Advice on DC Lights & Switches

RockKnoll · October 2021

Hello, I am helping a friend wire his off grid cabin. We are going with all 12 v DC lighting and a few other minor DC items. I am finding conflicting advice on switches. With LED lights the biggest draw any individual switch will see is 2.5 amps. I have read that high quality (spec grade) commercial switches (20 amp 120v) are fine, may just not last as long.

Also, we intend to buy conventional AC fixtures and use 12 v DC LED bulbs.

Will greatly appreciate any experience you can share. THANKS

BB. · October 2021

Yes--There is possibly an issue with DC power and switches...

For the most part wiring itself is similar between DC and AC (15 amp branch circuit in AC or DC needs a 15 amp breaker/fuse).

The major difference is that DC power is much more difficult to "switch"... DC current tends to sustain arcs "better" (such as DC arc welders are "better" than AC arc welders). For example:

https://www.youtube.com/watch?v=Zez2r1RPpWY
The above was done at higher voltage (220 VAC vs 220 VDC)... But the issue is present on almost all voltages (technically, it takes ~12 volts or more to create a sustainable arc). 24 VAC or VDC makes for a very nice arc welder.

So--You need to look at the switches, fuses, and circuit breakers to see if they are rated for AC or DC, or many times, the DC rated voltage and current is less than the AC voltage and current for the same switch/breaker/fuse...

For example, here is a typical Leviton Switch product specfication:

https://www.leviton.com/en/docs/Q-1054B_Specialty_Switches_Sell_Sheet.pdf

For the Same Family that may be rated for 120 VAC @ 15 amps, there is a different part number for a switch rated for 24 VDC @ 3 amps.

Circuit breakers and fuses also have DC or AC ratings... Some have ratings for both like Square D QO circuit breakers:

https://download.schneider-electric.com/files?p_Doc_Ref=0730CT9801

Voltage Rating The circuit breaker must have a voltage rating greater than, or equal to, the system voltage. When a circuit breaker clears an overcurrent, it is done in two steps. First, the current sensing system identifies the overcurrent and releases the tripping mechanism. This results in a parting of the contacts. The circuit breaker must then extinguish the voltage arc across the contacts. If the circuit breaker has the correct voltage rating, it can efficiently extinguish this voltage arc. QO and QOB circuit breakers are rated for use in the following voltage systems:
• 120 Vac
• 208/120 Vac
• 120/240 Vac
• 240 Vac
• 48 Vdc (10–70 A for 1 and 2 pole circuit breakers, 10–60 A for 3 pole circuit breakers)

Here you can see that the DC rated voltage is much less than the AC voltage rating (and not many "off the shelf" home breaker panels/components come with DC ratings.

Here is a long (boring) thread on switch/fuse/breaker ratings....

https://forum.solar-electric.com/discussion/353232/oversized-wire-and-breaker

The short answer for switches--Get good quality snap action types (not sure if "quiet" switches work as well on DC) and they will last for years on 12 VDC...

For DC fuses and breakers--You do want to look at both the DC (or AC) ratings, and there is the Amp Interrupt Current (AIC)... Standard home circuit breakers are rated for 10,000 AIC. That is the maximum current a pole transformer can output to the homes on its 120/240 VAC service drop.

For large battery banks, they can also output 100s to 1,000s of Amperes into a dead short. So looking at AIC that is >100s of Amps or more, is important for DC systems too. The size of the wiring also matters... An 18 AWG wire that runs 20 feet will not carry as much current in a dead short at a 4/0 AWG cable (which can carry much more current as it has lower resistance).

You don't want to use a low AIC breaker (such as automotive fuses) on a large battery bus.

For solar, as an example, our host NAWS (Northern Arizona Wind & Sun) has DC rated fuses and breakers specifically to address these issues.

https://www.solar-electric.com/residential/circuit-protection.html

And Square D QO series breakers are a good choice for smaller solar systems too (AC and DC sides).

-Bill

wellbuilt · October 2021

It’s very easy to just get a small inverter to run lites and other things , the the wiring is easy .
The led lite are pricey .

A small inverter dosent use a lot of power .

Just saying Good luck with your project 👍

RockKnoll · October 2021

Thanks, I'll check on those DC switches from Leviton. If they cost a lot more, I may just see how often I have to replace switches. A couple will be flipping an amp or two, other's a fraction of an amp. So some may last for decades.

I was planning on using a QO panel, glad to see that part will be easy, with usual parts.

thanks!

BB. · October 2021

You can look for other (not NEC/Home wiring) type switches with DC ratings... Or you can get regular toggle switches that are DC rated.

Or, for the most part, you will have pretty low current LED lights--And the switches will last for many years--And be very cheap and easy to replace (if ever needed).

12 volt wiring--It can be a real pain for other reasons (low voltage DC in general)... Voltage drop from a few amps going 30 feet--Can cause issues with the lights and other devices.

