I'm contemplating wiring a small garden shed with 12v RV LED lighting, and using a solar panel to trickle-charge a lead-acid battery. Fuse or breaker protection is of course a good idea.

Are 120v/240v AC switches, fuses, and circuit breakers generally safe to use with 12v circuits?

We've had a somewhat similar discussion about a year ago, but that was regarding 120v DC. For this discussion I am only referring to 12v (13-15v nominal).

At 12v DC I would not expect the same arc extinguishing problems compared to 120v DC, so it in general looks to me like all 120v AC or higher fuses, switches, and breakers should be directly compatible.

I'm looking for some expert to jump all over me and tell my why this would be wrong, wrong, wrong, as I'm trying to follow best safety practices here for wiring a building with 12v LED lighting.

The goal is to be using standard-looking wall switches and wiring, in case there is ever a desire to switch from 12v DC to 120v AC power and not completely start over again rewiring everything.

If an expert would recommend higher current rated switches for DC, I am fine with that.

12 gauge NM-2 carrying 12v DC should apparently use the same current limiting as 120v AC, at about 15 amps, and for 10 gauge, 20 amps.

Voltage drop may be higher with 12v in standard AC wiring, but it does not appear to be enough of an issue to worry about in a shed wired with 12v LED fixtures, with wire runs of less than say 100ft / (30m) max.

This voltage drop calculator says that for a 100ft run of 12ga at 15amps (max) the drop for 13.6v is 10.42v:

http://www.calculator.net/voltage-drop-calculator.html

I have tried the LED fixture with a 9 volt alkaline battery and it illuminates acceptably to me.

 

If the loads are mainly non-inductive you can use common AC switches, in most case fuses and breakers operate the same, the fuse voltage rating is for the sweep through rating for a certain voltage and breaker operate in a current/thermal manner.
Max.

 

Just be sure to install a proper DC rated fuse near the battery terminal that feeds the AC panel.
Something like this is needed for accidental short protection of the battery feed.

http://www.bluesea.com/products/5191

 

I just got shot down a couple of days ago on this. Something about AC contacts don't wipe themselves properly and DC switches do. I guess it would lead to a failure to contact through the microscopic oxide layer that 120 RMS can punch through and 12 VDC can't.

Melty wise, you're probably OK, but reliability wise seems to be the concern.

 

I just got shot down a couple of days ago on this. Something about AC contacts don't wipe themselves properly and DC switches do.

Personally I have found this more of a concern with high inductive loads.
And on Very high inductive loads, arc chutes and magnetic arc-blow out is often necessary.
Max.

 

The proper answer is NO.. Not to mention DC rated breakers are easy to get so why not use the proper device.

 

Regardless of what is the "correct" answer, I would go ahead and use the AC switches.

 

as I'm trying to follow best safety practices here for wiring a building with 12v LED lighting.

"best" safety practices would be to use properly rated components..simple as that..

 

It appears that what I am looking for are "dual rated breakers" or possibly "AC/DC breakers".

If DC breaker panels are commonly available for low-amperage consumer DC applications, I don't know where to find them, and dual-rated are not easy to locate either.

But I did find this today:

http://www.nmsu.edu/~tdi/pdf-resources/cc68.pdf
Home Power #68 • December 1998 / January 1999
Code Corner: Surfing through Breakers
by John Wiles

[....]
Dual-Rated Circuit Breakers

The Square D line of QO breakers and QO load centers are rated for 120/240 Volts AC, and they also have a lower voltage DC rating. When used in DC branch circuits (circuits from the battery to the load) they can be used on 12 and 24 Volt battery systems. Although rated for 48 Volts DC, the higher battery voltages (above 48 Volts) in normal charging and during equalizing pose stresses on these breakers and they should not be used in 48 Volt battery systems.

===========================

Also, the following is a not entirely fair comparison for 120v AC vs 12v DC....

Youtube: AC versus DC load breaking comparison with a knife switch
http://www.youtube.com/watch?v=Zez2r1RPpWY

But I guess it answers the question I was asking last year, about why not use 120v AC breakers used for 120v DC.

 

Square-D QO and QOB Miniature Circuit Breakers Catalog
0730CT9801R1/08, 2008, Class 730

http://static.schneider-electric.us...rs/QO-QOB Circuit Breakers/0730CT9801R108.pdf


Refer to Square D Data Bulletin 0601DB0401 for additional information on DC-rated circuit breakers.

http://static.schneider-electric.us...rs/QO-QOB Circuit Breakers/0730CT9801R108.pdf

Generator dc systems produce a short-circuit current with a rapid current rise that quickly reaches a steady state. DC battery systems produce a short circuit with a rapid rise to its maximum level that then decays as a function of the battery’s chemistry.

[...]

Systems that use batteries as a power source, such as uninterruptible power supplies (UPS), require special considerations when specifying overload protection. Typically, the protective device must interrupt overloads of 300–400% of rated full-load current in less than five seconds to prevent internal damage to the batteries.

 

My next question would be "So should I just use dual-rated AC/DC fuses instead? Get an old 120v/240v AC fuse box and use screw-in fuses?"

