Inverter Surge capacity and draw

solar_dave
solar_dave Solar Expert Posts: 2,397 ✭✭✭✭
in Off Grid Solar & Battery Systems #1
I am interested in knowing if the surge capacity on the typical TSW inverters drive the draw amperage up from the battery. For instance a 1000watt inverter with a 2000 watt surge doubles the battery draw for the time allowed by the inverter.

If that is true then would I need to double the wire gauge and fusing to support that draw? or is the time short enough to not required that sizing increase.
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  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    #2
    Re: Inverter Surge capacity and draw

    Most inverters suggest a fuse & wire size. I'd stick with that. IIRC, the MS-SS-300wants a 60A fuse on it's input for it's 600watts surge.

    I know my XA6048 ePanel has a huge honking DC breaker, which is not supposed to trip, for it's 10 sec 12KW surge.
    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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  • icarus
    icarus Solar Expert Posts: 5,436 ✭✭✭✭
    #3
    Re: Inverter Surge capacity and draw

    I think you need to wire (and fuse) for the surge capacity of the inverter. For example, the MS Suresine 300 can deliver 600 watts for a good long time,, 1/2 hour if i recall. 600 watts at 2 vdc is ~50 amps,, more if the battery voltage drops below 12 vdc. That is why I believe they call for a 60 Amp fuse and wiring for 60 amps on the input side.

    That said, I have mine wired on a 30 amp breaker, only because I never have a total load that comes close to exceeding ~ 200 watts,, or ~17 amps. I saw no reason to use such big wire,, and breaker. I think I wired it with #4 or maybe #2 AWG wire.

    Tony
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,439 admin
    #4
    Re: Inverter Surge capacity and draw

    Simple answer is that inverters have almost no input power storage (more than a cycle or two--lots of 120 Hz DC Current Ripple from inverters).

    So, you are correct that you have to size the wiring for voltage drop at maximum surge current... For example, a 300 watt 12 volt inverter with 600 watt surge:
      600 watts * 1/10.5 volts cutoff * 1/0.85 inv eff = 67 amp surge current

    So, if you choose to run the battery down to 11.5 volts under load, and 10.5 inverter cutoff... Your DC Power voltage drop (wire, fuses, breakers, etc.) maximum is ~1.0 volts.

    Note, this is not the maximum design continuous current which would be based on the 300 watt maximum continuous rating (generic inverter--Note the Morning Star Suresine is rated 300 watts but 600 watts for 10 minutes).

    I don't know the exact details of generic surge ratings for power systems... But, to give you an idea, I believe that a 15 amp 120 VAC branch circuit is rated for 40 cycles at 45 amps (less than 1 second surge).

    It is not unusual to assume that a circuit will support 5x rated load for surge current... Although, for low voltage DC power (like 12 volt), the headroom for voltage drop is very small and can become the limiting factor when designing wire gauge and supported length.

    Note that things get a bit more complex with non-DC and non-sinusoidal AC current wave forms (and poor power factor/badly out of phase AC current)--Because wire heating is based on I^2 * R self heating (double current equals 4x the power losses)--You can have further research/math to do in those cases.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • ChrisOlson
    ChrisOlson Banned Posts: 1,807 ✭✭
    #5
    Re: Inverter Surge capacity and draw
    solar_dave wrote: »
    For instance a 1000watt inverter with a 2000 watt surge doubles the battery draw for the time allowed by the inverter.

    It more than doubles the amp draw because the inverter efficiency drops drastically during surge power events. Our Xantrex inverters, which draw 375 amps at full load, from a draw of around 80-100 amps running normal loads, will peg the needle in the 500 amp meter on my bus hard against the stop when our 2 hp 240 volt well pump starts.

