Grounding the inverter question

ezbme · 09-10-2009, 11:25 AM

I am about to finish the off grid power in my bus & have a question about grounding my inverter... I have a Xantrex sinewave 1000 watt inverter with the instructions but am not sure if they want me to take the ground directly from the neg post of the inverter to the body of the bus or not. Is this the way it connects? I would imagine a ground to the wall would be sufficient as the bus is all steel? I am def a newbie electrician & am learning as I go...

Thanks,
Jonathan

bikeforthelight · 09-10-2009, 11:35 AM

thats what i would do jonathan....i mean the whole bus is grounded already, so grounding to the frame, floor, or wall should all be the same. i mean, most twelve volt lighting systems are grounded through the screws holding them to the ceiling, so i think it works either way.

icnivad · 09-10-2009, 02:10 PM

Right. Most likely this should be fine as your frame should have a pretty good ground. You might want to double check that your battery is grounded fairly well to the frame, though, as an inverter will draw significantly more power then the lights that the frame is typically used to ground. At 12v a 1000 watt amp should use about 83 amps, which requires a 2 guage cable. Make sure the entire circuit from battery to inverter and back again is at least using a 2 guage cable, and that your connection points where you've screwed into the frame are clean and rust/paint free.

Ob1 · 09-10-2009, 03:07 PM

Personally, I would cable it. No telling how well the body and frame connection points will conduct electricity. Add some corrosion to a high amperage circuit...

the_experience03 · 09-10-2009, 06:56 PM

For high draw circuits I prefer to ground directly to the battery. It is acceptable and normal to lose .1-.2 volts per connection so minimizing them is helpful in reducing current draw by increasing the available voltage for conversion since power (watts)=volts*amps

Sojakai · 09-10-2009, 09:09 PM

From what little I know about cars, and from what has been said previously in the forum here, buses are no different, just bigger cars...

Isnt the frame connected to the starter battery? According to a friend who lived in his van for about two months, grounding to the van body was fine since he was using the starter battery as his power, but he would have to run a grounding line if he got a battery bank, or the bank would ground into the starter battery and cause problems...

I really dont know much about cars... So, take it as someone who knows little being worried over mole hills.

Ob1 · 09-10-2009, 09:43 PM

Cable it. You are gonna run a "+" cable anyways. Keep it as short as possible, and dont put the inverter in the same space as the batteries. The guys who designed the inverter also wrote the instructions and tested the systems for reliability and safety.

Sojakai · 09-10-2009, 09:46 PM

I tend to pick up and retain odd bits of information pretty much at random... But if I want to scare people, I talk about "The Greatest American Hero". People have either never heard of it, or they remember it and feel old... I hear alot that I am "Too young to remember that show". Gotta love reruns in the late 80's...

Sorry about that, and now back to our Regularly Scheduled Topic...

ezbme · 09-11-2009, 09:24 AM

Thanks for all the info!

I picked up a grounding strap I am going to attach to the neg post of the battery & to steel on the bus & be done with it...

Redbear · 09-11-2009, 09:38 PM

You'd be surprised how easily a bad ground interferes with a 100-watt 2-way radio transmitter. It will really play havoc with a 1000-watt inverter. Since both 2-way radio dash units and power units are generally grounded to the body, I try and teach installers:

1. drill a fresh hole in good metal (I prefer to use a slightly undersized bit)
2. use a new sheet metal screw - #10 for a receiver, 1/4 inch for a transmitter, and
3. Always put a star-toothed washer under the head of the screw.

The factory frame-to-body straps aren't always good. My step-daughter had an old Chevy that would occasionally decide it was being hot-wired because of high resistance from a bad body strap (the battery minus was wired to the engine), so I just put a "transmitter ground" from the battery minus to the nearby fender. The electronic module became happy, and it ran fine.

Grounding to a bolt and nut will often not be a good connection, and just putting a high-current ground under an existing bolt that mounts something to the body is a recipe for trouble. This is possible even on connecting posts where there is a battery feed on another wire on the same stud. We had an '06 Freightliner stop charging because a positive stud under the frame that connected all the hot wires together stopped making connection to the alternator wire. When you have a stud connection joining two or more wires, you should probably take it apart once a year at least, clean all the terminals and hardware, and re-torque the nut(s).

Ob1 has it right. This isn't a dome light or stereo we're talking about. Mount the inverter as close to the house batteries as you can, and run the wires by the shortest safe route to the batteries. The only interruption in the wires between them should be a disconnect switch, if you use one.

ezbme · 09-11-2009, 11:49 PM

Thanks for the detailed advice & instruction Redbear!
Today I finished the off-grid power. What I ended up doing was to run short leads throughout the whole system. The leads on the batteries are short, the distance from the batteries to the inverter is less than 48", & I added a circuit breaker to the hot DC side. I used a 40A breaker as the cables are 4AWG & they fit this breaker (home depot electrician said it was fine to use). I did add a ground strap to the inverter to an area I drilled, sanded to bare metal & then bolted the strap on after spraying the metal with WD40 to protect it. The place I drilled thru is a steel lip that comes off of the wall that the seat belts mounted to & is very strong & thick steel (seat backs faced the wall). Are you saying I should remove this ground to avoid potential problems?

the_experience03 · 09-12-2009, 02:34 AM

I hate to be that guy and all, but 1000 watts AC is 1000 watts DC, To get wattage you multiply volts times amps. Perhaps a better way to say it would be to get current (amps) you divide wattage by volts. You inverter is capable of 1000 watts. 1000 watts divided by 14.4 (which would be your alternator going full tilt boogie, not the battery voltage with the engine stopped) would be nearly 70 amps. Worse yet is that that is assuming 100% inverter efficiency which is not the case. The point is that that 40 amp circuit breaker is grossly undersized. If you want to get anywhere near the 1000 watt rating of your inverter you're going to need at least a 100 amp fuse and more than likely you will need a 120 amp or larger fuse to handle the current at maximum inverter output.

