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Old 09-12-2009, 08:43 PM   #21
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Re: Grounding the inverter question

- good story sokaji! I have to see what I have as far as a fire extinguisher...
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Old 09-12-2009, 10:10 PM   #22
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Re: Grounding the inverter question

Quote:
Originally Posted by ezbme
Quote:
Originally Posted by Ob1
Your fuse and cable selection should be at least to 100% of the inverters rated power, not what your intended load is.
I think I have it covered as the inverter is rated: Output current (peak) 25 A. This is covered by a GFI outlet & the breaker box is in between the battery bank & the inverter protecting & allowing me to shut off the power... This wire gauge is good for what I am running.
Thanks for all the info & the electrical lesson!
I thought the discussion was about the breaker on the input. You list the output rating of 25A, and I'm going to assume it is only a 120V AC inverter, which would be equal to 250A at 12V DC assuming 100% efficiency (which isn't possible), so the max input drain would exceed 250A.

Still, you should have a really good fuse between the battery and the cable. Your undersized input breakers just mean they'll be tripping every time you exceed their capacity.

It looks to me like you're using a double breaker. Assuming you're using both for the load, I think that might give you 80 amps of input capacity, or about 8 amps of output capacity. However, if I'm wrong about that guess, and it is only using 40A on the breaker, your max output will be a bit under 4A of AC power.

This might be more than enough for your needs. Or, you might be resetting that breaker and cussing. In any case, adding a heavy duty fuse between the battery and the cable rated for the max input power of your inverter will make your setup much safer. As stated elsewhere, a chaffing of the wire can cause the wire to touch ground, which instantly turns your wire into an arc welder, and could start a fire.

just my opinion,
jim
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Old 09-13-2009, 04:01 AM   #23
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Re: Grounding the inverter question

Are we talking about the same thing here? I was assuming the breaker you were talking about was on the 12 volt DC wire coming from the battery and feeding the inverter. If this is the case then no, I don't think the breaker is big enough. If all you're running is the computer and such then you will never touch the rated capacity of the inverter so it probably is not going to trip. However, your inverter is capable of supplying far more out than that little breaker is capable of feeding in to it. It's not a hazard, but the breaker will trip causing your computer to die on the spot if you load it up.

40 amps DC*12 volts (as low as your battery might ever get...lower will make the results worse)

That's 480 watts of input power. But wait...the average inverter is only 90% efficient or so.

.9*480= 432 watts AC output

Your breaker is effectively turning your 1000 watt inverter into a 432 watt inverter...and that's 432 watts sustained OR surge most likely unless your breaker is slow to trip. It just seems like a waste to me, but then again, if all you're running is a SMALL desktop you'd be fine.
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Old 09-13-2009, 09:07 AM   #24
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Re: Grounding the inverter question

Quote:
Originally Posted by the_experience03
Are we talking about the same thing here? I was assuming the breaker you were talking about was on the 12 volt DC wire coming from the battery and feeding the inverter.
Your breaker is effectively turning your 1000 watt inverter into a 432 watt inverter...and that's 432 watts sustained OR surge most likely unless your breaker is slow to trip. It just seems like a waste to me, but then again, if all you're running is a SMALL desktop you'd be fine.
I should name my bus "The Inexperienced"...
I have had so many different subjects I had to learn about when getting our bus together & electrical was def one of them that I knew nothing about. I really still know nothing about electrical & the help I had gotten up to this point was not good advice (outside skoolie - home depot mostly).... I want to get this set up correct & for it to be safe & efficient.
So to sum this up, I need a larger breaker on the DC feed line to the inverter so I don't starve the unit of power correct? I don't understand why the output max on the instructions for the inverter states 25A?? That is one of the reasons I purchased the GFI I did.

Jim: "It looks to me like you're using a double breaker. Assuming you're using both for the load, I think that might give you 80 amps of input capacity, or about 8 amps of output capacity. However, if I'm wrong about that guess, and it is only using 40A on the breaker, your max output will be a bit under 4A of AC power."