But I have already found more resources indicating that's not an easy answer either.

http://www.nmsu.edu/~tdi/pdf-resources/cc67.pdf
Home Power #67 • October / November 1998
Code Corner: Focusing on Fuses
by John Wiles

Which Fuses are Suitable for DC?

Fuses suitable for DC fall into several types. Any fuse used in DC renewable energy circuits should have the DC ratings printed on the fuse or be shown in the UL Listing information in the technical data for the fuse. The technical data is available in the manufacturer’s catalog, data sheets, and even on the WWW. Beware that seemingly identical fuses from different manufacturers may not have the same DC rating even though they are given as exact replacements in the manufacturer’s cross-reference data. The DC rating and UL Listing should always be verified.

(Lots more in the article..)

 

If you are working on low voltage, If you visit a Auto wrecker, you can get a whole DC fuse and relay centre for a couple of $.
Max.

 

If you are working on low voltage, If you visit a Auto wrecker, you can get a whole DC fuse and relay centre for a couple of $.
Max.

yes... assuming your voltage is below 32V (some only 28V) as thats the typical rating for dc automotive fuses..

All auto parts stores will have "fuses" for low voltage DC..

All electronics parts stores..digikey/newark/mouser/farnell,etc.. will have dc rated breakers for a couple bucks each.. Many are panel mount with a simple hole or other cutout.. Just make your own "breaker panel" with a piece of metal/wood and some holes.. For example http://www.digikey.com/product-detail/en/QMB-102-00ENN-3BA/Q518-ND/1963973
but they have tons of different styles/voltage/current ratings,etc...

DC breaker panels can be purchased through solar, marine and telecom power distribution companies and will typically run in the $100-2000 range depending on your number of circuits and ampacities.

 

yes... assuming your voltage is below 32V .

The OP mentioned 12vdc...

Max.

 

The OP mentioned 12vdc...

Max.

Then good to go..

Thought that was just his light voltage though.. wasn't sure about the solar panel

 

From that last reference on fuses, auto fuses with solar power is not a good mix, and automotive fuses may fail to stop a short for solar power.

What About Automobile Fuses?

Automobile electrical systems are designed to operate in a different manner from stationary PV systems. In an automobile, the electrical loads are designed to operate when the engine is running and the alternator is charging the battery and supplying the loads. The alternator output is in the 14–16-volt range and the radios and lights are designed to run at about 12.8 volts. A voltage drop is allowed in the conductors to reduce the alternator voltage to that needed by the loads. With the exception of the starting circuit, most conductors to the lights, fans, and other loads have relatively high resistance. This circuit resistance includes the resistance of the steel body parts used for the negative conductor. The system is not designed to power the loads for any length of time on the battery alone when the engine is stopped—the result would be a dead battery. This high-resistance wiring limits the available fault current from the battery and allows the use of automotive fuses with a very low interrupt rating.

A PV system, on the other hand, operates for extended periods of time on the battery without charging from the PV. Voltage drop must be minimized since the batteries start out at only 12.6 volts when fully charged. To minimize voltage drop, larger, low-resistance conductors are used in PV systems. These low-resistance conductors allow higher fault currents throughout the circuits. These higher fault currents are substantially in excess of the very limited interrupt ratings of automotive type fuses.

Another factor in the use of automotive fuses is that most PV systems (except the very smallest) have several batteries with high short-circuit current capabilities when compared to the single automobile battery found in vehicles. While there are a few UL-Listed automotive fuses, most are not listed. Even those that are listed are listed for use only in vehicles, and are tested to standards for use in vehicles. They have none of the conventional interrupt and voltage ratings. Automotive fuses do not meet NEC requirements for installation in PV systems that come under the Code. For these reasons, it is inadvisable to use automotive fuses in renewable energy systems to meet NEC requirements.


So, probably a small Square-D load center with QO breakers is the best way to go, to have potential future AC compatability.

 

The Square D QO series breakers and load centers are the best choice I found when investigating similar questions for a lighthouse I work on.

Automotive supplies may work in a pinch, but can never be part of a system that meets any electrical code requirements, so that rules out every fixed structure (but your RV is fine).

Let me see if I can dig up the link to some great solar system guild lines: took me weeks to understand it all, it is that dense.

Found it. New Mexico State University publications.

Note not all the links work, keep clicking and you will get lucks on some and get the pdf (in 2 parts).

 

Automotive supplies may work in a pinch, but can never be part of a system that meets any electrical code requirements, so that rules out every fixed structure (but your RV is fine).

The Square D QO series circuit breakers and load centers are the best choice I found when investigating similar questions for a lighthouse I work on. Let me see if I can dig up the link to some great solar system guild lines: took me weeks to understand it all, it is that dense.

Found it. New Mexico State University publications.

Note not all the links work, keep clicking and you will get lucks on some and get the pdf (in 2 parts).

There is a removable cover on the front of a circuit breaker behind which will pass through an overcurrent protective device. Square D breakers all have this basic mechanism.

 

Hagar MCBs give their rated breaking capacity (usually 6 kA from memory) for up to 250V a.c. and 60V d.c. The thermal trip remains the same but the magnetic part normally responds to the AC peak current so they need about 40% more current to trip the magnetic part.

Realistically at 12V you're unlikely to have any real arc inturruption problems, at least at the currents you'll be using for lighting. It should all just work.