# PM42 wiring question



## Zootdad (Aug 31, 2013)

Considering utilizing a Digitrax PM42 for 4 sub-districts. Do I need to twist the wires from the PM42 to the track? The blocks are long....30' in either direction from the PM42. Any advice would be helpful.


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## norgale (Apr 4, 2011)

You can give each pair a little twist so that they will stay together and not get mixed up in other wires. Twisting the wire doesn't hurt it any but makes the wires somewhat shorter depending on how much you twist them. Pete


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## mopac (Feb 24, 2011)

Pete, I think he is talking about the 2 bus wires setting up radio interference on
long runs (30 or more feet). Rumor has it that if you twist the 2 wires this will not
happen. 3 or 4 twists per foot. I know of this but know nothing about it. I need to
learn also. I need runs of 37 feet so I need to find out about it also.

I could not see any harm twisting the wire. To be safe twist it.


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## Zootdad (Aug 31, 2013)

Thanks Mopac & Norgale for your replies. Hope others chime in. Mopac....you are correct with your assumption. I am new to DCC wiring and have read many articles on the subject for the last two weeks with frankly conflicting information. Perhaps the terminology is confusing me.

All articles recommend twisting the main bus...I understand that. However, I am building a large home layout, U shaped...if stretched out would be about 90' long, with numerous power districts and sub-districts. I am planning 4 main lines, with 2 large helix, that could run multiple consists of up to 4 powered units each. 

My confusion is, with multiple power districts, some diagrams I have seen show the main bus twisted, but the TRACK bus untwisted. I understand that for block detection, the track bus should be untwisted, but I am not planning any block detection. With no block detection anticipated, I guess the safest approach would be to twist all wires. With multiple isolated power districts, is a conventional main power bus even necessary???

Sooooo confused.


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## DonR (Oct 18, 2012)

Yes...twist those long lines. Without a twist the digital signals can
be inducted into other wires running parallel. Twisting causes
an 'out of phase' effect that prevents this crosstalk. The old
telephone and telegraph lines on the poles and cross arms used
insulators on special brackets to put a 'twist' on the lines for
this same reason.

If you have short lines with no wires running along side it's not
that important.

Don


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## tr1 (Mar 9, 2013)

*twisting wire pairs under thirty ft. not necessarywith DCC*

From what I learned from this discussion shorter runs of D.C.C. wire pairs(under 30'are not twisted together. Now are they red and white in color?Is the voltage going to the track A/C
voltage? Or a simulated A/C voltage (square wave signal)? And what gauge wire is used for the

anticipated current spikes on a derailment short circuit, on the layout? I know that
dc wire is blue. And I know the wire gauge is determined by the amount of current in the wire.
At this time, I'm unable to determine the correct gauge and color of wire to be used. In a D.C.C. system.Can somebody help me with this? Thank-you,
tr1


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## waltr (Aug 15, 2011)

The DCC power signals are kind of Square waves. This is why people talk about radio interference, signal integrity and twisting wires.
Read up on the DCC standards here in S-9:
http://www.nmra.org/standards/sandrp/consist.html

Any two different colors can be used for the DCC bus (I use red & white, 14ga stranded since this was available at surplus prices at a flea market). Two colors are not really needed but make it easier to get the correct connection to the rails without creating a short.
The wire gauge is also debated as you have seen. The rule of thumb is: The more current draw and the longer the run then large the wire should be. 
What is the max current output from the command/booster you are using?
I read you'll have about 37 feet of bus length. If using a 5 Amp booster then use a minimum of 16ga wire and prefer 14ga.
I twist the bus wires just to keep them as a pair for neater and easier routing under my layout.
This article make good sense in all their suggestions:
http://www.dccconcepts.com/index_files/dcclayoutwiringwire.htm


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