3kw Powmr 48v inverter -> transfer switch general questions

I have a couple questions about my setup before I connect everything and power it up...  

First, this system is on my home, which is connected to grid power.  My plan is to have a 3kw solar panel system on the roof, that is connected to a Powmr inverter / charge controller.  The Powmr is connected to a 48v 10kwh battery pack for storage.  The Powmr inverter is a 110v 3500w single phase inverter that I want to connect to a transfer switch.  I have already installed the transfer switch to the subpanel in my garage and connected 10 critical 110v circuits (bathroom, kitchen, etc)

In the documentation for the transfer switch, it says to use a Floating Neutral generator.  My worry is that I have no idea what the Powmr inverter is... I have checked their website and the manual, but I don't see anything.  Is there a way to test this?

As well, I assume that I can pick and choose which circuits will run on the inverter and which will run on mains, correct?  That way, my lights and small load stuff could be on the inverter, and the larger stuff could be running on mains.  Any problem with this?  

I have never used a transfer switch before, so I am just double checking and tripple checking before I connect the inverter to the transfer switch and switch one of the circuits from mains over to gen position.  



  • BB.BB. Super Moderators, Administrators Posts: 32,315 admin
    edited December 2021 #2
    Welcome to the forum Tremmert,

    I looked through the manual and other information at your link to the hybrid inverter... It does not talk about system grounding anywhere.

    What would I do? (this is your system--Verify my suggestions before doing anything--I have no special knowledge here)

    First, take your Ohm meter and measure the resistance between Chassis ground (metal bonding screw on exterior of chassis) (note: no external wiring connected to AC in or AC out connectors--Don't want genset or main home panel to affect Neutral+Ground connections) and:
    • On the AC Input power connector:
    • Check left terminal (labeled as GND in manual) and see that you have near zero Ohm resistance (ground=>ground---Should be "the same gnd)
    • Check right (#3 terminal) labeled as Neutral and you will have either near zero Ohm resistance (Neutral bonded to Chassis Ground) or you will have >>100's of Ohms of resistance (NO Neutral to Chassis ground bond).
    • On the 120 VAC Output power connector:
    • Check external Chassis Ground to right hand "Neutral" terminal (as labeled in manual). Either near zero Ohms or >>100's of Ohms of resistance (non-bonded N+G).
    Are you using AC power from Genset or AC/Utility Mains?

    Genset: In this setup, the genset should have "floating" Neutral (no N+G bond). Check with meter (or genset manual)--If there is a N+G bond, I would suggest to "lift" the N+G bond connection (larger gensets "assume" they are the only AC energy source and Bond N+G. Smaller gensets (and AC inverters) usually have Neutral floating (no N+G bond).

    AC Utility Power: Technically we treat AC utility power as "floating" Neutral--Even though it is usually tied to ground rod at the pole transformer.

    AC Main Panel for Home: Generally AC main panels have a screw that "bonds" Neutral Bus bar to Earth Ground (chassis or earth ground bus bar).

    The NEC defines that all AC loads go through L1/L2/Neutral wiring. If you have tow or more N+G bonds (say in AC inverter and AC main panel), then you have parallel current paths (Neutral in parallel with Green wire/conduit safety ground). And we do not want that. Floating Genset and AC Inverter Neutral and having only one N+G bond in the AC main panel is the typical method to address this issue.

    As a first pass--Multiple N+G system grounds in your home power system will not cause major problems.

    Second pass A--If you have two or more N+G system ground (parallel current flow)--It can cause weird effects... Such as causing circulating current in the Main AC panel sheet metal (transformer coupling from 2 parallel current wires going into sheet metal between two separate holes--Can induce current in sheet metal and cause heating in chassis sheet metal.

    Second pass B--If you have a N+G bond in the genset and N+G bond in the main AC panel, If there is a GFI (ground fault interrupte) outlet or similar--This looks like a "ground fault" and will trip the GFI on the genset.

    Regarding the Transfer Switch:

    I guess that your main AC panel is 120/240 VAC split phase power (standard North American home wiring).

    This inverter (and genset, if used?), is a 120 VAC single phase inverter. So you will only wire 120 VAC circuits for backup power (yes, is usually obvious, but just want to be clear).

    This transfer switch is a 120/240 VAC split phase type. You have two options--One is to only use 1/2 of the transfer switch and use 120 VAC wired to the "black" circuits (lose the "red" circuits).

    Or wire AC inverter L1 / N as" L1 to both "black" and "red" circuits--And only support 120 VAC loads. And this is probably what you will plan on doing.

    HOWEVER, this does run into an issue.

    For 120/240 VAC split phase power... The neutral current is the "difference" between L1 an L2 (i.e., if you have 10 amps on L1 and 6 Amps on L2, you have 4 amps on Neutral).

    Where this can be an issue... For example you can get 2 wire ROMEX (Black, White Neutral, and copper ground wire) or 3 wire Romex (Back, Red, White Neutral, and copper ground wire).

    With 120 VAC power... The Neutral is only carry the max rated current (14 AWG @ 15 Amp rated). Since Neutral current is L1-L2=N, that is fine.

