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Small Cabin Forum / Off-Grid Living / Designing & wiring for a 12 volt tiny house.
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SandwichBear
Member
# Posted: 4 Sep 2013 16:10
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I built an 8x12 tiny house 150ft from my parents' garage on their suburban property. Now I am ready to install a small 12v system. I have a question.

General Details:
100 amp hour daily need (approximately)
Four 12v 115ah batteries in parallel (to be purchased)
100 amp multi-stage converter (to be purchased)
12 circuit blade-style fuse box (Blue Sea; purchased)
300-400 watt cigarette-type inverter (in case of small 120v need)

Running permanent power from the main house is not an option. Solar is not an option. My plan is to charge the battery bank for a few hours every couple days.

My plan is to use a long orange cord from the garage plugged into an inlet on the house (house-style 3-prong). The inlet will be wired to the 100 amp charger - when plugged in, the batteries are charged efficiently. Then I can wrap up the orange cord until next time.

I will have 12v lights and switches, and cigarette-type outlets placed around the house. My laptop is the biggest power draw, and I will find a 12v plug for that. A small inverter will be on hand "just in case" something small/low energy/120v needs to be plugged in for a bit.

Does any of this raise red flags so far?

I am new to 12 volt systems. I have done a ton of reading, but I have no hands-on experience. My plan is to create a good plan, buy the parts, and wire everything carefully when I have full confidence that I am not being hasty.

Here is my real question, provided the above background information seems feasible: where do I ground my lights and 12v outlets? This question is possibly so elementary that I am embarrassed to ask, but I have not found an answer or tutorial to help. The fuse box has a + connection from the battery bank (to be fused at 125amp max.) Each circuit from the fuse box has one screw for the positive wire to run to the light/outlet. So I attach a ring terminal on 12 gauge wire, and run the other end to the load. Right? Then what do I do with the negative wire on the back of the load? Where does it go?

If anyone knows of a good tutorial for wiring a similar system, please let me know. I'm starting from absolutely zero knowledge and experience - but I had zero building experience, and the house I built looks great, so I know I'm capable of learning.

Any reply with specific information about where to run wires between a fuse box and a 12v light fixture - and how this system is grounded in a cabin setting - would be greatly appreciated. Any input about the other details of my proposed setup will be carefully considered, and also much appreciated.

Thank you.

VTweekender
Member
# Posted: 4 Sep 2013 16:40
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Sure sounds like a great and fun project!! It appears you want to set it up like an RV...

I can tell you that the converter you plan on using is made to power your devices and not charging batteries...they will not fully charge a battery..

You can read that on this link along with other info that may come handy to you http://www.marxrv.com/12volt/12volt.htm .. read both part 1 and 2 to get all info bits here and there that fit into your project..like wire sizes..

SandwichBear
Member
# Posted: 4 Sep 2013 17:30
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This is the converter/charger I was looking at:
http://amzn.com/B008B13NIQ

MtnDon
Member
# Posted: 4 Sep 2013 18:21 - Edited by: MtnDon
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Quoting: SandwichBear
Four 12v 115ah batteries in parallel (to be purchased)


Don't! MUCH better you use 6 volt golf cart batteries; two pairs in series to make 12 volts and then the two series strings in parallel. Better batteries and the fewer the parallel strings the better.

~~~~~~~

Quoting: SandwichBear
This is the converter/charger I was looking at:
http://amzn.com/B008B13NIQ


I did not read the specs but does it say what the absorb time is? It would be nice if the absorb time with that was user programmable. Also it would be nice if the voltage set points for bulk to absorb and then to float were user programmable, but that is probably asking for too much.

~~~~~~~

Quoting: SandwichBear
plan is to use a long orange cord from the garage


How long? What gauge? long cords incur voltage drop that can be problematic. I can calculate the cord gauge best suited for the distance and maximum avergae load if you supply that. (If you need help, that is.)

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NOTE: regarding the charger / converter. IF you had those 4 x 115 amp hr batteries in parallel the ideal maximum charge current would be C/10, where C= the amp hour capacity (460). That would be 46 amps. The 100 amps may be too high and may cause overheating in bulk stage charge IF there is no remote battery. temperature sensor option.

