Question About Shielded Wire

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TomS

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I'm working on converting a PM-932 to CNC and need some input on low voltage wire. I've searched eBay, Amazon and several CNC component suppliers on the web and I've found a significant difference in pricing among them for 18/4 shielded cable and 18/4 security wire with a shield wire. I've found security wire for as little as $56.00 for 500 feet including shipping. Shielded cable runs about $1.00 a foot and up. Is there a difference besides price? I've looked but can't find a specific designation for a stepper motor application. I'm not good at electrical/electronics so I may be looking in the wrong places. Any other considerations before I make the purchase besides price?

Thanks


Tom S
 
The biggest difference between them is likely the insulation voltage rating. the power cable is likely 300volt rated, while the security is probably 30volt, maybe 100v. Therefore, it will depend on your exact setup as to the type of cable that you can use, eg, if you are running your steppers at 24Vdc, you should be good to use the security wire. Anything more than that, and you need the other cable.

You should also check that the security wire has an actual braided/foil shield, as it would be pointless to use it without this feature.
 
Not all shielded cable is the same.
There are braided shields, wrapped shields and probably others.
Also impacting the price of a cable is the number of strands in each conductor. Higher strand count = higher flexibility.
 
My 2 cents - alarm wire is for low voltage use and is not the most flexible wire known - some alarm wires are solid wire and cannot be flexed repeatedly - different types of stranded wire cost differently because even though it has stranded conductors, the single conductor wire that makes up the wrap varies in AWG size and # of strands that make up the single conductor - for example, on 10 ga. stranded wire, you could have 7 strands of a # 24 wire wrapped on themselves with a twist - the total conductor would be equivalent to 10 ga - you could have 20 strands of a # 40 wire to make up the same diameter gauge. It also depends if the copper making up the conductors is of low oxygen content, etc.
Most of these things that make up the technical portion of the wire don't apply in your case, except to add extreme cost to the wire. If you are using this wire for a stepper motor, and are concerned about noise, purchase the amount of conductors that you need plus a shield. When connecting the shield, ground the shielding wire ON 1 SIDE ONLY (this could be to the chassis or a ground wire back to the panel - (not to the - signal side). If you bond both sides, you could induce what is called an eddy current which could flow between 2 different grounds slightly different in potential.
Hope this helps and is not confusing - I used to do a lot of instrumentation work in my earlier days before para-medicine!
 
Thanks to you all for your responses. I did some checking and most of the shielded security cable on eBay states it is for "limited power" circuits but they don't say what the max voltage is. My power supplies are 60V and 80V so I'm not certain this stuff will work. I'll do more digging but I'm now leaning towards paying more and getting cable that is compatible with my installation.

Thanks again.

Tom S
 
I just installed 16-4 speaker cable on 4 steppers for a system I'm building. Home Depot carries it. I haven't tried it yet, but I have a high degree of confidence that it will work just fine. It's not voltage rated, but speaker systems can easily go over 70 volts. My only real concern is the number of strands in each wire, It is only 7 strand wire, my preference would be 19 strand, much more flexible. I am using cat-track type wire guides with large radii and anchored the free ends to keep flexing to a minimum. I don't see any need for shielding the stepper power wires.

The limit switch wiring does need to be shielded, but there you are only operating at 5 volts, so any shielded cable would work. Home Depot carries 18-4 and 18-2 shielded alarm cable. Also I wire my limit switches as normally closed so that electrical noise has no effect on the operation, and if a wire breaks, the effect will be the same as the limit being tripped.
 
One difference to watch for is whether there is a single shield around all the conductors, or if there are individual shields around some, or each of the cables. There are many variations, so read the fine print.
 
Tony Wells said:
One difference to watch for is whether there is a single shield around all the conductors, or if there are individual shields around some, or each of the cables. There are many variations, so read the fine print.
Click to expand...

Thanks Tony. As you suggested I've been reading the fine print on most of the eBay and Amazon ads. Several of them list the conductors as copper clad aluminum or there is no description at all. I'm steering away from these. But as you might guess those that are stranded copper cost more.

Tom S
 
If the cables will be moving, you will want the most flexible stuff(highest strand count) you can get, as otherwise, over time, the wires will begin to break.

Also, a foil shield, though it gives the best initial coverage, is not designed to flex, and over time it can open up, decreasing the shielding effectiveness. A spiral=wrapped shield is more flexible, but it too can open up over time, A braided shield will be the most resistant to degradation from flexing.

There are cables that are made specifically for use on moving machines; last I looked, they are very expensive, and difficult to buy in small quantities.

Now. all that said, I wired my machine up using cable that I got from my job in the AV industry, with foil shields, and what i would consider medium strand count wires, and I have had no problems with it over the last 8 r 10 years.

As stated, the shield should be grounded at one end only, to avoid ground loop currents(not eddy currents).
 
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