DC load center for new small house

[openplanet]

Hello All--

First time poster here.  I'm building a small, off grid, passive solar house in Maine with a modest 1.2 kw array.  My question concerns the dc load center.

I plan to wire the house for both ac and 24vdc.  Each dual-gang outlet box will have two ac and two dc sockets.  All lighting will be dc.  Given that all dc loads will be low amperage (no circuit above 15 or 20a) I'm wondering why I should go to the expense of using breakers like Midnite's rather than an ATC fuse block such as one might find on a boat or RV.  And, assuming I use fuses, is there a good reason to go with something like Midnite's fuses and fuse holders on a DIN rail rather and ATC fuses?

As an aside, it's puzzling to me why there's relatively little information and discussion about dc load centers on line.  Any thoughts about this?

I greatly appreciate being able to take advantage of the collective wisdom and experience on this forum.

Best to all--

Paul

[Westbranch]

FYI Square D "QO"  CB's are rated to 48v DC as are the load centers.

[Robin]

I tried to design out DC load circuits about twenty years ago. The DC circuits for a house are not standard. There is no standard for DC sockets. The blade automotive fuses are not listed for use in residential wiring although I do not see why they wouldn't work?
As an engineer who designs systems to meet the NEC it is important to design around standardized parts as much as possible. People do use our little Big Baby and Quad boxes for DC loads all the time. They also use the MNDC15 for more circuits. Quite a few people even use our combiners. Like I said though, I try to not recommend DC circuits other than a light circuit in the equipment room. Today's inverters are efficient enough that people can use regular appliances. Keep in mind that DC circuits in the house may not meet code. I don't know if your house will be inspected or not. The use of automotive fuses will never pass inspection. Another thing to consider is fire insurance. A house that is built to code and has been inspected can have fire insurance. A non code compliant house may not qualify.

[Halfcrazy]

To echo Robin. Code compliant is definitely something to think about. Depending on the number of circuits I have used the MNPV6 and MNPV12 with circuit breakers numerous times as DC load centers. I much prefer Breakers as I can open them under load and I can reset them when I do something stupid.

Ryan

[openplanet]

Thank you Robin and Ryan.
My main reason for wanting to run DC loads as much as possible is to eliminate the inverter as a failure point.  It seems a pity for a whole RE house to be dead-in-the-water because of an inverter failure.
In any event, I will take your advice and use an MNPV12 with circuit breakers.
Thank you.
--Paul

[dgd]

My main reason for wanting to run DC loads as much as possible is to eliminate the inverter as a failure point.  It seems a pity for a whole RE house to be dead-in-the-water because of an inverter failure.

Inverters in general are pretty reliable. A simple insurance is to have a second perhaps smaller inverter for emergency use or spread your ac loads over two medium instead of one large inverter.
I would imagine this would be more economical than dual wiring a house for ac and DC loads.

Dgd

[laszlo]

Inverter longevity -- the warranty that most companies in the market provide is more than half the actual MTB of the device. So in general if you have a 5 year warranty on an inverter, which is pretty standard these days, one can expect the inverter to last about 10 years.

A/C power is also considered safer than DC because with alternating current the voltage goes to zero 60 times per second so the breaker has  chance to interrupt it before it starts a fire or causes harm.

[RossW]

with alternating current the voltage goes to zero 60 times per second so the breaker has  chance to interrupt it before it starts a fire or causes harm.

1. The whole world doesn't run at 60Hz.
2. Even if you are on 60Hz, the voltage passes 0V twice per cycle, this 120 times per second
3. Breakers don't wait for a zero crossing to trip

[laszlo]

The way I understand  it you can only get an electric shock during the positive half of the cycle, and so that is 60 times per second when the hot goes to 0V.  At any rate, rather than nitpick over this, the key point is that A/C power is much less likely to maintain an arc than DC power since the voltage is attenuated to zero in each cycle. To each his own, but it seems the consensus in this forum is to recommend code compliant AC wiring in the house for the following reasons:

-no DC outlets available
-expense of wiring for DC
-difficulty of obtaining fire insurance for house not wired to code
-DC is more likely to maintain an arc than AC
-you will not find an electrician who will install or service a DC wired house, or an inspector who will pass it

[RossW]

The way I understand  it you can only get an electric shock during the positive half of the cycle

Whoever taught you that is going to get someone killed. Either polarity will send you to your maker without fear or favor!

