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Q&A Spotlight

How to Wire for LED Lighting

Does the wiring in a house with all low-load LED fixtures look any different?

Would a switch to all-LED lighting running on low-voltage DC circuits make it possible to wire a house differently? [Photo credit: Rex Roof / CC / Flickr]

One of the benefits of using light-emitting diodes (LEDs) over conventional light bulbs is that they produce a lot of light without using a lot of electricity. This low current draw, and the possibility of using a low-voltage direct current (DC) circuit rather than a conventional alternating current (AC) circuit, has Caroline Di Diego looking for advice.

“I understand we are still very much in a hybrid AC/DC universe and AC has to exist for kitchen appliances and infrastructure appliances in the utility room,” Di Diego wrote in a 2013 Q&A post. “However, our client wants to take the step into LED.”

At the time, Di Diego was having trouble finding answers to “LED wiring 101” questions about wire gauge options, voltage regulation issues, and other quirks of running DC circuits.

The post is now four years old, but recent additions to this thread, and a surge of interest in the efficiency benefits of LED lighting, suggest the issue is very much alive. Is there anything new to be said about LED lighting and its implications for residential wiring? Let’s find out.

Don’t overthink the problem

There’s no need to do anything unusual for LED lighting, advises Thomas Stone. Don’t overthink the problem.

“My house was built in 1957 and has been converted to nearly all LED lighting,” he says. “What did I have to rewire? Nothing. After some experimentation, I can recommend Cree and Philips as reliable brands, but not the new low price-point versions.”

Charlie Sullivan thinks that’s good advice, and he helps Di Diego sort out the AC/DC current question. “Although inherently LEDs run on DC, the high-quality end-use products are all made to run on AC,” Sullivan says. “The current draw on an LED light on an AC circuit is small, so in theory you could use smaller wire than normal wiring for lighting, but in practice you can’t go smaller than is allowable with a standard 15-amp circuit breaker, so you’ll be wiring it the same as for any other lighting.”

Sullivan points out that LEDs are improving rapidly, so he would not recommend fixtures with built-in LEDs that lock the buyer into existing technology.

The AC/DC battle goes on

Thomas Edison promoted the use of DC electricty, an electrical current that runs in one direction, in the late 1880s. That was the standard in the early  years of electrification, according to an account from the U.S. Department of Energy called

Direct current had some advantages, but it was not easily converted to higher or lower voltages. Edison’s rival, Nicola Tesla, pushed AC (alternating current) instead. With AC, electrical current changes  directions — 60 times per second in the U.S. — and voltages can easily be stepped up or down with a transformer.

Tesla won that battle, and AC became the standard. But the war may not be over. Computers, LEDs, solar cells, and electric vehicles all run on DC, and DC still has its advocates.

“Engineers have been touting the advantages of wiring homes for DC power for over a hundred years, ever since Thomas Edison lost the AC/DC battle with Nikola Tesla,” writes GBA Editor Martin Holladay. “There are still a few engineers lobbying for DC wiring.”

That said, Holladay continued, DC advocates can’t even agree on voltage, “so I wouldn’t be putting all my eggs in one basket if I were you. Stick with AC wiring.”

Where to locate the transformer

Even with an all-LED lighting scheme running on DC circuits, the house would still need conventional 120-volt AC current for appliances and other plug-in devices. The lighting would need a transformer that converts AC to DC.

“My thought was to have a central device near the service panel instead of multiple transformers,” Di Diego writes, “That seems obvious, and [a] much cleaner design.”

Mike M., however, thinks it would be a better idea to run 120-volt AC power to areas where DC was needed and install transformers there.

“Even if you have a central transformer, you’d have to have a DC protection panel with fuses or breakers to protect all of the DC wiring,” he says. “You would also end up with two sets of wires running to each area, one for the AC receptacles and one for the DC lighting. I think this would be extremely cumbersome and more difficult to retrofit or alter later.”

Charlie Sullivan lists two other reasons why distributing the AC/DC conversion makes more sense than a centralized transformer. Lighting fixtures drawing 12 or 24 volts would require bigger wire than a 120 volt AC circuit, he says, “so distributing the conversion makes more sense in that respect.”

