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I wasnt entirely sure how to best word my title, but I am currently facing the challenge that while installing and wiring all my electronic components in their target location I need the cables to be significantly longer than they need to be when my installation is assembled in order to have enough room to perform all the soldering and contacts. To avoid having cables dangling around inside that box I would like to create one or two loops with those cables before the final assembly.

What will be the effects of doing this? The cables are 18 AWG with a thick silicone insulation and will have 5V DC with small currents < 500mA. The total length is roughly 50cm (1.6 feet) and when wound the length will be reduced to half of that. I am manly asking this question because in the army I was always told to completely unspool every cable that I use and was wondering if that is only applicable to those high voltage and high current cables or not.

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  • \$\begingroup\$ What is the total length of the cable, and how much longer will it be with the extra loops? \$\endgroup\$ Nov 9 '20 at 6:37
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    \$\begingroup\$ I have added that information to the question. \$\endgroup\$ Nov 9 '20 at 6:40
  • \$\begingroup\$ "and when wound the length will be reduced to half of that" - how does being 'wound' reduce the length? Please tell us how much of the cable is 'wound' and the diameter of the loop(s). \$\endgroup\$ Nov 9 '20 at 6:47
  • \$\begingroup\$ I am not sure how to best express it, the cable obviously doesnt change length when wound, but a 50cm cable is used to cover a 25cm distance. It will run for 10cm without winding on both ends, so 30cm will be inside the loops of approximately 4cm diameter (its done by hand, so slight variations of all those numbers are possible). \$\endgroup\$ Nov 9 '20 at 6:53
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Loops with a bit over 1 turn are similar to straight cable as far as current handling is concerned. 18AWG 2 core cable is rated for ~7 A, well above your maximum current, so heating should not be a problem.

Resistance is ~21 mΩ per meter, so the extra voltage drop will be ~ 0.5 A * 0.021 Ω * 0.3 m * 2 conductors = ~6 mV, which is insignificant compared to 5 V.

Inductance of the loops is about 0.2 μH, which will probably be insignificant in a cable carrying DC power.

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At 500mA there's no worry about cable heating. A common situation where unspooling cable matters is for a vacuum cleaner for example, where the cable contains as little copper as possible (because copper costs money) and is tightly wound on a spool inside the cleaner without any air cooling. In your case, no worries.

A loop of wire creates inductance, but a loop of cable which contains both hot and ground does not add inductance in series with your power supply, because the current in each wire is opposite, so that cancels out the magnetic flux. Low inductance wound resistors are made this way too. So you shouldn't worry about inductance either.

The winding will create common mode inductance, a bit like a ferrite clamp on the cable, which isn't a problem.

The only issue would be if the coil is wound around something that emits a magnetic field, like a transformer, or if it is wound along another wire going to a high current device like a motor driven by PWM. So you shouldn't bundle all the cables together and then coil them all in the same loop, that would create an air core transformer. Again, only a problem if cables or wires generating a strong magnetic field from AC current is bundled with sensitive analog signals. For 5V power supply, shouldn't matter.

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The answer could be divided into these parts:

  1. Current and power lost. A cable have a resistance and the power lost will cause heat and a winded cable have no means to dissipate the heat from the inner loops. This is unlikely with AWG18, 5v and 500mA.

  2. High frequencies. A loop is a inductance and can cause undesired effects. Without knowing what kind of circuit...

  3. Magnetic A electromagnets parts is a coil and a magnetic core. Is there any magnetic sensors that could be affected?

So with other words it depends.

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  • \$\begingroup\$ For 2: The cables are the ground, power and PWM signal for a servo. For 3: The closest electronic component is a raspberry pi that will be around 4 inches away from the loop. \$\endgroup\$ Nov 9 '20 at 7:45
  • \$\begingroup\$ As your description was quite general, I have outlined the problems that winding cables can cause in general terms. And as Bruce Abbott answer is also pointing out in more a factual way, the effect of the winding is probably insignificant. \$\endgroup\$ Nov 9 '20 at 8:58
  • \$\begingroup\$ Ground and power : no problem. PWM signal will be subject to pulse distortion : potential problem. \$\endgroup\$ Nov 9 '20 at 15:18
  • \$\begingroup\$ Errr... "high frequencies": DC! \$\endgroup\$ Nov 9 '20 at 17:34
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You can safely roll that cable up. Generally, the reasons to not roll up cables are:

  1. Heat from the cable resistance cannot dissipate.
  2. Some cables do not bend well (they have large minimal bend radiuses like co-ax and/or are generally prone to mechanical stress failure)
  3. Interference for signal carrying cables increases with each loop. Cables which may be shielded well enough when lying next to another cable may interfere when lying next to 100 cables which carry the same signal, phase-aligned.

None of the reasons applies here:

  1. The resistance of 1.5 feet of an 18AWG cable which has 6.4 Ohm/1000 ft is 0.0064 Ohm/ft; at 0.5A and 1.5 feet that results in a heat production of 0.5 V*0.01 Ohm = 0.005 W, which is hardly measurable, because the resistance of such a short cable is really small. (If you rolled up 10000 feet I'd become concerned.)
  2. A simple cable, especially silicon-insulated, can be bent any way you like (once, at least).
  3. This cable carries DC, as you say, hence no interference.
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