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Thread: inverter to AC breaker box (off-grid)

  1. #1
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    Default inverter to AC breaker box (off-grid)

    I just read this:

    13. How can DC-AC inverters be connected to multi-wire branch circuits?

    Do not directly connect the hot side of the inverter to the two hot legs of the 120 / 240 VAC electrical breaker panel / load centre where multi-wire ( common neutral ) branch circuit wiring method is used for distribution of AC power. This may lead to overloading / overheating of the neutral conductor and is a risk of fire. A split phase transformer (isolated or auto-transformer ) of suitable wattage rating ( 25 % more than the wattage rating of the inverter ) with primary of 120 VAC and secondary of 120 / 240 VAC ( Two 120 VAC split phases 180 degrees apart) should be used. The hot and neutral of the 120 VAC output of the inverter should be fed to the primary of this transformer and the 2 hot outputs ( 120 VAC split phases ) and the neutral from the secondary of this transformer should be connected to the electrical breaker panel / load centre.


    This is from a the Samlex website. I'm confused as I wanted to do just that, distribute power to various circuits in the cabin. So, is this just with this inverter (600 watt heavy) or Samlex or all inverters with plugs? If so, how do I run AC power throughout my house w/o runnnig a bunch of ext. cords?

    Are Samlex made overseas?

  2. #2
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    Default Re: inverter to AC breaker box (off-grid)

    The reason you shouldn't connect the inverter to both sides of a 240 split phase panel is that certain wiring methods allow 2 circuits to share a neutral as long as they are on different phases, thereby allowing the neutral to essentially carry double the current that any give size wire would be rated at. By being on different phases, the neutral load would never exceed it's rating because when one phase was peaking at the top of it's phase, the other was at the bottom of the wave.

    I am not explaining this very well, but I hope you get the idea.

    Now in the real world, in a small off grid cabin it seems to me that this wouldn't be an issue assuming that you have wired your building and your panel to account for this rule. A small number of branch circuits carrying small loads shouldn't be a problem, assuming that the neutrals are wired properly.

    I'm sure that some one will chime in with a better technical reason.

    Tony
    Please note, being a moderator does not add any weight to my opinions 300 watts Siemens/BP panels,plus a Sun 90,, making ~400. ~30 amps into Rogue MPT-3024, 450 ah of Trojan T-105, Morningstar ts300 inverter, a Tri-Metric meter.a collection of antique generators, plus 2 Honda eu-1000i's (also a BS2512 IX controller) and assorted other stuff!

  3. #3
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    Default Re: inverter to AC breaker box (off-grid)

    There are several issues.

    1. Inverter need to support a grounded neutral if you are following code--most MSW do not, many (most) TSW do.
    2. If you need 240 VAC--those appliances will see zero volts if the Black and Red hot leads are tied together (not a problem if all 120 VAC loads).
    3. You need to understand the wiring of your system... If you have a typical Black/Red/Neutral setup on a split phase 120/240 VAC branch circuit using 14 AWG wiring... Each wire can carry 15 amps (typical NEC). Black to White 15 amps (120 VAC). Red to White 15 amps (15 amps). And Black to Red 15 amps (240 VAC)

    When the Black and Red are split phase, the Black + Red current adds to Zero amps on the White wire (the split phase part--i.e., center tap of a transformer).

    If, your Black/Red are both the same 120 VAC phase with White... You could put 15 amps down Black and 15 amps down Red, and the White will carry back 30 amps and overheat.

    The breaker/fuse is only on the Black/Red branch wires. The White/Neutral does not have any fuses/breaker and can be overheated if connected to an inverter that can output, for example, 30 amps.

    However, if your inverter can only output 15 amps at 120 VAC total (or you use 1x 15 amp breaker to power both BL/RD wires instead of 2x 15 amp breakers, one for each BL and RD wire)--you will be perfectly safe.

    -Bill

    PS: You could also use two White Wires (Black+White #1 / Red+White #2) and be OK too... If you had a really big inverter with a really small breaker box--I guess you could overheat the neutral bus bar with the additive currents--normally the neutral bus bar would be designed to handle one leg's worth of current maximum).
    20x BP 4175B panels (replacement) + Xantrex GT 3.3 inverter for 3kW Grid Tied system + Honda eu2000i Inverter/Generator for emergency backup.

  4. #4
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    Default Re: inverter to AC breaker box (off-grid)

    The only reason the neutral will potentially carry more current is because the two independent inverters are not lock phased to be 180 degrees out of phase.

    In reality all neutrals to all outlets are brought back to the main breaker box neutral bus bar so the only place where more neutral current could occur is in the boxes neutral bus bar. Not much of an issue.

    In a normal grid fed box, the neutral line going back to the pole will not carry the sum of all the 120 v loads since some of them on either side of the neutral (L1,L2) will cancel their current contributions on the neutral cable back to the pole.

    Not much relevance to two inverters tied to L1 and L2 in a breaker box.

    Just make sure you don't have any 240 v loads unless the inverters are phase locked and inverted, like two stacked Trace SW series stacked inverters, and the inverters are neutral groundable.

  5. #5
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    Default Re: inverter to AC breaker box (off-grid)

    This was asked about 1x 600 watt inverter--I believe...

