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Power Bus Wire


petejones
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I was just about to purchase some 24/0.2 cable for my power bus when I remembered I have rolls of household electricity cable in the shed, from lighting to heavy duty mains cable. I'm guessing that would be OK to use, stripped of the grey insulation? Would single core or multi-core cable be best for the power bus?

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Yep household mains 3 core T&E power socket cable, strip off the outer Grey insulation and use the inner insulated main cores. Droppers can then be soldered to the main bus wires at the required points.

This tool or a similar one, is very useful for removing small parts of the insulation to allow the soldering of droppers.

HTH

 

https://www.ebay.co.uk/itm/8E58-Yellow-Pliers-Universal-Automatic-Wire-Stripper-Stripping-Alloy/303232200885?hash=item469a0c30b5:g:w28AAOSwl0pdNbH6

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This should be fine for permanent wiring, but you should really use stranded for wires that might be flexed around a lot, such as connectors between baseboards on a portable layout.  Solid wire may snap if bent too often, creating the sort of intermittent fault that's a pig to locate. 

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Solid core is perfectly OK if fixed in position, around and under the layout.

Multi-core might be more preferable for any trailing cable connecting the system/booster track output, to the layout.

 

It's also easier to use thinner multi-core for the droppers connecting the Power Bus to the track.

 

 

 

.

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9 hours ago, petejones said:

On a layout I purchased a few years back, the chap used rails for the bus; ie: they were stripped from the sleepers and fixed under the baseboard. The layout was only 6ft long, though.

Nickel silver is not a good choice for a conductor, although this should not become an issue with a 6' layout. Copper is a better alternative. Its electrical conductivity is about 16x that of NS & it is cheaper.

NS is used for rails because it stays clean, allowing good contact between rail & wheels.

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Solid core wire is used in houses because the cable is held completely immobile by the fixings and mounting systems and there is no vibration . The large amount of cable runs and hence the cost being the deciding factor 

 

in my view it has no place on a model railway nor should solder be used as the mechanical method of attaching droppers to it. 

 

I can never understand how people spend 1000s on rolling stock and yet scrimp and scrape on what is an important part of any layout , the wiring and connector systems 

 

use a suitable tinned multi strand cable wherever possible and leave T&E for your cooker ! 

 

Edited by Junctionmad
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12 minutes ago, Junctionmad said:

 

I can never understand how people spend 1000s on rolling stock and yet scrimp and scrape on what is an important part of any layout , the wiring and connector systems 

But look at the miserly approach to control systems, which affect every locomotive!

 

There are glamorous items like locos, less glamorous like rolling stock, and then there is the rest. 

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28 minutes ago, Junctionmad said:

Solid core wire is used in houses because the cable is held completely immobile by the fixings and mounting systems and there is no vibration . The large amount of cable runs and hence the cost being the deciding factor 

 

in my view it has no place on a model railway nor should solder be used as the mechanical method of attaching droppers to it. 

 

I can never understand how people spend 1000s on rolling stock and yet scrimp and scrape on what is an important part of any layout , the wiring and connector systems 

 

use a suitable tinned multi strand cable wherever possible and leave T&E for your cooker ! 

 

 

Yes, you're absolutely right! However, I'll still use T&E when under the fiddleyard. You never know when I'll need a cuppa....

 

Yes, I'm only joking! I'm using Post Office twin paired.....

 

 

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2 hours ago, Junctionmad said:

Solid core wire is used in houses because the cable is held completely immobile by the fixings and mounting systems and there is no vibration . The large amount of cable runs and hence the cost being the deciding factor 

 

in my view it has no place on a model railway nor should solder be used as the mechanical method of attaching droppers to it. 

 

I can never understand how people spend 1000s on rolling stock and yet scrimp and scrape on what is an important part of any layout , the wiring and connector systems 

 

use a suitable tinned multi strand cable wherever possible and leave T&E for your cooker ! 

 

 

Sorry but that just makes no sense. If you have a permanent layout, the bus wire is fixed and immobile. Solid core wire from T&E works perfectly well for the DCC bus as many users, me included, can testify.

 

And what exactly do you mean by "nor should solder be used as the mechanical method of attaching droppers to it."? I use both soldered and "scotchlok" connectors to attach wires to the bus that feed into multi-connectors and all work absolutely fine.

 

Perhaps if you have a portable layout where the wire is often exposed to being snagged then perhaps multi-core is better. 

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Blast, I have been doing wrong for all these years (together with most of my friends) - make you wonder how we get the amazing reliability and complete lack of faults using the wrong wire and making our connections incorrectly.

