wiring pure and msw inverters together?

summitdweller
summitdweller Solar Expert Posts: 28 ✭
in Off Grid Solar & Battery Systems #1
How does one go about wiring both a small 12v 300W pure sine wave inverter and a 12v 1500w modified sine wave inverter for the big stuff? The 1500W inverter is also the charger (not used that often). The 1500W is really only used for the vacumn and a few other things that I only use when the sun is shining. The 300w pure would be for the TV and laptop etc.

Thanks
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #2
    Re: wiring pure and msw inverters together?

    Basically, first you need a common Bus Bar for + and - connections on your battery bank. This could be long brass bolt, flattened copper water pipe, or even true bus bars of some sort.

    Terminal Bus Bar - Long $14.35
    wind-sun_2149_63612818 Terminal bus bars for wiring up systems, arrays, whatever. Available in 3 colors: Red ...

    Terminal Bus Bar - Short $12.85
    wind-sun_2149_63303692 (Midnite Short Busbar) Shorter version of the insulated MNTB bus bars. Available in 3 colors - Red, White, & Black - please specify when ... are useful for PV+ input on the narrow OB E-Panel and for a separate PV- bus bar for charge controllers that require them, and a variety of other ...
    If you have parallel batteries, you can follow this web page for proper wiring/balancing of loads:

    www.smartgauge.co.uk/batt_con.html

    Once you have a reliable method for connecting to your battery bank, you just need a fuse/circuit breaker and wiring rated to manage the load...

    Run one set of wires to the small inverter from the bus connection (fuse/breaker on + battery supply lead).

    And run a heavier set of wires to the large inverter (plus fuse/breaker on + lead)....

    Remember, the fuses/breakers are there to protect the wiring from short circuits.

    If your inverters do not have on/off switches, you might want to use a circuit breaker instead--both for safety and for on/off.

    Remember, you are talking about some pretty hefty currents here... A 1,500 watt 12 volt inverter running 85% efficiency, and 10.5 volt battery cutoff with a 1.25x NEC safety rating for fuse/breaker/wiring:
    • 1,500 watts * 1/10.5 volt batt * 1/0.85 eff * 1.25 NEC safety = 210 Amp circuit
    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #3
    Re: wiring pure and msw inverters together?

    On the DC side there is no problem running two different inverters of any type off one battery bank. Just remember the idle current of each inverter will be drawing down the batteries. Shut off the big inverter when not in use. Of course each should have its own appropriately size fuse too.

    On the AC side never, ever connect two inverters' outputs unless they are specifically designed for "stacking".
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  • icarus
    icarus Solar Expert Posts: 5,436 ✭✭✭✭
    #4
    Re: wiring pure and msw inverters together?

    How are you going to wire them? Through a AC load center. The MSW needs to be wired (typically) in a way that keeps the neutral ungrounded or it will go poof, so if you wire them through a common load center, one inverter energizing each buss of a 240 load center for example, you have to ensure that the neutral buss bar is not grounded. The Suresine 300 doesn't really care if the neutral is grounded or not.

    You could also wire them through separate load centers, or simply hard wire one for the every day loads, and power msw loads with a cord. (Make sure you fuse the battery side of inverters).

    Please note, as noted in a following post, in the scenario I am suggesting, each inverter would power only the dedicated circuits on each 120 vac buss. In other words, each inverter would power only certain circuits. If you wanted to power any circuit (or all) with either inverter you would need a transfer switch

    tony
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  • summitdweller
    summitdweller Solar Expert Posts: 28 ✭
    #5
    Re: wiring pure and msw inverters together?

    I do have 4 98 amp hour batteries and looking at the wiring in parallel website they are on the diagram 2 may change that after looking at that site. The msw is already wired with the correct fuse from the batteries to the ac load center that is in the 5th wheel trailer. Sooo I need the buss bar then connect the 300w pure sine with What size wire? fuse? + side from the batteries to it-then where do I connect to the AC load center as the msw is already connected? Yes I would definately turn off the msw most of the time that is one of the main objectives.

    Thanks again
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #6
    Re: wiring pure and msw inverters together?