Also, Sullivan adds, power supplies have standby losses. If there is a central power supply it’s going to be on all the time, so standby losses are higher.

“Another disadvantage of a central system is that if it fails, all the lights quit,” Sullivan says. “Having one light ‘burn out’ is much more tolerable, especially given that the power supply is not something you can buy at a local hardware store.”

Small gauge conductors are not very practical

Laurel Davidson wonders about the possibility of using “power over Ethernet” cables to run LED ambient lighting, task lighting, and possibly operating DC-powered appliances such as a television.

Davidson says that Cat 5 cable with 24-gauge conductors can safely carry 360 milliamps at 50 volts.

Yes, Holladay replies, but remember that 360 milliamps at 50 volts equals just 18 watts, enough for one or two LED lamps.

“Contrary to popular belief, low-voltage DC wiring (for example, wiring for 12 volts or 24 volts DC) needs to have a bigger wire gauge, not a smaller wire gauge, than 120-volt AC wiring (assuming loads of the same wattage),” Holladay says.

The smallest wire gauge allowed by code for residential AC circuits is 14-gauge, Trevor Lambert says, which can handle 15 amps.

“For a circuit of, say, eight LED light fixtures at 12 watts, that’s 96 watts,” he says. “At 24 volts that is only 4 amps. So you could certainly get away with 18-gauge wires for that application. However, it’s not clear whether that would actually be cheaper. The volume of sales of 14/2 wire makes it pretty cheap. At best the difference is going to be barely more than negligible.”

Holladay is very familiar with the inconvenience of running two types of cable. “My off-grid house is wired with two voltages in every room,” he says, “120 volts AC and 12 volts DC. The AC wiring is all 12 gauge, while the DC wiring is 10 gauge. Needless to say, the DC wiring cost me more than the AC. But wiring an off-grid house is a little different from the subject at hand — a few LED light fixtures.”

Our expert adds this

Let’s hear from Peter Yost, GBA’s technical director:

BuildingGreen has great resources for LED lighting and DC power systems within its . (Note: BuildingGreen takes no advertising. Its business model relies on subscribers paying for full access. But it does have quite a bit of information in front of the paywall.)

BuildingGreen’s collection for lighting is usefully broken down by lighting type: pendant lighting, recessed lighting, task lamps, wall lamps, and replacement lamps. The good news is that there are LED products available for every type of light fixture. Yes, BuildingGreen is tuned more for commercial buildings, but pretty much every type of lighting can be (and is) used in residential buildings.

A big move forward for LED lighting was the advent of modular LED lighting, where key components of the lighting can be replaced rather than the whole fixture.

Back in 2008, just about every lighting manufacturer was jumping into the LED market; there was a lot of confusion and there were low-quality products on the market. Fortunately, programs such as EPA’s Energy Star that you can use to help separate the wheat from the chaff with LED lighting.

There is also a ton of information on residential LED lighting within the Rennselaer Polytechnic Institute (RPI) Program for the Evaluation and Analysis of Residential Lighting ().  Especially useful is the division.

While it is true that “…our analog-like AC power is increasingly at odds with our digital DC world” — the quote comes from an August 2016 BuildingGreen feature article, “The Death and Rebirth of DC Power”– moving completely away from AC is not going to happen for quite some time. And deciding at just what level to convert to DC (for certain loads, for certain end-uses, for subsystems) is certainly not “settled” for residential buildings.

Can we look to commercial buildings for crossover technology regarding DC power systems? There is an organization working on moving our commercial building electrical systems (and even microgrids) from AC to DC power: the . EMerge has a e that will address both existing and new home hybrid power system standards.

13 Comments

  1. Aedi | | #1

    It is strange there is no mention of USB throughout. The newest USB standard supports up to 100 watts (20V at 5 amps), and is the standard used for a good portion of consumer electronics. The technology is also rapidly improving, and I expect to see 24V (120 watts) supported in the near future, and eventually far more. Since the cable also includes the capacity for data transfer, it can be used to deliver variable amounts of power depending on the device (assuming the device supports it). I'm not big on the whole "smart house" thing (at least, not the current fashion of insecure devices owned and controllable by a third party spying on you for advertisers), but if that nonsense catches on, even silly things like light bulbs and refrigerators will have a use for that data connection.