 

Maybe we are just lucky, or perhaps your advice is questionable and isn't 'tinned wire' just copper wire that been run through a solder bath to make it easier for soldered connections?

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8 hours ago, Junctionmad said:

proper multistrand tinned overall stranded wire is the staple of electronic hook-up wire.  I see no reason to second guess generations of engineers 

 

Exactly - for hook-up wire; (except plain stranded copper is actually far more common than tinned).

Internal equipment wiring & interconnection is a different application than power distribution.

 

There are various classes of cable structure for different uses. Installation cables are very different from "hookup wire".

 

 

There is nothing at all wrong with single core for fixed distribution whether it's fixed to a wall or a layout base, as long as it is not being regularly flexed.

 

The feeds to that and the dropper connections are stranded so absorb any slight movements.  It's a good practical and functional setup.

 

[Designing & manufacturing industrial electronics control & power systems for over 40 years].

 

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What concerns me is that anyone reading this thread might have the impression that all that is needed for the DCC bus is hook-up wire. Whereas an analogue DC system is likely to be no more than 12v @ 1 amp, DCC systems can have much more power and that power is on full all the time. 12v @ 1 amp is 12 watts of power: my DCC system is 16v @ 5 amps which is 80 watts of power - nearly 7 times as much and enough to illuminate a couple of 40w incandescent light bulbs. Hook-up wire is fine for a 12w bus but not for an 80w bus which needs to be treated as power distribution. 

 

The main requirement for DCC bus wiring is to ensure any short circuit is detected immediately by the command station, so that power can be cut off before any damage is done. Shorts are often caused by a loco derailing on points, and the short occurs when full power flows through the loco's pickup wires. These are not designed to carry full DCC current, except momentarily. If the DCC bus wiring is too thin at this location, voltage drop can occur resulting in the command station not seeing the overload and hence leaving power on. Say, for example, only 50w is being drawn: within literally a couple of seconds the heat generated will seriously damage the loco. Same effect happens with solenoid point motors if this amount of power is applied for a couple of seconds.

 

My DCC bus is 2.5mm solid core from T&E. Its advantage is that it's readily obtainable from DIY stores at around £1 per metre. It's not flexible and is rigidly fixed to the underside of the baseboard. I have droppers on every piece of rail, taking care with turnouts, ie droppers on the stock rails etc. When wiring for DCC it's important to do the "coin test" to verify the adequacy of the wiring. This means simply placing a coin across the track at various places around the layout, especially turnouts, and ensuring the command station cuts out immediately. Only then is it safe to run trains.

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"Hook up wire" is a nickname for fine stranded wire than can take regular flexing - it does not define the physical size or current ratings of such wire.

 

That's why I do not like the phrase or use it except in replies to posts mentioning it; cable & wire references should always specify the cross sectional area or wire gauge, to be really useful.

 

See the size and rating chart here, as an example - anything from half an amp rating to over 200A rating, just in that series..

 

 

 

hook-up-wire-current-carrying-capacity.png

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To emphasise the point made above, here is another chart which actually uses the term 'Hook-up Wire' for sizes up to 1 AWG.

A second chart would be referred to for conversion between AWG and stranded wire nomenclature (e.g. 24AWG is nearly equivalent to 7/0.2)

 

 

Current carry chart 1.PNG

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2 hours ago, RobjUK said:

See the size and rating chart here, as an example - anything from half an amp rating to over 200A rating, just in that series..

 

Current rating is meaningless unless you know the context. What is crucial is the resistance per unit length and the maximum tolerable voltage drop at the load.

 

Illustrated by an extreme example: A cable that drops 5 V at 5 A over 10 metres in a 240V (the 5V drop is neither here nor there) circuit would be useless in a 12V DCC system with a 10m bus length (most of the voltage is lost across the bus).

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19 minutes ago, Crosland said:

Current rating is meaningless unless you know the context. What is crucial is the resistance per unit length and the maximum tolerable voltage drop at the load.

 

Illustrated by an extreme example: A cable that drops 5 V at 5 A over 10 metres in a 240V (the 5V drop is neither here nor there) circuit would be useless in a 12V DCC system with a 10m bus length (most of the voltage is lost across the bus).

 

Are you saying, if I have a 5 Amp DCC system, a layout that is 100 ft long and I don't want a voltage drop of more than 1 volt along the length of the layout, then I should be looking for wire with a total resistance of 0.2 ohms (1 volt / 5 Amps)?  This would therefore mean that I should be choosing bus wire with a resistance of 2 ohms per 1,000 feet, which based on the table RobjUK posted above means the DCC bus should be 12 AWG.  Based on the table Gordon H posted above, this would have a cross sectional area of 3.32 mm2 and should be rated at 34 Amps.  Is that correct (ignoring the issue of higher resistance on the droppers and at soldered joints)?

