Two System voltages, one DC disconnect enclosure.

Saggys

Before scouring the code book I thought I would ask the people that do frequent off grid installs.
I was wondering if it is code legal to have two inverters of different voltages on the same disconnect enclosure with two battery disconnects? I know when you stack inverters you are doing this but they operate on the same battery bank, I am wondering if I can have a 24 volt battery bank and inverter and 48 volt battery bank and inverter on the same DC disconnect enclosure that has two battery breaker locations? The reason I ask is I have some space limitations and this would help out. I am thinking with proper labeling this might be acceptable?

BB.

For battery voltages @ 60 VDC or less--Those are "similar" low voltage systems... Same enclosures OK. For 120/240 VAC, "different" type of power, and that would be a separate "box" from the DC wiring (or same box with metal divider between the two electrical system "type").

The DC systems should have "common" DC Ground (both negative buses from the two busses tied together, and tied to "safety ground" bus--And to the Ground Rod. If the two battery banks are "isolated" from each other--Then they have separate grounds--And they should have separate Electrical Boxes (you want a "short" from either + to go to metal box to green wire ground to DC bus return to Battery bank (either one) to blow fuse/breaker on the + wire from Battery + bus to AC inverter + DC input.

Of course--If you don't have any inspections--In theory you could run AC and DC in the same wire runs/boxes (of course, all AC green wire grounds connect to the "common" ground somewhere (such as the Ground Rod/Cold Water pipe grounding).

-Bill

mcgivor

The electrical code requirements applicable to solar predominantly apply to grid tied installations, there are many fundementals which can be applied to offgrid as good practice, even if not covered specifically by the scope of the electrical code itself, such as having all insulation within an enclosure rated for the highest voltage within and not having feeds from multiple sources without physical separation in a form of a barrier.

 Since both 24V and 48V DC fall under the extra low voltage ripple free value of 120VDC, I would think a barrier along with clear labeling would suffice. Although the voltages are low, batteries can provide extremely high current, not nessersary a shock hazard, but a possible short circuit hazard to those not expecting a secondary source.

Dave Angelini

Is this going to be inspected by a building department? Code is code on or offgrid in the US and most of North America.

 If yes, it can vary but the components of the power system have to be listed and some inspectors might want to see the listing that shows how and what was in the testing.

example  Outback and Schneider get their listings with specific equipment in the test. The power distribution panel that they both make connects to the inverter and contains the AC and DC switching and over current protection. I have had to replace items like that in a home resale to bring it up to code.

 Always ask the inspector what they want from you to be able to get the certificate of occupancy or complete the job. Then feed them the paper. Good Luck!

Saggys

Thanks for the replies, the DC disconnect is a listed Xantrex DC250 that has space for up to two 250 amp battery breakers and is set up for stacking two Xantrex inverters (2005 vintage gear) when using SW conduit boxes on the DC end of the inverters. The separate battery banks will each reside in the same outdoor battery enclosure. Each inverter will feed a separate building and the AC output of each inverter will have its own AC output feeder breaker panel. 
BB brought up an interesting point on grounding and bonding: I guess I drive ground rods at each building (separately derived systems) and bond neutrals at the switchboards for the AC side. Then bond equipment grounding conductors and battery banks negative to the DC bonding block in the DC disconnect.I will have two ground points but they will be bonded together.
This will probably never get inspected, however I am a licensed electrician and always do my best for a proper install.

mcgivor

Don't the breakers in the DC 250 enclosure connect to a common bus? 

Saggys

Upon further review I think I am going use two totally isolated systems, I do not want two ground points in this install.

Saggys

mcgivor, they are cable connected, no bus that I am aware of.

BB.

The ground rods--They are only there (mostly) for lightning grounds.

You need a common safety ground for the two systems if the AC power goes into the same (single) building/electrical system. You need a short circuit from either Inverters' AC output to go to safety ground and find its way back to the correct source inverter.

If the two AC power systems "never meet" in the same structure, then separate safety grounds are fine.

-Bill

Saggys

Got it Bill, the two AC systems will never meet, but their safety grounds will when the AC and DC bonds meet at the bonding block, that is the way the DC system will be tied to the grounding electrode (ground rod) Trying to follow the ground once and bond everything else.

Saggys

While looking for examples I found this in Outbacks site under wiring diagrams. On this particular drawing I see no physical connection between the AC equipment grounding conductor and the DC equipment grounding conductor and the battery bond is through the GFPI. The panelboard shows the typical neutral ground bond and at that point it is usually tied to earth. Still a little confused as to why there is no physical connection between the two other than the bus bars screwed to the enclosures?

BB.

