Solid- or stranded-core wire for internal instrument wiring - Page 1
Forum section: electronics / beginners

author: matthuszagh
author_rank: Frequent Contributor
timestamp: June 20, 2021, 02:47:53 pm
content: I'm trying to decide whether it's preferable to use solid- or stranded-core wire for internal instrument wiring (i.e., inside the chassis, between PCBs, or between chassis-mount connectors and PCBs, etc.). My initial impression was that solid-core wire would be preferable in this context. The main reason for this is that I believe the rigidity would be an advantage here since the wire is never intended to move, and it can be easily manipulated to take a specific path between two points (e.g., along the chassis). My understanding is that solid-core wire also has other improved properties over stranded-core wire such as greater ruggedness and higher electrical conductivity. It's also my impression that it's only preferable to use stranded-core wire when you want the increased flexibility (e.g., for most test cable).I recently opened the Art of Electronics - The X-chapters and this is discussed at the very beginning of the book. One passage states:QuoteBut later, it is stated:QuoteThe first passage appears to say that stranded wire is better in this context, but the second (though less direct) seems to be in favor of solid-core wire. My initial opinion was more in line with the 2nd passage.IIRC though, a lot of internal instrument wiring does seem to be stranded core.What do you prefer for single-conductor internal instrument wiring? Are there specific reasons to opt for one or the other that I haven't mentioned?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 20, 2021, 05:15:16 pm
content: If your instrument is manufactured once, drenched in potting compound and never serviced, absolutely, solid wire is ideal.Other than that, right, fine, it doesn't need to be as extreme as potting, but vibration from transportation and ordinary handling will shake the cabling and fatigue solid wire. You need quite a lot of mounting to prevent that. And if nothing else, assembly and service will flex it plenty.It's also redundancy from stress raisers. Stripping solid wire correctly can be difficult. Nicked wires are prone to failure. Stranded, you don't care much because even if you nick or cut a few strands, you still have more leftover. (Not so many more with mere 7 strand, something to think about; 19+ strand is much better in this regard.)It may also be a pure matter of practicality: high current cables are best made from welding cable (lots of fine strands), or flat braid, easily manipulated by puny human hands. Else, bus bars and brackets, made from sheet or bar copper, are an opportunity, but the design needs to be very well considered as they are a complete, inflexible, mechanical component. You can build a cable harness in some minutes, but these things can take days or weeks of NRE. (I mean, simple point-to-point links you can cut and drill in minutes, but preparing drawings for actually purchasing parts, takes some engineering.)Tim

author: tooki
author_rank: Super Contributor
timestamp: June 20, 2021, 05:39:59 pm
content: Quote from: matthuszagh on June 20, 2021, 02:47:53 pmAccording to my boss, using solid wire has largely gone the way of the dodo for internal wiring. It’s more susceptible to damage from flexing, and most crimp connectors are made for stranded.In DC, the cross-sectional area is all that matters regarding conductivity. So in this regard, absolutely no difference between stranded and solid. In AC of high enough frequency for the skin effect to become relevant, stranded wire would actually besuperiorto solid. (To the point that litz wire is made for high frequency stuff, where each strand is individually insulated to maximize skin effect. Or the “cable” connecting a TV transmitter tower to the RF power amplifier on the ground: it’s not stranded, but it’s not solid either: it’s a hollow pipe. No point in filling the core with metal if the power only flows along the skin anyway!)Solid definitely makes sense where you need wire to keep its shape, like PCB bodges.I’m not entirely sure why the mains wire in our walls is still generally solid.My understanding is that stranded is better for most applications, and that in the past, cost is what kept solid wire in use. Nowadays, the price difference is negligible, so stranded has come to dominate.

author: radar_macgyver
author_rank: Frequent Contributor
timestamp: June 20, 2021, 06:12:51 pm
content: Quote from: tooki on June 20, 2021, 05:39:59 pmMostly this, from the instrument-build perspective.In an industrial wiring cabinet where everything uses screw terminals or spring-loaded contacts, one could potentially use solid wire. However, I've never seen it done, my speculation is that when such an assembly is subject to vibration, the rigid solid wires transfer more energy to the terminal and work themselves loose over time. Stranded wires have a ferrule crimped over them to avoid the effect of working loose from under a screw terminal's clamp when the strands "settle".For in-wall power applications, I suppose it's cost that drives the use of solid-core. Also, most electrical outlets and accessories use screw terminals, which when used with stranded wire without a ferrule can work loose over time.Solid-core CAT5/CAT6 has slightly lower loss than stranded, that's the reason it's used between patch panels and wall jacks.Quote from: matthuszagh on June 20, 2021, 02:47:53 pmI believe this refers to 'bodge wires' on PCBs; when you see these on professionally made boards (eg: some PCBs in Agilent instruments), they are often glued down to avoid flexing the solder joint. #26 seems a bit coarse for this - I would use #30.

