Inverter/Genset Transfer Switch - 120v Single Hot to 120/240v connections

gengwallgengwall Posts: 19Registered Users ✭✭
Hi folks. Continuing on from my previous post on grounding, the next conundrum I am facing relates to the use of a manual transfer switch with 120v inverter as "utility" and connecting to the standard 120/240 panel. Again, I searched the forum but none of the posts I read hit specifically on this issue.

For reference, here is the inverter I have:

And here is the transfer switch that I have:

I have read previously that you can take the hot from a 120v source like the inverter and simply jumper the hots in the main panel to energize both sides of the panel. I guess the first check is to make sure that is kosher.

In my case, where the inverter would be connected to the utiity side of a 120/240 transfer switch and with the generator having true 120/240 connectivity on its side of the switch, do I/can I just make that "jump" between hots there in the transfer switch? Or am I stuck only having one hot side in my main panel when using the inverter? (There are worse things, of course, but it is only a 6 circuit panel so being knocked down to three would be inconvenient...but...see below).

As an ancillary topic, I had planned to feed the charging input to the inverter from a 20A breaker in the main panel, but that won't do because I would create a loop if I forgot to turn that breaker off when the panel was fed by the inverter. The breaker would have to be fed only by the generator with no chance of being fed by the inverter. Which may actually answer my question above. Maybe I only want one side of the main panel energized from the inverter and then energize both sides when I'm switched to generator so I can charge the batteries through a breaker on the gen-only side. Anyway, the inverter specs say this related to AC input: "AC INPUT: 54 amps at 120VAC with full inverter and charger load (20A max charger-only / combined input load to support charger and AC output is automatically controllable to 66%-33%-0% based on AC output load". Not sure what "full inverter and charger load" means. Does it mean that the genset could simply run through the inverter alone and if on, the inverter would use it to both supply AC power and charge the batteries? Or is it assuming AC input is coming from a utility source, not a generator? I would not attempt to use it as "full inverter and charger" - I just want to have the option to charge the batteries so a 20A circuit should be sufficient according to the specs, I think. And since the output side of the inverter would be disconnected at the transfer switch when switched to generator, I think that all I will get is charging (because no load on the AC output). So I should be ok, right? Or am I completely daft? Should I skip using the charging side of the inverter?

This is just me thinking out loud now. What I should have probably done is gotten a transfer switch with generator specific breakers instead of one that was just a single A-Off-B breaker. But oh well.


  • BB.BB. Posts: 27,444Super Moderators admin
    You can get a 6 pole 120 VAC (240 too) transfer switch for less than $300 (boy, I must be getting old, that still sounds like 2x as expensive as it should be):

    And I did the same thing with my house, wired up 120 to both L1 and L2 on the generator side of the transfer switch, and can only run my 120 VAC loads that way (I only have 120 VAC loads). My genset is also pretty small (1,600 watt)--So, it is safe as is.

    However, if you wire up a >>1,600 120  VAC genset this way, you have to look closely at your home's wiring. You may find an L1 (black lets say) + white going to some where else in your home. And an L2 (red) + white going elsewhere--That is perfectly safe.

    However, you may find a 120 VAC circuit pair of L1+L2+white (back+red+white) going to a set of 120 VAC loads... That is the problem. With L1+L2 tied together, the white "return wire" can carry 2x more current than it is rated for.

    One 120/240 VAC split phase wiring, the white only carries the "difference" between L1 and L2 (zero amps if L1=L2, or a maximum of L1 amps if L2=0 amps). When you jumper L1+L2, the current does not "cancel" as both L1 and L2 are on the "same phase"--And the white carries the sum of the current for L1+L2.

