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DC power cord questions

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#1 ylem


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Posted 24 March 2013 - 12:57 PM

I'm upgrading my SVP to goto the manual states it needs 12vdc 2a minimum. I was thinking of making an extension cord so I can run it off my vehicle, I was wondering how long and what gauge would be possible?

Not wanting to start a Tesla vs Edison debate :) but would DC go, say 50 or even 100 feet?

I really dont want to mess with power tanks and batteries, I am willing to run AC outside, but would like the DC cord for dark sites.

Any thoughts?

Thanks, Jeff

#2 mclewis1


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Posted 24 March 2013 - 01:43 PM

Jeff, Yes you can certainly extend a DC connection over that distance. First don't bother with an extension to your existing cable, instead make up a new cable that's suitable for the distances involved. What you need to be concerned about is the voltage drop across the long wire length. There are two variables to deal with this ...

1) Starting voltage, most 12v DC power supplies actually put out 13v or so, so you have a little bit of lee way. Any idea where the SVP starts to loose it's mind? Celestron hand controllers for example start to have problems down under 10.5v or so.

2) Thickness or gauge of the wire, thicker is better and will have less voltage drop over distance.

At 50' 18 gauge wire would be fine. At 100' I'd prefer 16 gauge wire. You could look for a spool of inexpensive 16 gauge speaker wire - the stuff with the transparent insulation over silver and gold colored wire. The insulation will probably get fairly stiff in the cooler winter months and shouldn't be used in freezing conditions but for your location it shouldn't be a big problem. If you are concerned about the flexibility have a look for some better quality (and more expensive) "lamp cord" with more flexible insulation.

Soldering 16 gauge stranded wire into small power connectors can be a bit tricky. You usually have to remove a few strands to get things to fit nice and snug before soldering. The cigarette lighter style plug on the other end is usually quite a bit easier, and another option is to use a pair of medium size clamp on connectors and go straight to the battery. If you opt for the clamp on connectors you should also mark the connectors in such a way that you can easily notice even in the dark which side is positive vs. negative. Wrapping some extra tape around one handle is one choice. You just want to make this as idiot proof as possible because you really don't want to get these reversed.

If we use the example I suggested above (100' of 16 gauge wire, starting with 13V and needing 2amps) then you'll see something like 11.4v at the other end of the cable which should be fine for your SVP.

#3 SkipW


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Posted 24 March 2013 - 02:02 PM

The issue is voltage drop. Here are approximate DC resistances for 1000 feet of copper wire of various guages:
AWG Ohms
18  8
16  5
14  3
12  2
So, with a 50-foot extension cord (50' X 2 conductors = 100' of wire), drawing 2A, you're looking at more than 1.5V loss with 18 Ga (8 Ohms/kFt * .1 kFt * 2A = 1.6V), one volt with 16Ga, and a bit over and under .5V for 14 and 12 Ga., respectively. There will be some additional loss in in the connectors, too. You'd probably be OK with a 14 Ga or heavier cord for 50 feet. If it were me, I'd probably go for 12 or larger wire (lower number) but it gets more bulky, heavy, expensive, and harder to work with as you go larger.

[Edit] mclewis1 is faster than me. Looks like the same info, but I'm more cautious. I think a fully-charged 12V lead-acid battery is slightly less than 13V with no load; "12V" power supplies (and car charging systems) are nominally 13.7V. 14-Ga lamp cord may be a good compromise on cost and performance, though. Speaker wire tends to be overpriced compared to lamp cord, which is just as good for this (avoiding the whole audiophile wire debate), or "landscape wire" which is designed for low-voltage outdoor lighting, may be nearly ideal.

#4 ylem


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Posted 24 March 2013 - 05:56 PM

Thanks guys, this gives me the info I was looking for.I remember from college how dc cannot be transmitted over long distances without "booster stations and batteries", the history of Tesla and Edison's fued is fascinating.

Thanks, jeff

#5 Billydee


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Posted 24 March 2013 - 08:47 PM


Be sure to buy a backup starter battery for your vehicle just in case you run your car battery down.


#6 frolinmod


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Posted 24 March 2013 - 09:28 PM

For a 100-foot 12V exterior temporary use power cable I'd use a 10 AWG heavy duty power cord from the hardware store with the ends cut off and replaced with Anderson power pole connectors. I'd just ignore the ground wire.

I don't actually ever run 12V power that far. But I do run it 50-feet and in that case I use 12 AWG stranded speaker cable from monoprice.com. Once again with Anderson powerpole connectors on each end. It's very inexpensive and it's flexible EXCEPT when it gets cold, like down to freezing or below. It's bad inflexible stuff in the cold.

Polarwire has power cable that is excellent in the cold, but it's both very bulky and very expensive.

#7 SkipW


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Posted 25 March 2013 - 08:53 PM

Nikola Tesla's AC justifiably won that argument, but the long distances they were talking about were in the dozens to hundreds and thousands of miles.

frolinmod: why ignore the ground wire? Just parallel one or the other (but not both!) of the two conductors and reduce the resistance of one of the runs by half. It's been paid for and has to be carried around and handled, so use it! :)

Also, if you use stranded wire that's too large to solder to your connector, feel free to clip off and discard some of the strands so you have a much smaller gauge at the connector. Won't hurt a thing.

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