Taking one circuit off grid and grounding

micahvmicahv Registered Users Posts: 17
What is the best way to isolate one circuit from my house in the main breaker box so I can power it with my PV system I'm cobbling together? This system comprises of 205W Kyocera panel, Morningstar MPPT CC and Inverter and a couple of the CostCo 6V Batteries.


http://tinyurl.com/ykpw8bn picture of my house circuit box.


I identified one circuit in my house that I feel I should be able to power with my system. So now I want to take my inverter output and tie it into that circuit. My plan is to get a separate breaker box to house that circuit as I don't think I can isolate this circuit in my existing household breaker box?

My next question is grounding. This is still confusing to me. So I will have a equipment ground and a conductor ground. Then at some point these two are bounded together and then go to earth. So looking at my existing household circuit breaker one can see that equipment and conductor ground is bounded together. Something I keep wondering is why do they no just run 2 wires from the pole if they are just bonding 2 together in the box? Doesn't this effectively make them the same?

Anyhow, so for my little system I will have an equipment ground running from the frame and rack of my panel into my midnight solar 125A MNDC breaker box. The Morningstar Charge Controller is DC Negative ground, so this is where I will tie all my grounds together and run to earth? Negative conductor and equipment tie into ground bus? Can the earth ground from my midnight solar breaker box go to my house circuit breaker ground? I read about some issues with inverters and grounding wrong?

I still don't understand ground since it all seems to tie together at some point and run to earth....and yet it is so complicated, one of the biggest sections in the NEC. When I look at Chap. 11 of my Photovoltaic Systems, Fig 11-30 it shows a picture of a grid tied system with equipment and conductor ground all bonded together. It sounds simple, tie all my ground wires together and run to earth, but sounds like it isn't this simple?

Thanks again in advance.

Comments

  • icarusicarus Solar Expert Posts: 5,433 ✭✭✭✭
    Re: Taking one circuit off grid and grounding

    You need a transfer switch, like those made by Gen-tran. http://www.gen-tran.com/

    You could also use a transfer switch like this:http://store.solar-electric.com/ioitauactrsw1.html

    I'm not sure the latter is UL/NEC compliant.

    As for grounding and the NEC, you are very correct. It is very complicated and is tied both to system performance, as well as system safety. (I don't profess to know much about it). I do know that there are issues between neutral wiring (which is generally grounded) as well as system grounding which is also grounded. There are cases that I have seen when one aspect of the code seems to conflict with others. You best bet is to consult a good EE or you local electrical inspector.

    Tony
  • BB.BB. Super Moderators, Administrators Posts: 32,008 admin
    Re: Taking one circuit off grid and grounding

    For safety sake, you probably need to leave the neutral and green wire grounds/bonds as they are and just pull the one "hot lead" and connect it to a transfer switch (like suggested by Tony).

    Part of the grounding issue will be addressed by the type/brand/model of Off-Grid inverter you purchase...

    Most MSW (Modified Square/Sine Wave) inverters do not have isolated outputs. If you connect one of the AC leads to ground (i.e., bonded neutral to earth ground) AND connect one lead of the battery bank to ground (typically the Negative Lead)--there will be a dead short through the inverter that will cause it to go "pop".

    Most TSW/PSW (True/Pure Sine Wave) inverters have isolation between the AC output and the DC input... So, a TSW inverter will be able to connect you your transfer switch+grounded neutral without any problem.

    Green wire bond the case of the transfer switch to the AC Panel safety ground and all should be fine. The green wire should be heavy enough to handle the maximum battery output power rating (generally defined by the circuit breaker or fuse on the battery + output)--normally a minimum of 6 awg.

    For the DC side, with a TSW inverter, you should run a green wire ground from the battery to the ground rod/cold water pipe--In one location only. The problem becomes if you have multiple grounds between battery negative, and some other devices (such as 12 volt radios/transmitters) where their chassis (and antenna) leads are intended to be ground return (such as for mounting in a car which uses the chassis as both a ground and a power return lead).

    Pick your inverter and look through its manual and/or post its information here and somebody can probably help you find the information. In some cases, you may have to call Tech Support for the answers.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • RCinFLARCinFLA Solar Expert Posts: 1,398 ✭✭✭✭
    Re: Taking one circuit off grid and grounding

    What inverter do you have or intending to get?

    If you have not got one yet, get one that allows neutral grounding. Other decision is whether to get a sinewave or modified sinewave inverter.

    The power company actually brings in three wires. The two insulated lines are the 240 vac feed. The support, uninsulated cable that the two 'hot' lines are wrapped around is the neutral return. Think of the neutral as the center tap of a 240 vac transformer. It carries some of the 120 vac load return current.

    Code requires a single grounding point of the neutral at the main breaker box, point of entry to your house. All 120 vac plug neutral lines (white wires) come back to the neutral bus bar at the breaker box.

