over amps on multi branch

ws9876ws9876 Solar Expert Posts: 372 ✭✭✭
if you wire a sine inverter directly into a mains panel,disconnected from grid, and you have multi branch circuits you can have trouble with excess current on the multi branch neutral wire. But you would be ok if you never exceed the amp rating of the breaker even when using 2 different outlets..,yes?? I mean if you never exceed 15 amps then the single neutral wire,12-14 ga. will handle that ,yes??? THis is all providing that the inverter is ok for doing this,ie ground issues etc..

Comments

  • froggersixfroggersix Solar Expert Posts: 35
    breaker isn't on neutral its on hot so if you connect two hots to inverter and both go back through same neutral then each hot is protected but neutral isnt. like using 30 amp hot to 15 amp neutral.
  • Ethan BrushEthan Brush Solar Expert Posts: 231 ✭✭
    ws9876 wrote: »
    if you wire a sine inverter directly into a mains panel,disconnected from grid, and you have multi branch circuits you can have trouble with excess current on the multi branch neutral wire. But you would be ok if you never exceed the amp rating of the breaker even when using 2 different outlets..,yes?? I mean if you never exceed 15 amps then the single neutral wire,12-14 ga. will handle that ,yes??? THis is all providing that the inverter is ok for doing this,ie ground issues etc..

    if you put both legs of the multiwire on the same breaker you would be the proper and code compliant way to go assuming you didnt actually need the full 30/40 amps. Due to the likely diversity of the loads and the fact that the"14 gauge 15 amps, 12 gauge 20 amps" rule is an additional "small conductor" de-rating which is much less than the actual table 310.16 ampacity used for most other conductors, I would almost certainly be fine on separate breakers anyway.
  • inetdoginetdog Solar Expert Posts: 3,123 ✭✭✭✭
    This really only comes up if you take a breaker panel which was originally wired for 120/240VAC with two hot buses of opposite polarity. That is the only situation in which a MulitWire Brsnch Circuit (MWBC) would be allowed in the first place.
    And then you feed it (via a transfer switch) from the output of 120V inverter without using a transformer, so that both buses are connected to the same hot output from the inverter.
    If you are going to do that, you will need to rewire all of your MWBCs to have two separate neutrals. Doubling the size of the neutral will also resolve the safety issue, but will still not be code (NEC) compliant.
    SMA SB 3000, old BP panels.
  • BB.BB. Super Moderators, Administrators Posts: 29,518 admin
    Remember that main panels are designed for 120/240 VAC split phase power from the utility (in North America, we have a Line A and a Line B connection, plus the ground bonded neutral). Since the Line A and Line B are 180 degrees out of phase, when you connect loads from Line A to Neutral and Line B to Neutral (120 VAC for Line A to Neutral, 120 VAC for Line B to Neutral, and 240 VAC from Line A to Line B), the neutral only carries the "difference between" Line A and Line B current (10 amps on Line A and 8 amps on Line B, means that 10-8=2 amps flows on Neutral).

    If you have only a 120 VAC inverter and connect Black from the Pure Sine Wave inverter to Line A+Line B (A and B shorted together), if you have any common neutrals in your house wiring (Red+Black+Neutral + Green/Bare copper ground), then Line A + Line B will add and the common current will over current the neutral wire.

    Similar if you have two 120 VAC PSW inverters that are not "Stacked" (designed for 120/240 VAC operation). If you connect one inverter to Line A + Neutral, and the second to Line B + Neutral, the inverters do not coordinate the phases and you cannot guarantee the Line A and Line B are 180 degrees out of phase to each other--So it is unsafe to use a common neutral in this configuration too.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • ws9876ws9876 Solar Expert Posts: 372 ✭✭✭
    anyone have a creative way to bypass MBCs and install new outlets thru closed walls...??
  • solar_davesolar_dave Solar Expert Posts: 2,343 ✭✭✭✭
    ws9876 wrote: »
    anyone have a creative way to bypass MBCs and install new outlets thru closed walls...??

    wouldn't it just be easier to put a center tap transformer in?
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