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Author Topic: 12v and 24v dc wire sizing ? What guage to use ?  (Read 3789 times)
scanzel
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« on: November 23, 2012, 09:35:48 AM »

For my 12 and 24 volt wiring would it be better to do 14 guage or 12 guage for all wiring. About 90% of the runs will be in the 15 to 20 ft range with some shorter. This will be used for roof vents, tv antenna and 12/24 volt lighting. I am hoping to do all led lighting.
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Steve Canzellarini
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« Reply #1 on: November 23, 2012, 09:44:57 AM »

I actually did mostly 10 gauge with some 12 gauge.  I wanted to be certain I wouldn't have any voltage drops.  My runs probably average 25 feet.  I think the only place I used 14 gauge is for the fans on my heat exchangers.  The fans draw less than 1 amp and only have 18 or 20 gauges wires on them.
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Brian Elfert - 1995 Dina Viaggio 1000 Series 60/B500 - 75% done but usable - Minneapolis, MN
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« Reply #2 on: November 23, 2012, 12:20:51 PM »

The current capacity for low voltage DC and typical AC 120 volt wiring is about the same.  14 gauge for 15 amps, 12 gauge for 20 amps, 10 gauge for 30 amps is the guideline I use, and it's kind of overkill.  There will be essentially no voltage drop on the length of runs you would see in a bus.  Use higher gauge if you want, it won't hurt anything, but it's a waste of money unless the wire you get costs the same.  For your LED lighting runs I would use 18 gauge, thinner starts to get a little delicate, but LED's draw such a low current.   http://www.powerstream.com/Wire_Size.htm

 Brian

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« Reply #3 on: November 23, 2012, 12:39:49 PM »

My apologies.  Please disregard what I wrote below as it is incorrect.  I did use 8 AWG, but the load is only 11 amps, not 25 amps.  (25 amps was using 12 volt bulbs.)  I can't recall why I used 8 AWG right now.


I disagree that voltage drop is not an issue on a bus.  My headlights are 24 volt (on purpose!) and they use somewhere around 25 amps on high beam.  I ran 35 feet of wire to the batteries.  If I had used 10 AWG I would have lost almost 2 volts.  I had to go to 8 AWG to minimize voltage drop.
« Last Edit: November 23, 2012, 06:42:51 PM by belfert » Logged

Brian Elfert - 1995 Dina Viaggio 1000 Series 60/B500 - 75% done but usable - Minneapolis, MN
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« Reply #4 on: November 23, 2012, 01:19:27 PM »

I was saying that voltage drop on LED light runs and 1 amp fan runs are inconsequential.  Headlights aren't what I thought we were talking about.  I gotta question your math, though.  24 volt headlights drawing 25 amps would be 600 watts of headlights.  My high beams are  way under 300 watts total, probably 260 watts (65 watt halogen times four).  600 watts of headlights is a lot of lights...  Also, I calculate the voltage drop at 25 amps and 24 volts over 35 feet of 10 Ga. wire to be .87 volts, based on 0.9989 ohms per 1000 feet  from the chart I posted the link for.  That just happens to be the chart I use, it might be wrong.

That said, I totally agree that one should calculate or measure the voltage drop for headlights and upgrade wiring as needed.  No disagreement there from me!  I won't say it's wrong to over-size house wiring either, I just don't agree that it adds anything to the quality of the install.  My 24 volt bus uses mostly 16 Ga. and 18 Ga for routine chassis wiring, and only uses larger for specific purposes.  It does use 10 gauge wire for 30 amp chassis wiring, though.

Brian
« Last Edit: November 23, 2012, 01:23:13 PM by bevans6 » Logged

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Len Silva
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« Reply #5 on: November 23, 2012, 01:22:10 PM »

I also have to disagree with Brian.  There is a BIG difference between low and high voltage wiring.  The voltage drop is a function of the wire size (resistance) and the current.

For instance, a 15 amp load ten feet from the source (twenty feet of wire), using 14 ga. wire will drop 0.76 volts.  That is not a big deal at 120 but it is significant at 12 volts.

With a 12 ga wire, the drop would be 0.48 volts and with a 10 ga. wire it would be 0.30 volts.

At minimum, I would go up at lest one wire size for the same load at 12 volts as compared to 120 volts.  Two sizes up would be better.
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« Reply #6 on: November 23, 2012, 01:34:20 PM »

Len, I said that ampacity of wire was very similar between 12/24 volt DC and 120 volt AC wiring.  That is a true statement.  I then said that voltage drop over 14 gauge wiring over short runs with loads in the 1 amp and below range were inconsequential, which I clarified in my second post, my first post was not very clear. I stand on both those points.   The main point, on which we all agree (I think) is to engineer the wiring to suit the loads.  Anyway, use whatever wire you want to, It makes no difference to me.

Brian, OUT.
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« Reply #7 on: November 23, 2012, 01:49:49 PM »

No problem Brian.

There may have been some confusion if someone interpreted what you said to mean it's OK to use the same common wire sizes as you would for 120 volts, that is 14 ga for 15 amps, 12 ga for 20 amps, 10 ga for 30 amps etc.