    So, L1/L2 tied to an inverter, with a common neutral, that is less than 1,800 watt output with 14 AWG wire (15-20 amp rated) is fine.

    -Bill
    20x BP 4175B panels (replacement) + Xantrex GT 3.3 inverter for 3kW Grid Tied system + Honda eu2000i Inverter/Generator for emergency backup.

  6. #6
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    Default Re: inverter to AC breaker box (off-grid)

    Quote Originally Posted by BB. View Post
    This was asked about 1x 600 watt inverter--I believe...

    So, L1/L2 tied to an inverter, with a common neutral, that is less than 1,800 watt output with 14 AWG wire (15-20 amp rated) is fine.

    -Bill
    All very good info. I've paid so much attention to DC over the last year that I haven't really done my home work on AC. I just assumed I would take the 3 wires from a plugged in "cord" on the MSW cheap inverter and split the black to the buss bar on one side, the neutral to the ground and the ground to the ground. And then run my AC circuits (power accounted for) throughout the cabin. Then when I get a 2nd TSW inverter I would do the same except to the other buss.

    Now I'm not sure I can do that. I'm not sure about the phase issue and don't want to "blow up" my inverters let alone burn down the house.

    Is there a basic AC book that pertains to inverter power?

    And now I have to think about "wiring" in the generator too to charge batteries, run pump, etc.

    I got a lot of HW to do.

  7. #7
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    Default Re: inverter to AC breaker box (off-grid)

    You won't have a problem with current carrying capability on the neutral.

    Your biggest potential issue is whether the Samlex MSW inverter you have is neutral plug side (the wider prong blade) groundable.

    Most MSW inverters have an H-bridge output with no isolation on the HV DC boost ground to battery ground. This allows some of the electronics on the AC H-bridge driver side to be run from the battery. A cheaper approach then having an separate, isolated secondary side generated low voltage DC source to run the H-bridge MOSFET's gates.

    The result is when you ground the neutral there will be an AC voltage to ground on the battery terminals. You will get a shock if you touch negative terminal of battery when standing on the ground. If you also ground the negative terminal of battery the inverter fuse will blow or inverter will blow out.

  8. #8
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    Default Re: inverter to AC breaker box (off-grid)

    I agree with RCinFL,

    Quote Originally Posted by bobdog View Post
    13. How can DC-AC inverters be connected to multi-wire branch circuits?

    Do not directly connect the hot side of the inverter to the two hot legs of the 120 / 240 VAC electrical breaker panel / load centre where multi-wire ( common neutral ) branch circuit wiring method is used for distribution of AC power. This may lead to overloading / overheating of the neutral conductor and is a risk of fire. A split phase transformer (isolated or auto-transformer ) of suitable wattage rating ( 25 % more than the wattage rating of the inverter ) with primary of 120 VAC and secondary of 120 / 240 VAC ( Two 120 VAC split phases 180 degrees apart) should be used. The hot and neutral of the 120 VAC output of the inverter should be fed to the primary of this transformer and the 2 hot outputs ( 120 VAC split phases ) and the neutral from the secondary of this transformer should be connected to the electrical breaker panel / load centre.
    Really need to know the exact model number of the inverter (and, if handy, a link to the manual).

    From the above, it appears that the inverter output is isolated and/or safe to ground reference (the AC neutral) because the warning talks about "Neutral" connections and overheating--and no warnings at all about grounding issues.

    However, because this just an excerpt of the manual--we don't know what other warnings there may be...

    Floating the "neutral" (not ground referencing the AC neutral) output of an inverter in a small cabin electrical system should not be a problem (even if the inverter supports a grounded neutral).

    The bigger issue is if you use a "standard" breaker panel--the neutral connection is not a floating bus bar--which means that the box, as supplied, is not safe to use with a "floating neutral" as the metal of the box can become energized as one leg of the inverter.

    So--a person ether needs to make up their own isolated neutral connection, or determine if the inverter supports a ground neutral or not.

    Unfortunately, equipment manufacturers are not always clear if grounded neutrals are allowed or not (even good companies like Honda and their very nice eu family gensets do not clearly sate if they support grounded neutrals).

    If you don't know for sure if an inverter supports a grounded neutral + grounded battery terminal--don't do it... Lots of sparks and smoke are possible.

    -Bill
    20x BP 4175B panels (replacement) + Xantrex GT 3.3 inverter for 3kW Grid Tied system + Honda eu2000i Inverter/Generator for emergency backup.

  9. #9
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    Default Re: inverter to AC breaker box (off-grid)

    I think most of the new breaker boxes have insulated neutral bus bar.

    This is required in a normal auxilary box setup to meet code of single ground bonding point back at the main breaker panel. The floating neutral in auxilary box is white wired over to neutral bus bar in main box which has the single point ground in main box case.

  10. #10
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    Default Re: inverter to AC breaker box (off-grid)

    Shows how long I have been inside one of those.

    Are the insulated neutral bus connections insulated to the normal 600 VAC as the rest of the box's hot leads will be?

    -Bill
    20x BP 4175B panels (replacement) + Xantrex GT 3.3 inverter for 3kW Grid Tied system + Honda eu2000i Inverter/Generator for emergency backup.

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