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Online Voltage Drop Calculator For Figuring Feeders

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Online Voltage Drop Calculator For Figuring Feeders
Posted by Mister Mikado on Saturday, October 17, 2020 4:27 PM

Online voltage drop calculator.  I set it for copper, pick wire size, 12 volts, DC, single conductor.  I put in .25 amps for a modern loco, is that correct?  At that current, even 18 gauge can go far without a big drop.  There's lots of technical info lower down. -Rob

https://www.calculator.net/voltage-drop-calculator.html

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Posted by betamax on Saturday, October 17, 2020 8:41 PM

It's fine for DC or 60Hz AC, but for DCC things change.  Suddenly the DC resistance isn't the problem, the inductance of the wiring becomes a concern. Smaller wire, more inductance. 

Often the issues related to wiring become the subject of over-thinking. The load is moving, current will be flowing in more than one set of feeders, and the length of those feeders won't be long enough to have a real impact.

Simple rule: If the wire gauge number increases by three steps (i.e. 16 to 19), the area of the conducter is halved, and the resistance doubles. 

A quick way to determine DC resistance of a wire: 10AWG = 1 Ohm per 1000 feet, so 13AWG is 2 Ohms/1000ft, and so on.

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Posted by Overmod on Saturday, October 17, 2020 8:52 PM

betamax
It's fine for DC or 60Hz AC, but for DCC things change.  Suddenly the DC resistance isn't the problem, the inductance of the wiring becomes a concern. Smaller wire, more inductance. 

I believe you mean 'impedance', not 'inductance'.

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Posted by betamax on Sunday, October 18, 2020 3:48 AM

No, inductance.  The reactive component is much larger than the resistance at this point. Managing it determines the impedance.

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Posted by Overmod on Sunday, October 18, 2020 4:06 AM

I had thought that parasitic capacitance was a far greater influence on impedance than self-inductance (especially if inductive reactance decreases the more skin effect is present at higher effective frequencies).  But my practical experience is limited to AF in that respect.

Here is a link to a calculator for wire self-impedance at higher frequencies.

https://www.ampbooks.com/mobile/amplifier-calculators/wire-inductance/calculator/

Note that this, unlike many others, calculates values for many relevant wire gauges simultaneously, saving the need to tediously change values for comparison.

I do not know if there is a 'peak' effective modulation rate for a particular DCC installation; this itself might make for an interesting discussion of 'practical design limits'.

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Posted by gregc on Sunday, October 18, 2020 5:07 AM

betamax
No, inductance.  The reactive component is much larger than the resistance at this point.

can you quantify this or does it remain voodoo?

greg - Philadelphia & Reading / Reading

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Posted by betamax on Sunday, October 18, 2020 9:47 AM

Overmod

I had thought that parasitic capacitance was a far greater influence on impedance than self-inductance (especially if inductive reactance decreases the more skin effect is present at higher effective frequencies).  But my practical experience is limited to AF in that respect.

Here is a link to a calculator for wire self-impedance at higher frequencies.

https://www.ampbooks.com/mobile/amplifier-calculators/wire-inductance/calculator/

Note that this, unlike many others, calculates values for many relevant wire gauges simultaneously, saving the need to tediously change values for comparison.

I do not know if there is a 'peak' effective modulation rate for a particular DCC installation; this itself might make for an interesting discussion of 'practical design limits'.

 
Parasitic capacitance comes into play when you twist the wires together. Amateur radio hobbyists would do that to create small capacitors for tuning purposes.
 
PicoFarads at 60Hz have reactances in Megohms. DCC signals are in the audio range, but as a pulse, they are loaded with harmonics to make it just that much more interesting.
 
Inductance is related to wire diameter. Smaller wire, more crowded the flux becomes. As current increases the Counter-EMF will increase (AKA BEMF). Skin effect is negligible at low frequencies. 
 
The simple solution: Keep your bus and feeder wires close together to cancel the inductive properties of the wire. Since feeders tend to be short and only carry a fraction of the total current available, there is no need for analysis paralysis.  
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Posted by Lastspikemike on Sunday, October 18, 2020 10:38 AM

So twisting the pair of feeder wires together from the bus to the track  is a good idea? A bad idea? Or makes no difference?

Twisting the bus wires together?

Separating the bus wires by how much if you don't run them "close together "?

Alyth Yard

Canada

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Posted by SeeYou190 on Sunday, October 18, 2020 10:55 AM

betamax
Since feeders tend to be short and only carry a fraction of the total current available, there is no need for analysis paralysis.  

No need at all.

Yes

- -
So twisting the pair of feeder wires together from the bus to the track  is a good idea? A bad idea? Or makes no difference?

Twisting any type of communication wires is always good practice.

If it makes any difference or not is debatable, but it is so simple, why not just do it? It takes very little effort, and will do no harm.

-Kevin

Wink Happily modeling my STRATTON & GILLETTE RAILROAD. A Class A line located in a personal fantasy world of semi-plausible nonsense on Tuesday, August 3rd, 1954.

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