question about switch panel and fuses

1 Posts
Sep 16, 2009 11:45 am
question about switch panel and fuses

Hi there,

I am putting together a small 12V electrical system in my cabin. I am looking for advice on how to properly wire my lights into the battery bank. Right now I have 4 15W fluorescent lights that run to a marine switch panel. I am wondering how I properly size the fuse in the panel. It is 10Amp right now.

Maybe a marine switch panel is not the best tool for the job either? As far as direct wiring to the 12V I have a cellphone with smoothtalker booster on one switch, cambridge soundworks model 12 speakers on a switch, a VHF radio on a switch, and the lights on a switch. Is there a better way to wire that stuff into the battery bank?


« Last Edit: Sep 16, 2009 11:51 am by Bob Christensen »
Sep 19, 2009 03:52 pm
Re: question about switch panel and fuses

"I am looking for advice on how to properly wire my lights into the battery bank."

At the risk of sounding somewhat anal, that word "properly" opens a door to all sorts of opinions and, like belly buttons, everybody will have one.

The word "safely" could even open that same door.

There is the National Electrical Code (NEC) however, which would most likely apply here because your cabin would be considered a dwelling.

Things to consider are; element (for example; copper, aluminum, steel) AWG. (wire size) and type of wire insulation and their voltage and amperage ratings, means of overcurrent protection as well as physical protection of the wire and, a disconnect means. One other thing to consider with low voltage dc that is very important is voltage drop.

To give you an idea of what all that means.
NM-B 12/2 - Inner conductors are type THHN, rated 90 degrees Celsius, 600 volts. Solid strands. Outer jacket is PVC rated at 75 degrees Celsius. Ampacity of the product is limited to that for 60 degrees Celsius rated conductors per the National Electric Code Article336. Dwellings not exceeding 3 floors above grade. Exposed or concealed wiring. May be fished through walls, ceilings, and masonry blocks. Use for new wiring or replacement wiring. Only for use in normally dry locations. U.L. Listed. /2 = 2 Insulated Wires with Bare Ground. /3 = 3 Insulated Wires with Bare Ground. 
Romex NM-B 12/2 has a maximum rating of 20 amps BUT! at 12 vdc there can be significant voltage drop over, even over a short distance. As an example if this Romex NM-B 12/2 was used on a 12 vdc lighting circuit with one, 12 vdc 50 watt incandescent bulb that was 20 foot away, in wire legnth from the battery, there would be about a 3% voltage drop.
A lot of folks try to stay 2% or lower especially when power is coming from a battery that is discharging the whole time the light is on.
Switches must have UL listed ratings as well for use on direct current circuits. Those cheap alternating current switches at home improvement stores will not last and can be dangerous on dc circuits.

Think about how a conventional homes 120/240 ac breaker panel works. There is the main power supply (the utilities) going through a main breaker or over current protection device then all the branch circuit going out into the home from there each one protected with an overcurrent protection device sized according the wire size which is sized according the load.
You'll want to follow this example. There is the battery then a main and branch circuits. All overcurrent protection must be rated for use in direct current circuitry and be sized for the wire used which is sized and typed according the load and its location.

I could write a book on all of this, wait, there are allready books written on this subject. Psssh! Go buy the book. Smiley
Check Alt-E Store's, book store. You'd think they would have one for low voltage dc wiring. Oh! Check out
What the hey, here is one of my favorites.
It says it all at the end of the first paragraph.

Sep 19, 2009 04:14 pm
Re: question about switch panel and fuses

You know, I followed my own advice and checked Alt-E Store, and  even though they have quite an arsenal of knowledge in renewable energies, I could not find a book on wiring a home with direct current circuits.
Here is someone else take on the subject of wiring a home with low voltage dc power.
Keep in mind that the NEC is revised every 3 years.
PS. More to come as I find them.
Sep 20, 2009 10:50 am
Re: question about switch panel and fuses

Its amazing! I only searched for a total of 2 to 3 hours since my last post and, I could not find any book(s) about wiring a home or cabin for 12 vdc. Its not a common practice, that may be one reason why. Perhaps another piece of advice might be more practical for you would be to go to Alt E Store and research various items and there ratings. Things such as but not limited to fuse's and breaker's.
Knowledge is power!
I learned things about wiring a home for 12 vdc from The School of Hard Knocks, if you catch my drift. Theirs is not a structured, regimented curriculum but one of trial and error and ojt. My system started with a short piece of 10/2 UF wired directly to my trucks battery and hanging out of the grill with a 120 vac female cord end. From the house was 10/2 UF with the corresponding male plug. Just inside this went across one pole of a two pole 30 amp fused disconnect with a 15 amp fuse. The old glass screw in one time type fuse. From there NM-B 10/2 went to 3 different porcelain pullchain lamp holders each with a 12 vdc 50 watt incandescent bulb. This began my learning of the truth about cranking batteries vs. marine deep cycle batteries vs. true deep cycle batteries and how each has its own proper application. Theres that word again.
How did I know to use 10/2 as opposed to any other, with reference to voltage drop? I didn't. But I did know that #10 was good for 30 amps. The wire and the disconnect were all free, salvaged, all I had to purchase was the fuse and and male and female cord ends as well as the round nail up plastic boxes, lamp holders and bulbs as well as wire staples. Ohm's law told me that 50 watts at 12 volt equals 4.17 amps and that times three is 12.51 hence the 15 amp fuse. We learned eventually (and several cranking batteries later) how many and how long we could burn the lights and not have to push start my truck the next morning. This evolved in a system with an high output alternator on my truck with a battery isolator and a couple of marine deep cycle batteries and bigger wire (6/3 SO cord) with a 50 amp range receptacle and plug, a 60 amp fused distribution panel with 40 amp main fuses and a few more lights.
All of that has since been abandoned and removed.
Now battery recharging is all done with PV and household power is all 120 vac from inverters except for a few dc lights and a cigarette lighter type receptacle for recharging certain things. All sized and protected in accordance with the NEC I would like to add. I did have some advantages in the fact that both of my parents were/are knowledgable of of electricity and electrical wiring. I grew up hearing phrase's like Ohm's law and potential to ground. I myself have become an electrician in the fields of residential, commercial, and industrial. Mostly industrial motor control trouble shooting as a way of earning a living. PV is my main intrest though and I never stop trying to learn and gain experience in that field. But, with reverence to the immortal Luke Skywalker, if there is a bright spot in the PV universe I am in the place farthest from it. Unless you include my own private experience, a PV installation where (in my youth) I was an awning rigger installing the frame and PV modules (no wiring) to the side of a building, a 2 day workshop at the NCSU Solar House where John Wiles was the guest speaker, my occupation as an electrician, various books and magazines (self education), and the internet, I have no practical experience in the field of PV. Something I can only hope will change one day before I am too old.
And now you know, the rest of the story.
You know, it would seem that just living a life of various experiences and self education would make one eligible for an earned degree equivalent to a structured and regimented degree. Perhaps one day, when the acquisition of wealth and power are not the primary factors ruling our lives such things will be. As Syndrome put it, "When everybody is super powered, nobody will be."
351 Posts
Sep 20, 2009 01:39 pm
Re: question about switch panel and fuses