IF 4 typical golf cart batteries were used in series / parallel the amp hour capacity would be approx the same.

~~~~~~~

Another Note: Unless the charger is left connected all the time the equalization charge stage will never be activated. It depends on being connected to input AC for 7 continuous days and then it equalizes every week. And an every week equalization is too frequent. Too frequent use of equalization rots batteries.

~~~~~~~

Quoting: SandwichBear
where do I ground my lights and 12v outlets?


Each circuit has a positive and a negative. If there is not a Negative Buss bar/connection point, than you need to make one and run a negative line from there for each circuit.

In addition at one point, and only one point in the system, the negative should be connected to an actual grounding wire that then connects to an 8 ft copper clad grounding rod that is hammered into the earth. That is the ideal, but many 12 VDC systems get along without that when they are not having to meet electrical codes.

The cabin wiring can be the usual Romex type used for AC. The ground wire in that would be connected to the ground buss connection. When using Romex the black is considered + in a DC system, the white is the negative and the bare copper, sometimes green, is the ground.

~~~~~~~

rayyy
Member
# Posted: 4 Sep 2013 19:00
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I think you need a small battery charger and a couple of marine deep cycle batteries.If all you are powering up is a laptop and a couple of lights.A small 12 volt fuse block with fuses and 12 guage romex can be used .

SandwichBear
Member
# Posted: 4 Sep 2013 21:05
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Thanks MtnDon. In response:

1) Golf cart batteries. Agreed - now looking at 6v Deep Cycle wired for 12v

2) No "absorb time" listed for the charger. I would only plug it in every 2 days or so, ideally, so the batteries wouldn't be equalizing all the time. Is there a better option other than this device?

3) Re: orange cord. Right now I have a fairly flimsy one that runs a saw by the building site. It is just shy of 150'. I can get a beefier one for the charger, but haven't chosen one yet. I could also examine the possibility of buried wire for part of the run, and plugging in at a junction closer to the garage. (Running an extension cord across the driveway to a relay to charge batteries.)

MtnDon
Member
# Posted: 5 Sep 2013 01:31
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Quoting: SandwichBear
1) Golf cart batteries.


Sams Club / Costco have great GC-2 pricing at their stores that carry batteries. Not all do.

~~~~~~~

Quoting: SandwichBear
2) No "absorb time" listed for the charger. I would only plug it in every 2 days or so, ideally, so the batteries wouldn't be equalizing all the time. Is there a better option other than this device?


Google IOTA battery chargers. When the IQ4 option is selected you get a similar 4 stage charge system. Their absorb is 8 hours which may be excessive, IMO, but I have used theirs and like them a lot. They have many models with different amperage outputs.

Honestly theirs are probably much the same as the one you posted. Iota is US made last I knew. They can also be stacked; 2 same model chargers used in parallel (with an accessory) to double charging capacity.

~~~~~~~

Quoting: SandwichBear
3) Re: orange cord.


To use the Iota 45 charger as an example... It draws a maximum continuous current of 11 amps AC. Using that and the 150 foot distance, my calculator says I should use a wire of 10 AWG. That would keep the voltage drop down to less than 4%.

12 gauge @ 5.5% and 14 gauge @ 8.5%.

islandguy
Member
# Posted: 5 Sep 2013 08:05
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My entire cabin uses exactly the same system..., we run a single 12v gel cell deep cycle battery to power all the lights and the 12v water pump. We recharge the battery by running 15 a cords about 150' and recharge at about 8 amps. I used regular wiring and light fixtures, but use 12 volt light bulbs, available from rv dealers. The system works very well, and we have been happy with it for several years. During summer, we can run the cabin for a couple days without charging the battery at all, then take it home with us, and charge it there.

Dillio187
Member
# Posted: 5 Sep 2013 11:58 - Edited by: Dillio187
Reply 


Quoting: MtnDon
Quoting: SandwichBear
Four 12v 115ah batteries in parallel (to be purchased)


Don't! MUCH better you use 6 volt golf cart batteries; two pairs in series to make 12 volts and then the two series strings in parallel. Better batteries and the fewer the parallel strings the better.



what he said. Skip all the parallel batteries, and use 4 golf cart batteries to get your 440ah bank. wire the bank like shown for series/parallel to equalize the draw across the parallel strings.

Battery diagram

SandwichBear
Member
# Posted: 5 Sep 2013 12:55
Reply 


Thank you for the replies. I am looking in a better direction at batteries now, and I have some added confidence that my proposed system is not far off the mark.

About the ground issue, however, I'm still fuzzy. I don't have any big electrical loads, but it still bothers me not to do something the "right" way, or an absolutely safe way.

So, ideally I would use the 8ft copper bar hammered into the earth. (If this is overkill, is there a different option?) At the bar, I use a screw clamp to attach some 10 awg wire that runs up through the floor? On the other end of that wire, I attach a terminal, and run the negative wire from all of my 12v outlets and lights to the terminal? That closes the circuit, and I have a safely grounded setup?

Or, do I run all the negative wires to a bus that goes to the ground on the charger? Then I ground the charger to the copper bar?

This is one of the projects where I'll know a lot more afterward, and feel ridiculous looking back. I appreciate the patient responses.

MtnDon - you helped with the first round of silly questions about my roof and rafters, and your input here is making you the MVP for the next round of questions as well. Thank you for spending time on this.

MtnDon
Member
# Posted: 5 Sep 2013 14:32
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Since this is a 12 VDC system, the ground wire and rod may be considered overkill. Virtually anybody can touch both positive and negative leads in a 12 VDC system and feel not even a tingle. Virtually no chance of shock. That can not be said once the DC voltage gets around 48 volts.

So you could most likely safely build the 12 VDC system with positive and negative wires and not worry about a ground. (The lawyer that lurks in me makes me use those 'qualifier' adjectives.