    If you have 120 VAC single phase L/N from inverter tied to L1/L2/N, the current in the neutral is L1+L2=N ... So if you have 15 amps in L1 (black) and 15 amps in L2 (red), then the "return current" in the White Neutral is now 30 Amps. And 30 Amps exceeds the 15 amp rating of 14 AWG wiring.

    Since both 2 wire and 3 wire ROMEX circuits (and older knob and tube) are common in homes--You have to check and see what circuits you plan on supporting for emergency backup to ensure you don't over current a common neutral in 3 Wire Romex. (2 wire ROMEX will not be an issue here).

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • tremmerttremmert Registered Users Posts: 8 ✭✭
    I can't thank you enough for such a thorough and detailed response.  I enjoy learning this type stuff, and it sure can be tough to find good help online.. (or locally!)

    I ran a few tests as per your request:

    Resistance from Chassis Ground to:
    • AC Input - Ground -- 0 Ohms
    • AC Input - L1 -- Infinite Resistance
    • AC Input - N -- Infinite Resistance
    • AC Output - Ground - 0 Ohms
    • AC Output - L1 -- Infinite Resistance
    • AC Output - N -- Infinite Resistance
    I plan on using Mains as the AC input into the inverter (and battery power as well).  When the battery bank is low, my solar panels will keep them charged.  At night, if the batteries are low, then Mains power will recharge the batteries if depleted.

    So based on your description, I believe that I have a "floating neutral" inverter.  As well, I checked in my main and sub panels, and it appears that only my main panel has the N-G bond.  So I assume all is well here. 

    Correct, my main AC panel is 120/240 VAC split phase power.  My plan was to run L1 to both "black" and "red" circuits inside the transfer switch.  Actually, I was just going to install a jumper between L1 and L2 inside the l14-30r plug that will plug into the transfer switch (from the inverter).

    I am still a little confused on the amount of current coming back on N.  Is there a way to verify this?  For example, if I remove the cover to the sub panel, is there anything that I can look at to identify my wiring?   (Attached is a photo)

    Thank you again for your help and detailed descriptions, it is greatly appreciated.
  • BB.BB. Super Moderators, Administrators Posts: 32,315 admin
    So far so good... Floating Neutral in AC inverter. If you have an backup genset--Need to confirm that it has floating Neutral (or figure out how to lift the N+G bond in genset--In theory, if you use the genset as a portable, you are supposed to reconnect N+G bond--Although, many smaller than ~3.5 kWatt gensets and AC inverters default to floating output).

    For a 14-30 NEMA plug--30 Amp rating so you should use a 10 AWG "jumper" between L1 and L2--I do that on the male plug that goes into the 120 VAC genset outlet (as this will alway be 120 VAC and no danger of shorting a 120/240 VAC outlet (say a bigger genset with 120/240 VAC output).

    To identify ROMEX... I believe that White ROMEX sheath is 14 AWG, and Yellow ROMEX sheath is 12 AWG (I am not an electrician). 2x wire with ground:

    And 3x wire + gnd ROMEX looks like this:

    Again, I am no Electrician... But from what little I have seen is BLK+White is 2 wire ROMEX, and BLK+RED+White is 3x ROMEX. And the B+R+W 3 wire having a common neutral being the "issue" with possible over current in the White/Neutral.

    Make sure you match the 14 AWG @ 15 Amp wiring with your 15 amp Transfer Switch Circuits and 12 AWG @ 20 Amp wiring with 20 Amp TS circuits (If you can).

    You have to trace back to the ROMEX sheaths to see which sets are Paired. And that L1/L2 pairing of B+R needs to trace back to your breakers too... You do not want two breakers on L1 to feed B+R+W sets in ROMEX (same issue, possible overheating of Nature).

    The link to your photo has "expired", so I cannot confirm (the details) that the breakers are wired correctly for B+R pairs (L1+L2, not L1+L1 or L2+L2 breaker pairs).

    You can post photos directly to the forum too.

    All this make sense? I/We want you to be safe. Ask questions if not making 100% sense.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • tremmerttremmert Registered Users Posts: 8 ✭✭
    Hi Bill,
    Hope you had a great New Year!  As well, thank you again for the detailed response, it is greatly appreciated...

    1) I do have a genset (9kw), and I did verify that it is also a floating neutral.  

    2) I am not home to verify, but below is a photo.  I will take a closer look at the Romex when I get back to the house and report back..

  • tremmerttremmert Registered Users Posts: 8 ✭✭
    Looking at the above photo, it appears that I only have one circuit (lower left) that would be L1+L2=N (this is my electric dryer).  I don't plan on having this circuit connected to the transfer switch.  Also, I only see 2 red wires, one for the main bus bar and the other going to the electric dryer circuit.  Is my thinking correct?
  • BB.BB. Super Moderators, Administrators Posts: 32,315 admin
    I agree. Looks like the balance of your wiring is B+W+G... No other "common" neutrals to worry about.

