Please verify grounding and bonding plan for this system

[7GD]

Hi forum folks,

Long-time reader here, first-time poster. Thanks for the many years of great insight you've all provided on this forum!

I'm designing an off-grid solar PV and battery system that will supply power to a laundry shed and two individual yurts. The 1.44 kW solar array will be mounted on the roof of and the ~27kWh 48V battery bank, charge controller, AC-to-DC battery charger (for backup generator charging), and inverter supplying the washer and dryer will be housed within the laundry shed. I am then running 48VDC supply wiring to a metal-enclosed "power center" mounted on a 4x4 just outside of each yurt (yurts are located approximately 120' from the laundry shed, in opposite directions), which will then power the almost entirely DC yurt end-uses (lighting, refrigerator, DC computer charger, USB outlets) via a 48V to 12V buck converter and a DC fuse block with the exception of a single 375W 48V inverter at each yurt that will provide power to a couple of outlets if ever needed. Each yurt will draw max 17A @48V from the battery bank, and the laundry shed will draw max 62A @48V from the battery bank.

Here is a link showing a schematic of the design - only one yurt is shown, since the other is the exact same.

My main question is about bonding and grounding. After reading the stickies on this forum about the subject, and other research, I am preliminarily thinking that I will:
-Ground the solar array frames directly to a ground rod located the shortest possible distance away (directly underneath closest corner of array).
-Bond the charge controller enclosure, AC-to-DC charger enclosure, and inverter enclosure to the negative terminal bus (directly connected to negative battery terminal), and run a ground wire from that bus to the same ground rod.
-Install a separate ground rod outside each yurt enclosure, and ground the metal enclosure. Bond the negative 48V supply terminal to the enclosure.

Does this sound right? I've read some places about installing two ground rods - would you recommend that I do this at each location (so 6, total?). And any other feedback is welcomed!

Many thanks!

[Mike90250]

multiple Ground Rods should be tied together with heavy gauge cable, to prevent Step Gradient in case of a fault or lightning (below pic)

But first, why are you complicating things with DC and AC power distribution ? You will likely have code issues

Is your clothes dryer gas or electric? electric heat pump ?

Have you considered stacked inverters that will shut the unused capacity down ? Step-Voltage_potential.jpg

[7GD]

Hey Mike,

Thanks for the reply. I'm trying to maximize system efficiency by matching source to end use with the fewest conversion steps possible. PV panels supply DC, batteries store and supply DC, LEDs require DC, computers and other computing and communications devices require DC, and there are highly efficient refrigerators now utilizing DC brushless motors. Eliminating inversion and conversion losses feels highly desirable to me, and has allowed me to get away with a significantly smaller PV array and battery bank than would typically be required.

The dryer is a gas dryer with a 1 hp variable speed motor. No heat pump - passive solar cooling and heating structural siting and design.

To make sure I'm understanding your recommendation - you are suggesting that I should run a heavy gauge wire along the length of the underground conduit run from the shed to each yurt to connect the ground rods at the shed with those at the yurt power center panels? What gauge is heavy? Largest gauge in this system as spec'd is 2awg.

Thanks again!

[Mike90250]

I think #6 solid is the recommended wire to tie ground rods together

Are you in a lightning prone area - if so, your yurts may need Franklin Rods

And my view on the current sexy crop of DC appliances, is they are pricey, complicated and expensive. consider another $500 of panels instead of DC fridges. Panels are CHEAP now and while I'm not an advocate of wasting electricity, sometimes a few % conversion loss is better than the $1400 fridge that went bad after 2 years.
And make sure the energy consumption estimates are based on the same factors, when you can get a 900wh 22CF energystar fridge or for the same money, get a 11cf danfoss based fridge that consumes 500wh.

There seems to be a high complaint rate on the DC deep well pumps, that cost several grand & need a fancy control box, yet I installed a conventional well pump because a replacement is a couple hundred and any pump vendor has them in stock.

These are my opinions, others may agree or disagree, but look at all the options before you commit

[7GD]

Thanks Mike, for your responses! 6awg bare copper it is. Just run it in the PVC conduit alongside the 48V red and black supply from the shed ground rod to the yurt ground rods? and am I right in that, in addition to grounding the negative terminal bus in the shed to the shed ground rod, I also want to ground the negative 48V supply terminal in each of the yurt power center enclosures to the neighboring yurt ground rod? I'm reading that you want to only have one GEC in a system, but since the yurt control centers are so far away (120') from the main GEC, wondering if that still applies. Perhaps connecting the ground rods with the 6awg copper takes care of this.

And I hear you about the relatively unknown lifespan of commercial-size DC appliances - though I do have an Engel RV/van fridge with a danfoss compressor that absolutely rocks, and is running 8 years later as well as the day I bought it. I am willing to give them a try though in order to minimize resource use. And it's not just the $500 in panels I saved - it's the smaller charge controller, inverter, battery bank, AC-to-DC charger, etc. This whole custom system cost me less than 50% of the system that was quoted to me by a "design-over-the-phone" conventional everything-inverted off-grid system supplier - and I will save more than that in 10 years when I have to replace the batteries and other components.

I am grateful for your thought-provoking input!

[MichaelK!]

One other thing that I could add is that your solar array is very small for a battery bank of that size. I'm assuming that your bank is ~562Ah in size. Assuming you want to charge at least at 1/10C, that would work out to be 56A X 52V charging X 1.25 loss factor = 3640W, more than double what you are installing. I have that sized battery in my 24V workshop system, and I'm charging that with 2000W of solar. At a minimum, I'd recommend at least doubling your solar input.