~~~~~~~

If THIS is the fuse panel you bought it appears the upper terminal set is for the negatives. Is that correct? If so, that's good; positives and negatives all have a source point.

Then IF you decide to use Romex wiring and use the bare ground to ground things you need another buss for the grounds. Ideally this would be contained within the same enclosure as the positive & negative.

8 ft copper clad rods are the standard used.

The idea with the ground buss is to have a single ground buss that conponents like the charger, to connect to. All the circuit grounds would come home to it. Anything added in the future like a solar charge controller would also be connected to the ground bus. An inverter with an external ground lug would also connect to that ground buss.

Then one single connection from the negative DC buss to the ground buss.

But as stated the inherent extremely low risk associated with 12 VDC makes the ground wiring nearly superfluous. That does not mean that you can get careless when handling wires that may be live. A couple of batteries do have enough capacity to weld things together and perhaps cause explosions.

SandwichBear
Member
# Posted: 5 Sep 2013 15:10
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Re: fuse box - the one pictured is the one I tried to order, but the one I actually got doesn't have the negative bus. Should I return it, or will it still work?

Thanks for explaining the risk factor of 12v - I am beginning to understand these concepts, and continue to review the information from the books that I have on hand. (For the record, I use qualifier adjectives constantly in normal speech and writing, so when I claim authority on a subject, I have credibility - to those who know me in real life, anyway. Using qualifiers is a good practice, even for non-lawyers.)

Back to wiring a 12v light. The positive wire from the fuse box goes to the light's positive lead - and the negative wire from the light goes back to...
1) ...the negative bus on the fuse box, if there was one, and then that circuit is good to go. (?)
2) ... a bus bar that I buy separately and wire to the negative terminal on my bussless fuse box, and then that circuit is good to go. (?)
3) If using Romex, the black internal wire goes to the light's positive lead, white goes to the negative lead (and back to the negative bus) and the green wire goes to a ground bus that leads to the (pretty much superfluous) copper rod. (?)

In the case of Romex use, the ground bus [attached to 10awg wire leading to the 8' rod] should be next to the fuse box so the three internal wires are conveniently leading to a similar location in the house. Right?

Sorry if it seems like I'm dragging out a simple question. I want to be clear as day before I set this up. I appreciate your plain-language explanations and patience.

MtnDon
Member
# Posted: 5 Sep 2013 18:13
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Quoting: SandwichBear
Re: fuse box - the one pictured is the one I tried to order, but the one I actually got doesn't have the negative bus. Should I return it, or will it still work?



It boils down to needing some method, some place to tie all the negative leads together. The idea of a buss is that if for any reason it becomes necessary to remove one wire at any time, it permits that without requiring other wires to be disrupted, as would happen when several wires are simply connected with wire nuts.