    The two 240 vac hot lines are commonly called L1 and L2 lines. Your breaker box has two rows of hot bus connections for the breakers to connect to. L1 feeds one hot bus, L2 feeds the other. The breakers allow connections to either L1 or L2 for the 120vac loads and L1 & L2 for 240 vac loads. Usually the 120 vac outlets are attempted to be distributed evenly between L1 and L2 connections.

    120 vac loads should be all black wires, sometimes you find red wire used. 240 vac loads are red and black wires. White wire is neutral return line that connect to the neutral bus bar off to side of box. Green wires or bare wires on Romex are ground wires from outlets that are connected to box ground bus bar.
  • icarusicarus Solar Expert Posts: 5,433 ✭✭✭✭
    Re: Taking one circuit off grid and grounding

    RC makes a good point that I had forgotten. (and Bill)

    Most cheap inverters have a "floating" neutral, and they go up in smoke right now if you wire them to a grounded neutral buss bar/panel. I know of no MSW inverters that permit hardwiring to a grounded neutral.

    There are a few small true sine wave inverters (Morningstar sure sine and some Exeltechs) that require grounded neutral. (there may be others as well).

    Also, consider what loads you are going to put on the circuit. I suggest that you bite the bullet and buy a good inverter now and avoid damage that may be caused by a MSW inverter. (Many loads don't like MSW including SOME electronics and many motor loads.)

    Tony
  • micahvmicahv Registered Users Posts: 17
    Re: Taking one circuit off grid and grounding

    Thanks for the advice, why a transfer switch if I am trying to totally isolate the one circuit from the utility line? I thought a transfer switch was to allow one to toggle between two power sources without interruption. For my project I just want to dedicate this circuit to PV only and if I run out of juice no big deal. This way I can get a better feel for off grid and real energy usage.

    As for my inverter I got the Morningstar SureSine 300. Looking through the manual it appears that the neutral ground is "bonded" to the earth ground which is then run to earth via a 4# wire. The AC feed uses 12#. A diagram in the manual shows a conductor running from AC neutral to Earth ground terminal on the DC side(not sure what conductor on DC is getting grounded), is this done internally or do I need to connect these myself.

    I'm still confused by grounding and the how equipment and conductor ground are even different if they are bounded together. Doesn't bonding make them effectively the same conductor? So if AC neutral out from the Inverter is bonded to the Earth ground of the Inverter can I just run this 4# cable from the Inverter Earth ground to my ground rod, or ground bus in main breaker box? I guess some of the confusion is messing with my existing breaker box, I should remove all wiring from my main breaker box for this one circuit and place in a separate breaker box, and then run the ground wire, to ground rod? or main breaker box?



    Thanks again
  • Solar GuppySolar Guppy Solar Expert Posts: 1,973 ✭✭✭
    Re: Taking one circuit off grid and grounding

    The requirement of #4 wire for the ground is a misprint in the instructions for the SureSine, using the same Gauge wire as the AC connections is fine.

    As for isolating a circuit from the main panel, it can be done but is not NEC compliant for a number of reasons but at least with the SureSine its a true Sine Wave unit with galvanic isolation so you don't have this its blows up issue with ground connections on input and output.
  • icarusicarus Solar Expert Posts: 5,433 ✭✭✭✭
    Re: Taking one circuit off grid and grounding

    As SG suggests, it is not code compliant, but if all you wish to do is energize the one circuit with Pv it would be pretty simple. Pull the conductors off the breaker, make up a joint and send a wire out of the breaker box to a suitable sub panel, with the grounds landing back in the main box.

    Personally, I think it would be simpler (and cleaner) to install a simple transfer switch, that way you can have both worlds.

    A 4 circuit gen-tran switch is not very expensive.

    Tony
  • micahvmicahv Registered Users Posts: 17
    Re: Taking one circuit off grid and grounding

    So if I use a transfer switch, I would run a line from the circuit breaker in my main breaker box and then a line from my inverter to the transfer box and then the house line from the breaker box. I could then just turn the main breaker off to only allow for PV power to the circuit....and if I get desperate I can just turn the breaker back on?

    So for now/temporary grounding while I play with this stuff without connecting to the house wiring just hooking an outlet up to the inverter. How is the best way to ground, I notice my water faucet has a ground wire from Cable TV wire would it be ok to attach to this while I play around with my system? I'm only concerned with 2 seperate earth grounds in an electrical system if the grounds are in the same system?

    Thanks again,

    Micah
  • icarusicarus Solar Expert Posts: 5,433 ✭✭✭✭
    Re: Taking one circuit off grid and grounding

    A transfer switch is pretty simple. It allows input from one of two sources (two imputs) to feed one output. So the circuit would look like this. One side of the transfer switch would feed from your main panel. When the transfer switch was on the "panel" side, it would feed from the panel, through the switch, and then on to the load. The other side would feed from your inverter (or genny) and when switched to that side, it would disconnect from the panel, connect from the inverter, through the switch and on to the load. The grounding would all carry through the switch to the main panel such that the neutrals would ground through the main panel while switched to the main panel, or through the inverter when switched through the inverter. The grounds would continue to be made up through the main panel.

