240V inverter neutral current rating

I am trying to determine which article of the electrical code applies to the neutral conductor rating when connecting a 240V inverter to the grid. Some inverters have a neutral lug but will work without any connection to it since it is not really a split phase output and no current flows thru the neutral. The 240V output from the inverter can connect directly to the L1/L2 terminals on a service panel. I was told this by Fronius and SunnyBoy tech support. The neutral can be connected if desired or required by code but carries no current.

If a neutral is connected how would one determine the ampacity of the neutral conductor. For example if the 240 volt L1/L2 output required a #8 Cu conductor does the neutral need to also be #8 or could it be a #10 or smaller.




  • BB.BB. Posts: 27,689Super Moderators admin
    Re: 240V inverter neutral current rating

    For a 120/240 VAC split phase set of loads--Your branch circuit neutrals need to be sized to the Branch Breaker/Hot wire sizes since return current = source current.

    Assuming you have a PSW (pure/true sine wave) Inverter with isolated output tied to an isolated sub-panel... You can tie the inverter center tap to the AC Main Panel with the same gauge/capacity cable as the Inverter's main output.

    But comes the "it depends/complicated" questions. Is the stand a lone panel always separate from the Main Grid/AC Panel? Then you could, instead, tie the sub panel Neutral+Ground bonds together, and run a ground back to AC main panel ground or the main Ground Rod.

    If the sub-panel is sometimes powered by the Grid (transfer switch, or AC inverter with transfer switch)--Then the Neutral from the AC panel needs to be the "same rating" as the Black+Red breakers/cable to the AC Inverter (or transfer switch) input. And the Green Wire should be carried through from the Sub-panel ground to the main panel ground. And the sub-panel Neutral should be floating.

    It gets even more complex because some AC Transfer switches/inverters provide their own local neutral+earth ground bond (when running off grid) and smaller gensets (less than ~3 kW or so), have floating neutrals (and larger gensets generally come from the mfg. with Neural+Earth Frame bonded in the genset).

    Clear as mud?

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • inetdoginetdog Posts: 3,121Solar Expert ✭✭✭✭
    Re: 240V inverter neutral current rating

    A 240 volt only inverter which has a neutral for reference only will be a grid tie inverter only.
    Any imbalance between the loads on the two phases will just be made up by the utility.

    You definitely need to pay attention to load balancing and provision of a solidly connected neutral when you try to produce 240V for off grid operation (in the US anyway, where 120-0-120 single phase three wire 120/240 is common.) There are several good threads on load balancing, autotransformers, etc.
    SMA SB 3000, old BP panels.
  • TroySmith80TroySmith80 Posts: 10Registered Users ✭✭
    edited April 9 #4
    I'm just learning, but have a similar question as OP. 

    It seems that Bill's response is tangential to the question, since it doesn't seem to address grid-tie. 

    My question, is just if it's safe/legal/functional to run a 240v inverter (or, more likely, several in parallel) to the L1 and L2 without connecting a neutral line. The primary reason i'm interested in doing this in cost savings on wire. I'm working out how i want to setup a ~20kW array that will probably be sited 300 to 500 feet from the service drop. In the research i've done so far, it looks like triplex wire (L1, L2, G) is substantially cheaper than quadruplex (L1, L2, N, G), like more than just the expected 25% cheaper. I don't want to pay for more wire/conduit than is necessary.

    Does the inverter need the N line to be able to properly match the grid power frequency and such, or some other "reference" purpose as inetdog says? If it is still required, can it be MUCH smaller than the current carrying L1 & L2?
  • EstragonEstragon Posts: 2,615Registered Users ✭✭✭✭
    Personally, I'd ask the utility what they expect/permit in hooking up to their grid, and work with that.
    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
  • mcgivormcgivor Posts: 2,059Solar Expert ✭✭✭✭
    Estragon said:

    Personally, I'd ask the utility what they expect/permit in hooking up to their grid, and work with that.
    Good answer 

    The utility is generally responsible up to the point of demarcation, the main disconnect, rules and regulations which apply up to that point are different from those on the consumer side, or within the dwelling, which must conform to the electrical code for safety reasons. The purpose of the neutral is to carry unbalanced load, emphasis on load, feeding into a grid is not a load, so perhaps the neutral is redundant. Please note I'm not suggesting one way or the other, just attempting to rationalize the situation.
      1500W, 6× Schutten 250W Poly panels , Schneider 150 60 CC, Schneider SW 2524 inverter, 8×T105 GC 24V nominal 

  • BB.BB. Posts: 27,689Super Moderators admin
    My older (first) GT inverter did not have a neutral connection. My replacement did and the installer did not run a neutral but just a wire from chassis ground to the neutral connection of the inverter.

    For my GT inverter, the neutral connection is just a voltage sense line (no appreciable current flow). It is (I guess) required by the newer codes to detect a failed neutral/earth bond and will shut down if the 120/120 voltage to neutral falls outside of specifications (I guess if the voltage is outside of ~100 to ~132 VAC range?). The intent is for the GT inverter to shut down if there is "anything wrong" with the voltage or frequency of the AC Mains.

    So, functionally, the neutral wire does not need to be sized to the L1/L2 size. And you can use the ground wire as a neutral substitute in some situations (don't know if inspectors will accept).

    The practical side of an installation with a long cable from the GT inverter to the Mains panel (and utility transformer)--We have seen problems where a person has a relatively high AC line voltage (over 250-255 VAC) and the voltage "rise" of sending power from the AC inverter over the long cable run can cause the GT inverter to shutdown due to over voltage (GT Inverter will shutdown at ~260-264 VAC--Although some GT inverters have a service person adjustable shutdown point). 3% voltage "drop" is considered normal max for AC wiring... 3%*264 VAC = ~7.9 VAC... And if your AC mains are over ~250 VAC, it can be an issue.

    One way around the issue of long wire run is to install the GT inverter at the home/AC Mains panel, and run high voltage DC from the solar array to the GT inverter... You can run a nominal 300-400 VDC from the array to the GT inverter over long distances without the voltage drop trip issue, and running at higher than ~240 VAC nominal, you can use smaller diameter cables (save copper costs). This works well with central GT inverters, but not for micro GT inverters (micro inverters are installed one per solar panel at the panel, and AC branch circuit ran on roof).

    There can be other code/safety issues (sending DC solar power long distances, issues with remote shutdown, arc fault protection, lightning, grounding)... Many times, it takes several paper designs to work out the optimum costs and safety issues.

    Have you worked out with the utility their system issues (in California, >10 kWatt solar systems are in a different class and have different rate plan and sometimes connection requirements). Have you looked at your local utility's solar rate plan? Utilities are starting to work hard at reducing the amount of money they will give you for solar power (especially if your solar output exceeds your monthly/yearly kWH energy consumption). And if you are commercial power customer (such as farm), different rate plans again (and items like poor Power Factor penalties, max power flow limitations/billing steps, etc.).

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