Inverter/Transfer Switch

m_gdavis
m_gdavis Registered Users Posts: 3
in General Solar Power Topics #1
I am helping a friend set up his Utah Cabin. He is set on getting a 7KW generator for his cabin (this will only be used on weekends). He likes lots of power. I am looking at the Xantrek Freedom SW 3012 inverter. He plans on using 4 batteries.

My question is do any of you see a problem with using a external 100 amp transfer (instead of the inverters switch) to switch between the inverter power and the generator?

Also what are your thoughts about the Xantek Freedom SW 3012? I did search on it but found nothing.

Thanks for your help.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #2
    Re: Inverter/Transfer Switch

    Welcome to the forum.

    No problem switching loads between generator and inverter using an external transfer switch instead of one built in to the inverter/charger (if so equipped).

    I'm not familiar with the SW 3012 but from the numbers it's 3kW 12 Volt. Not exactly a match for a 7 kW generator and someone who "likes lots of power".

    Before he goes broke buying the wrong sort of equipment maybe we could have a look at his estimated maximum Watts and daily Watt hours? If he hasn't got those numbers he's headed for "Ready, Fire, Aim!" and the resulting consequences.
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  • m_gdavis
    m_gdavis Registered Users Posts: 3
    #3
    Re: Inverter/Transfer Switch

    Thanks.

    I tried to get him to figure out how much watt hours he need but he doesn't have fixed uses. He currently has a nice trailer with generator, batteries and DC lights and plans to wire the cabin like a trailer as a start (DC lights, propane refrigerator, and add AC outlets for TV's and other lights. The reason for only the 3000w is the 12V system for the DC lights and other appliances. We didn't want to run a 24 or 48v system (inverter). I hope that makes some sense.
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  • stephendv
    stephendv Solar Expert Posts: 1,571 ✭✭
    #4
    Re: Inverter/Transfer Switch

    While there's no real problem with using an external transfer switch, it won't be as nicely integrated as the internal relay in the inverter. Using the internal relay he could program the inverter to auto-switch over on a) too low battery voltage and/or b) too much current draw. It effectively means that he won't have to start the generator manually. This functionality is in most of the more well known inverters outback, xantrex xw, magnum etc, I don't know whether the freedom sw supports it.
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  • icarus
    icarus Solar Expert Posts: 5,436 ✭✭✭✭
    #5
    Re: Inverter/Transfer Switch

    Ready, Fire, Aim!". 3 kw from 12 vdc is is ~250 amp. We really suggest you look at a 24 or 48 vdc system, as the battery wiring is simpler, the wiring more efficient, and more importantly, if you are going to drive 3kw out of 12 Volta you are gng to need a huge battery. I suggest, as did 'Coot, get a better handl on the loads. All design considerations strt with the loads,, all else is R,F,A!

    My hunch is that if this guy is power hungry and uses a 7 kw genny, he is going to use all of the 3 kw of the inverter, and likely kill his batteries in the process. What on earth does one need to run in a Cabin that requires 7 kws or 3 for that matter. remember, cons action is a cheap lesson, as if he can reduce his loads, both total and peak, he will be able to buy a much better system, much cheaper.

    Tony
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #6
    Re: Inverter/Transfer Switch

    Remember, a 7 kW inverter with a 3kW inverter and transfer switch--Normally, you would wire up a maximum of 3kW of loads to the inverter/transfer switch and the more than 3kW loads directly to the 7 kW genset (say everything in the house to the 120 VAC inverter, and a big/deep well 240 VAC well pump to the genset).

    Note that large inverters need large battery banks to power the loads/starting surges...

    For example, a C/8 discharge rate (healthy rate of continuous discharge for a flooded cell battery):
    • 3,000 watts * 1/12 volt battery bank * 8 hour rate of discharge * 1/0.85 inverter eff = 2,353 AH @ 12 volt battery bank

    If he wants to support a 3,000 watt surge, then a C/2.5 rate of discharge maximum is suggested (again for flooded cell):
    • 3,000 * 1/12 volts * 2.5 rate of surge discharge * 1/0.85 inverter eff = 735 AH @ 12 volts

    And the amp rating for 3,000 watt 12 VDC inverter wiring would be:
    • 3,000 watts * 1/10.5 inverter cutoff * 1/0.85 inverter eff * 1.25 NEC derating = 420 Amp rated wire and breaker/fuse

    To recharge the battery bank, you would want ~5% to 13% rate of charge (nominal). 10% of a 2,353 AH battery bank would be 235 amps. Or 10% of 735 AH would be 74 Amp recommended charging current.

    Also, it is nice to size the genset to the amount of charging current you will be supplying to the battery bank... A 3kW inverter system would need around a 2-4kW genset (depending on rating of AC battery charger and the eventual size of the battery bank).

    Generator fuel efficiency (kWH per gallon of fuel) typically begins to fall dramatically at 50% of rated output (i.e., fuel flow levels off at 50% of full load from 50% load to 0% load). If the generator is going to run a lot to recharge the battery bank--Sizing the AC charger and the generator to the battery bank charging needs can save a lot of fuel (i.e., use a 2kW genset to recharge the battery bank vs a 7kW could save 1/2 to 2/3 of the fuel used to recharge the battery bank).

    Normally--I think many of us here would suggest a small battery based solar pv system (even 300-600 watt range) and then run the genset for larger loads... Possibly even two gensets. A quiet 1-2 kW inverter/generator (Honda/Yamaha/etc.) for "smaller" loads. And the 7kW for large loads (shop loads, saws, well pump, etc.).

    But, in the end, it is very difficult to design the "optimum" solar power system without knowing more about the loads. Using a Kill-a-Watt type meter to measure the AC loads (per day, by season if appropriate) would be a big help.

    Depending on the needs--Could even simply start with a Honda eu2000i inverter generator and a kill-a-watt meter. Run for a few days to a month or so purely on the genset and log the daily power usage (kill-a-watt meters need to be read before genset is turned off).

    Will very quickly identify how much power is really needed to run the operation.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • m_gdavis
    m_gdavis Registered Users Posts: 3
    #7
    Re: Inverter/Transfer Switch

    Thanks for the info. He will be using the large generator for shop tools along with the inverter/charger. He also has a honda 3000 that I told him to use but he was unimpressed with its ability to run his tools. There will not be many loads in the house and he understands the fuel cost issues of a big generator.

    Thanks for all of your advice.
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  • icarus
    icarus Solar Expert Posts: 5,436 ✭✭✭✭
    #8
    Re: Inverter/Transfer Switch

    Just an FYI, I run my entire shop on a 2900 watt Mitsubishi genny. Granted, I am not running more than one tool at a time, and not running a welder. Table saw, chop saws, bench grinder, surface planer and all other others run just fine! Anything that will run on a 15 amp circuit, which is nearly all 120 vac tools will run on a EU 3000!

    Tony
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