I do have double 40A breaker but only on one side... again the help of the Home Depot "electrician"... Man I got screwed listening to these guys. How would I go about splitting the DC cable to pass through both 40A breakers? OR should I just return the breaker & get a new one? What input amp breaker do you guys recommend? What size should I get for the GFI???
Thanks for the help so far...

Jonathan
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Old 09-13-2009, 10:49 AM   #25
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Re: Grounding the inverter question

You have to think of 2 different systems coordinated together.

You have a 1000 watt inverter, right? According to that calculator link, at full inverter output it will draw 100 amps from the 12VDC side, and will take 2ga wire if the batteries are 8 feet from the inverter. So, fuse the 12VDC side at 100 amps, and cable it according to the distance covered.

Now, the output of the inverter is 25 amps. That output is probably protected by a circuit breaker within the inverter, check on that. I would use a marine or RV breaker panel and set up 2 circuits; a 10 amp and a 15 amp. Layout each leg with your present needs in mind, and future ones as well. Is a dorm frig, window AC or other goodies in your future? Best install wiring for them before you put the walls up! As far as GFCI wiring goes, I have no experience there

Ever plan on hooking up to shore power? You will need to set up an AC transfer switch. Or, you could set up a single AC inlet receptacle to feed the circuit breaker panel, and switch from an exterior cord to inverter cord. Never have them both wired in at the same time.
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Old 09-13-2009, 11:11 AM   #26
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Re: Grounding the inverter question

Thanks OB1...Your info helps make it easy to understand!
I shoulda referred to skoolie first. I have been so short on time getting this setup because of our moving tomorrow but that is no excuse...

The inverter has a built in transfer switch with hard wire for shore power so that should cover me in that department. I will change out the 40A for a 100A when we get to NM. The GFI should be sufficient for now as the bus will serve as a business office, off grid till we built a pole barn. As far as future needs the bus will be gutted again & then I will layout & plan the electrical as it will change completely from the present state.

Thanks again everyone for being patient with "the newbie"!

Jonathan
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Old 09-13-2009, 11:29 AM   #27
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Re: Grounding the inverter question

Built in transfer switch? Nice inverter! Wish mine had that!

Good luck with the move!
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Old 09-13-2009, 12:12 PM   #28
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Re: Grounding the inverter question

With a 25 amp fuse that inverter has some surge capacity! Very cool. Nominal output would be between 8-9 amps depending on if it is 110 volt AC or 120 volt AC output.

The key is just to remember which side of the equation you're working with. Watts are watts so that makes it convenient to deal with. 1000 watts on the AC side is 1000 watts on the DC side. You arrive at wattage by multiplying volts times amps. Since on the DC side you are only running 12 volts, but you're running 120 volts on the AC side it requires 10 times as much current (or amps) on the DC or input side to feed the output. I promise it will all make sense one day.

A car audio shop is a good place to be looking for the 12 volt stuff (including advice) that you need.
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Old 09-13-2009, 01:36 PM   #29
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Re: Grounding the inverter question

A couple of points:

1. There is nothing wrong with having a fuse/breaker that is rated for LESS than the capacity of the wire - it is there to protect the devices, and the wire will hold up to any load that doesn't blow the fuse or trip the breaker.

2. NEVER protect a WIRE with a fuse/breaker that will supply more current than the wire ('downstream' of the protection) can handle.

3. Especially in a mobile installation, with vibration and grounded metal all around, always put the fuse/breaker as close to the source (batteries) as possible, to protect against fires if the insulation is compromised and the wire shorted. This is especially true if the wire is near hot or moving parts, or must pass through a metal panel. Notice how the breaker for some of the loads on your starting batteries is right next to the batteries: http://i175.photobucket.com/albums/w...s/DSC06827.jpg A typical plan is to have a 60, 100, or 200 amp breaker at the battery, depending on the wire size, to protect the main feed. Then you can have smaller fuses/breakers at the distribution points based on protecting each of the loads.