Ob1 · 09-12-2009, 07:23 AM

Ditto on the big fuse.

Rub through the positive cable insulation without a fuse {mounted very close to the batteries, you get a welder...or a fire. Depend on corroded body panel and frame connections for the ground, same deal.

That is the one thing I dread about my bread truck RV, fire.

2 nice extinguishers on board, fore and aft.

ezbme · 09-12-2009, 08:53 AM

Quote:

Originally Posted by the_experience03

I hate to be that guy and all, but 1000 watts AC is 1000 watts DC, To get wattage you multiply volts times amps. Perhaps a better way to say it would be to get current (amps) you divide wattage by volts. You inverter is capable of 1000 watts. 1000 watts divided by 14.4 (which would be your alternator going full tilt boogie, not the battery voltage with the engine stopped) would be nearly 70 amps. Worse yet is that that is assuming 100% inverter efficiency which is not the case. The point is that that 40 amp circuit breaker is grossly undersized. If you want to get anywhere near the 1000 watt rating of your inverter you're going to need at least a 100 amp fuse and more than likely you will need a 120 amp or larger fuse to handle the current at maximum inverter output.

Bear with me - I am trying to understand what is going on here... The power setup I have is strictly off-grid solar like I mentioned & it will not be connected to the bus power/alternator in any way...It will be a seperate system completely... I will only be running a computer (the highest powered piece of equipment) & a few other office items, nothing else. Do I really need to have a 100 amp fuse inline?? 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. Oh, & yea, I already have the fire extinguishers in place!

Ob1 · 09-12-2009, 09:22 AM

You have to account for all the energy stored in the battery, and the large cables the inverter require. The large cables will handle a much higher amperage.

A smaller wire will heat up, even smaller it will burn like a light filament then melt. So, you use big cables to avoid the heat and voltage loss.

The big cables can handle the amperage without heating up. Having them unfused is dangerous.

Ob1 · 09-12-2009, 09:52 AM

here is an online inverter calculator that will give you an idea of your needs, and what cable to use. looks pretty close to most specs I have seen, but when it comes to cabling I believe in overkill.

http://www.skingcompany.com/portable_power/calc2.htm

ezbme · 09-12-2009, 03:41 PM

Quote:

Originally Posted by Ob1

here is an online inverter calculator that will give you an idea of your needs, and what cable to use. looks pretty close to most specs I have seen, but when it comes to cabling I believe in overkill.

http://www.skingcompany.com/portable_power/calc2.htm

I checked it out & if I am reading it correctly the draw will only be 13 amps at any given time with everything running... That is pretty low.

Ob1 · 09-12-2009, 05:29 PM

Your fuse and cable selection should be at least to 100% of the inverters rated power, not what your intended load is.

Sojakai · 09-12-2009, 06:21 PM

I am curious, what size Fire Extinguishers did you get? My guess would be the normal 10lb extinguishers. You might want to get a 30lb to back up your smaller ones. I know that the average person will never have to use their extinguishers, but in the off chance that you do, its better to have at least 1 BIG extinguisher to fall back on when you discharge the little ones.

My father use to know a Firefighter named Brian. Brian would go to companies and teach the employees how to put out fires so that they could do something to contain them while the Fire department was on their way... So, one hot LA day, he sets up a small pavement fire out in the parking lot of a business and in the course of his teaching shows how to put it out with a small extinguisher without problem. He resets it, and hands a new extinguisher to an employee and tells them to put it out, and for two or three fires, this works. Too bad that the heat of the day (around 90), plus the fires heat, have heated the tar in the pavement... So the last group uses their extinguisher to put out the fire, only it doesnt go out. She they call for help.

Brian grabs a fresh extinguisher, and empties it into the fire to no effect. Goes back to his truck and grabs another one...and another...and another. Seeing that he has a problem, he calls the Dispatcher and tells them that he needs One (1) Fire Truck to come to him to put out a "Small Parking Lot Fire". The Dispatcher types this in, and the computer says that its an industrial area and requires that they dispatch EVERY fire truck in the valley... Brian, just putting down the radio, started hearing siren after siren start up... A line of trucks shows up shortly after. When the lead truck pulled up, the Fire Chief steps off and walks over to Brian to find out whats going on. Brian explains and points to the 20 or so spent extinguishers that he used.

The Chief turns, walks over to Brian's truck and opens the back, where he pulls out a 50lb Extinguisher. Carries it to the fire and with one long squeeze of the handle, puts out the fire. Brian was heard to have muttered "I forgot that I had that..." to the Chief when he asked him why he didn't use it.

Moral of the story: 1) Have a large extinguisher handy even if you think that you will never use it. 2) When calling in a fire at an Industrial Complex, Expect alot of Peer Witnesses to see your screw-up.

Brian also did a demonstration about how Gasoline is hard to ignite... He took a cup of gasoline and dropped a lit cigarette into it while the room gasped in shock. When nothing happened, he continued his explanation while he lit up another, grabbed an eye dropper of gas and put a drop into a large stoppered bunsen burner, and started shaking it. After shaking it for about a minute, he pulls out the top stopper and drops in the cigarette and BOOM.... Little bits of paper and tobacco falling to the ground...

No more than Thirty (30) seconds later, People from around the building start showing up with first aid kits and wanting to know what the "Explosion" was...

Brian has a Reputation.

ezbme · 09-12-2009, 07:41 PM

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!

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