not see the intended voltage.

 

The voltage rating on cable refers to the insulation breakdown voltage, which you allude to but, even for lowly hook-up wire, this is generally in the 100s or 1000's V range. Thin conductors are perfectly safe for mains, they don't need to be "mains rated", depending on the application.

 

Think of an old fashioned transformer. The low voltage secondary wiring can be thicker than the mains wiring as it carries more current.

 

There was one very well known DCC retailer (I'll spare their blushes) who used to multiply the insulation breakdown voltage by the supposed current rating to come up with an enormous power rating for their cable :banghead:[I've discovered my favourite emoji]

 

 

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Andy and Andrew,

 

I was referring to previous comments regarding "mains-rated" cable and the perception that it automatically better because it is mains cable

 

What I should have said was:

Just because it is mains rated, doesn't mean that it is automatically good for a DCC bus.

 

There is no way I would use 0.5mm mains cable for a DCC bus. 2.5mm is a different matter.

 

As both of you say - core size is the real factor for current capability - and that I have no problem with.

(I used to work with 1000A and have seen what it will do to a 6mm cable when earthed......................)

 

Hence the examples with different cable sizes for different mains devices for those that think all "mains cable" is the same.

Edited by newbryford
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