Hooking up two 120 V inverters to sub panel

techntrek
techntrek Solar Expert Posts: 1,372 ✭✭✭
I wavered between posting this here or in the off-grid forum, figured this is "advanced".

I currently have a 240 V inverter hooked up to a sub panel, for emergency backup use. I don't have any 240 V or multi-wire branch circuits on this panel. So, if the existing inverter decides to take a long vacation I've considered using two smaller inverters hooked to each leg as a replacement. Used PSW inverters/UPSs are available a dime a dozen which is why this is appealing. I was lucky to get my used split-phase inverter so cheaply, I haven't seen many since at the price I paid.

Without 240 V or MWBC, there isn't a strict need for the phases to be in sync, but there are some issues I'm aware of. Highly doubtful this would be to code, duly noted. I also would have to be careful to not overload the neutral buss, since the phases will at times be in sync which will double the load on the shared buss. However, since I wouldn't use more than 2000 watt inverters (16.6 amps per leg) I wouldn't come close to the panel's rating so no big deal.

Any other drawbacks? Odd harmonics/capacitance issues as the phases sync up and then lag over time? Open invitation for alien invasion? :p
4.5 kw APC UPS powered by a Prius, 12 kw Generac, Honda EU3000is

Comments

  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    Re: Hooking up two 120 V inverters to sub panel

    It's going to be a lightning rod for high tides in your area. :roll:

    But you forsaw the neutral buss. You may discover a 240V doorbell transformer somewhere in a wall. How the inverters may feel about that, I don't know.
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
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  • techntrek
    techntrek Solar Expert Posts: 1,372 ✭✭✭
    Re: Hooking up two 120 V inverters to sub panel

    Well, I know that is on my attic circuit, 120 V, and not on my sub panel, so all good there.

    I'll make sure I raise the house on stilts first if I ever do this. Since I'm already @ 750' above sea level that'll give me 756' before I start to sweat.
    4.5 kw APC UPS powered by a Prius, 12 kw Generac, Honda EU3000is
  • dwh
    dwh Solar Expert Posts: 1,341 ✭✭✭
    Re: Hooking up two 120 V inverters to sub panel

    Is the sub-panel standalone or is it tapped off the main panel?
    Are you using a transfer switch?
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: Hooking up two 120 V inverters to sub panel

    Is your inverter output isolated or not? Typically, TSW inverter output are floating (transformer isolation) and (again typically) MSW inverters do not have isolated outputs.

    If one of the outputs (of a non-isolated inverter) are tied to ground (and the battery bank is grounded) to make a ground neutral configuration--the inverter(s) will blow.

    If you tie the two outputs of non-isolated inverters together (common bus like in a 120/240 VAC split phase system)--You might also blow an inverter (I am not sure--but I would not try it on my $$$ equipment).

    Ground/common neutrals/TSW/MSW/standard 120/240 VAC split phase systems have all sorts of issues on wiring. If you have non-isolated inverters--I would keep their output separate from each other and not share any wiring/breaker panels at all.

    As always, check the manual (and many cheaper inverter manuals do not say much if they are isolated or not on their input to output). Some expensive hardware does not say anything either (like Honda euX000i family gensets).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • techntrek
    techntrek Solar Expert Posts: 1,372 ✭✭✭
    Re: Hooking up two 120 V inverters to sub panel
    dwh wrote: »
    Is the sub-panel standalone or is it tapped off the main panel?
    Are you using a transfer switch?

    Actually its a sub-sub-panel. Service panel, to transfer switch, to genset sub-panel, to bypass switch, to inverter, to inverter sub-panel.

    edit: I should add its a double-conversion inverter (repurposed high end UPS), so no transfer switch for the inverter just a bypass. Two actually, an external manual one and a manual/automatic built into the unit.
    4.5 kw APC UPS powered by a Prius, 12 kw Generac, Honda EU3000is
  • techntrek
    techntrek Solar Expert Posts: 1,372 ✭✭✭
    Re: Hooking up two 120 V inverters to sub panel

    BB - I just double-checked the manual and it is bonded internally. I need to head to bed, I'll re-read your post tomorrow with some sleep under my belt. Can't focus right now. :D
    4.5 kw APC UPS powered by a Prius, 12 kw Generac, Honda EU3000is
  • techntrek
    techntrek Solar Expert Posts: 1,372 ✭✭✭
    Re: Hooking up two 120 V inverters to sub panel

    BB - I'm not sure why a non-isolated inverter would be a problem. Bonding it when it isn't now shouldn't cause a short, its highly doubtful its positively grounded inside, so bonding it won't complete any short circuit. I've seen generators and inverters that provide access to the bond strap so you can remove it if the situation or code requires it.

    I'm not talking about tieing the two outputs together, that would definitely cause magic smoke to rise. Just the neutral.
    4.5 kw APC UPS powered by a Prius, 12 kw Generac, Honda EU3000is
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: Hooking up two 120 V inverters to sub panel

    The non-isolated Inverter creates direct electrical connections (via the switching transistors) directly between the battery bank and the AC output.

    If you ground reference the battery bank (negative ground is typical) and ground reference one lead of the inverter's AC output (to make a "neutral")--there is a dead short from the battery positive lead through the internal of the inverter with a return path back through the grounded neutral to the negative battery lead.

    If you parallel connect one lead (aka "Neutral") of two MSW/non-isolated inverters (that are not electronically synchronized), I would believe that there will be part of a cycle where you have a dead short through the one "common AC wire" as the two inverters "beat" (slowly frequency drift at 60.0x Hz) across each other.

    If the two inverters do not use a common battery bank then their input is isolated. However, the batteries must be floating too (not negative or positive ground referenced)--And that makes your +/- battery leads "hot" with respect to ground (if the inverter AC "Neutral" is ground referenced).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • techntrek
    techntrek Solar Expert Posts: 1,372 ✭✭✭
    Re: Hooking up two 120 V inverters to sub panel

    Sounds like I might do better hooking one inverter to both legs.
    4.5 kw APC UPS powered by a Prius, 12 kw Generac, Honda EU3000is
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: Hooking up two 120 V inverters to sub panel

    Tie both halves of the AC bus together and make a 120 VAC breaker panel... Remembering now that the neutral currents all add (instead of the A+B summing to zero for equal current flow)... So, if this was a 100 Amp @240 VAC panel, it would now be 100 Amp@120VAC instead (and not 200 Amp@120 VAC--staying with the 100 amp max rating of the neutral bus bar).

    But, if you are not dealing with more than 2,000 Watt inverters--Then you really have the choice of wiring everything with 12 AWG wiring and using switches instead... A 20 amp breaker will never open (if the inverter is working correctly).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • techntrek
    techntrek Solar Expert Posts: 1,372 ✭✭✭
    Re: Hooking up two 120 V inverters to sub panel

    Well, I already have the sub-panel in place, hooked up to my existing 6 kw inverter. Better to use breakers still because a dead short will still flip them, and when I added this panel I converted the breakers for the bedrooms to arc-fault breakers.
    4.5 kw APC UPS powered by a Prius, 12 kw Generac, Honda EU3000is