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 2015 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 “War of the Currents.”
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 Product Guidance tab. (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 jumped in with standards 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 (PEARL). Especially useful is the Solid State Lighting 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 Alliance. EMerge has a residential standard initiative that will address both existing and new home hybrid power system standards.