    I am not quite sure I understand your use of the AC Load Center...

    Basically, you cannot connect the AC Output of any inverters together to one AC line (except with specific inverter models designed to share AC outputs).

    So, unless you use something like an AC transfer switch (manual or automatic)--You will end up with two separate AC outlets. One to the 300 watt inverter, a second for your 1,500 watt inverter.

    I don't know what AC inverter/model numbers you have... But running the same calculations as my previous post:
    • 300 watts * 1/10.5 volt batt * 1/0.85 eff * 1.25 NEC safety = 42 Amp circuit
    Using the NEC wiring table with typically available wire, 8 AWG would be able to carry 55 amps at normal temperatures. NEC is pretty conservative--But for solar being safe and avoiding excessive power loss/voltage drop is important--especially with 12 volt batteries which have little headroom for voltage drop.

    You should be keeping the inverter DC power cables as short as practicable. Long cables (more than a few feet) typically need thicker cable to avoid excessive voltage drop.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • summitdweller
    summitdweller Solar Expert Posts: 28 ✭
    #7
    Re: wiring pure and msw inverters together?

    I now have my AC wires coming out of the msw to the main AC circuit breaker box. With the new setup I would have AC wires coming out of both inverters going to a manual transfer switch then have the wires going to the main box? I am assuming I would only need a 30amp switch is that correct? Then just flip the transfer switch and turn on/off the correct inverter? Is there a so so high tech cheaper transfer switch?
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #8
    Re: wiring pure and msw inverters together?

    You have a manual transfer switch like this:

    Midnite Solar 30 Amp 240 Volt dual AC transfer switch

    Or an automatic one (simple) like this:

    Iota ITS-30R Automatic AC Transfer Switch 30 Amp

    Wire the automatic switch to transfer when the 1,500 watt inverter is turned on (one option).

    The other would be to wire up two sets of outlets--leave the current load center for your 1,500 watt supply and a second set wired to points where you would use the 300 watt TSW supply (cell charger, laptop, TV, radio, some lighting, etc.).

    It is probably not very likely that you would use the same outlets for large/small loads.

    By the way, what 300 watt TSW inverter are you looking at?

    The MorningStar 300 watt TSW (600 watt for 10 minutes) has a "search mode" which cuts way back on the Idle Current -- Basically, it looks for >6 watt 120 VAC load, then turns on.

    It also has a DC inhibit input that allows you to turn the inverter on/off with a simple 12 volt low current switch.

    Morningstar SureSine, 300 Watt Sine Wave Inverter 115VAC

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • summitdweller
    summitdweller Solar Expert Posts: 28 ✭
    #9
    Re: wiring pure and msw inverters together?

    The Moringstar is the one I am looking at. I might do the two outlet idea the other way as only the microwave and the vacumn really need the msw, and I have a very small place, then just turn off the circuit breaker that goes to the microwave and voile I hope, as the switch is what $100 ish. Thanks
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  • summitdweller
    summitdweller Solar Expert Posts: 28 ✭
    #10
    Re: wiring pure and msw inverters together?

    okay I put a signature thing on this so you know what I got
    Thanks again
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #11
    Re: wiring pure and msw inverters together?

    The Xantrex includes a transfer switch (generator/inverter)... That would work very nicely when you need to use the genset to recharge your battery bank or provide more power than your bank can supply.

    By the way, have you tested the DR with your genset to make sure it charges your battery bank? Some Inverter/Chargers don't like gensets with wondering frequency (don't know anything about the DR specifically--just a common issue with some some inverter/chargers).

    Yea--it is kind of hard to find a cheap two pole transfer switch (two pole transfer switch required for use with MSW inverters--you should not even share the AC neutral line either).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • icarus
    icarus Solar Expert Posts: 5,436 ✭✭✭✭
    #12
    Re: wiring pure and msw inverters together?

    The Suresine 300 is a great little inverter.

    Tony
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #13
    Re: wiring pure and msw inverters together?