As you said, the ampacity of the wires remains the same, they won't overheat and there is no danger.  The difference is that the voltage drop is the same and as a percentage of the beginning voltage, it is much higher with the lower voltages.  The same 1 volt drop at 120 volts is meaningless but at 12 volts it can be a problem.

I agree, if we are talking about 1-2 amp loads, it doesn't matter.
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« Reply #8 on: November 23, 2012, 02:23:09 PM »

Len and Brian, I over-reacted and was over aggressive.  I apologize.  Sorry about that, been dealing with insurance companies all day, and that is always going to make you crazy.

Brian
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« Reply #9 on: November 23, 2012, 06:48:06 PM »

I was saying that voltage drop on LED light runs and 1 amp fan runs are inconsequential.  Headlights aren't what I thought we were talking about.  I gotta question your math, though.  24 volt headlights drawing 25 amps would be 600 watts of headlights.  My high beams are  way under 300 watts total, probably 260 watts (65 watt halogen times four).  600 watts of headlights is a lot of lights...  Also, I calculate the voltage drop at 25 amps and 24 volts over 35 feet of 10 Ga. wire to be .87 volts, based on 0.9989 ohms per 1000 feet  from the chart I posted the link for.  That just happens to be the chart I use, it might be wrong.

I just used my headlight example because I recalled the numbers although I realized I was wrong on the number of amps.  It was only 12 amps, not 25 amps.  25 amps was if I used 12 volt bulbs.  For the life of me I can't recall why I used 8 AWG because 10 AWG would probably have been fine although I rechecked and it is actually 40 feet of wire, not 35 feet.  I had figured on needing 6 AWG wire for 12 volts which is why I went 24 volts for my lights. 

Going with 8 AWG wire was not cheap so I must have had a good reason for doing so.  I spent about $80 on a used crimper and another $20 or so on 8 AWG terminals in addition to higher cost for the wire.
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Brian Elfert - 1995 Dina Viaggio 1000 Series 60/B500 - 75% done but usable - Minneapolis, MN
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« Reply #10 on: November 23, 2012, 07:59:48 PM »

Sounds like y'all runing solid wire, me being the village idiot, I ran all stranded wire everywhere and next gauge above what was actually needed as I never knew ahead what all I might need to add later on that circuit.
Always figured the day would come when running an extra wire might be difficult once you have the walls in, ceiling up, an example, I should have run an extra circuit for a 3rd roof A/C, never did.  Another hind sight issue. Not bright like y'all.
I am not the bright bulb who never has hind sight issues, I see em daily.
Ya gotta laugh a little more I think.
Dave
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Len Silva
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« Reply #11 on: November 24, 2012, 06:32:55 AM »

Brian,
8 ga. is about right for that application.  You must calculate based on the total wire length (even if you are using the frame for return).

So, 24 volts, 12 amps, 80 feet and 8 ga. wire is a drop of 0.6 volts.  10 ga. would have resulted in a drop of 0.96 volts.

Electrically, it makes no difference if the wire is solid or stranded, same ampacity for a given wire size.
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« Reply #12 on: November 24, 2012, 06:44:46 AM »

I'm 99% sure I used only 40 feet in my calculations.  I thought only the positive mattered in length, not both positive and ground/negative.  I personally never use the chassis as a return path.

This thread is about DC and I can't imagine anyone using solid for DC.  I used stranded for both AC and DC myself.  Solid wire (Romex typically) meets code for AC and lots of RV manufacturers use it.
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Brian Elfert - 1995 Dina Viaggio 1000 Series 60/B500 - 75% done but usable - Minneapolis, MN
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« Reply #13 on: November 24, 2012, 07:12:16 AM »

Brian, I think your application (batteries in the rear of the bus, presumably) is a great one for a bus bar application.  In my bus, DC is distributed from the batteries to bus bars with 1/0 cable (for 24 volts) to the main distribution points, then quite short runs to the loads.  The headlights are run from the front bus bar, and the wire lengths are about 3 feet and 8 feet, respectively, for left and right.  I did the same thing for house wiring, with a fairly heavy run (twice anticipated load capacity) to a distribution box that acts as a fused bus bar.  I also home-run grounds for house wiring, less so for chassis wiring since it is already 100% chassis return stock.

Brian
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1980 MCI MC-5C, 8V-71T from a M-110 self propelled howitzer
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1972 NTM MK-4 B/SR
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« Reply #14 on: November 24, 2012, 08:04:39 AM »

My bus does have a whole electrical panel up front with places to connect up to chassis 12 volt and 24 volt.  The problem is the manufacturer did not use heavy enough wire in my opinion to add the headlights.  I was already seeing some voltage drop without the headlights running.  I choose to run a line directly to the batteries for the headlights and use solenoids switched by the original headlight wires.

My house DC bus bars are also at the rear of the bus.  I guess I never thought about putting another set of bus bars in the center of coach to make runs shorter.  I only have one house circuit at the front of the bus so no reason for a bus bar there.  My house bus bars are fed by 4 AWG cables that are very short.  I don't have all that many 12 or 24 volt circuits so I figured 4 AWG is plenty since the 12 volt is protected by a 100 amp fuse and the 24 volt by a 50 amp fuse.  I use 4/0 cables to my inverter.
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Brian Elfert - 1995 Dina Viaggio 1000 Series 60/B500 - 75% done but usable - Minneapolis, MN
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