Same reason applies to ground busses. We do not want to interrupt grounds to other devices when we service some other one. Again, very important in higher voltages, not so much in 12 VDC.

~~~~~~~
Quoting: SandwichBear
Back to wiring a 12v light. The positive wire from the fuse box goes to the light's positive lead - and the negative wire from the light goes back to...
1) ...the negative bus on the fuse box, if there was one, and then that circuit is good to go. (?)
2) ... a bus bar that I buy separately and wire to the negative terminal on my bussless fuse box, and then that circuit is good to go. (?)


Either will work.

~~~~~~~
Quoting: SandwichBear
3) If using Romex, the black internal wire goes to the light's positive lead, white goes to the negative lead (and back to the negative bus) and the green wire goes to a ground bus that leads to the (pretty much superfluous) copper rod. (?)


Yes. The NEC does not directly address wire colors in DC circuits. But the AC convention that white is always neutral in AC more or less carries over to the negative wires in DC. Thus the DC positive becomes black. A ground is a ground. (Not to be confused with or by those folks who call the DC negative in a vehicle a 'ground'. That can lead to confusion, IMO.)

Red can be used as a DC positive, but using black as a negative, as many do, goes against the "what is white" school of thought. The electricians I talk with all agree on the concept of DC white = negative.

I carry that through to the battery connections, using all black wires/cables and then using 6 inches of white tape at the end/connection point of every negative.

~~~~~~~
Quoting: SandwichBear
In the case of Romex use, the ground bus [attached to 10awg wire leading to the 8' rod] should be next to the fuse box so the three internal wires are conveniently leading to a similar location in the house. Right?


Ideally all three (pos, neg, grnd) would be in the same enclosure, but since we don't have an ideal situation, separate but close by should be fine. Be nice if there was a cover door or removable panel to keep curious fingers away and safe.

SandwichBear
Member
# Posted: 5 Sep 2013 20:01
Reply 


Thank you again. I'm almost fully understanding this, I promise (and hope).

I am leaning toward using Romex because I like that the three wires are sheathed together. Seems easier. So I've established that black will make the positive part of the circuit and white for the negative. Do I connect the green/ground wire along with the white/negative wire to the lead at the light? Just cap them all together? (With the other end of the green wire going to the ground bus).

Additionally, I've found that my fuse box has no negative bus or negative connection of any sort. I'm considering returning it in exchange for one with a negative bus. BUT - if I stick with this fuse box, where do the white wires go? A bus connected to a negative battery terminal?

If a fuse box has no negative connection points, where do all the negative wires go in an auto setting?

A custom box will be built to contain the components of this setup. At very least, there will be no visible wires within curious-finger-range. A good point.

MtnDon
Member
# Posted: 5 Sep 2013 20:19
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Quoting: SandwichBear
Do I connect the green/ground wire along with the white/negative wire to the lead at the light? Just cap them all together? (With the other end of the green wire going to the ground bus).


At the fixture end if there is no ground lug on the light or whatever, just leave the bare wire tucked out of the way where it can not accidentally contact the pos or neg. In an AC circuit many receptacles, etc have a green ground screw. With a lot of DC stuff there will not be any ground lug/connector. If you use any metal boxes connect the bare to the metal box, as in AC.

I have used some Thinlite fluorescent and have grounded the metal enclosure of the lights to the bare wire.

~~~~~~~

Quoting: SandwichBear
where do the white wires go? A bus connected to a negative battery terminal?


That would work. You can buy buss bars for std home service panels in varying lengths. Sold as a ground buss. If you make an enclosure from a non conductive material you can screw/bolt one of them to the case for the negatives.

One of many...

~~~~~~~

Quoting: SandwichBear
where do all the negative wires go in an auto setting?


Items in an motor vehicle have a pos and a neg going to each electrical item. Eventually all the negatives will connect back to the main negative lead to the battery in some manner.

SandwichBear
Member
# Posted: 5 Sep 2013 21:00
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Thank you very much for this input. I think I have enough to proceed. If I have future questions, I should be able to provide photographs of the real stuff in question.

MtnDon, you've done it again. I'm grateful.

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