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Old 05-06-2015, 11:44 AM   #1
Mini-Skoolie
 
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Electrical Question (Grounding)

Hey All,

Couple of school bus conversion first timers here that were hoping the community might be able to help out with a wiring question.

In the image linked/embedded below, you can see our AC distribution box.

There's two things we're unsure about:

AC Feed
Firstly, our system would not run when we had the black input wire (Not shown) inserted into one of the two insertion points on the Live bar. To solve this, we put a 8 gauge piece of wire between these two points, inserted the Live wire alongside one of them and, Presto!, it worked!

Our question really is, is this safe?

Grounding
Our other conundrum is how best to ground the circuit. As this type of panel is sometimes meant to be used as a sub panel, the neutral and ground circuits are meant to be separated. However, in our case, we will be using it as our main breaker. In this instance, they advise using the green screen (In the neutral bar) to bond the neutral bar to the box. Once this is bonded to the box, it is essentially connected to the ground bar, right?

So to ground the entire system, we want to run a wire from the ground bar to the chassis of the bus. BUT, the bus chassis is already acting as a DC neutral circuit from the lighting on the bus, we think (?). Could we damage our inverter if we end up bonding the neutral bar to the box, which in turn bonds to the ground, which in turn is linked to the chassis, which has a DC neutral circuit?

Sorry if I'm not being clear enough. This stuff is really confusing!


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Old 05-06-2015, 01:35 PM   #2
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Please read the last 10 pages of this thread first, Then the rest later.

http://www.skoolie.net/forums/f10/ho...ac-448-11.html

Nat
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Old 05-06-2015, 02:09 PM   #3
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In your box you need:
* A proper fitting where the Romex is entering. Masking tape is not sufficient abrasion protection and you need the strain-relief/support anyway.
* Yes, the green screw is designed to provide neutral-ground bonding. This type of box can be used for anything, and that's how you do what you're trying to do with it.
* You need a neutral/ground connection on that top lug on the neutral bus bar.

I can't tell from the photo - does this box have two live bars? You have a dual breaker in there and most are designed for 240V, where one leg goes to each 120V bus. Is that what you're planning to use as your main breaker?

If so that's part of your confusion. It looks like you're wiring your bus for 120V but you have a 240V box and breaker. So you have two bus bars in the box, and the breakers alternate between them at each position. Bonding the two busses together WILL work, but:

* You're not supposed to use a green wire for this bonding. Change it to black or wrap it with black electrical tape.
* The wire is too small (I think - I can't quite tell what size it is).
* Route bonding wire BEHIND the circuit wires (since the circuit wires are the ones you want to maintain the most).
* That's the wrong breaker. That kind is designed to let one side trip but not the other. In a main panel like this if the main breaker trips it's supposed to trip ALL the way. They make a metal bonding clip that goes over the two breaker switches on a tandem like this that forces them to "common trip".

You can absolutely damage your inverter if you get this wrong, but not usually just because of the wiring. Usually it'll happen because of something like a bad ground at the site hookup or a phase issue when you fire up your generator. Ground is supposed to be 0V potential everywhere - AC, DC, nuclear power plants, wherever. More important, ground is supposed to have "unlimited sink capacity". This means you can fork a bolt of lightning through it and it can absorb it all. Realistically, most grounds aren't THAT big (Earth ground is the only one that can do it) but the point is to remember how current flows in these systems.

In AC, current flows bidirectionally (alternating...) but only in the hot and neutral wires. The ground is an emergency-only wire and current is not supposed to flow in it. In DC, current regularly flows through the ground - it's often the return path for a circuit. However, currents are also typically much lower, so even a large piece of metal will register as 0V potential even when a circuit is on and using it as a return.

Go read the thread Nat talked about and also Google "Poop Sheets". It's not 100% of what you need to know, but it's a huge start.

You could also do worse than picking up a copy of the National Electrical Code. I'm assuming based on the photos of the parts you're using that you're in North America but even if you aren't this is still pretty legit for most of the components you'll be using. It'll be a book about an inch thick and sort of floppy - should be $20-$30. It VERY specifically covers EXACTLY how main and subpanel boxes are supposed to be wired.
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Old 05-06-2015, 05:49 PM   #4
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Suppose the breaker slots were numbered from 1 to 6 starting at the top. The two-pole 30A breaker at the top fills slots 1 and 2. Above it there's a lug barely visible, and beside slot 6 there's another lug and a bus bar visible. I'll bet that if you lifted all the breakers off you'd find that bar with its lug at the bottom and the one at the top have fingers interleaved so that each one serves alternating slots. In other words, the "hot" connected to the top lug serves slots 1, 3, and 5, while the "hot" connected to the bottom lug serves slots 2, 4, and 6 (ie they alternate like taskswap also guessed).

