Wiring inverter into breaker box questions

PorkChopsMmm
PorkChopsMmm Solar Expert Posts: 189 ✭✭✭✭✭
in Off Grid Solar & Battery Systems #1
Sorry if this has been covered in depth, I have tried to search these forums and the internet and not come up with much. I am trying to figure out the best way to hook my inverter up to my breaker box.

I have a completely off-grid setup 3 240watt panels, a 12V battery bank of 8 55 amp hour batteries, an MX60 MPPT charge controller, and a Xantrex 2.5 2500 watt prosine inverter/charger. All of this equipment is purchased and has been in use for months while I build my cabin -- it works great.

We built an offgrid cabin on land we own and I am finally getting to finish the interior. I have been working with a retired electrician who has helped do all of the wiring up to code. We installed a 200 amp Square D breaker box under his recommendation that if I ever get grid power someday I will already be set up for it and expanding electrical needs. He has some background in offgrid power as he ran his farm and several others using wind turbines in the 1970's using railroad batteries (a great story, but I digress...).

His suggestion for wiring my Xantrex inverter into the breaker box was to run the AC out to the leads on the breaker box and use a "bridging wire" to connect the 2 120 circuits. I have seen this set up before but I apologize if I am not using the right terminology. The electrician didn't know that my inverter has 2 AC outs... so 2 120 connections. Should/can I use both of these connections at the same time to power my electrical panel? If so, are there any special considerations for hooking up the 2 120 lines to the 240 power inputs within the panel? If it makes any difference, I was going to sink 2 grounding rods (per the electricians advise) on the outside of the house and wire them up to the grounding run in the breaker box.

From the user manual for the inverter it looks like the largest gauge wire I can hook up to the AC out lines on the inverter is 10 gauge. Would using 12 gauge Romex be a problem? The distance from the inverter to the breaker panel will be less than 10 feet.

Any thoughts or advice? Thanks in advance!
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #2
    Re: Wiring inverter into breaker box questions

    Are you sure you have to 120 VAC outputs? Just a quick look at the spec. and there should be an AC in and AC out (grid/generator AC in, inverter AC out).

    Bridging that two bus bars so that you have 120 VAC common to both is OK...

    Just remember one issue... Many times people use a 3 wire Romex (black/read/white-neutral/ground). For house wiring, it is assumed that there is 240 VAC between black and red (they are 120 VAC which is 180 degrees out of phase). That means the neutral will never carry more than the current of the black or red wire (the white/neutral wire carries the difference in current between black/red).

    With your bridged bus bars--Black and Red will be in-phase, and the white/neutral will carry the sum of the current and can carry more current than it is rated for... So, in your home, you should never use the 3 conductor (B/R/W) Romex with a common neutral.

    Your inverter is not that big--but if you tie in a large 120 VAC genset, then you can overload the common neutral.

    The other possible issue is that if you ever use a 240/120 VAC genset--bridging the bus bars will prevent you from supporting 240 VAC appliances with that panel until the bridging is removed.

    Not right or wrong--just more of an issue for 120/240 and proper "neutral" wiring support.

    Grounding the Neutral/box/green wire to earth is normal NEC code... Just confirm that you can "ground the neutral" for your specific model of inverter (sorry, I don't have time to look it up in the manual right now).

    In general, TSW (true sine wave) inverters can support a grounded neutral (and MSW inverters, in general, cannot)--but you need to confirm before powering things up to save an expensive mistake.

    Using heavy gauge wire in a solar RE system is usually a good idea... It keeps losses down and better supports surge currents.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • PorkChopsMmm
    PorkChopsMmm Solar Expert Posts: 189 ✭✭✭✭✭
    #3
    Re: Wiring inverter into breaker box questions

    BB, thanks for the reply. Below is an image from the user manual and a link to the manual itself.

    dzf5go.jpg

    Manual:
    http://www.xantrex.com/documents/Inverter-Chargers/PROsine-3/ProSine_25-30-Owner-Guide(445-0096-01-01_Rev-A).pdf

    It shows the single AC in... which you are right is fed via my generator and the 2 AC outs. I purchased this inverter used and it had Romex tied in to all 3 hook ups. I used the Romex from the AC in and attached a heavy gauge extension cord to it to get power from the generator. I am left with two bare Romex wires. I just hooked up a single outlet to one of the wires to power my tools for now.

    I could do the single 120 line into the break box and use a bridge wire but I was wondering what other off-griders do who have an inverter with 2 AC outs? I don't have any plans to ever run 240 off of the breaker panel.

    Thanks for the help!
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