4. Not all circuit breakers are DC RATED. Some or all Square D brand 'QO' model breakers are. Your helpful store man may not have steered you wrong on the model. Always check the labeling on any breaker before using it on DC. An AC only rated breaker may overheat on DC.

5. Since watts are volts times amps, a rule of thumb repeated over and over here is that it will take ten times the current at 12 volts to provide equivalent wattage to a 120-volt load through an inverter. With inverter loss figured in, eleven might be a better multiplier. As The Experience calculated, 40 amps at 12 volts will give you about 432 watts, or about 3.6 amps at 120 volts. (The 4 amps I posted earlier was a rough figure without any inverter losses built in.) If that is all you will ever need, no problem. But I suspect the day will come when you will want to plug in something substantial, even for a few minutes. If that happens, you will curse yourself for not up-sizing the DC breaker.

6. It's probably not a good idea to double up the dual 40-amp breakers on the same load to try and get 80 amps. It can be done, and the breakers are already paid for, but without load balancing there is no guarantee that each side will handle 50% of the load. It is true that splitting the load will give you somewhere between 40 and 80 amps before tripping, but there is no way to find out what it will be until your particular installation is complete, and with a steady load you measure current with a clamp-on meter on each breaker feed.

Here's why: Lets say the sum of the resistances of the input and output connections, jumper wires, plus the internal resistance of each breaker adds up to 0.04 ohms on the right and 0.02 ohms on the left. These are minuscule resistances, but the difference will cause two-thirds of the current to flow through the slightly lower resistance on the left, and one-third will go through the higher resistance on the right. The left breaker will trip at 40 amps, so you will get only 20 amps on the right side before the cut-off. This gives you only 60 amps total. High-power amplifiers and other devices with transistors operating in parallel generally have low-resistance, high wattage resistors feeding each one, so the resistance of each balancing resistor is much greater than the variation between devices and causes the current to even out.

Since you already have a (DC Rated?) enclosure for Square D breakers, why not swap out the double 40 for two individual breakers. One could be the supply for the inverter, and the other could be a master disconnect for 12-volt loads off of the house batteries, now or later. DC lighting??
Quote:
I did add a ground strap to the inverter . . . . . Are you saying I should remove this ground to avoid potential problems?
Quote:
I have read & talked to others that have told me they keep their off-grid setup completely seperate from the bus power, including no grounds whatsoever.
The AC safety ground MUST connect to the inverter when the inverter is the operating AC source, either through the third prong of an output outlet, or by a ground strap at the inverter. The same is true of a generator. The DC plus and minus can be isolated from ground if desired. But here's something to think about. If the third prong ground is tied to the DC minus inside the inverter, and the DC hot accidentally contacts the bus chassis in any way, all the energy in the batteries will go through your inverter to return to battery minus.

Ever see the insides of a radio after someone tries to start a diesel locomotive with a breaker open that disconnects the battery bank minus from the chassis? I have, more than once. When you use the case of the radio and the radio ground wire to (try to) turn over the big engine, the results ain't pretty.

Here's a plan: with the battery bank disconnected, measure the resistance from the inverter DC minus input to its AC safety ground. If it shows any connection, I would definitely put a high-current strap from battery minus to the chassis, to provide a high-current path to protect the inverter wiring from "oopses." If there is no connection, and you are confident your solar system is also ground-isolated, then you should have no problem leaving the house DC system isolated from ground.

But you may want to add DC devices in the bus someday, right?
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Old 09-13-2009, 10:34 PM   #30
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Re: Grounding the inverter question

Man thanks redbear & all for taking the time to write all you did... I am shoulder deep in bus loading & am taking a 5 minute break every now & then... I haven't been able to keep up with this topic but believe me, I will read it & learn it once we are back online & chilling out in the buffice in NM!!

THANKS EVERYONE for all the great info! I AM learning something about electric! I am sure I will have plenty more questions tho...
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