Using standard AC fixtures for DC applications

Question

I am building a small off-grid 12V solar system and looking for ways to connect DC lighting. I found many 12V DC light bulbs with a standard E26 base and wondering about an appropriate receptacle and switch.

Can I simply use something like this?

(Performax™ White Plastic Pull Chain Lamp holder / Part PPIWD7488)

Is there any issue with a pull chain switch and/or receptacle designed for AC power? Would any standard AC light switch coupled with an E26 base be safe or do I need to find a DC rated switch?

Answer 1

There is one concern, that means you need to derate switches etc. for DC use:

Because AC passes through zero 100-120 times per second, any arcing in the switch is rapidly suppressed (and there is arcing as you break contact). Switches, relays etc are thus rated for a lower current for DC than AC. Consumer products like the one pictured tend not to specify a DC rating, however typical switches for building into equipment do. This example (you'll need to read the datasheet to see that much detail) is rated for 20A at up to 277VAC, but only 7A at 72VDC and 14A at up to 36VDC. There are other limits on the current, but these are the relevant ones.

The amount you need to derate varies with the contact geometry. Many light sockets rated for incandescent mains bulbs should be fine with 12VDC LED bulbs, because of their much lower current - but the much lower voltage makes up for that.

Any fuses in the system - and you should have fuses to protect the wiring in a 12VDC battery system - should be specified for 12VDC. Luckily these are common in automotive applications.

You do have an alternative source of parts though: Caravans/RVs use 12VDC with lead-acid batteries (and sometimes solar panels) and there are plenty of LED lighting products designed for that application. After all, with LEDs, you shouldn't need to replace the bulbs so integrated fittings are fine.

Answer 2

I'd be wary of that lamp holder with a built-in pull chain.

As has been said in other answers, switches designed for AC need to be derated when used for DC, though it's not always clear by how much. Furthermore, a 12V DC light will draw ten times more current than a 120V AC light of the same power rating.

Wall switches are generally rated pretty generously, since the manufacturer doesn't know what loads will be connected to them. The built in pull chain on the other hand is likely built on the assumption it will only ever control a single bulb, so it's likely to have much less margin.

Answer 3

For a dollar more you can get ceramic ones that will behave much better in an arcing scenario, which is an omnipresent risk with DC.

But yeah, this is kinda what you do with low voltage 12V and LED bulbs. 12V is such a low voltage that it tends to be pretty docile. 48V on the other hand…

That said I agree with Jay613 that Edison sockets are a foolish way to set up low voltage lighting. There are many, many "bulbless LED fixtures" which are more compact and elegant than Edison screw-in LED types, which are annoyingly expensive for what they are. Not least they can conform to the wall or ceiling and be 1 inch thick, instead of being this ungainly thing sticking out. Heck, they make LED strips which are $15 for 16 feet, and can be cut and spliced every 1-4 inches depending on density. We really do have the option to break out of Edison's mold.

And I wouldn't want to rely on the pull chain switch, which is simply not rated for DC. I'd prefer to use spec-grade or better wall switches.

If you're using AC wires to wire it (great idea, the fat copper section really helps with voltage drop), remember DC - is required to be marked black (or be black) and DC + is required to be marked red. Don't use /3+gnd cable, use /2 and mark the white red.

You might consider switching the negative unless some other constraint prevents this. That is to conform to the standard for 12V LED PWM dimming and RGB/RGBW LED lighting, where the positive is common.

Apropos to nothing, Square D "QO" panels are UL listed for up to 48 volts DC. All others are AC only.

Answer 4

There's no issue. I used to build ice houses with the same hardware. It was never a problem.

As long as the current stays well under the rating, all good. (You may need to calculate back from wattage, which is what's often specified, or just use your bulb's wattage.) There is some difference in current between DC and AC, but it's only a concern if you're pushing the ratings. With modern lighting you probably won't get remotely close.