One battery bank - two inverters - separate distribution panels - safe?

NicaSol
NicaSol Registered Users Posts: 32 ✭✭
in PV Installers Forum - NEC, Wiring, Installation #1
Greetings-
I would like to clarify some discussions in existing threads on this forum (if required for reference).

My good old Xantrex 2524 inverter failed a week or so ago (error code 05G-Input Relay Failure). Given the serious civil unrest in Nicaragua (very unsafe to travel) I have no chance of getting it to a repair facility. The best I can do locally is to buy a Samlex PST 1000 24. Since this will not meet the AC load of the existing system I was thinking about buying two to feed two reduced-load panels and, in if one inverter should fail, still have the second to run critical loads - security system, pool pump (3A on low speed), internet radios/routers.

A search led me to a few threads (above) in this forum about running multiple inverters off one battery bank. The consensus was that it can be done by connecting the inverters to their own (two) separate distribution panels. It was stated in one thread (and intimated in the others), that when the OPs' were asking about combining inverters, the reason for separate distribution panels is that if unsynced invereters were connected in parallel - positive to positive-neutral to neutral - to boost the amperage output to better meet the load of the one existing distribution panel, and if the AC output cycles (and other things?) did not match (were unsynced), one inverter could back feed into the other, likely destroying one if not both.

What I would like to clarify (for 2 inverters):
>the positive of each inverter connects to the positive breaker bus of its respective panel;
>the negative/neutral of each inverter connects to the neutral bus of its respective panel;
>the ground of each circuit connects to the ground bus of its respective panel;
>in a "main" panel, the ground and neutral buses are normally(?) bonded;
>the bonded ground/neutral buses are connected to a grounding rod;
>I believe, that even with independent ground rods for each distribution panel, the rods would have to be bonded;
>with, effectively, one grounding rod for both the neutral & ground of each panel,
   there is a connection between the neutral terminals of the two inverters.

1) Does this setup result in two "main" distribution (breaker) panels? Is that allowed? Safe?
2) With the two inverters' neutral terminals connected (but not the positives), will this damage one or both inverters?
3) Do I have this right? Is there an alternative solution?

Thanks for looking at this inquiry and, hopefully, your advice.

Alan


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  • mcgivor
    mcgivor Solar Expert Posts: 3,854 ✭✭✭✭✭✭
    edited July 2018 #2
    The correct terminology for the output would be live ( or often referred to as hot) and neutral. If the inverter has no intrinsic neutral, by bonding one leg to to ground establishes a neutral, if two inverters have one leg bonded the other legs will now be the live with respect to ground, but out of phase from one another, therefore need to be isolated. The common neutral will not connect the outputs but merely share the same potential need to provide a reference. 

    Read the manual, specifically regarding the establishment of a neutral  and chassis grounding, not all inverters allow bonding of one output leg to ground, as it could be done internally using chassis ground to maintain a single point required for certification UL, CSA and so forth. If in doubt Samlex tech support is very good and will provide a definitive answer. 
    1500W, 6× Schutten 250W Poly panels , Schneider MPPT 60 150 CC, Schneider SW 2524 inverter, 400Ah LFP 24V nominal battery with Battery Bodyguard BMS 
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,599 admin
    #3
    As I read your questions and answers. Yes, you have everything correct.

    The one thing you do not need to do is drive another ground rod unless you need it (ground rods are generally used to dissapate lightning energy and static discharge).

    If the two inverters/panels were a large distance apart, then a second ground rod could be good for lightning control (lightning follow the shortest path to ground... A long cable run, in excess of 10-20 feet horizontal), and the lightning will probably find a different path to ground.

    Connecting the two ground rods together (and to the negative battery bus) is used to ensure you have a low resistance path for short circuit current back to supply power (and trip the DC or AC circuit protection).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • NicaSol
    NicaSol Registered Users Posts: 32 ✭✭
    #4
    This is what Samlex said over a couple of email exchanges based on a slightly modified version of my OP.

    Samlex: Connecting multiple inverters to a single battery bank is never a problem assuming the bank is large enough to handle the current draw which 1300 Ah is.

    Samlex:If each inverter is cabled with appropriate sized wires and circuit protection (fuse) there should not be any issue.

    Samlex:As for AC outputs on multiple inverters, - as you mention these can not be paralleled unless the inverters are designed especially for this purpose. The PST series can not be AC paralleled. You must have separate AC load circuits for the two inverters.

    Samlex:As you have “two reduced-load panels” I assume you will be wiring one PST to its own panel. In this case, yes each panel is a main panel with its’ own distribution, and will have a neutral to ground bond and a separate earth connection. Yes, under these conditions this is safe.
    Me>there are 2 AC distribution panels, one for each inverter. If these are considered main panels, is the neutral/ground bound for each panel made in the panel (panel's neutral bus bonded to panel's ground bus), or externally at, say, the earth ground rod? Page 38-8.5.2 & page 39-8.5.3 are confusing (to me) depending on whether we are talking about the panels being main or sub panels.
    Samlex:In your case each inverter is independent and isolated feeding its own panel which is also be independent so each will be a main panel with its own independent AC distribution, neutral bus bonded to its own ground bus.
    Me:>"a separate earth connection" (your last paragraph) (not plural, I think) - does this mean one grounding rod for both AC distribution systems? The inverters and AC distribution panels will be beside each other. If 2 grounding rods, how separate (distance-wise) would the earth connections/rods have to be? And if 2 separate rods, wouldn't there still be the chance of a bond through the earth, thus a bond between the neutral AC terminals of the inverters (as it would also be with just one earth rod) and is this a problem? And doesn't the NEC specify just one common point of earth grounding?
    Samlex:Each panel will have a earth ground connection. This ground could go back to the same rod or different rods, more for connivance than anything else. Since each AC system is independent they can not interact. The way to look at this in respect to NEC would be as two separate buildings.
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