    The wiring should be sized according to the inverter manufacturer recommendations. But from my standpoint, the bigger the wire to the inverter, the happier it is.
    --
    Chris
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  • waynefromnscanada
    waynefromnscanada Solar Expert Posts: 3,009 ✭✭✭✭
    #6
    Re: Inverter Surge capacity and draw

    If I remember right, when i got my first SS-300 back a few years ago, it called for a 100 amp fuse, which hit me as way overkill, so I used 60 amp and never had a problem.
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  • solar_dave
    solar_dave Solar Expert Posts: 2,397 ✭✭✭✭
    #7
    Re: Inverter Surge capacity and draw

    Thanks guys, thinking about add one to the Volt, for those rare outages. It can supply 175 amps off the HV battery to the 12 Volt system when in the "ON" state. The fuse box has a spare 80 amp connection but if I want more than a 500/1000watt inverter it will have to be wired and fused properly to the battery. I figured as much.

    I just want to cover my deep freeze and fridge mostly. I suppose I could just rotate them to control the load side and use like a 600 watt TSW on the 80 amp fused location.

    was thinking this might do the job nicely.

    http://www.solar-electric.com/xp-600-12.html
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  • TheBackRoads
    TheBackRoads Solar Expert Posts: 274 ✭✭
    #8
    Re: Inverter Surge capacity and draw

    to get double the capacity is only 30 bucks more...........

    http://www.solar-electric.com/exxp12vol11w.html

    PS.. NAWS stole my pic for that :p
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  • solar_dave
    solar_dave Solar Expert Posts: 2,397 ✭✭✭✭
    #9
    Re: Inverter Surge capacity and draw
    TheBackRoads wrote: »
    to get double the capacity is only 30 bucks more...........

    http://www.solar-electric.com/exxp12vol11w.html

    PS.. NAWS stole my pic for that :p

    Yeah saw that, would also require fusing setup and direct connect to the battery. The smaller inverter can be attached to the fused accessory output.
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  • TheBackRoads
    TheBackRoads Solar Expert Posts: 274 ✭✭
    #10
    Re: Inverter Surge capacity and draw

    Just don't utilize the inverters full potential and I'd think you'd be fine... but in the rare case you needed full power you could manually hook it direct to a 12v battery(s). But that's just my $0.02 :p
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  • solar_dave
    solar_dave Solar Expert Posts: 2,397 ✭✭✭✭
    #11
    Re: Inverter Surge capacity and draw
    TheBackRoads wrote: »
    Just don't utilize the inverters full potential and I'd think you'd be fine... but in the rare case you needed full power you could manually hook it direct to a 12v battery(s). But that's just my $0.02 :p

    Point well taken. I will consider it.
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  • 2manytoyz
    2manytoyz Solar Expert Posts: 373 ✭✭✭
    #12
    Re: Inverter Surge capacity and draw

    Wanted to add that it's important to use the manufacturer's recommended size fuse. Inverters have internal fuses too, but tend to be a major pain to get to. If you oversize the external fuse, causing an internal one to blow instead, your fun begins at this point.

    Fat wire is a good thing, too big a fuse, not so much. Just my observation.

    A coworker bought a used inverter, and taking it apart was much more of a pain than expected. Lots of screws, knowing which ones to pull so the guts didn't fall apart in the process was a challenge. Ended up calling Xantrex. That's another long story, but here you can see the internal fuse...

    dscn3343.jpg

    dscn3344.jpg

    dscn3345.jpg
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  • techntrek
    techntrek Solar Expert Posts: 1,372 ✭✭✭
    #13
    Re: Inverter Surge capacity and draw

    Is it that small black thing near the center of the frame marked "Mega 225 A"?
    4.5 kw APC UPS powered by a Prius, 12 kw Generac, Honda EU3000is
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  • 2manytoyz
    2manytoyz Solar Expert Posts: 373 ✭✭✭
    #14
    Re: Inverter Surge capacity and draw
    techntrek wrote: »
    Is it that small black thing near the center of the frame marked "Mega 225 A"?

    That's it. IIRC, there was another fuse deeper inside the case.
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