    A caveat in case this issue was missed or forgotten:
    If you're going to have an MSW and a TSW inverter sharing AC neutral (as with a common AC box) there must not be a neutral-ground bond on the TSW inverter or else it too will be "shared" with the MSW inverter, and they don't like that (to put it mildly). Keep both AC outputs completely isolated from one another is, in my opinion, best.
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  • icarus
    icarus Solar Expert Posts: 5,436 ✭✭✭✭
    #14
    Re: wiring pure and msw inverters together?

    Noted in post #4 but never an issue to reenforce a critical issue.

    Tony
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  • Paul54
    Paul54 Registered Users Posts: 18 ✭
    #15
    Re: wiring pure and msw inverters together?

    I have been using double pole double throw AC coil relays to do change over of selected house loads. My inverter, a no name Asian import, is a true sine but does not like it's neutral grounded. I come out of my main grid breaker panel and break the branch circuit with the relay. The normally closed contacts are the grid power with the wiper arm being the branch circuit. The normally open contacts are the inverter feed. The AC coil is fed with the inverter's AC output. The result is that when I turn on my inverter it automatically pulls in the relay coils and switches the branch load circuits to inverter power. When I shut the inverter down it automatically drops back to grid power. I have a switch in series with each coil that lets me "arm" or disarm each individual branch circuit. The relays being double pole, switch both the hot and neutral legs of the branch circuit. This also isolates the grounded house neutrals from the inverter. Like others have stated before, you must keep the neutrals isolated. When I was putting in this system, I "accidentally" found two branch circuits that were sharing neutrals downstream of the breaker panel. Whether you build your own transfer switch or buy one, carefully check that each branch circuit is truly isolated, that no hots or neutrals are shared or no neutrals have been grounded downstream of the breaker panel. A good quality relay with 20 Amp contacts is about twenty bucks. You can add them one at a time as needed. They work well for adding multiple inverters to your house circuits. But remember, when mixing inverters and or grid power, complete isolation is the key.
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  • summitdweller
    summitdweller Solar Expert Posts: 28 ✭
    #16
    Re: wiring pure and msw inverters together?

    I know it has been awhile I sort of understand the neutral ground idea. So if I have one inverter the suresine 300 go to the transfer switch and the msw also go there, turn off the not used inverter, transfer to the right (on) inverter then go to the single ac breaker box to power all circuits will this work or does the neutral ground issue come into effect? The ground for the system is from the breaker box to ground no where else. Also what size inline fuse from batteries to suresine300?

    Thanks I need to do this as now I have a computer that hates the msw.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #17
    Re: wiring pure and msw inverters together?

    The short version is if the MSW inverter has either output wire connected to any wiring that is bonded to ground anywhere it can go up in a puff of smoke.

    It is perfectly normal for the one and only neutral-ground bond to be in the breaker box. The MSW inverter doesn't care where it is, it only cares if it makes an electrical connection with it and ends up feeding high Voltage back into itself (because the case is often bonded to negative and/or the negative to ground).

    The only way to prevent the trouble with a transfer switch is if the switch changes both hot and neutral and the neutral-ground bond is on the output of the sine wave inverter and is not part of the AC wiring if the switch is set for "TSW" input rather than "MSW" input.

    Usually a computer doesn't care about the quality of its input Voltage as they have great power supplies. The same can not be said of monitors, however. If you have an inverter that actually messes up a computer, get rid of it; it must be awful junk.

    The MS 300 Watt inverter uses about 30 Amps max on continuous but can draw up to 60 under surge conditions. As such, the manual calls for a 100 Amp fuse or breaker.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #18
    Re: wiring pure and msw inverters together?

    I am not sure--You want to have a transfer switch between two inverters as sources (MSW and TSW)?

    I would probably just wire a separate circuit (if you can) from the TSW to the loads you want on the TSW inverter.

    Regarding grounding... Technically, the "problem" with most MSW inverters is if the battery is grounded and the AC "white wire" is also grounded. That will cause a short circuit through the middle of the inverter--and possibly ruin the MSW inverter.

    So, you could "float the battery bank" (both + and - leads which now become "hot" with respect to ground) with the MSW inverter white wire grounded--However, not recommended. Many things assume that the battery bank is ground referenced and you could, for example, get the metal chassis of the inverter "hot" (if it is connected to the battery "-", or if you have RT 232 communications between a 12 VDC device and something else, etc.).