For example, say you want to send 5 amps on 14 AWG wiring 30 Feet @ 12 volts--And you want (a suggested) 0.5 volt maximum drop:

https://www.calculator.net/voltage-drop-calculator.html?necmaterial=copper&necwiresize=0&necconduit=steel&necpf=1&material=copper&wiresize=0.4066&resistance=1.2&resistanceunit=okm&voltage=12&phase=dc&noofconductor=1&distance=30&distanceunit=feet&amperes=5&x=59&y=8&ctype=nec

Result

Voltage drop: 0.93
Voltage drop percentage: 7.77%
Voltage at the end: 11.07

So even using 14 AWG house wiring and a reasonably 30 Feet, sending 5 amps (5 amps * 12 volts = 60 Watts) is almost 1 volt drop... Not great (typically we aim for 1% to 3% maximum voltage drop). Either larger AWG wire (~10 AWG) or higher voltage (24 VDC or 120 VAC) or less power to the remote "lamp/device" is needed.

Using higher voltage (such as 24 VDC) makes things a bit easier wiring wise (smaller wire and/or can send more current longer distances)... And there are quite a few 24 VDC (or 12-24 VDC) lights out there---.

Or as WellBuilt suggests--Just use a relatively small AC inverter (such as 300 Watts) and work with 120 VAC... Standard 'house wiring' components from Home Depot--And you can now send power easily 100's of feet without excess voltage drop, and use "reliable" AC rated switches.

-Bill

PS: For LED lighting... I would suggest you get a few different "LED Lamps" + Ballasts and see how they work for you... Amount of light, color of light, and some are very sensitive to voltage fluctuations--I had some CFL bulbs that would have an irritating "blink" with unstable 120 VAC line voltage).

706jim · October 2021

My cottage is wired for 12 volts DC as well as 120 volts AC. Do yourself a favor and forget the 12 volt wiring. It's a waste of time for several reasons. Get a small inverter (even 150 watts) and run regular lamps.

wellbuilt · October 2021

^^^^^^^
this is what I’ve found , I started with 12v and it’s a pain .

Even a 12v radio sucks a bunch of power .

I use a marital job radio , it uses 3 watt on ac power .

I could not see a picture of any dc tv ac tv uses 20 watts .

My cabin is wired for AC power I could bring in a line and just hook up .

706jim · October 2021

Not only are there concerns with the switches, but there really is no decent standardized 12 volt socket. All of the ones you usually see are based on the ancient cigarette lighter socket that has very poor conductivity to that silly spring loaded positive terminal.

BB. · October 2021

Anderson Connectors are used for 12 VDC:

https://powerwerx.com/anderson-power-powerpole-sb-connectors

And you can get bus and fuse systems at HAM radio suppliers for Anderson/power pole connector systems.

https://duckduckgo.com/?q=power+connectors+for+ham+radio&iax=images&ia=images

-Bill

tabbycat · October 2021

Most rv converters use breakers for the AC wiring and fuses for the DC wiring. You avoid the complexity of the DC breakers this way.

RockKnoll · November 2021

Thanks all for offering your advice!!! My friend has pretty minimal needs, and is not one to take advice well.

I have it set up to switch all wiring to AC if that turns out better in the end. Will be curious.

Thanks!

checkthisout · November 2021

I have a mix of 12v and 120v lighting.

You can use normal household switches. They are fine and will last forever without issue.

There are more tiny light options in the 12v range and you don't have to use junction boxes and such...just wire it up like an RV.

BB. · November 2021

I will disagree that 12 VDC power on 120 VAC switches will last a long time... I have a few (now older) LED reading lamps with DC switching to turn the LED on or off... And over the years, several of the switches have failed--And I did replace them with 120 VAC light switches--Which have worked OK for another bunch of years, but appear to be slowly going bad (LED blinking a bit until the switch is thrown again).

And the part of avoiding J-Boxes... In NEC (national electric code), you are not allowed to "bury" electrical connections in a wall or ceiling. You need accessible J-Boxes for debugging and such (if ever needed to fix a bad connection, etc.).

Could you bury J-Boxes and connections in the wall of an RV or Cabin for 12 VDC or even 120 VAC--No inspector, nobody to make you do it over again. But if ever needed--It is a pain to work on buried/bad wiring practices...

Had a sub-contractor electrician (decades ago) that turned out to not have given up the old drug habit. Had to redo all of his stuff--And found other issues years later that I had to figure out. Was not fun (or cheap for the contractor to have fixed).

Oldy but goodie... Spiders on drugs and their webs (yes, the wiring looked like some of these webs):

https://rarehistoricalphotos.com/nasa-spiders-drugs-experiment/

-Bill

checkthisout · November 2021

RVs have connections in the walls for the 12 volt stuff. Yes they are not behind wall panels but you make a hole in the wall material, run your wires and splice to your device and then tuck the wires and connectors back in the hole and mount the device. They can also be behind surface mount stuff with the backing being the wall material.

This different than 120v stuff which must be inside a rated box.

120v snap switches work excellent for DC lighting and also do not look cheesy like DC switches do.

They will not fail within the ones lifetime and if they do they are 50 cents a piece at Home Depot.

MarkP · December 2021

I have a 48 volt Dankoff water pump. It came with instructions for hooking it up to a SquareD 240 VAC pump controller. When you take the cover off that you can see the actual contacts clearly. The controller switches both legs of the 240VAC. The Dankoff instructions had you wire the 48VDC+ leg in series through both contacts. Anyhow the moral of the story is to use a double pole switch and route the current through both poles to get twice the stopping power.

I highly recommend getting an appropriately sized inverter and going with all off the shelf 120VAC devices.

Advice on DC Lights & Switches

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