12 Volt DC Wiring - Ground Fixture or Not?

Started by redside, December 02, 2018, 02:00:31 AM

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redside

I am going to be wiring my cabin soon and have decided on a combination DC/AC system.  I am thinking of using a progressive dynamics https://www.progressivedyn.com/rv/power-centers/inteli-power-4000-series/ AC/DC distribution panel to keep things clean.  It has a built in converter for charging the battery bank and a 120V 30 amp main in for the generator.

Do you wire a 12v system just like you do 120V with a return ground from each fixture back to the distribution panel?  Everywhere I look there are only two wires in a DC circuit: the positive and the negative unlike an AC circuit which has a hot, neutral, and ground wire.

I think code allows 48v and less systems to not be grounded, but isn't this a safety/fire hazard?





MountainDon

I believe the NEC rule is that is the battery bank is 24 volts or less, then a separate DC ground wire is not needed.  But that is not the end of the story.  If a non-grounded DC system is used then the NEC also states that both the positive and the negative wires need overcurrent protection; fused = or breaker.  So to me, it is less bother to simply run ground wires for all the low voltage DC as well.

How much DC do you plan on?  I went through this 10+ years ago and decided the only DC I really needed at the time was a circuit for the DC ceiling fan (at the time the only way to get a highly efficient ceiling fan) and an RV water pump, plus three lights for use when/if the inverter craps out. That became very handy after we were struck by lightning. We charge phones, computers via the AC as there are so many other things that require AC.

You did not mention if this was an off-grid solar powered installation.  If so, are you designing in a good lightning arrestor and ground system?  We didn't give a lot of thought to that initially. We had an arrestor at the panels. We made extensive additions after the strike. Midnite Solar has some great surge suppressors.
Just because something has been done and has not failed, doesn't mean it is good design.


redside

There will be one big copper rod driven in to the earth that the system will be ground to.  I will have 2 6 volt golf cart batteries, a small 100w solar panel/controller, and that is it for now.  I will have a yamaha 2000 to run ac loads when needed.  My power needs are very small.  The DC will run a ceiling fan, lights, usb outlets for charging phones/laptop, and an rv water pump.  I was initially going to go with 120V, but there are so many 12 volt lighting LED products now and the costs are reasonable (they even have 12 volt edison base light bulbs now).  It just seems to be less costly and more efficient to go DC.

I don't quite understand how a fuse on the negative side does any good, but I am sure there is a reason and I'm not that smart when it comes to this stuff.  That said I agree that it is just simpler to run a separate grounding wire back to the distribution panel to protect in case of a ground fault.  This would also allow me to change over to 120V AC in the future if I decide to without rewiring which is a huge plus.  I guess I will need to buy a grounding bus and install it in the DC panel.

If going AC with an inverter is superior then I will be grateful if anyone makes that suggestion.  I looked at the costs of inverters which are not insignificant. You also lose 10% efficiency plus powering the inverter.  Lots of times I will be charging smart phones/tablets and it just seems so inefficient to do it off of an inverter.  All that said, it really doesn't seem like much of a costs to fire up the generator and charge to battery bank to 90% when needed.  I think most battery chargers will recharge a battery bank fairly quickly to 90% which would get me through the rest of the weekend.  Maybe I am overthinking this whole thing and should just go AC. 

MountainDon

Sometimes it's just a personal choice.  Everyone's outlook at their power system, their needs, and their wants vary.

We took a couple of years to plan it out. Over that time the electrical wants grew.   We grew to want to be able to power a number of things. Pretty much every day we are at the cabin we use the Blendtec blender, the electric kettle, the microwave and maybe the toaster oven. Those require 120 VAC and I don't like having to go out to start and then stop a generator. We had a Yamaha inverter 2800 type. No electric start available on that. We had owned it a few years already. Oh, also my wife was not at all looking forward to having to start the generator herself if I was not available. And I didn't want to buy an electric start generator at the time.

I decided on an Outback inverter ( and charge controller, plus the integrating hub). Not cheap but it has a great charger built in to charge with power from the generator. Plus it has a sleep function. When sleeping and nothing is using 120 VAC  the inverter uses milliamps of power. When a 120 VAC item is turned on the inverter starts almost instantly. Both it and the charge controller, as well as most everything with electronic controls, were killed by our lightning strike. That's why I asked. If you are in an area prone to lightning, lightning protection should be on the must-have list. Lightning grounds are an adjunct to the basic grounding roids the AC or DC power delivery systems need. We have several rods and a couple of foot square copper plates buried for the lightning protection system.

There is nothing inherently wrong with doing just DC, as long as appliances like what we use a lot are not a part of the mix.  If you do both AC and DC be sure to keep them separated and marked so at some time in the future somebody does not unknowingly screw a DC lamp into an AC powered socket.

Just because something has been done and has not failed, doesn't mean it is good design.

redside

#4
All good information.  Thanks Don.


Dave Sparks

I would add that the choices can be influenced by the long term goals. Are you going to live there someday? Is there market value in building a "normal"
home and its requirements.
"we go where the power lines don't"