I wasn't nitpicking for the sake of it - but false information (if repeated often enough) "becomes true!". (Well, it is believed to be true (and repeated by more and more people, thus perpetuating the myth).

No argument that 240V AC is less dangerous than 240V DC to the average punter though!

[vtmaps]

I plan to wire the house for both ac and 24vdc.

Stick with AC for the sake of efficiency.  Of course, the inverter represents a loss of efficiency, but running low voltage through your wires also represents a loss of efficiency.   

You will have a much more extensive and inexpensive choice of efficient lighting, fans, etc if you go with AC.  The most efficient exhaust fans you can buy are powered by AC but have DC motors.  Power supplies can be very efficient, so it is usually advantageous to run AC through the house and convert it to low voltage DC at the fan. 

--vtMaps

[openplanet]

Thanks everyone for the input.
So...this forum proves its usefulness right out of the starting gate, and I must reconsider my decision about dual-wiring.
I wonder if anyone has done any kind of survey to see what percentage of off-grid systems wire dc-direct to lights and other loads (like a SunDanzer 'fridge).

[Westbranch]

Never heard of a survey yet. 

I still have a 1 room cabin that is DC wired but some of the furthest from the Bank get a bit more than 5% loss, no issue with regular quartz halogen but can be a problem with some LED's with tight operating range tolerances.  see avatar...

The new full size place will be 110v AC

[Thirteen]

with any system it needs work to keep it running properly. So install a single battery 12v along side of the big bank with a small inverter and 12vdc lights so when things do go wrong you cna have power to wokr on the main bank. If it is during the winter and things go bad you would be able to have a couple of lights to work with. You could always hookup a truck to charge the small battery. Safety first. 13

[zoneblue]

There are several of us going the mixed ac/dc route. For me its a no brainer, i have an interest in electronics, we have no kids, live in a tiny place, and live on 3kWh per day.

For most of my loads (at least 90% of the base loads) all the gear is low voltage dc.  Why convert from  24 to 240 then back to 12?

Over the years of living in motorhomes, ive accumulated 24V stuff that is in the upper range of efficiency. We use lots of Dc converters, big and small, which are way more efficient than inverters, in the 95% range.

Our base loads are:

- fridge 24v 70 watts about 40/60 duty cycle
- lighting (about 75 watts of LED). This runs on 24V. Each fitting draws about 800mA at 25volts.
- one custom atom server draws about 15 watts, runs on 19v via a 24-19v converter.
- two laptops run on 12 and 19v , with their own converters, about 12 watts each.
- one desktop with 24" screen. Monitor is 12 Volts, 15 watts via dc converter. CPU has a custom 12-30V power supply board, and runs on 19v converter. Draws 20-40 watts.
- two 2088 waterpumps, one from the stream, one from the tanks. 24v, 30 watts each.
- an electric fence energiser, 12 volts, trivial.
- phone chargers x 2, via 24-5 usb converter
- NiMH, lithium and nicad chargers for various tools, all runs on 12 volts via dc converter.
- cordless phone, still working on that, but its 5.7V using a custom converter.

The only things we turn the inverter on for are power tools, kitchen appliances, and the washing machine.  Personally we like not living in a soup of 50 hz emf hum, although ill grant you all those switch mode power supply probably arent "silent" either, but i tend to put tthem in metal boxes, and use extra capacitance on the input and outputs to try to keep the dc bus clean.

Building Biology says that the earth has several low frequency resonances around the 10 hz range, that are critical to our health and well being. Hence the desire to avoid drowning them out with a much stronger 50hz signal.

As for cabling i run 6 guage from the main dc load  center to 3 sub centers. We got a great deal on the 6awg it was like a dollar something a meter. For branch lines ill use 6mm2, but none of the branchs having fusing over 5amps. So theres no cable loss.

For ac theres only two circuits one to the kitchen and one to the workshop/laundry. Id say about  a quarter of the wiring that we have installed for dc.

But if you really have 20 amp dc loads then yes, maybe you oughta go ac.