In the Outback drawing, they do not address grounding at all... The AC inverter may or may not have a "chassis to neutral bond". Have to research that (as always, check the manual for the exact inverter you want to use).

The solar GFPI system--That is a whole 'nother kettle of fish. In my humble opinion, the Solar GFPI system (using a 1 amp breaker between system and safety/earth grounding, that breaker trips the main solar breaker) was a terrible "hack" trying to address the very real problem of "arc faults" (DC current sustains arcs much better than AC current--Meaning that shorts/poor connections in the solar array can start roof fires). The "GFPI" was intended to disconnect the solar array if there was a short between Solar +/- and the metal (grounded) racks.

There is a big discussion about the issue from a few years ago I can link too.. But my suggestion was that the using Fuse/Breaker for DC Ground Fault detection is actually very dangerous (for various reasons) and actually against Code/Good Electrical Practices. My suggestion is to "ditch" the DC GFI ground fault "stuff" (don't use the breaker, wire around fuses in some equipment).

Some manufacturers have better addressed the problem. Some of Midnite's MPPT controllers have an Arc Fault detection system (monitors the solar wiring for Arc Fault electrical signatures) and MorningStar which actually designed and sells a "true" DC Ground Fault breaker system that is similar to the AC GFI breakers that are out there.

-Bill

Dave Angelini

Disagree and this is an old one Bill, agree on that  The drawing is how Schneider and Outback meet code for these installations. It works fine and I have never seen anything that that even remotely could be called dangerous. This backed by 100's+ of this exact wiring installation. These inverters do not have a chassis to neutral bond. None of Outback and Schneider do from the factory. They both do this for all of the different requirements for worldwide installation.

It seems to me that the Saggy is asking a new question compared to the title in this thread of 2 different inverters in the same distribution panel. Since he said code is not an issue as this is being done safely but not exactly as the equipment was listed for and uninspected.

Midnite does not make an Inverter and so that is apples and oranges on how they choose to wire a power system. Kind of unfair but valid.

Saggys

Yes Dave you are correct, this question has kind of morphed into, does the DC equipment grounding conductors and the AC equipment grounding conductors bond together to the grounding electrode (ground rod)? Or do you simply wire as shown in Outbacks Drawing, with the two grounds separate, with or without the GFPI? With the 2 different types of voltages this is more complicated than installing a typical home service.

BB.

I know know that Dave and I disagree on the use of fuse/breaker between DC negative ground and Green Wire system safety ground... As I recall, he has had multiple "ground sense" fuses blow (from lightning and such?) and finds the system are rarely (if ever) damaged.

Not trying to put any words in Dave's posts--And I respect he is an expert in the Off Grid Power field--Certainly much more than I ever could be called.

And Dave knows more about how the various name brand inverters are wired on their AC outputs (none of them do a chassis/neutral bond)--Which is the correct way to default the wiring when you have (possibly) multiple AC sources (utility, AC inverter, AC genset, shore power, and in North America, the default Neutral+Ground bond usually in the main AC Breaker panel).

Yes--The DCGFI is "mostly" tied to the solar charging source side of the equation--As it was intended to reduce the chances of solar panel & wiring fires due to "arc fault"--But really + or - shorts to chassis mounts/green wire grounds. The reality--Unless there is an Arc Fault detector somewhere in the DC system (Midnite has one in their MPPT Classic charge Controllers)--The Ground Fault Sense fuse does not do anything directly related to arc faults (other than if there is an Arc Fault from DC wiring to earth grounded metal).

However, because most all (that I know of) Solar Charge controllers' do not have transformer isolated outputs--They DC negative "connection" connects throughout the entire DC wiring system--From array, through charge controller to battery bank to AC inverter DC input... So, just like in AC neutral+ground bonding--There is a similar set of issues with DC Negative+Ground bonding.

So having a clear wiring diagram that addresses "DC Green Wire Grounding" between metal chassis for various "black boxes", mounting frames, j-boxes and panels, and DC negative+green wire bonding (through DCGFI fuse/breaker/sense circuit or not) is important.

-Bill

Dave Angelini

For Offgrid (critical power system) I run an AC panel ground to a rod and power system equipment ground to a rod. I connect the 2 rods and might even add more grounding like a UFER or cold water metal out to a well or cistern. I often run 2 inverters for people who want a back-up system with AC bypass in the PDP, not unlike what you are doing. Many new Lithium batteries do not even have a ground connection. Just (+) and (-).

We all have the same goal here, I think, and Bill is has a good point on GFCI that I just do not share. No big deal!

Saggys

Thank you guys, so simple its complicated. Thank you for the clarification Dave, that is what code shows, but it is easier done on paper than the field.