    This looks like a sub panel off of your main panel (i.e., garage sub panel or similar?). The "mains" feed cables do not look very heavy... Do you have enough power (30+ Amps @ 120/240 VAC) to power your current/planned loads off this sub panel? The electric dryer itself could be 20 Amps...

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • tremmerttremmert Registered Users Posts: 8 ✭✭
    Yep, you're correct.  This is a sub-panel located in my garage.  I am not sure what gauge that wire is, but this was installed when we built the house (2010).  So far, we have never had any issues.

    I have a Emporia power monitor connected on the main panel outside so I can see exactly how much power is being consumed on each leg.  With the transfer switch connected to 10 circuits, I am expecting a maximum of 1kw on each leg.  I don't have any plan of connecting things like the water heater or electric dryer...

    So overall, I assume your opinion is that I am safe to proceed and wire up the transfer switch?

  • BB.BB. Super Moderators, Administrators Posts: 32,315 admin
    No red flags...

    Of course doing this yourself may be controlled by your local building department/licensed electrician requirements...  :#

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • tremmerttremmert Registered Users Posts: 8 ✭✭
    Actually, I do have one last question...  This transfer switch is only switching over the hot and not neutral.  So that means if the mains is ON, there would be utility voltage and my generator voltage on the neutral.  Is that correct thinking?
  • BB.BB. Super Moderators, Administrators Posts: 32,315 admin
    edited January 2 #11
    I believe that the generic transfer switches only switch L1 and L2... Neutral would not normally be switched (for RVs and Boats with shore power/genset optional power, there are transfer switches that do switch neutral/ground bonding).

    Is "there voltage" present... To measure voltage, you need two points to measure it at.

    In North America, we "bond" Neutral+Ground in one location (details are different discussion). That means that "Ground" is zero volts, and now Neutral is bonded to Ground and has zero volts too (relative to cold water pipes, electrical boxes, etc. in house).

    The general answer is there should be Zero Volts on the Neutral of the house wiring, and the Neutral from the genset is also connected to house Neutral, so Zero Volts too... And since the genset input to the transfer switch, and the attached genset, share the same Neutral/White wire--They should all be near Zero Volts (and sharing the green wire safety ground too). Note that Neutral has current flow, so you can have upwards of a few volts of "voltage drop" between various neutral wires and "ground".

    In practice... Trust but verify. Assume any/all electrical connections are hot until you have verified zero volts.

    Some examples.

    Digital Volt Meters (DMMs) have very high input resistance (1-10 MOhms or so)... That means that you can have a static electric charge or even acting like a radio antenna for simple piece of wire otherwise unconnected can look like it has a non-zero voltage (note that AC vs DC settings on volt meters can give different "answers"). But virtially zero current available (more or less, 5 mAmps (0.005 Amps) or less is "safe" for humans.

    Zero Volts grounding can be different in different areas of a large building. A somewhat common issue with large builds, these different grounds can actually drive several amps of current between two grounds (have seen old RS 232 "dumb terminals" burn out their digital ground to safety ground PCB trace burn on (such as between Terminal 120 VAC "ground", and Printer 120 VAC "ground").

    In an industrial setting, I have gotten shocked from two different ground rods ~60 feet apart... >60 VAC between two ground rods. This was a salt water aquarium on San Francisco Bay Fill with multi HP (large) salt water circulation pumps... Enough AC leakage current to ground to create a shock hazard between the two ground rods.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • tremmerttremmert Registered Users Posts: 8 ✭✭
    Bill, I can't thank you enough for sharing your expertise.  If you ever make it out to Texas, I would love to buy you a beer.

    I will keep you updated on my progress.

    Take care and happy new year!
  • BB.BB. Super Moderators, Administrators Posts: 32,315 admin
    Thank you for the kinds words and offer to visit. It has been a decade since I have done much traveling--But hope to do more in the next few years.

    Let us know how it all works out for you (the good, bad, and the ugly.)  :)

    Best wishes,

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • mike95490mike95490 Solar Expert Posts: 9,511 ✭✭✭✭✭
    I like the soft wire insulation as the wire bundle leaves the armor cable raw edge.  Eventually that will get exciting.  Unless there is an invisible plastic bushing on the end of the armor
    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 ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister ,

  • tremmerttremmert Registered Users Posts: 8 ✭✭
    edited January 15 #15
    Hello Bill,
    Hope you have been well.   I do have one more question for you....

    Tonight I decided to try the transfer switch.......    

    If you look at my photo above, you will see a GFCI outlet directly below the Transfer Switch.  I wired an extension cord to a L14-30r plug (connected both L1 and L2 inside the plug) and plugged it into the Transfer Switch.  Once I plugged it into the GFCI outlet, the GFCI immediately tripped.  

    All loads on the transfer switch were in the MAINS position.   (I recall reading in the manual that the "generator" must be a floating neutral.)
    I am still super hesitant in connecting my solar inverter to the transfer switch and though this would be a good, "safe" test to confirm all is well.  

    Any thought on the GFCI tripping?  Could this mean I have something wrong?
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