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Thread: transfer switch for off grid power

  1. #1

    Default transfer switch for off grid power

    I recently purchased a manual generator transfer switch (like one here: http://reliancecontrols.com/ProductDetail.aspx?30216A ) and wanted to hook it up to where I have my off grid inverter power feeding into an IOTA ITS 30-R automatic transfer switch wired to the normally closed connection. My generator would be wired to the normally open connection, overriding solar power when started.

    The output of the IOTA switch would then feed into the Reliance manual transfer switch and connect to the six breakers in my main panel.

    My question is how do I wire the Reliance transfer switch for neutral and ground when connecting to my main panel? My main concern is creating a grounding loop from solar power (DC is grounded at negative terminal) conflicting with the transfer switch neutral and ground tying into the main panel.

    I'm sure someone has done something like this before, so any suggestions would be appreciated.
    Last edited by czyhorse; June 18th, 2011 at 8:12 PDT.

  2. #2

    Default Re: transfer switch for off grid power

    Must be having a senior moment as I don't see why you need two transfer switches to change your AC feed between generator and inverter.

    A transfer switch has one output and two inputs. The output goes to the AC breaker box. One input goes to the inverter's output, one goes to the generator's output.

    If you have one of the inverter/chargers it has AC IN which means it has a built-in transfer switch.

    Ground wiring should not be affected by any transfer switch, as all grounds should be connected together at all times and terminate in a single Earth ground point. The only time you get in trouble with this is using an MSW inverter which can not have the AC neutral-ground bond. In that case you leave the bond out and let the neutral "float". Check to make sure the generator doesn't already have this bond in any case, as you don't want two such connections.

    Again, I don't understand what you're trying to achieve with two transfer switches. Is grid power available as well?
    Four 175 Watt panels, OB MX60, 232 Amp hrs, OB 3524, Honda eu2000.

    Ohm's Law: Amps = Volts / Ohms
    Power Formula: Watts = Volts * Amps

  3. #3

    Default Re: transfer switch for off grid power

    Sorry...forgot to clarify.

    My main breaker panel is set up for utility grid power. I'm attempting to have a central switching point to divert power to selected breakers when utility power is lost. The manual switch is to divert from utility power, and the IOTA is to switch from solar power to generator power when necessary.

    My off grid is solely for emergency backup.

    Hope that makes sense....

  4. #4

    Default Re: transfer switch for off grid power

    Yes, that fills in the missing pieces.

    So your manual transfer switch will change the critical load(s) between utility power and emergency back-up. The auto switch will change the emergency back-up feed between generator and inverter. Or you might want to have the auto switch change the critical loads to back-up and manually swap between inverter & generator. Whichever works better for you.

    Now, what have you got for an inverter? That will also be important to the equation.
    Last edited by Cariboocoot; June 18th, 2011 at 9:11 PDT. Reason: Add option
    Four 175 Watt panels, OB MX60, 232 Amp hrs, OB 3524, Honda eu2000.

    Ohm's Law: Amps = Volts / Ohms
    Power Formula: Watts = Volts * Amps

  5. #5

    Default Re: transfer switch for off grid power

    My inverter is an Exeltech XP1100. My solar and utility grid systems have separate ground rods though, therefore I don't know if my plan will work.

  6. #6

    Default Re: transfer switch for off grid power

    Okay, so not an inverter-charger. You could really automate this project with one of those.

    Are you sure 1100 Watts is a large enough back-up? That auto switch has a 60 Amp capacity (for two sides) on 120 - that's 7200 Watts. You should make sure that the Exeltech isn't trying to feed that much when the emergency power kicks on. Basically your inverter is capable of handling "one outlet worth" (not even full capacity there). What are you trying to protect? Refrigerator?

    Sorry to sidetrack.
    So you'll hook up the auto switch to handle power coming either from utility or emergency sources.
    The emergency sources being either the inverter or the generator, which you'll select manually with the other transfer switch.

    Keep in mind the utility, auto switch, and second transfer switch are designed with two poles for handling 240 VAC whereas the inverter is only 120 VAC - one pole. Which your generator is I don't know. I mention this because if you have the Exeltech's output connected to the wrong input side on the auto switch the wrong thing, or perhaps nothing, will be powered up when it changes over. You do not want to try to supply both sides of the 240 input on the auto switch with the inverter's 120 VAC output.