“Maybe a marine switch panel is not the best tool for the job either?”

As Thomas pointed out, the answer to that is dependent on your criteria for your system.
Is it Workable and safe ? Or, Code compliant (NEC), workable, and safe ?

Your marine switch panel is not code compliant for a dwelling. Neither are most RV devices.
578 Posts
Sep 21, 2009 10:59 am
Re: question about switch panel and fuses

for folks doing 12v wiring, here are the most common units that they use.  both enclosures are listed to meet code.

depending on the size of the conductors, you may have trouble getting anything above 2awg into the breakers or potentially busbars.  these are the most popular choices, but not necessarily the best for your situation.

square D qo gear is rated for 12vdc as well as 120/240.  I believe that to use this enclosure for 6 circuits, you have to jumper over from what would have been two separate legs of 120.

with 8 small breaker slots, the outback enclosure is larger and more flexible in its implementation.  you will have to add a positive busbar, as i believe that it comes without one, assuming a jumper from the bottom of the large inverter disconnect circuit breaker that is used in typical off grid systems because of 2/0 and larger conductors and their 3/8ths lugs.

12v wiring is not that common anymore as a permanent code compliant solution.  issues such as voltage drop, large conductor sizes, listed equipment (light switches are a nightmare to find), and ability to have anybody but the installer work on it easily are the biggest challenges.

generally, I have found that by the time the time and expense of doing things correctly (code compliant) at the 12v dc level, the costs in time, money, effort, and efficiency, are more than made up for in creating a standard AC system with an inverter.  the wiring instantly becomes smaller, more standardized, and the ability of an electrician to work on the system in the installers absence increases substantially.  only in the smallest of systems (this one may apply) do we find that the 12v systems are the absolute win win.

hope that helps,

altE staff

"Making Renewable Do-able"

Tel: 877.878.4060 x107  or +1.978.562.5858 x107
Fax: 877.242.6718  or +1.978.562.5854
99 Posts
Sep 25, 2009 09:25 pm
Re: question about switch panel and fuses

Now battery recharging is all done with PV and household power is all 120 vac from inverters except for a few dc lights and a cigarette lighter type receptacle for recharging certain things. All sized and protected in accordance with the NEC I would like to add.

I'm currently wiring my home for DC (in addition to AC).  I'm using SquareD QO panels and breakers, which are rated for DC in addition to AC (individual panels of course, you don't want to mix them).  I'm curious as to what the NEC has to say about "a few dc lights and a cigarette lighter type receptacle". 

I'm planning a loop of DC lights as backup in case the inverter goes out.  And I want to have DC receptacles in several rooms for basically anything that would normally use a wall wort / power brick.  That includes any kind of chargers and most electronics. 

Is there any kind of standard DC receptacle type other than the cigarette lighter type?  I couldn't find any.  How do you mount one of these in a standard wiring box?  Do you just splice the wire in a concealed/enclosed box and then drywall around a wire sticking out of the wall?  Or are there such receptacles (perhaps for the RV market)?

Also, do you happen to know of any indoor 12VDC motion/occupancy sensors?  I'd like to have my DC lights triggered by occupancy.  They will provide baseline ambient lighting while AC lights will provide task lighting.
351 Posts
Sep 26, 2009 12:49 pm
Re: question about switch panel and fuses

Cigarette lighter plugs and receptacles are not code compliant. If you want to be totally compliant, you will need to use different receptacles and then make an adapter to the cigarette type device.

You can use a number of 125V or 250V receptacles for your DC system, as long as they are not used elsewhere in the house and they are labeled.  The NEMA 6-15r is one that is a good one. It is rarely used in households.
It is also available in a duplex configuration, but the single is generally easier to find. It is available at Lowes or Home depot.

I’ve seen a few people use the NEMA L5-15r or L5-20r twist lock receptacles.

Finding light switches is a problem, if you want them to look like light switches.  If you don’t mind metal toggles switches, then they are easier to find. Code requires an AC-DC General-Use Snap Switch.

A code compliant, 12VDC motion switch, that is fluorescent compatible ?? Good luck.

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