    I would not use a water pipe ground as a temporary ground unless there was no other alternative. I would use either the house ground at the main panel, or a separate ground rod.

    Icarus
  • homerramirezhomerramirez Solar Expert Posts: 102 ✭✭
    Re: Taking one circuit off grid and grounding

    your question is simple, the answer should be too. :p

    1. trace the circuit and make sure does not branch to any other oulet.
    2.- turn off breaker and remove the circuit hot, neutral and ground.
    3.- install a small breaker box (15.00 at H.D.) and add a small 120v plug on line side of breaker box extended to inverter
    4.- install a 2nd 8ft. ground rod for the off grid circuit and run a ground to breaker box
    5.- do not bond your neutral to ground (use the neutral bar)and wire your ground to metal box and the plug that will connect to inverter.

    why install an small breaker box? to protect yourself and property
    why install an off grid designated ground rod? utility grounded conductors get bonded to grounding conductor at breaker box, ground faults will find the shortest way to ground, at the speed of light is hard to tell if you inverter and off grid circuits are on it's way.

    I hope this helps.;)
  • dwhdwh Solar Expert Posts: 1,341 ✭✭✭
    Re: Taking one circuit off grid and grounding
    RCinFLA wrote: »
    120 vac loads should be all black wires, sometimes you find red wire used. 240 vac loads are red and black wires. White wire is neutral return line that connect to the neutral bus bar off to side of box. Green wires or bare wires on Romex are ground wires from outlets that are connected to box ground bus bar.

    Well, yes and no.

    120vac and 240vac load conductors can be any color except white, gray or green.

    Grounded conductors (neutral), of size #6 and smaller can be either white or gray. Larger than #6 can be white, gray or have three white stripes.

    Equipment grounding conductors can be bare, green or green with a yellow stripe. (If they are even made of wire - metallic conduit can also serve as an equipment grounding conductor in some situations.)

    For identifying legs on 3 phase, the colors are black, red and blue or orange. On single-phase (regular 120 or 240) no differentiation is needed.

    Two-wire Romex and MC normally contains black, white and bare (or green), while three-wire Romex normally contains black, red, white and bare (or green).


    In the OP's pic, one leg of the 240v supply (presumably from the meter socket which is not in the picture) is identified with a blue stripe, and the neutral is identified with two white stripes. The blue is not needed and there should be a third white stripe. It looks like the orange romex has green ground, and the white romex has bare ground.


    In general, I agree with Homer Ramirez' post.


    Micah,

    What you are dealing with is two "seperately derived" systems. They need to be kept *completely* separate. That includes hot, neutral and ground. It would be different if you were using a grid-tied inverter, but you aren't.

    The romex feeding the circuit that you wish to move to the inverter - it may run through some electrical box where there is another circuit as well. It is possible that the installer twisted the equipment grounding conductors together from two different romexes.

    So, when you disconnect the hot, neutral and bare wires from your main panel, do some continuity checks to make sure that they are in fact separate.

    As for bonding your inverter's neutral to ground - it depends on the inverter. Some require that bonding, some will blow up if you do it. Follow the manual for your specific inverter.
  • bryanlbryanl Solar Expert Posts: 175 ✭✭✭✭✭
    Re: Taking one circuit off grid and grounding
    What you are dealing with is two "seperately derived" systems. They need to be kept *completely* separate. That includes hot, neutral and ground. It would be different if you were using a grid-tied inverter, but you aren't.
    good point but what gets confusing is that there are three types of ground (earth, frame, circuit).

    I'd think the scenario given in the OP could be handled with a DPDT switch to allow the circuit to be fed from either of 2 sources. This sort of transfer switch is most common. Frame and earth grounds are usually not switched (I am not sure of the code here, though) but just all connected via the green wires.

    The neutral to frame ground bonding should occur at the source (inverter, genset, ...) but that is usually only required for large (> 5kw) sources.
  • dwhdwh Solar Expert Posts: 1,341 ✭✭✭
    Re: Taking one circuit off grid and grounding
    bryanl wrote: »
    I'd think the scenario given in the OP could be handled with a DPDT switch to allow the circuit to be fed from either of 2 sources.

    It could IF the inverter has a bonded neutral. If it's a floating neutral, just doing a DPDT flip-over to the inverter could "possibly" lead to the inverter's neutral being bonded to ground - which could be bad for the inverter.

    If the OP makes certain that the circuit in question is isolated unto itself before installing the switch and flipping it, AND makes sure that the frame of the inverter is grounded (or not) according to the inverter's manual, then sure, a DPDT could work.
Sign In or Register to comment.