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I am setting up a design for a small submersible ROV and planning to try cat5 cable to power the various components. I have found the ratings for the cable, however I have a question about water vs heat dissipation. Apparently, I can get approx .5-.7 amps of power transmission safely for the motors, but would I be able to push it a little more to 1amp if the cable is submerged in cold northern lake water? The cable would be 100ft and I have factored in voltage drop due to resistance. Thanks for any advice.

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  • \$\begingroup\$ You can always push it a bit harder - the question is will it be worth the risk? How come you ended up with CATV twisted pair cable for your application? What is wrong with more standard cables? \$\endgroup\$ – Joren Vaes Apr 13 '17 at 13:58
  • \$\begingroup\$ Just accessibility and price. I can get 100ft for 20 bucks. I'm all ears for a better rated cable with similar quantity of insulated wires. Cheers. \$\endgroup\$ – Archaeus Apr 13 '17 at 14:18
  • \$\begingroup\$ Part of the cost equation for the cable is will your ROV float to the surface if the cable breaks (e.g. a conductor melts)? \$\endgroup\$ – Andrew Morton Apr 13 '17 at 19:18
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    \$\begingroup\$ Yes, its design is to be naturally buoyant in case of malfunction. \$\endgroup\$ – Archaeus Apr 13 '17 at 23:30
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Nobody will give you any sort of guarantee.
It may or may not work, you have to try this experiment.
It is hard to say that the heat will not build up in the middle of the cable. There is an isolation between water and wires itself. So it will isolate heat transfer from the wires to the water. Yes, cold water will help with cooling. But will it be enough to keep the wires at acceptable temperature? Hard to say. My guess is yes, it will work. But as I said, no guarantee.

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  • \$\begingroup\$ That science did cross my mind so I appreciate the feedback. \$\endgroup\$ – Archaeus Apr 13 '17 at 17:41
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Yeah, 1 Amp should be ok.

General rule about most maximum ratings is that the manufacturer will actually set it lower than the actual safe rating of the device. However, this is only the case for most devices, as there are exceptions to this rule when dealing with the extreme cases (for instance: HV step-up/down transformer sub-stations which actually do display their actual safe rating so they can get the most use out of it possible).

Given that you will be also using it in a freezing cold lake should remove any remaining risk of it overheating as well, so you basically have two thumbs up from me.

A message to all newbies reading this post:

Just because I said that Archaeus can use more amperage on his cable than the rated limits state is ok, there are a couple reasons why you should not try to do this yourself [yet].

  1. I took into account his current setup and situation

There are certain situations where you can and can not use more power than the rated safe limits. In his case, there were a couple conditions that drove me to my conclusion: he was only using .3 Amps more than the safe rating, he was using it in cold conditions, the item was a cable, etc...

  1. Overloading a cable is not the same thing as overloading a multi-core CPU.

Moral of the story: Complexity is the enemy of above-rating loads.

  1. We have safe rating limits for a reason.

Applying more than the limit in general isn't a very sound engineering practice, although it can suffice in a pinch. Always try to find a solution that has both safety and efficiency in mind.

 

Ok, i'm done. Time to grab some more coffee.

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    \$\begingroup\$ @Archaeus Hey, no problem. Happy to help. \$\endgroup\$ – gnimmargorP Apr 13 '17 at 17:55
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As others have said, it might be a fairly safe risk - you are not wildly over the ratings. A couple of points that do occur though.

This type of cable comes in various qualities, materials and gauges. At worst perhaps, 26AWG aluminium - at best 24 AWG pure copper - which WILL handle 1.4A in multicore (7-24 cores) at RATED levels.

https://www.engineeringtoolbox.com/wire-gauges-d_419.html

What is the maximum voltage rating for the cable? Have you considered using a higher transmission voltage and then using a simple DC>DC converter inside the sub. If your cable has a working voltage of (for example) 90v then even at just 1A, you could then drop that to perhaps 12v onboard, and obtain the best part of 7.5A at 12v onboard using just a single twisted pair.

If you do not need THIS level of power and stick to 48vDC, then step down DC-DC converters can be bought online for about £2 / $3 this will take the 48vDC and turn it into 5V, 12V - whatever you want basically.

So 48VDC > 12V 4A or 5V at very nearly 10A - all on a single pair.

My other point I am sure you have considered but I mention in the 100-1 chance you had not, is of course make sure you use the cable designed for patchleads (multistrand) and NOT fixed wiring (single core conductors). I am guessing you will NEED that cable to be resilient to being flexed on an ongoing basis.

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