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#134061 - 10/17/02 09:20 AM Why cables look like they do
C-H Offline

Member

Registered: 09/17/02
Posts: 1508
Loc: Stockholm, Sweden
Hi!

I had an e-mail discussion with a cable manufacturer recently since I wondered why cables look like they do. I was rather surprised at some of the answers I got.

Solid vs. Stranded. The price difference is only 2% and the manufacturer didn't seem to like solid wires. In their opinion, the only reason to use it was that it was easier to use in some terminals.

I only see solid 1.5 and 2.5 mm2 (#16 and #13) cables in the shops. To my surprise, I was informed that the manufacturer made much (otherwise identical) stranded cables in these sizes. It is sold to camper manufacturers. (!)

Question for the ECN members: Do you prefer solid or stranded?

I also asked why european cables are have a 70°C rating as opposed to the 90°C rating used in North America. There were two reasons: It's cheaper making 70°C 2.5mm2 than 90°C 1.5mm2. In addition, 90°C cables are rather stiff. (They have to add a stabiliser to the softener to keep the softener from leaving the cable at the higher temperature.)

Question for ECN: Are North American cables e.g. Romex, stiff?

I'm starting to think that cables look the way they do simply because nobody questions their design.

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#134062 - 10/17/02 11:13 PM Re: Why cables look like they do
Trumpy Offline

Member

Registered: 07/05/02
Posts: 8540
Loc: SI,New Zealand
Definitely, stranded, it is easier to twist two or more wires together and to a certain extent gives you a better connection, as you have more conductor area touching the insides of the connector of your choice.
By the way, I commonly use both types,but stranded wins with me, hands down.
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#134063 - 10/18/02 01:09 AM Re: Why cables look like they do
nesparky Offline
Member

Registered: 06/21/01
Posts: 650
Loc: omaha,ne
For residential and small commercial I prefer solid wire. It's easier and cheaper to terminate on switches and outlets.
For larger commercial and industrial I use stranded wire. Most of these places have a lot of vibration from equipment.
I do not consider romex to be stiff. Larger sizes are just harder to handle size 8(appprox. 5.486 mm) or 6 (approx. 6.452 mm). ( size 8 is limited to 50 amps @ 75C and size6 is limited to 65 amps @ 75C.)

[This message has been edited by nesparky (edited 10-19-2002).]
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#134064 - 10/18/02 06:57 AM Re: Why cables look like they do
pauluk Offline
Member

Registered: 08/11/01
Posts: 7693
Loc: Norfolk, England
Romex-style cables sold in Britain are solid cores for sizes 1 thru 2.5 sq mm.

Pre-metric cables (before 1970) were stranded in all but the smallest size. The old 3/.029 and 7/.029 sizes are much easier to work with given the cramped space in many British boxes, even though the physical cable size is actually slightly larger than with their modern counterparts.

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#215540 - 05/30/15 11:26 AM Re: Why cables look like they do [Re: C-H]
LongRunner Offline
Member

Registered: 05/30/15
Posts: 34
Loc: Albany, Western Australia
Hi. I know this is an old thread, but the topic is no less relevant today than it was back then so I don't see why not.

C-H was probably right about it coming down to people's unwillingness to question the authorities. Now, I'll examine the dimensions of some common flexible cords as specified by UL (I don't have access to the standards documents themselves - too expensive for anyone not in the trade frown - but I managed to retrieve the dimensions of many of the cords from a few manufacturers' websites)...

Compare SJT to ST (or, indeed, their rubber or TPE-insulated counterparts), for example. The 18AWG and 16AWG versions of both have the same inner insulation thickness of 0.03" (0.76mm), with only the outer sheath being 0.06" (1.52mm) thick on ST (except for the 5-conductor variants which further increase it to 0.08" (2.03mm)) versus 0.03" thick on SJT. The category voltage is up to 300V for SJT (and SVT, SPT and NISPT), but 600V for ST. Then compare them to the European H05VV-F, which has a category voltage also up to 300V on any phase, but up to 500V between phases in the cord; its construction is comparable to SJT (or actually has a bit thinner inner insulators in the 0.75/1.0/1.5 sq.mm variants). So H05VV-F is suitable for 231/400Y (or 277/480Y although I don't know of a region actually using the harmonised cables with that voltage) but SJT isn't? If you don't need the extra abrasion resistance and aren't worried about "by-the-book" inspectors then I guess you could use SJT for 277/480Y, save a little cash and be happy.

NISPT-2 is another oddity; what exactly is the point of making a cord with nice thick inner insulators, but a thinner (and therefore easier-to-break) outer sheath than even that of European H03VVH2-F?

But it's SPT-3 that's the real case of "why bother?"; the 18AWG and 16AWG variants have an insulation thickness of 0.06" which is the same as the sum of the inner+outer insulation thicknesses for SJT (as already mentioned), so I think it's a fair comparison. But the 14AWG and 12AWG versions apparently had to increase the insulation thickness to 0.08" and 0.096" (2.41mm) respectively, so these "household use only" cords ended up using as much as or even slightly more PVC than the "suitable for commercial use" SJT. So I can't say I'm surprised that it has no near-equivalent elsewhere in the world. (Mind you, that whole "residential/commerical" distinction was almost entirely stupid to begin with, and I can only say I'm glad it's more-or-less exclusive to North America. But from what I see of them, the UL never were any good at making logical standards.)

