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Today I saw a Sony camera charger that claimed to provide 8.4V/1.7A output but had a cable going to the device being powered with 18AWG (0.824mm2) cross-section and insulation rated at 300V (yeap, three hundred volts). It was composed of two single-wire components each insulated separately and being about 3 millimeters thick (measured over the insulation). Components were aligned in parallel and welded together which resulted in about 3 by 6 millimeters cross-section.

AFAIK such beefy cable could be used to power a several hundred watts desk lamp anywhere in the world (even in Europe where mains is 230V) and having such bulky (and not very cheap) cable looks quite unreasonable unless it is necessary.

Why would a tiny low-power charger be shipped with such beefy cable?

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    \$\begingroup\$ ESD might provide a chance for more than 300V to be seen between the center-conductor and the outside of the wire insulation. \$\endgroup\$ – Joel B Nov 2 '11 at 13:47
  • \$\begingroup\$ Errr - a European 200W desklamp will draw half the current that a similarly powerful US device would. It can therefore afford to have a thinner cable than the US device. \$\endgroup\$ – Martin Bonner supports Monica Mar 28 '19 at 14:33
  • \$\begingroup\$ Yes, you need thinner wire for higher voltage but you need thicker insulation for higher voltage too. Because insulation is on the outside it adds much more to cable cross-section. \$\endgroup\$ – sharptooth Apr 1 '19 at 8:05
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Because they had such cables in stock at the moment? Maybe they were able to buy them cheaply. Some manufacturer may have overproduced them and was looking to get rid of the stock or they were designed for another product that turned out to be a failure. They may have same power supply case for several products and just change the electronics inside. Some of them may require more power than others.

Also note that 300 V insulation isn't that much. It could be more expensive to look for cable with lower insulation ratings. For example ALL professionally made serial port cables I have have 300 V insulation and we all know that 300 V isn't anywhere near the voltages used in RS-232 ports.

Another point could be the bulkiness of the cable itself. While the cable may be overkill for electricity transmission, consider usual environment in which the cable would be used. You can't assume that the user of the cable will be able to handle the thin cable well and that the environment may will be friendly to thin cables. Remember there are pets that may want to eat the cables, someone may trip over the cable, the cable may be bent many times during it's lifetime and so on. In such cases it may be better to provide a good quality cable and just using one which is thicker may be the cheapest solution.

Another point is the appearance of the product itself. You didn't mention the exact camera model, so I can't dig up any information about it, but sometimes the sufficient cable may look and feel cheap or weak and that may leave bad overall impression of the product. It's certainly cheaper to just put in a good quality cable in order to impress the customers a bit more.

So to sum this up: I don't see a valid reason to use this cable from the point of view of physics, but keep in mind that engineering is applied physics and that makes the business side of the problem important to engineers.

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  • \$\begingroup\$ I'll accept this one. Days later I saw three different IBM laptop chargers - they supplied current at least 3 amps at at least 16 volts, so indeed the cable is just overkill. \$\endgroup\$ – sharptooth Nov 7 '11 at 6:47
  • \$\begingroup\$ I think this answer misses the point that it's Amperes, not Volts, that matter for cable thickness. Consider the 40A 5V wiring in an old PDP-11 chassis, or the 120A 12V wiring for car starters and whatnot. \$\endgroup\$ – Jon Watte Aug 28 '13 at 16:32
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    \$\begingroup\$ @Jon Watte OP asked why a cable with higher than needed insulation voltage was used in the product and I answered. I don't see how that misses the point. At no point did I mention that the cable was thick primarily because of its voltage rating. Furthermore, voltage rating does affect the overall thickness of a cable, since higher voltage cables tend to have thicker insulation for same type of insulation material. At no point did I claim that current carrying capability of the cable does not affect its thickness. \$\endgroup\$ – AndrejaKo Aug 28 '13 at 16:45
  • \$\begingroup\$ OP also asked why 18 AWG wire was used. So, you answered half the question. \$\endgroup\$ – Jon Watte Aug 29 '13 at 15:49
  • \$\begingroup\$ @Jon Watte I also explained that for stated current, such thick wire is not really needed. I don't see how someone who wasn't a part of the design team could provide a better answer why the cable wasn't more cost-optimized. \$\endgroup\$ – AndrejaKo Aug 29 '13 at 19:11
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1.7A isn't exactly low power. As you note, the several-hundred-watt desk lamps run on 230V, so their current requirements are much less - They could actually be lower ampacity than your charger! For example, this is almost 4x the power levels of USB.

As you probably know, it's the current that determines the size of a cable, not the power. I calculate that your cable could carry up to about 8 amps, which is a little more than 4x the rated capacity of your charger, which may put out higher amperage under short-circuit conditions. In a short circuit condition, you'll want a fuse or PTC in the charger to cut out, rather than burning the cable.

It's not really overkill.

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    \$\begingroup\$ The current determines the minimum size of the conductor. The voltage determines the size of the insulation. There are many other factors, of course. \$\endgroup\$ – Jay Bazuzi Jun 15 '12 at 15:37
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Two things come immediately to mind - the first is that larger diameter cables give less voltage drop. I've got some really crummy USB cables, for instance, which have enough voltage drop that my USB powered HDs won't work with them. There may be some reason that they wanted to keep the voltage up, and therefore they went with heaver-than-you-expect cable.

The other thing is that cable (like many other things) costs less in bulk. They may well have decided that using the same cable for lots of things is cheaper than designing specific cables for every item. This might produce some overly bulky cables at times, but if they can manage to buy 2 miles of one style cable per month instead of 1 mile of two different cables, there's likely to be some cost savings there. If nothing else, it's almost always a savings if the factory doesn't have to handle two different parts.

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Conductor thickness is determined by a combination of operating current, length and how much volt drop you are prepared to tolerate. Robustness can also play a part.

Insualtion thickness is determined primerally by robustness. It doesn't take much plastic to stop a few hundred volts, but if that plastic is worn away then it's useless at any volage. Mains cables do often to have thicker insulation than ELV ones but that is mostly because the consequences of failure are higher.

Economies of scale also play a role, plastic is pretty cheap, so making large gauge cables with thin insulation makes little sense when you can just make the wire with mains-rated insulation and sell it to everyone.

It's hard to be sure what exactly drove the choice in this case but I guess they looked for a widely made cable that was robust and would give a low volt drop.

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