    So, nominally, for the MSW inverter setup. Ground the battery negative terminal and float the two AC wires (both are now "hot").

    If you have the AC home wiring now floating (no grounded neutral), you can connect the TSW inverter through a (recommended) two pole transfer switch (with the MSW on the other set of connections) and power everything relatively safely.

    For most TSW inverters, you can ground one of the 120 VAC outputs and call it "white"/"Neutral". As always read the manuals in detail.

    Regarding fusing, the manual should have the DC fusing requirement, but the back of the envelope calculation would be:
    • 600 Watts max power * 1/0.85 invrt eff * 1/10.5 battery cutoff * 1.25 NEC fuse/wiring/breaker derating = 84 amps minimum

    And round up to next standard wire/fuse/breaker sizes.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • inetdog
    inetdog Solar Expert Posts: 3,123 ✭✭✭✭
    #19
    Re: wiring pure and msw inverters together?
    Cariboocoot wrote: »
    The short version is if the MSW inverter has either output wire connected to any wiring that is bonded to ground anywhere it can go up in a puff of smoke.

    It is perfectly normal for the one and only neutral-ground bond to be in the breaker box. The MSW inverter doesn't care where it is, it only cares if it makes an electrical connection with it and ends up feeding high Voltage back into itself (because the case is often bonded to negative and/or the negative to ground).

    Every time I see this kind of reference, I cringe. What is actually happening is that the output is being shorted out, as BB stated. There is no "feedback of high voltage" involved at all.
    The results are what count (magic smoke being lost from the inverter) but the terminology counts in recognizing the problem and finding a solution (if any).
    It is, however, true that if the battery is not grounded and so the system does not smoke, but the MSW output "neutral" is grounded, there will be high AC voltage to ground on the battery leads and perhaps on the case of the inverter. In that limited sense, the AC is "feeding back." And the magic smoke may be emitted by you instead of the inverter.
    SMA SB 3000, old BP panels.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #20
    Re: wiring pure and msw inverters together?
    inetdog wrote: »
    Every time I see this kind of reference, I cringe. What is actually happening is that the output is being shorted out, as BB stated. There is no "feedback of high voltage" involved at all.
    The results are what count (magic smoke being lost from the inverter) but the terminology counts in recognizing the problem and finding a solution (if any).
    It is, however, true that if the battery is not grounded and so the system does not smoke, but the MSW output "neutral" is grounded, there will be high AC voltage to ground on the battery leads and perhaps on the case of the inverter. In that limited sense, the AC is "feeding back." And the magic smoke may be emitted by you instead of the inverter.

    Every time I see this kind of reference I cringe.
    Shorted output to input; output is higher Voltage; inverter fries.

    The next person who insists on explaining things in a complicated way to someone who wants a simple answer (newcomer) is going to get the Nac Mac Feegles sent to his house! :p
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  • inetdog
    inetdog Solar Expert Posts: 3,123 ✭✭✭✭
    #21
    Re: wiring pure and msw inverters together?
    Cariboocoot wrote: »
    Every time I see this kind of reference I cringe.
    Shorted output to input; output is higher Voltage; inverter fries.

    How about shorted output to ground. Anything is higher than ground. Output circuit fries (taking rest of inverter with it).....
    SMA SB 3000, old BP panels.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #22
    Re: wiring pure and msw inverters together?
    inetdog wrote: »
    How about shorted output to ground. Anything is higher than ground. Output circuit fries (taking rest of inverter with it).....

    Nope.
    Shorts can only occur within a circuit. Ground is not part of a battery circuit unless it is connected to it. Which is where the problem lies: negative input on inverters normally is connected to the safety ground. Tie one side of the AC out to ground and you've connected it to the input. Lack of isolation within the unit causes the problem. Components designed for 12 VDC in nominal really don't like 1/120th second of 10X their rated Voltage.

    In fairness, not all MSW inverters suffer from this. They are not all alike inside. They don't all have the same input or output specs. Some of them can work this way.

    These days it's just easier to go with sine wave and eliminate all the potential issues.
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