    While it is far from practical to wire a house using the latest standard of USB cable today, in a future where DC is more prominent I fully expect it to be the standard for power delivery. After all, there are already power outlets with built in USB ports available.

    1. Jon R | | #2

      Doing such low bandwidth communications over wire is dead - it will all be wireless. It's already sometimes more cost effective to use wireless for control (vs long runs of 14 gauge copper). $6 for my most recent wifi power switch. Not to mention that people are going to increasingly expect to say "turn on outside lights" from almost anywhere vs finding the right light switch.

      1. Aedi | | #3

        You are correct, wireless is most definitely here to stay. But if a hard data connection is already there for things like power management, I'm sure they'll find a use for it.

  2. Walter Ahlgrim | | #4

    What does the code book say?

    I do not have it memorized but though there were some class 2 limits less than 50 Volts, 2 Amps and 100 Watts

    Are there UL listed DC panels?

    Walt

  3. Christopher Welles | | #5

    I'm currently looking at 24VDC on local (room level or few rooms) distribution basis. At least for lighting and some building automation purposes. That seems to strike the best balance between safety, reasonable resistance losses, usefulness, available equipment, and cost. It's common in industrial automation, passive POE, and is used by EMerge's Occupied Space Standard (for offices). I believe class 2 circuit limits allow for 4 amps (100 watts). Not massive, but useful for all sorts of low voltages purposes, and can be utilized regardless of what direction various standards go.

    48 volts and higher power limits would probably end-up needing some type of negotiated power delivery, like POE or USB-PD. That's going to be much more expensive, more fragile, and more likely to be obsolete on a much shorter time frame than would be reasonable for residential construction. Because of that, it's going to be at least 50 years before it would move into residential.

    That's not to say I'm against using POE for something like security cameras, or have a number of USB-PD outlets. Just that it wouldn't be built into the backbone of a typical house.

    Regarding wireless control, I suspect you'll see a lot more hard-wired connections than you might be expecting. I'm sure wireless will be mixed in, but when things go mainstream, it needs to work reliably for more than 20 years without it all being ripped out and scrapped.

    I've had WIFI for over 20 years (first access point cost me $1500 :-( ), and I regularly update my router and lots of other equipment. I have a couple of dozen LIFX bulbs, POE switches, Kevo, Alexa, and a mix of random bits and pieces. Nothing is ever as reliable as it needs to be. It all needs to be periodically updated or replaced. This is not the future for most people.

    Sub-system communication using something RS485 based seems reasonable. Maybe DMX for local lighting control? Ethernet / WIFI makes sense when coordinating major sub-systems, but I would never want any shared network involved in critical communication (like a make-up air damper tied to my range hood).

    1. Trevor Lambert | | #6

      I don't know what any codes say about it, but I wouldn't try to pass 4A down an ethernet cable. Even if you used two pairs of 23AWG, the voltage drop for a 30' run would be over 10%. I think the heat generated would be substantial. POE is typically for low power applications, like a phone or camera, maybe a single light, not general lighting.

      1. Christopher Welles | | #7

        Yep. For 24VDC, I'd think more 12 or 14AWG, just like typical AC wiring. Martin's 10AWG certainly makes sense given that he's using 12VDC.

        POE++ apparently goes up to 100 watts (at 48V), but I believe that uses 4 pairs of wires. Ubiquiti is pushing their UniFi LED system that uses POE for general lighting. I'm not a fan of the idea.

