Voltage Drop in cables

[Scott]

Ok, I'm awaiting the arrival of a 15A 13.8V transformer and ideally, I'd like to keep it in my shed where I monitor my image capture and run the cables out to the mount (saftey first and all that). Assumming I draw 10A (overkill) and the run is 20mtrs, what size cables do I need to avoid voltage dropping to an unacceptable level?

Sorry for my inability to explain myself

[Tinker1947]

I did in the past run a 30' cable from a Maplin 13.8v PSU the handset on the NEQ6 showed 13.2v this was thin twin core (1.5mm) cable the round white stuff, don't know if this helps.......

[Horwig]

Just checked some cable manufacturer's data. 1.5mm cross section cable comes out at about 15 ohm/Km, double it for two runs, and factor in the length of your cable. If my maths is any good that's a 6 Volt drop at 10 amps for 20 metres of cable.

6mm cable is a lot better at 4 Ohm/Km, but still not great at 20M and 10Amps.

I know that 10A is a worst case figure, but it's going to be a lot easier (and cheaper) to get that supply closer to the scope/mount. The other way is to run mount and camera etc on separate cable runs.

Huw

[Scott]

It all helps mate . 

[Astroboffin]

Hi,

I run 10m from warm room to pier, with 1.5mm twin and earth mains cable (as I already had a roll of it) it is 13.8v and drops to 13.2v at pier, so in theory if it's 0.6v drop over 10m then over 20m you should still get over 12v, which would be fine.

Although there will be a scientific formula for working this out, as if doubt it drops the same amount all along the cable, it may well be that as you go over a certain length it may drop at a higher rate.

I am sure others will explain this better, as it will involve knowing exact cable size, starting voltage and current draw, and this is a bit beyond me.

I can't speak for any other cable other that the one I mentioned, it will vary with different cables.

I have tested mine drawing up to 12amps, from my 15amp bench supply.

Regards

AB

[steppenwolf]

Over 20 metres, at 10A load, your voltage would drop from 13.8v to nominally 12.5v if you used copper wire with a cross sectional area of 5mm² (roughly 2.5mm diameter) so I'd reckon that this would be your absolute minimum cable diameter. I'd very surprised if you actually drew that amount of current though!

I'd rather see the PSU nearer to the 'scope and protected with an RCD.

[Scott]

Hmm, perhaps I'm over thinking things. probably easier to have a water tight box pierside 

I would say the only way I'd come close to 10A would be if I ran the laptop off it

[Alien 13]

It is allways better to have the PSU closer to the load and if possible run seperate wires from the PSU to the low current loads that way any voltage loss does not effect the more delicate equipment and reduces any load modulation/crosstalk effects.

If the PSU has 4 wire outputs (plus/minus and sense wires)  then this can help if a single long cable is the only option.

Alan

[Davey-T]

I'd rather see the PSU nearer to the 'scope and protected with an RCD.

 I agree, better to run 240v to the obs'y (safely of course) as I believe 240v " travels" better over longer runs / thinner cables than 12volt.

Dave

[Scott]

 I agree, better to run 240v to the obs'y (safely of course) as I believe 240v " travels" better over longer runs / thinner cables than 12volt.

Dave

Sadly, it's not an obsy, rather a permanent pier although a waterproof enclosure with a 240v supply is still the obvious choice I guess

[Davey-T]

I used this stuff, OK as long as you're not planning on burying it,

Probably get it cheaper on internet.

Dave

http://www.qvsdirect.com/2-5mm-3-core-28a-50m-drum-hituf-cable

[Horwig]

 I agree, better to run 240v to the obs'y (safely of course) as I believe 240v " travels" better over longer runs / thinner cables than 12volt.

Dave

It's a matter of power law, P=IxV, so for a constant power, if you increase the voltage by 20 fold (12v to 240v), then the current drops by a factor of 20. Then use Ohm's law to calculate the voltage drop in the cable , V=IxR, and you see that the length of cable for a percentage volt drop increases by the same factor of 20.

Hope that makes sense, never was good at long explanations.

As to running separate cables to kit, that's how my obsy's wired. Mount, camera and dew heaters have their own cables from the bench supply, less chance of crosstalk.

Huw

[Gina]

I use is this cable between warm room and pier (just under 5m) but you wouldn't be able to use 20m of it at 10A as the volt drop would be 7.4v by my calculation.  I use separate cables for mount and imaging rig.  I agree with the others - use mains to your pier with a waterproof case and RCD at the indoor end.

[Bubbles82]

I am planning to use two lots of 2.5 twin and Earth from the PSU to the pier, which will then be split between the the six 12v sockets ie 3 12 volt sockets per run of cable hope this helps

Sent from my iPad using Tapatalk

[Horwig]

I am planning to use two lots of 2.5 twin and Earth from the PSU to the pier, which will then be split between the the six 12v sockets ie 3 12 volt sockets per run of cable hope this helps

Sent from my iPad using Tapatalk

It then helps to group kit into 'dirty' and 'clean'. Because of the cable resistance, any switching, or variable load, e.g. mounts and hew heaters, will put voltage bumps in the supply coming out of the cable, so to isolate the sensitive kit, camera, 12v usb hubs etc, they would benefit from being on a different cable run to the dirty stuff.

All of this is relative, if the cable runs are short, and the loads benign, then there is little to gain. It's a bit like HiFi speaker cabling, you can go really overboard, and see no difference.

Huw