Generator Question for an Electrician....

buggeyesbuggeyes Registered Users Posts: 15 ✭✭
We have a backup generator for our off grid system. The plug coming from our system to that generator is a Leviton L6 250V 3 wire plug with a ground (G), and 2 hots (X) & (Y). When we checked we saw that our system has the green on G, the white on Y and the black on X.

We want to make an extension cord just in case our back up generator needs to be serviced.  The wire from the house is cut exactly to the bolted down backup generator with no room for play - so an extension cord is what we figured we would need to make. 

Our backup backup generator offers 2 different plug options, a 3 wire 120V and a 4 wire 120/240V (looks like Leviton L14). Since our current generator supplies our house with 220V we figure we need to make the extension cord to go from our 3 wire house plug to the 4 wire 250V

So how do i wire this? I bought a 4 wire cable - a female L6 (for our house system to plug into) and a male L14 (to plug into our backup backup generator) The L14 offers a ground, neutral and 2 hots. 

If someone could please enlighten me as to how to properly wire that would be awesome. Just want to make sure I do things correctly as I am still learning all this electrical stuff. I would almost assume that I should ignore the neutral and just hook up the ground and 2 hots on both plugs.
Thanks!!

Comments

  • oil pan 4oil pan 4 Solar Expert Posts: 767 ✭✭✭✭
    edited December 2016 #2
    Chances are your generator isn't setup correctly at all for building power tie in.
    The generator comes from the factory with the neutral and ground bonded on the generator somewhere.
    This is correct for stand alone power, if just have some things plugged into the generator.
    That's how they are intended to be used.
    To do a generator tie in the neutral ground bond needs to be severed.
    So there is no right way to tie in because the generator and house are not wired correctly.
    Your house shouldn't have a 3 prong plug for generator tie in. It should be 4 prong, 4 wire.
    I have the neutral ground bond on a light switch for stand alone or tie in, and a 4 wire tie in for my house.

    Solar hybrid gasoline generator, 7kw gas, 180 watts of solar, Morningstar 15 amp MPPT, group 31 AGM, 900 watt kisae inverter.

    Solar roof top GMC suburban, a normal 3/4 ton suburban with 180 watts of panels on the roof and 10 amp genasun MPPT, 2000w samlex pure sine wave inverter, 12v gast and ARB air compressors.

  • EstragonEstragon Registered Users Posts: 4,496 ✭✭✭✭✭
    I'm not an electrician, and if I tried to pull a permit for my cabin in the boonies I'd likely just get funny looks.  That said, it seems to me you need to wire both generators into an appropriately rated transfer switch (gen 1 / Off / gen 2).   I think without the switch you'd be backfeeding a generator.  Don't know what that would do, but probably nothing good.  

    You'd wire the backup gen plug with just the two hots and ground, same as the main generator.  The ground will be tied to neutral (one hopes) in the main panel in the house for 120v loads there.  Assuming the generators and main panel are close together, local ground at the generator should be at or very near ground potential at the panel.  Any 120/240v loads (eg. 240v stove with 120v clock) in the house would use local neutral.

    As for lifting the neutral-ground bond on the backup generator, I'm not so sure.  Grounding generally, and generator grounding in particular often confuses me.  I think you could leave the bond in place so any 120v loads plugged in locally to the generator would fault overcurrent devices in the generator.  There would be no neutral going from generator to house, so the neutral-ground bond would be local to each location.

    Anyone see a problem with such a setup?
    Off-grid.  
    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
  • BB.BB. Super Moderators, Administrators Posts: 32,319 admin
    Roughly, smaller gensets (i.e., 3 kWatt or so portables) tend to have green wire ground isolated from white wire/neutral (floating neutral).

    Larger gensets tend to be thought as the source for all power, which many times, plugs directly into the genset itself. So they have green wire and white wire bonded together (and the frame of the genset is supposed to be tied to a ground rod).

    AC mains for homes/businesses, tend to have the Green wire and White wire bounded together in the main panel (one point) (technically, I believe that most pole mount transformers have green+white bonded at the pole to a ground rod at the base of the pole--And assuming that a single transformer connects to 2-5 local homes, the actual green wire+white wire bonding does take place at multiple locations (pole and 1 for each home). So we do "lie" about the single point grounding of AC mains systems...

    You do want a single point of green+white bonding so that you have the ability for a short circuit (such as Hot to Green wire/earth ground) to find a path back to the white wire and trip the breaker.

    What you do not want is to have a (for example) a green+white wire bond at the genset, Hots+white+green wire to main panel, then another green wire+white wire bond in the main panel. That configuration would let current share between the green wire and white wires (parallel conductors). We do not want normal load currents to flow in the green wire--Only "fault" currents to flow in the green wire.

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