  4. Dennis Miller | | #8

    I guess if Edison and Tesla sparred over whether AC or DC was the better approach, then we are in good company if we do the same. I frankly don't see the big advantage right now to wiring a house with DC at all. For most of us we want to be able to plug in a vacuum cleaner or blow dryer in most any room. The AC wiring accommodates these power-based devices quite well and many motors get their rotation from the rotating magnetic field produced by AC applied to the stator coils. Electronic stuff (where electricity carries information) all runs on DC. But at what voltage? Your computer alone has several different DC voltages inside. Other electronic gadgets run at differing voltages. So DC wiring would require converting from one voltage to another. Of course this can be done, but then why not just skip the DC wiring and just convert AC to DC? Probably costs about the same and is equally efficient using switching power supply designs and with very low standby power. As we build our new house this year, unless someone comes up with some really convincing arguments, I'm still going with the typical AC outlets everywhere because I can get anything to work with that. There may be all kinds of DC devices coming out now and in the future, but i'd be concerned as to which ones will still be around in 15 years. I'd rather not wire for DC and then find the industry ultimately didn't adopt the same standards that I did.

  5. Dennis Miller | | #9

    When the article talks about "transformers" I hope it is not referring to the simple DC power supplies that simply use a transformer and a rectifier circuit. I hope it refers to switching type power supplies that convert from AC or DC to whatever you need on the output. These supplies still use transformers but generally much smaller and lighter than the old linear supplies because they use a completely different methodology to generate the desired output voltage. As far as I understand energy regulations are tight enough now on "wall warts" that the simple linear supply no longer is manufactured because it's not as efficient, both in operation and in standby.

  6. User avater
    Nils Bird | | #10

    Where are the battery powered lights with remote switching when you need them? I'm hoping that in the short term there will be battery powered lighting that you peel and stick anywhere with no need for wiring at all. It seems to me that with incandescent lighting I had to replace bulbs every couple of months or so. Obviating the need for wiring in the ceiling would be worth the extra effort needed to replace or recharge batteries.

  7. User avater
    Ethan ; Climate Zone 5A ; ~6000HDD | | #11

    I tested a bunch of bulbs (Cree, Philips, IKEA). I was particularly interested in warm dimming (color temp warms as light dims. Surprisingly, the best bulb I found was IKEA. One thing you have to remember in any case is to adjust the dimmer settings.

  8. Kurt Hanushek | | #12

    Stick with the standard medium Edison base and candelabra base sockets with 120 volt ac for lighting. There is over 100 years of installed fixtures. They are not going to change anytime soon. They started for incandescent bulbs, they handled cfl's just fine and if you have looked at your local big box store, there are now a myriad of LED bulbs using those two standards. If in ten years, there is a new lighting technology, that can be screwed in to the same fixtures, whether they are brand new or over 100 years old.

    The circuitry to convert ac to the appropriate dc voltage is included in LED light bulbs that sell for a buck or two. providing dc voltage would require similar circuitry if the voltage provided did not match the requirements of the lighting device and is not a standardized item on the shelves of every store selling lighting products. The only readily available lights using dc is LED tapes and ropes which normally are used in only a few limited places.

    Still in doubt, think of the hassle of finding bulbs for fixtures that use specialized fluorescent or halogen bulbs.

  9. David_Piranesi | | #13

    I'm interested in converting knob and tube overhead lighting to 12VDC using (attention Kurt) Edison-base LED bulbs that I believe are for boats. There are many 12VDC Edison bulbs on Amazon, though I've never heard of any of the brands for interior lights (e.g. Ashalight, Petronius, Tento...). There's one by Feit that might be OK for outdoor, named as a pool/spa light.

    It seems to me that the old knob and tube wiring gauge would be sufficient to handle current for LED lighting only, though I suppose I should pencil that out and I haven't yet. Also, it would need overcurrent protection in case someone screwed in the wrong thing, not sure how I would do that. There is also the matter of ensuring that the 12V circuit is in fact isolated from the 120 (I found a neutral the other day that's looped into the overhead circuit from somewhere else...)

    Well, I haven't moved forward with the 12V idea yet, but I wanted to pipe in that for overhead lights only, it might make it easier and safer to insulate above the ceiling. Seems to me people used to insulate over knob and tube without worry. But now they worry and don't do it. I suppose it's not a problem with the original wiring, it's that over the years people have modified that wiring, I can see some of that has happened.

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