Anyway, it's interesting to note that the (modern) Australia/New Zealand TPS cables insulate the earth conductor as well as the circuit conductors (avoiding the use of add-on sleeves, at the expense of a somewhat wider and costlier cable), and that their earth conductors also have at least 7 strands even when the circuit conductors are solid (most likely so that the earth is the last to break if the cable is abused by repeated bending).

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#215549 - 05/31/15 12:58 AM Re: Why cables look like they do [Re: C-H]
LongRunner Offline
Member

Registered: 05/30/15
Posts: 34
Loc: Albany, Western Australia
Come to think of it, I don't think there's a good reason to bother with the textile-braided cords on clothes irons (etc.) anymore (besides tradition), as there has been a technique to modify PVC (cross-linking the molecular structure via carefully controlled irradiation) not to melt for many years now. Hook-up wire with such insulation (abbreviated as XLPVC) is actually fairly common for internal wiring of equipment (UL AWM style 1430 or similar); I'm quite sure they wouldn't be using it if it wasn't cost-effective, especially in the current economy (well, it's more of a false economy than anything else to tell the truth, but that's another story). It even has the designation "V4" (where "V" is standard PVC, "V2" is high-temperature PVC for up to 90C, and "V3" is PVC for cold conditions) under the harmonised coding system (so H05VV-F for example would become H05V4V4-F), or so I read.

I'm pretty sure a cord using it would be more reliable than the braided type, at any rate (given that the braided cords are particularly prone to kinking - over-tensioning the conductors and in some versions also revealing the inner coloured insulators - and at least some braid types will eventually fray from normal use pretty much regardless of how well-treated). And guess what? Some versions of the braided cords feature XLPVC inner insulators anyway!

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#215563 - 06/03/15 07:36 AM Re: Why cables look like they do [Re: C-H]
Texas_Ranger Offline
Member

Registered: 12/17/01
Posts: 2343
Loc: Vienna, Austria
I think one of the reasons for choosing braided flex for irons was the added flexibility. PVC sheaths can be ridiculously stiff and that's annoying for something that's moved around a lot.

I definitely prefer solid for two reasons: most continental European (except Italian) manufacturers don't list their equipment for the use with stranded wire at all so you'd have to pigtail in each box (and use extra-deep boxes as officially switch boxes are rated for either one device OR connectors, e.g. Wago or choc blocks) and stranded wire has a greater outer diametre than solid because the strands are round and take up more space than one solid conductor. That's most noticable if you try splicing stranded wires using choc blocks.

In Austria and Germany everything for fixed installations up to 6 mm2 is solid and you can get 10 mm2 solid too. 10 mm2 and up are usually rigid stranded, each strand being roughly the thickness of a 1.5 mm2 solid conductor.

Stranded wire is occasionally used inside panels but rarely anywhere else.

There's no need to twist any wires these days, switches and sockets usually have push-in terminals (the last manufacturers to retain screw terminals were a few DIY-grade botchers where the plastic body of the sockets would give or even break before you could properly tighten the screws) and splices are exclusively made using Wago connectors - 273 series for solid up to 2.5 mm2, 223 for mixing stranded and solid. I haven't seen a professional use the once popular choc blocks for ages (except for connecting pendant fixtures) but I suppose a few conservative ones still might. 6 mm2 and up are usually spliced using fixed terminal blocks or connectors that fit on a DIN rail.

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#215569 - 06/04/15 03:36 AM Re: Why cables look like they do [Re: Texas_Ranger]
Trumpy Offline

Member

Registered: 07/05/02
Posts: 8540
Loc: SI,New Zealand
Ragner,
With the case of ironing appliances and other such heating appliances, are cords not sheathed in silicone rubber, in Europe?
What I mean is, any appliances that have directly exposed heating surfaces, where the cord could come into contact with the heating surfaces?
Sure, if you want to leave an iron sitting on it's cord, you get all you deserve, but using 75C or 90 PVC flex around hot surfaces, just seems really stupid.

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#215576 - 06/05/15 03:05 PM Re: Why cables look like they do [Re: C-H]
Texas_Ranger Offline
Member

Registered: 12/17/01
Posts: 2343
Loc: Vienna, Austria
Some irons might have silicone sheathed cords but traditionally it's cotton braided synthetic rubber-isolated conductors (H03RT-F). No PVC anywhere to be seen.

Cheap soldering irons often seem to have PVC (H03VV-F) and hot plates, toasters and the like usually have H05RR-F (natural rubber).

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#215581 - 06/05/15 07:51 PM Re: Why cables look like they do [Re: C-H]
LongRunner Offline
Member

Registered: 05/30/15
Posts: 34
Loc: Albany, Western Australia
Well, from my own huge stash of cords saved from the landfill, the older cords (by manufacturing date) are generally more flexible than recent production, even comparing PVC to PVC. So it seems that the manufacturers have had to cut down on the plasticiser content lately to stay price-competitive (and it does at least beat many of the "alternatives"). (I don't even notice the difference in flexibility between 3G0.75 and 3G1.0 cords, by the way.) Perhaps the rubber-cable guys took that as a cash-in opportunity, too? (Even if the rubber itself holds up, it provides relatively little protection of the underlying conductors from oxidation, which can increase resistance and result in overheating even at normal loading.)

I agree that silicone rubber (as better soldering irons usually use) would probably be the best overall choice for irons (very flexible, doesn't fray or kink easily, and can withstand higher continuous temperatures as well), albeit at higher cost.


Edited by LongRunner (06/05/15 07:54 PM)

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