author: bob91343
author_rank: Super Contributor
timestamp: June 20, 2021, 09:25:23 pm
content: I am reminded of my first repair job in the 1940s. A Zenith console radio lost sensitivity.I discovered a broken wire between the antenna post and the bandswitch. It was a straight solid wire and lacked flexibility from the rear chassis flange and the switch wafer. Since it was insulated, I didn't think it was faulty but that turned out to be the problem, an invisible break inside the jacket.Flushed with success, I went on to become an engineer.

author: mansaxel
author_rank: Super Contributor
timestamp: June 20, 2021, 10:04:43 pm
content: Solid is asking for trouble.Stranded is the way to go.You've been given all relevant reasons upthread.

author: vk6zgo
author_rank: Super Contributor
timestamp: June 20, 2021, 10:57:30 pm
content: Quote from: tooki on June 20, 2021, 05:39:59 pmIt looks like a "hollow pipe", but it is actually a rigid or semi-rigid coaxial cable.QuoteSolid wire was quite useful in vacuum tube circuitry with "point to point" wiring, as a solid wire connection between reasonably nearby devices could be short & reasonably self supporting.

author: bob91343
author_rank: Super Contributor
timestamp: June 21, 2021, 12:10:46 am
content: Historically, solid is a bit cheaper than stranded, and so catches the attention of the bean counters. It even takes less time to strip and solder.

author: james_s
author_rank: Super Contributor
timestamp: June 21, 2021, 12:14:54 am
content: Definitely stranded for chassis wiring, it's much more robust in terms of dealing with flexing.

author: rstofer
author_rank: Super Contributor
timestamp: June 21, 2021, 01:43:38 am
content: I have no idea how many millions of miles of 30 AWG solid wire was used to wire-wrap digital systems.https://en.wikipedia.org/wiki/Wire_wrapI use solid wire for short jumpers on PCBs and breadboards but everything else is stranded.

author: WimWalther
author_rank: Regular Contributor
timestamp: June 21, 2021, 04:06:26 am
content: My general rule is solid for connections that dont have to flex or handle vibration on any regular basis. But do take care to avoid nicks, thougb this can be just as important with stranded wires.

author: tooki
author_rank: Super Contributor
timestamp: June 21, 2021, 10:38:41 am
content: Quote from: vk6zgo on June 20, 2021, 10:57:30 pmCoax can have a stranded or solid center conductor. Or a hollow one, which is what I’m talking about. I’m not talking about the shield.

author: vk6zgo
author_rank: Super Contributor
timestamp: June 21, 2021, 12:46:53 pm
content: Quote from: tooki on June 21, 2021, 10:38:41 amYour post was unclear-----the part which is readily visible is the outer conductor, which to a lay person, just looks like a big "pipe".

author: tooki
author_rank: Super Contributor
timestamp: June 21, 2021, 02:47:22 pm
content: Quote from: vk6zgo on June 21, 2021, 12:46:53 pm1. But I’m not a lay person.2. My description was perfectly clear. I didn’t say it “looks like” a pipe, I said itisa hollow pipe, because I know for a fact that they are. I described only the construction, and didn’t mention appearance at all.3. Given that a solid conductor and a hollow metal pipe look the same from the outside, the fact that I said they are hollowinsideshould have given you a clue that I know more about them than their external appearance.For those who don’t know what I’m talking about, see the attached picture. “Air core coaxial cable” is the magic search incantation to find more if you’re curious.

author: mansaxel
author_rank: Super Contributor
timestamp: June 21, 2021, 03:46:50 pm
content: Quote from: tooki on June 21, 2021, 02:47:22 pmHow convenient, one can pour the Radio down the hole!(I do know about the skin effect...)

author: tooki
author_rank: Super Contributor
timestamp: June 21, 2021, 05:08:45 pm
content: Quote from: mansaxel on June 21, 2021, 03:46:50 pmAnd if the radio waves or electrons get stuck, you can flush them out with soapy water! 🤣