    Grounds. I don't know how you have it wired now, but there's no reason I can see that both systems can't share the same Earth ground point. A separate post for the inverter isn't necessary; you should be able to connect its ground to the house system. If anyone knows why not, they'd better speak up now before he follows my advice and possibly does something wrong.

    Some of this stuff is a bit hard to explain over the internet.
    Four 175 Watt panels, OB MX60, 232 Amp hrs, OB 3524, Honda eu2000.

    Ohm's Law: Amps = Volts / Ohms
    Power Formula: Watts = Volts * Amps

  7. #7
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    Default Re: transfer switch for off grid power

    The key thing is the neutral -> ground bond. That should be done in the main panel. It should NOT be done anywhere else.

    So make sure the neutral and ground are not bonded in the Reliance box (they almost certainly won't be, but make sure).

    Make sure the neutral and ground are not bonded anywhere else - not at the generator and not at the inverter. Depending on the generator, neutral might be bonded to ground at the factory. If so, then you need to "lift" (disconnect or remove) that connection or bonding jumper.


    Then there is the inverter...

    According to the Exeltech XP1100 manual:

    http://www.exeltech.com/downloads/xp600-1100manual.pdf

    "WARNING: The AC neutral lead is bonded to chassis through the barrier terminal strip connector located on the back of unit. Chassis must be bonded to earth ground through the external ground connector located on the rear of the unit.
    (See Appendix C)"


    You DON'T want the neutral bonded to ground in the inverter because it is already happening in the main panel. Appendix C shows a "bonding strip" installed across the neutral and ground terminals in the inverter. You need to remove that bonding strip.


    As for the grounding...the manual also states:

    "The Negative or Positive terminal of the battery (DC Source) must be bonded to earth ground. It's recommended that it be to the same earth ground used for AC ground."

    No problem.



    So you lift the neutral-ground bond in the inverter and run the hot and neutral to the Iota. You run a ground wire from the ground terminal to the ground bus bar in the Iota.

    Same with the generator - make sure neutral and ground are not bonded, run the hot/neutral to the Iota and run a ground wire from the gen frame to the Iota ground bus. The gen frame is required to be grounded if you are feeding a building with it.


    Then you run hot/neutral from the output of the Iota - along with a ground from the Iota's ground bus - to the Reliance and hook up the Reliance to the main panel according to the Reliance manual.



    The only question is the ground rods. You say there are more than one and the DC negative is grounded to the second rod. If that second rod is far away at the PV array then no problem. If it's right next to the main panel ground rod, then you should really have everything going to just the one rod.

  8. #8

    Default Re: transfer switch for off grid power

    I am planning to do a similar thing. While I don't have my panels yet I do have my manual generator transfer switch which ties into my homes panel and will control 6 circuits. My plan will be to run these circuits on solar every day and go to grid only if I have to. I've hopefully determined my panel needs based on the estimated daily demand of these 6 circuits. Just as a note of clarification, I plan no use of a generator I will just hook my inverter into this switch instead.

  9. #9
    Join Date
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    Default Re: transfer switch for off grid power

    Quote Originally Posted by sprintman View Post
    I am planning to do a similar thing. While I don't have my panels yet I do have my manual generator transfer switch which ties into my homes panel and will control 6 circuits. My plan will be to run these circuits on solar every day and go to grid only if I have to. I've hopefully determined my panel needs based on the estimated daily demand of these 6 circuits. Just as a note of clarification, I plan no use of a generator I will just hook my inverter into this switch instead.
    Is there a automatic transfer switch made that would basically use PV power until grid power is needed?

  10. #10

    Default Re: transfer switch for off grid power

    Quote Originally Posted by tmarch View Post
    Is there a automatic transfer switch made that would basically use PV power until grid power is needed?
    If you've got grid, why would you have off-grid PV power? It's really expensive per kW hour.

    Grid-tied solar works seamlessly with the grid. All household needs supplied by PV if enough is available, grid power used if not, surplus PV power sold back to grid if allowed by the utility.

    But yes there are inverters which could be set up to utilize the grid as a "generator" and cut it in if battery Voltage falls below a set point.
    Four 175 Watt panels, OB MX60, 232 Amp hrs, OB 3524, Honda eu2000.

    Ohm's Law: Amps = Volts / Ohms
    Power Formula: Watts = Volts * Amps

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