Small inverter: chassis ground per ABYC

[ThanksLori]

I plan to install a small inverter (700W or less). There is no other AC on the boat and I will use only the receptacles built into the inverter. A very simple installation...maybe.

I am also trying to comply with ABYC. The inverter chassis ground cable must the same size or one size smaller than the DC positive feed.

So I bought a 700W inverter - marketed for use on boats - and the chassis grounding lug is tiny; way to small to properly fit a cable connector meeting the ABYC requirement. I looked at other small "marine" inverters online and they all seem to have similar tiny little grounding lugs.

I really was hoping to avoid spending $1k+ and in the process end up with a larger inverter than needed. I saw a suggestion somewhere to "sand off" the paint on chassis, drill a bigger hole, and just fasten the ground cable with nut-and-bolt (with a StSt washer between chassis and bolt).

What do you think? is this workable? Any other ideas or suggestions for a brand/model that capable of meeting ABYC without modification? Many thanks!!

[pnw2022]

This seems to be a common problem. I recall reading a pretty lengthy article somewhere (maybe MaineSail) about most inverters having undersized ground lugs and basically non-compliant. A few pictures of some actual installations as well. Unclear why some (most?) are built this way, perhaps for the RV or home market or something the smaller grounding wire size is allowable? The boating market is pretty small in comparison to the overall market.

I don't have a solution, other than probably sanding and expanding the hole seems practical. I am not an electrician, your question just reminded of that old article on this topic.

[CharlieJ]

OP: Your alternate method is spot on except for the stainless steel washer step. Stainless steel adds significant resistance to a circuit. Simply remove paint, insert a bolt or machine screw and attach the chassis ground lug to it. Easy peasy.

[pnw2022]

Separately - I was actually just looking through wiring docs for my boat today. This is a 2009 Hunter 41DS. On a lark, took a peek at the inverter wiring and it actually shows a smaller gauge wire for the charger/inverter chassis ground, and a smaller ring at the inverter side for the chassis ground.

INVERTER (120V)
2/O RED 9' 3/8" 3/8" INV TO STUD
2/O YELLOW 8' 3/8" 3/8" INV TO GND
1/O GREEN/YELLOW 8' 3/8" 1/4" INV CASE TO GND

[ThanksLori]

Thanks, pnw and CharlieJ. My guess is there are lots of inverter installations with too-small chassis grounds...
Fair winds -
M

[s/v Jedi]

It says you can use one size smaller so what is the problem?
Afaik, 1/0 is one size smaller than 2/0.

[pnw2022]

Originally Posted by s/v Jedi

It says you can use one size smaller so what is the problem?
Afaik, 1/0 is one size smaller than 2/0.

Ahh - I didn't know that, makes sense.

[FlyingScot]

The ABYC requirement is one size smaller than the required size. If you oversize the other cables for minimizing voltage drop, you do not have to oversize the chassis ground cable.

[CharlieJ]

FlyingScot:
A case ground one size smaller than the B+ is an exception that is allowed with other conditions. You are generally correct but there are detail.

[Bowdrie]

The OP is speaking about a 700 watt inverter.
What are all these posts about 1/0 and 2/0 wire?
I reckon that knowing the length of runs might help in the decision-making process.
PS: probably help in the fuse/breaker selections also.

[sailingharry]

I hadn't though about the problem you identified. I went and looked at my inverter, a 3kW Victron. The case is 1/8" aluminum. I suspect the aluminum will melt before the 3/0 case ground gets hot! a 1/8" bus bar would never be rated for 400A.



In your case, the case ground is much smaller. A 700W inverter can pull maybe 100A. A #6 is good for 120A, I suspect you could probably get away with a #8 (80A, so ABYC approved for a fuse to 100A). So connecting that to your case is fairly easy.


But check my related thread on where to land the other end of that case ground. ABYC is completely silent on the matter (they say were to put it, but use words that have no definition -- and using meaningless words is essentially being silent).
https://www.cruisersforum.com/forums...er-291203.html

[CharlieJ]

@sailingharry #11

ABYC A-31 Chargers and Inverters

I suspect the aluminum will melt before the 3/0 case ground gets hot!

The point of the case ground is to provide an alternate return path from a metallic case should a fault develop in the equipment to allow enough current to open a fuse/circuit breaker in the B+ supply. Otherwise the AC safety ground would be the only return path and it surely would melt under fault conditions!

ABYC A31 Chargers and Inverters:

31.7.5 DC Grounding Connections
31.7.5.1 The DC grounding conductor (see FIGURE 1, FIGURE 2, FIGURE 3, and FIGURE 4) shall:
31.7.5.1.1 be connected from the metallic case or chassis to the engine negative terminal or its bus...

[sailingharry]

Charlie,


The issue is that no where is "the engine negative terminal or its bus" defined (not in A-31 or E11). On my other thread, I drew up the attached drawing. Electrically, terminals A, B, C, D, or E are identical. Wires A-B-C are being removed (undersized). My new ground will be the yellow wire, reducing length by 2/3 and reducing the number of connections (my shunt has an open terminal, so no stacking required).



Which of those terminals (A-E) are "the engine negative terminal or its bus?" They are all electrically identical. Is it legal to install my case ground as a 16" wire between the case and the spare negative terminal in the inverter (shown as the green wire)? This is no different that connecting both wires at the other end (at the shunt -- but is the shunt even legal, and ONLY C is legal?).


My problem is this seems too simple. If this is all that is needed, it would make a LOT of sense for Victron to make that connection internally and solve the problem -- and ensure that even beginners get this part right. But if it's not OK, why not?

[CharlieJ]

@sailingharry #13:

Is it legal to install my case ground as a 16" wire between the case and the spare negative terminal in the inverter (shown as the green wire)?

No.

In my explanation at #12, a better word than “alternate” would be "redundant". That is the intent of the Standard and what is shown in A31 Figure 3.

BTW, the NEC also requires a separate case ground although much smaller than the ABYC requirement . This is currently under discussion/review at the ABYC.

[GordMay]

See ➥ https://www.cruisersforum.com/galler...r&imageuser=79