Inverters and house wiring

[Madscientist267]

In the quest to determine how exactly to go about utilizing RE in my house, I am leaning against grid-tie primarily due to headache and cost factors. At the same time, I don't want to spend a whole bunch on more wiring...

That being said, what are the general guidelines for using some of the existing wiring on an inverter rather than the mains? The idea is more or less to wire all of the lights (which are now CFL), and potentially some outlets (for floor lamps etc) on an inverter, maybe on it's own panel but manually switchable to the main panel if need be.

Thing of it is, with the power levels that I'm intending to use, I don't really even see the need for breakers or anything else on the 110 side to protect any house wiring.

The inverter that I'm looking at couldn't pump enough current to cause a problem if it came up - 1400VA wouldn't even trip a 15A breaker... But I have a feeling average code would likely require me to keep all the pointless breakers to distribute the load rather than just have the DC side of the inverter fused...

Truth is, I don't even need that much power (lights peak out at ~300W calculated total), but there are two reasons I would use the bigger inverter. First, it has plenty of reserve capacity if I did need it. Second, it's the smallest sine inverter I have. I'd rather not run modified sine through the house - harmonics and all...

What do I typically need inverter, protection, integration and distribution wise in this arrangement? I realize I would have to ultimately look at code, but this is a feasability check. I don't want to cut corners, but don't want to spend frivolously either.

Thanks ahead of time...

Steve

[TomW]

mad;

What I did and do is:

Pull the circuits you want out of your breaker panel connect to good heavy duty plugs. Run an outlet from the breaker to the outside of the panel. Run good, heavy duty outlets from your inverter to same area. Mark well. Now all you need to do is swap the plugs from grid fed outlets to inverter outlets. Manual but foolproof and cheap. Oh, your inverter outlets should be protected with a breaker but you cannot easily use the ones in the box.

Pretty simple. Not sure if it is code compliant but the inverter never connects to any grid equipment so it should be fine?

Tom

[ETech]

Your inverter is only 1400 watts 120 Volts @ 11.6 Amps.

1. 1400 Watts of power can create a nice electrical fire.
   
2. It takes less that 0.01 Amps throught you heart to kill you.
   
   

The circuit breakers are very inexpensive if it could save your life or prevent a fire.

You can route your lights to a seperate (sub) breaker box.

Set up a switch over box or plug arrangement (as previously suggested) to feed your Sub-box. Switch over boxes are used for back up power sources. They are available in both manual and automatic versions.

Clearly label this box as to it's power source, so power can be safely cut for repair.

I suggest leaving some lights on mains only, just in case of inverter failure.

If using an automatic switch-over box, it could be wired to return to mains if the inverter failed.

Please make whatever you do safe!

Just my two cents, Etech

[scottsAI]

Steve,

Breakers preform couple services.

First is protect the wiring to prevent it from burning down your house.

Secondly isolation. Problem or a fault or need to work on a circuit, open breaker. Safety.

My 6 hole sub panel cost $22, hard to buy a box to connect things for less. Breakers $4 each. Nice to have a distribution point to add on to over time. Careful shopping, some boxes were very expensive, have no idea what they offered over the box I got. Confirmed what I did was to code.

Point to ponder

If wiring is not to code, and a fire happens. Insurance company can possibly deny claims regardless if wirings fault or not. More than one sad story of insurance company denying a claim on a technicality.

Have fun,

Scott.

[SteveCH]

I have a small house, running off-grid and it always has. I wired my place for both 12 v. and 120 v., so there are two sets of circuits throughout. [I would probably just go all 120 v. now, but back in the early 80s, things were somewhat different and options fewer.] Anyhow, since my lighting, refrigeration, ceiling fans and some other uses are 12 v., my 120 v. is basically computers, TV stuff, occasional drills or kitchen things [mixers, grain grinder, etc., and some radios]. My 120 v. useage is not huge, and I have been able to combine my 120 v. circuits into two bigger circuits, each of which has one standard circuit breaker, the type you buy at Home D. or wherever for $4. I run my inverter out to a small, two slot breaker box with those two breakers, divide the current there, the box was maybe $10 a year ago. The inverter has its own breaker, of course, but this gives me more flexibility if I need to cut off an outlet or wall switch for repair, etc., without shutting off the inverter and the entire house for 120 v.

For grid-tie, and for obvious reasons, you would be scrutinized to the max for anything you'd do, so I agree with avoiding that option. It isn't an option for me as there are no power lines within a mile. But draw up your circuits into a diagram, figure out how much current each needs [maximum], and figure out how many main breakers you'd need. They are very cheap, so a lot of them would be reasonable, too. I just don't have that many circuits or that much 120 v. use to make me want more than my two.

But the main thing is, your inverter will be the source, metaphorically like the service line on a grid home. You can treat that elec. coming from the inverter just like the service line. Just take the lines from the inverter, run them into the breaker box, divde them into however many you need to send one to each breaker, and you are in business. It took me a couple of days to set mine up that way, as I had to go thru all my 120 v. circuits and figure out how to divide them. I chose a time I was home alone and could shut off my inverter and so on.