 

I guess what I'm not clear of here is what is the "maximum tolerable voltage drop at the load" and how should that figure be calculated?

 

My proposed layout won't even 50 feet in length and my intended bus wire is multi-core wire that is around 2.5 mm2, so I'm not anticipating issues personally, but it would be good to better understand the rational behind the selection of particular cross-sections.

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3 hours ago, Crosland said:

Current rating is meaningless unless you know the context. What is crucial is the resistance per unit length and the maximum tolerable voltage drop at the load.

I totally agree, I've emphasised that myself before.

 

The table post was purely to put across the fact the "hook up wire" is a general classification (mainly in the USA) and not any specific size or rating of wire.

 

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5 hours ago, Dungrange said:

 

Are you saying, if I have a 5 Amp DCC system, a layout that is 100 ft long and I don't want a voltage drop of more than 1 volt along the length of the layout, then I should be looking for wire with a total resistance of 0.2 ohms (1 volt / 5 Amps)?  This would therefore mean that I should be choosing bus wire with a resistance of 2 ohms per 1,000 feet, which based on the table RobjUK posted above means the DCC bus should be 12 AWG.  Based on the table Gordon H posted above, this would have a cross sectional area of 3.32 mm2 and should be rated at 34 Amps.  Is that correct (ignoring the issue of higher resistance on the droppers and at soldered joints)?

 

I haven't checked your maths, but that's the idea. Remember to account for the resistance in both the feed and return. Your 100 ft layout example has 200 ft of bus wiring.

 

5 hours ago, Dungrange said:

I guess what I'm not clear of here is what is the "maximum tolerable voltage drop at the load" and how should that figure be calculated?

 

How long is a piece of string :) ?

 

With DCC you want to minimise the voltage drop for consistent running at all parts of the layout but most of the time the loads (the loco) are distributed around the layout and it's unlikely you will be drawing the whole 5 A from the booster to the farthest part of the layout. Under normal running voltage drop is not a great concern.

 

When it becomes critical is if there is a short circuit. If the wiring resistance is too high then the booster will not see a high enough fault current and may sit there slowly cooking itslef and/or other components. Hence the "coin" or "screwdriver" test. A deliberate short at the farthest point from the booster should cause an immediate shutdown.

 

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1 hour ago, Crosland said:

I haven't checked your maths, but that's the idea. Remember to account for the resistance in both the feed and return. Your 100 ft layout example has 200 ft of bus wiring.

 

Thanks - that's a good point, which I have to admit I had overlooked.  I guess the key point is that there is no 'correct' bus wire for all DCC layouts as it depends on the size of the layout (and the amperage of the command station).  The larger the layout, the larger the DC bus wire should be.

 

1 hour ago, Crosland said:

With DCC you want to minimise the voltage drop for consistent running at all parts of the layout but most of the time the loads (the loco) are distributed around the layout and it's unlikely you will be drawing the whole 5 A from the booster to the farthest part of the layout. Under normal running voltage drop is not a great concern.

 

I'm assuming that since Voltage (V) is the product of the Current (I) and the Resistance (R) the voltage drop (delta V) is a function of the current actually being drawn by the locomotive (say 0.5 Amps) rather than the 5 Amps that the DCC system can provide.  On that basis I can see why voltage drop wouldn't be a big issue under normal operation.  If the locomotive is closer to the DCC command station, the resistance will be lower and with a limited number of locomotives, so will the current draw. 

 

1 hour ago, Crosland said:

When it becomes critical is if there is a short circuit. If the wiring resistance is too high then the booster will not see a high enough fault current and may sit there slowly cooking itself and/or other components. Hence the "coin" or "screwdriver" test. A deliberate short at the farthest point from the booster should cause an immediate shutdown.

 

Does that therefore mean that the wiring should be designed for the "fault current" rather than the capacity of the Booster?  That is, instead of saying "I have a 5 Amp booster" and using 5 Amps to determine suitable wiring, I should seek to find out at what current draw the command station or circuit breaker shuts down.   I'm assuming that a 5 Amp system doesn't necessarily shut down at 5.01 Amps?

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6 hours ago, RobjUK said:

I totally agree, I've emphasised that myself before.

 

The table post was purely to put across the fact the "hook up wire" is a general classification (mainly in the USA) and not any specific size or rating of wire.

 

Thank you , I use the term ( I did a lot of product design in the us 

as to mean a stranded interconnect wire of any suitable current capacity 

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