So, I'm very curious about this distributed 28vdc you have in your house. Care to spill the beans? What guage wire did you distribute it over? Whats the source? What do you use to step down the voltage (say, for example, to 12v)?
Well, none of the following is a recommendation. I'm a geologist, not a licensed electrician. If you take your electrical advice from someone whose profession is to break rocks and splash around in dirty water you may get what you deserve ;-)
"LOW VOLTAGE" IS NOT SYNONYMOUS WITH "SAFE".
LOW VOLTAGE WIRING CAN BE AS DANGEROUS AS HOUSEHOLD AC.
IN PARTICULAR, HIGH-CURRENT SOURCES LIKE BATTERIES CAN AND HAVE CAUSED FIRES AND SERIOUS BURNS AND WORSE.
ALL LOW VOLTAGE SOURCES NEED TO BE CURRENT-LIMITED.
The wiring varies, but where I could, I pulled up from the basement 4-conductor w/ ground 10 AWG NM ('romex') that I special-ordered through my local Home Depot. The electrical inspector --- apparently not a fan of low-voltage lighting -- begrudgingly remarked "At least you didn't use undersized wire" when approving it. He remarked that he's seen a lot of places where folks incorrectly thought that 'low voltage' meant 'low current' too ....
Unlike AC wiring where two 'hot' conductors on different phases can share a single neutral conductor, each DC 'hot' requires its own neutral if the full 30A rating of the 10AWG wiring is needed. Hence the atypical four-conductor cable. With nominal 24vdc, the maximum rated power over this cable is 2 X 30 Amps x ~27 volts ~= 1600 watts - slightly less than a single 15-amp (14 AWG) 110VAC circuit. Obviously power-handling is not low-voltage wiring's strong point ;-) This cable is fully National Electrical Code (NEC) and UL-compliant for up to 220 VAC 30 amp and so could be re-purposed if needed for retrofits in the future. I pulled a pair of these cables to the kitchen ceiling, piano room ceiling, and to the second floor when I installed a 220VAC load center, ethernet fiber, CAT5e, shielded twisted pairs for audio, pipes for solar/hot-water heating, and replacements for the aluminum wiring of the 2nd-floor HVAC and laundry room.
Elsewhere I have used other smaller (14AWG and 12 AWG) NM ('romex') and 2-conductor, stranded 14 AWG code-compliant CL2 cable commonly used for in-wall installation of speaker wires. The latter is plenty good enough for most all non-lighting, electronic needs. Locally there is multi-conductor wiring from a security system that was installed in the early 1980's that I have repurposed in a few places. There is also direct-burial-rated 10-conductor 18 gauge wire in places too.
Dimming of conventional, commercial halogen 12vdc track (ceiling) lighting is through surplus 24-->0-12vdc variable output DC-DC converters from (and in) a DMX-512 lighting controller custom-made by www.dfd.com that was originally used to light a floating Disney display/exhibit/thing. I was originally thinking of doing homebrew bare-wire 'artsy' 12vdc halogen installations but for now have opted for boring commercial tracks. In the kitchen the 12vdc is accent and task lighting to supplement the existing ceiling cans which become CFLs. In the piano room, they are primarily to illuminate the artwork.
Rapid advances in LED technology may cause our conversion from halogen 12vdc to LEDs to be sooner rather than later which would likely obsolete the voltage-control dimmers.
Where isolated grounds are needed for electronic gear, I use isolated DC-DC converters at point of use. EBay typically has a variety of converters with 18-36vdc ("24vdc") inputs. The distributed PC's use isolating nominal 24->12vdc converters that in turn power picoPSU-120 ATX and similar power supplies http://www.logicsupply.com/products/picopsu_120 . This allows for _local_ 24vdc PC backup on top of the intrinsic DC backup with the simple addition of two sealed 12vdc lead batteries in series, a trickle charger and diodes. (The picopsu PC power supplies are simply astounding IMO ;-)
The main DC power sources are located in the basement and are from a pair of UL-listed Inteli Power 9100-series converters that are designed to both/simultaneously charge their associated big golf-cart 12vdc deep-cycle battery and provide power. An 80-amp PD9180 powers the 0-12vdc (the 'lower bank') and a 60-amp PD9160 powers the nominal 12-24vdc bank. So an additional 20 amps of 12vdc is available compared to 24vdc.
Plans are to add two 7AHr 12vdc sealed lead batteries and associated small supply/chargers to provide 48vdc output for Power Over Ethernet (POE). The nominal 48vdc output will need to be voltage regulated because the combined battery voltage is actually ~ 4x13.6 =~ 54.4 vdc which exceeds the National Electrical Code limit of 50 volts for some classes of 'low-voltage' wiring. This will be done through multiple monolithic voltage regulator IC's that also provide current limiting. Last I knew, the IEEE-802 POE limit was ~13 watts, with 26 and 52 watt standards being worked on.
Important parts of the infrastructure remain to be finally installed. I purchased, but haven't yet decided to use, a 110 VAC circuit breaker panel with built-in low-voltage DC fuse panel that is marked as UL-514 listed. The AC circuit breakers would provde local control of the PD9100 power supplies and other HA-related uses. The DC fuses would provide DC power limiting. It looked great in the pictures but I discovered it was Chinese-made when I received it. I'm trying to determine whether there is an American-made equivalent.
Wherever practical, the plan is to have both auto-recovery, current-limiting devices as well as fuses and(or) circuit-breakers on each circuit so that the system can recover gracefully form a momentary condition without creating the need for a manual re-set that might not be possible in un-attended operation. Current-limiting and ground isolation are the _technically_ challenging parts. At least so far ;-) Installing the cabling involved much physical labor in a house with mostly solid brick exterior and interior walls including the hard, dirty work of chiseling, and drilling and uncomfortable crawling on my belly through places with 185 years of accumulated dirt.
Most of the DC wiring infrastructure is in place, and parts purchased, but with some final design decisions and installations pending. I'll post some pictures when it looks more presentable. The likelihood is that some parts will always be a work in progress adapting to the tasks and purposes to which it is put.
To re-iterate, I am a hydrologist, not a licensed electrician. So the preceding is a description of what _I_ am doing and is not advice to others. You do what you do at your risk, not mine. That should be clear enough, right?
Remember: "LOW VOLTAGE" IS NOT SYNONYMOUS WITH "SAFE".