    You cannot tell what wiring you have in your home until you look closely.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • gengwallgengwall Posts: 19Registered Users ✭✭
    Luckily, there is no wiring yet. I haven't wired the cabin so I will make sure I don't have any multi-branch circuits (that's what you are referring to, right?). My main question, though, is can I jumper L1 to L2 on the inverter side of the transfer switch (genset already comes into transfer with separate L1 and L2). 
  • BB.BB. Posts: 27,444Super Moderators admin
    Yes, you can jumper L1+L2 on the inverter side of the transfer switch--That side will be isolated when the transfer switch is off (i.e., you don't want to short L1+L2 on the main panel side if you have a 120/240 VAC source (grid, genset, etc.).

    Yes, multi-branch circuits is what was I was warning about with L1+L2 tied together and the the excess current on the neutral if both L1 and L2 are loaded.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • gengwallgengwall Posts: 19Registered Users ✭✭
    Great! Thanks. Now the even more important question I have to ask myself is do I want to jumper them. If I do, I get all the circuits in my main panel available whether I'm using generator or inverter power. But then I can't feed back to the inverter charging input with AC from the panel - I would have to go separately from the generator which would mean another cord, plug, wall breach in the building, etc. In the long run I may leave one side of the main panel as generator only but it is good to know that I have the options.
  • hdoylehdoyle Posts: 2Registered Users
    Hi. I know this is an old thread but I hope someone is still around to help out. I'm in the same situation. I have a 10 circuit transfer switch in place for our 5500W generator already. The transfer switch is a Connecticut Electric EmerGen Switch model 10-7501A. 7500 W 30 AMP input. Two of the circuits provide 220V to our AC unit and the rest are used for 110. So i also want to install my Samlex Solar PST-1500-12 PST Series Pure Sine Wave Inverter through this transfer switch. I'm following this thread enough to NOT want to jumper the two hots in a 4 wire cord. Would it be safe if i wire the three wires from my inverter output to just 3 wires in the 4 wire cord that plugs into the inverter? So basically I just get to use 5 of my 10 circuits (really only 4 since I wont turn on the 220v when running on the inverter)?

  • EstragonEstragon Posts: 2,327Registered Users ✭✭✭✭
    IMHO, it would be safe (if I'm understanding what you have in mind properly).

    Not really a safety issue, but when running the genny, all 120v loads will be run on one leg of the 240v. If the loads aren't very big, it shouldn't matter much, but if you run a lot of big 120v loads it might be hard on the generator.
    Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
    Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
  • hdoylehdoyle Posts: 2Registered Users
    edited June 6 #8
    Sorry I could have been clearer. I have a four prong 120/240 cable I use to connect my generator to the transfer switch. I need to create a new seperate cable to connect my inverter to the same transfer switch. Since my inverter hard wire output is standard 3 wire (black, white, ground) for 120v but the outlet for my transfer switch is a 4 prong to provide two hots for 240v, I'm thinking I can only safely provide power to one hot prong on the four prong plug. Thus only one side of my transfer switch will be active. Which is fine with me as long as it's safe. To be really clear I'm not touching the wiring of the transfer switch itself.
  • BB.BB. Posts: 27,444Super Moderators admin
    Assuming your wiring is designed to carry both the hot current (black, or black+red) current and the return (white/neutral) current, and you don't have any 240 VAC appliances you want to run from the inverter, you could tie both black+red together in the inverter->transfer switch plug just fine.

    Or, you can rewire the breaker box to have only the circuits you want to run on the (for example) Black/L1 hot lead when running from the (smaller) AC inverter. And wire the non-op on Inverter circuits to the L2 bus bar. Your choice.

    Of course--You want to ensure there is no combination of Transfer Switch and AC inverter to main panel connections where you could connect the AC inverter and/or AC genset and/or AC mains together.

    Assuming your transfer switch goes before the Main AC panel and your Genset/AC inverter input is connected there (and AC mains disconnected by transfer switch). You should be fine.

    If you are not sure, ask more questions, or get somebody to help to ensure you are safe (as always, we are just responding to what you have written--Want to make sure you and your family are safe).

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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