[dnix71]

Most house wiring ties all the neutrals together and bonds them to ground.

Separating branches from the existing breaker panel may not be possible. Small inverters float neutral. Their outlets aren't polarized and grounded like your house.

If you have a small setup, maybe you could wire outside lighting to the battery/inverter setup or wire a branch to your work bench for occasional use.

[laskey]

The Electrical code isn't normally about what's practical.  It's about maximums and wosrtcases and safety.  You can bet that to do what you want to code you'll need a transfer switch just like if you put in a gas gen, and the work will have to be done by an electrician, or at the very least inspected by your friendly neighborhood electrical inspector or your fire insurance will certainly be void.  Insurance companies don't get rich by writing a lot of cheques.

Cya,

Chris

[Madscientist267]

I must admit, I hadn't thought about it really in terms of insurance. I really only weighed logical and practical.

My figuring was, any short that would trip a 15A breaker would cause the inverter to go belly up before reaching the 15A mark, inherently protecting the house wiring, even better than a breaker. But, I suppose they don't necessarily see it that way.

Thanks for pointing the insurance issue out; that could potentially become a very bad mistake indeed. There's no way I could afford to have a disaster hit and be denied the insurance to procure another roof over my head.

So, with all that being said, might it be better to go with a DC scheme? Or am I going to run up against the same thing? I'd probably still use 24V for this. I remember reading something somewhere about 48V being a limit for DC without inspections being required, but if that could get me into trouble with an insurance company down the road, I don't want to go that route either.

To me, it would almost seem more dangerous running 12 or 24 volt wiring through the house since the current flow required to be of use in this range is several times higher. Even with the thicker wire, wherever a short does happen, insane amperage could flow before a fuse or breaker interrupted it, potentially leading to enough heat at the location of the short to start a fire, right? The demonstration with the screwdriver across the terminals has never quite left my memory, and so I'm already paranoid enough with the cabling running the 10 feet to the bank for the computers...

I think it ultimately comes down to cost, safety, and what the wife will let me get away with (imagine that, a wheel is in fact round!). As long as the first two are in check, she'll probably go for the rest. But she also wants everything to be 'transparent'; she already moans and groans about a cat5 cable that runs through the hallway to the livingroom... haha I don't even see it laying there anymore...

Thanks for the help so far...

Steve

[WineGuy]

I had the thought of connecting the neutrals of the two outlets on the inverter, and using it as somewhat of a 220 source feeding a "normal" household breaker box.

Somewhere I read that the neutrals of an inverter should not be connected. This seems odd since the FRAME has a ground connection. Probably has something to do with the phases not being sync'd? Does anybody have any more info on this?

[Ungrounded Lightning Rod]

Actually those inverters have a 170ish volt power supply and switch the "hot" and "neutral" between that and ground alternately (with a both-grounded period between the two one-hot periods for "modified sine wave").  So both sides are "hot".

You'd be safer to wire your 120V circuits as if they were 240V.  Put the hot to one "hot" of the panel and the neutral to the other "hot".  Use a 220V tied-double-pole breaker to the hot and neutral lines to the outlet and you're in business.

Don't know how that would go over with the code guys.  (For starters, all three lights would go on when they use the "check the outlet for proper connection" tool.)  But it would be the right thing to do.

Downside:  GFCI breakers would be a pain unless you can find a 240V version.  (I can think of a hack with two singletons but it's an awful kludge and may not open quickly enough for safety.)  And a GFCI outlet may not open the neutral side of the line and if so it would NOT be safe.  B-(

[Ungrounded Lightning Rod]

See my post above for an explanation of the neutral issue.  Such inverters get their output grounding by connecting the "frame" (or one side of the battery) to one side of the power supply output on the DC side of the H-bridge.

[Madscientist267]

I've also seen a couple of inverters where the ground pin will trigger the inverter's protection circuitry by firing a triac or SCR and cause instant shut down when it sees some current on it, maybe as if to mimic GFCI. I figure this is probably as good as one can expect from something that is not expected to be truly grounded. In the circumstances that I would expect to trigger such a thing however, many of the small inverters can't power those loads for crap anyway (ie drills etc).

I won't test my theory to determine if it is fast enough to keep from killing me, but a 100k resistor between either hot or neutral and 'ground' will trigger it. Seems to be mostly on the smaller inverters (<500W). On some without this 'feature', the ground hole is either tied to 'neutral' and DC ground, or is just there connected to nothing, probably more as a convenience to the user.

The other thing I have considered, while we're at it, is to use an old UPS as the inverter instead of the high-side H-bridge type. Even though they are less efficient, there are reasons to go that way, one of which is that the transformer can be connected in any way deemed necessary; remember that a ground in an outlet is really only there as a shunt, hence the tie with neutral in the panel.

Thing is, what will they let me get away with code wise?

Steve