It looks like this panel is built so that it can be a main panel or a sub-panel. Depending on the choice, there may be some parts that must be discarded.

Breakers "normally" have line applied through some kind of blade terminal in the back that connects to those big bus bars. However, some breakers are listed for reverse use. The 2-pole 30A breaker may be that kind, and may be supplied to use as a main if desired. (or maybe you bought the tandem 30A breaker separately and installed it. I can't tell.)

Maybe if you can answer the following questions, then we can give relevant advice on how to configure the panel.
  • How does the inverter fit into the picture -- should it supply all the loads in the bus, or just some?
  • does the inverter have a built-in breaker so you don't need another "main" downstream from it?
  • Is the inverter the only power source, or is there another (ie shore power)?
  • 120 V or 240 V shore power?
  • How many amps for the shore power? (15, 20, 30 are common for 120 V; 50 A is common for 240 V)
The thread nat_ster linked to is a great one. IMHO its AC grounding advice might be summarized like this:
  • if your bus draws power from an external system ("shore power" like a campground pedestal, a receptacle at home, etc): you should use a shore power cable with conductors for hot (two hot in the case of 240 V split-phase), neutral, and ground. The green or bare ground conductor should attach securely to the bus body; the white neutral should be kept insulated from the bus body. Hot and neutral should go to every AC-powered device, and also every device should have a ground connection either to the bus body or through a dedicated ground wire.
  • if your bus supplies its own power (generator or inverter usually): The AC-powered devices should be wired as described above. The one difference is that in this case, there should be an easily-identified connection between ground and neutral in exactly one place. It might be that green screw in the breaker panel, or it might be located on the generator/inverter.
It gets tricky because often people want to live in both worlds. In such a case, the ground-neutral connection must be done in such a way that it is connected when the generator/inverter is supplying, and it is disconnected when shore power is supplying. Ideally this connect/disconnect would be built into the transfer switch or hookup cables so that it is automatic and correct every time.

I hope I got all that right... the slightest mis-step and I'll be barbequed by the forum electrical posse! (just kidding)

As far as the AC/DC ground concern with the inverter: check its installation manual. You may find that the manual specifically says the AC ground should be connected to the DC ground. My old modified sine xantrex unit does, and in my build thread I describe how I had a 50-something volt shock when I was playing with the equipment on the bench in the shop and laid one hand on the inverter chassis while the other touched one of the battery connections... ah-hah! That's why they said those points should be connected!
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Old 05-06-2015, 09:22 PM   #5
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looks like the lug top left is for the nuetral and the one on bottom is line, 120 v box.pull the breakers and test for continuity.
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Old 05-06-2015, 09:26 PM   #6
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Quote:
Originally Posted by superdave View Post
looks like the lug top left is for the nuetral and the one on bottom is line, 120 v box
I agree about those two lugs. But look closely above the stack of breakers, it looks like there's one more lug just barely visible up there. The upper end of the green wire appears to land in that one. That, and the way the one bus bar on the left seems to have alternating branches, make me think it's a 2-pole/240 V box.
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Old 05-06-2015, 10:22 PM   #7
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Yeah, I have a box similar to (not identical, but close) this. I think family_wagon is right. There are two hot lugs and one neutral. The ones at the top and bottom-left are both hot, feeding a pair of 120V circuits for 240V in the box. Bonding them produces 120V on all six circuits.

I still believe you need the common trip clip (or a breaker that does this internally) on the main breaker.
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Old 05-06-2015, 11:01 PM   #8
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To me it looks like the 30A breakers already have a black plastic clip between the two handles.

If it were me wiring that panel for 120 V only, I'd pull out the double 30A and install a single 30A in one of the slots. Then I'd jumper the two bus bar lugs together, and feed the shore power cord into the single 30A breaker's terminal. That puts a single 30A breaker as the main breaker with the other 5 slots running downstream from it.

My concern about having a common-trip breaker pair in a panel set up for 120 V is that if it's done wrong, they could end up in parallel with each feeding half the circuits in the panel so that it takes as much as 60A before both trip together.
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Old 05-07-2015, 01:15 PM   #9
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You are all incredibly helpful. Thanks a bunch everyone.
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Old 05-07-2015, 02:20 PM   #10
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The inverter you linked over there looks like a nice choice. Its manual mentions that it has an automatic ground-neutral bond relay that switches depending on whether the inverter operates or passes shore power through. Nice. But if you're feeding it from a generator, be mindful that the inverter will have that bond relay open -- and generators usually don't come from the factory with the bond in place, either, so you'd probably need to add that bond to any generator you use.
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