I'm looking for some input regarding the ground (negative, not neutral) connection for my 1800 watt inverter. I need about 10 of wire to run it back to the battery or I can ground it to the motor home frame with less than 2' of wire. I suspect that the frame will provide a lower resistance return to the battery than any reasonable size wire; however, not knowing the resistance of the frame, it hard to be certain. Thoughts?

The issue is to ensure a line-to-chassis leak gets earth and chassis grounded. Together. The case of the inverter is the point of interest.Shunting of noise atmospheric emissions to earth is also a consideration. Most inverter noise is carried by battery negative and positive power wires.

The frame does have low R. The missing info is how long and what gauge the battery neg to frame wire is. If also 2 ft, then that leaves only 6 ft difference wire vs frame.A complication is the inverter's chassis neg wire to frame, if used. That needs to go to a different frame spot from the neg input wire. the chassis ground is for reducing radio and TV interference.For just 10 ft, if you use fat wire for the neg and pos inputs it would not make much difference in voltage drop running the microwave IMO, but how much is too much? Don't know.For just wire economics for the least voltage drop you can use the frame for the neg and double up on the pos input. (Parallel both 10ft wires on the pos, and use the frame for the neg.) Now you will have way less R on the circuit.EDIT I posted a few years ago, how I added a copper water pipe to the neg path in parallel with the frame for my converter-battery run in the 5er. Got R down to hardly any. Then I added a fat pos wire to parallel that path, and got R down to even less.Point of no returns is when the reduced R lets the converter do its rated amps. Reducing R further is a waste because the converter is current limited.There must be an equivalent point with the inverter. Say when you can already run the MW at 50% battery SOC long enough to do whatever just before it alarms at 11v.No point reducing R more if you don't need to run the MW when the batteries are below 50%.

Something about using the frame for high amperage just doesn't seem right.

2oldman wrote:Something about using the frame for high amperage just doesn't seem right.Why not? It's done practically all the time for starter motors.

I measured the R of my 5er's frame to be 0.013 ohms. Adding the copper pipe got that down to 0.0014 The pos path was reduced to 0.006 with wires.The R stays the same. More amps means more voltage drop.Attaching the linking wires to the frame is the "weak link" where corrosion at the frame can develop

Inverter grounding to batt or chassis | Good Sam Community

BFL13
Explorer II
Dec 10, 2018
More like that here--same wording in many places--looks like Go-Power inverters are Magnum? Or they have the same manual writer! (GP converters are PowerMax)

https://gpelectric.com/wp-content/uploads/2018/05/78158_MAN_ISW_RevD.pdf
jharrell
Explorer
Dec 10, 2018
BFL13 wrote:
I think you are right from reading the manual where they use "chassis". It is true they all use the frame but separately, sort of.

See page 12 diagram. There is a "system" ground to the frame AND the neg from the battery to the inverter neg intake. Also there is an inverter chassis ground (equipment ground) to the frame.

Also something in there about the accessories like the monitor ending up as the neg path if you are not careful.

http://www.magnum-dimensions.com/sites/default/files/MagDocs/64-0089%20A%20Owner%20Manual%2C%20MSH3012RV%20Series.pdf

Diagram on page 26 makes it very clear:
jharrell
Explorer
Dec 10, 2018
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time2roll
Nomad
Dec 10, 2018
pauldub wrote:
OP here. I'll have half my battery bank on the curb side of the motorhome and the other half on the street side. To wire the batteries in a balanced fashion will take extra wire length. This is most of my motivation to ground the inverter DC return directly to the frame. I'll use a tin plated lug which is galvanically compatible with the steel frame. I can also use a chrome plated bolt so that the bolt will be galvanically compatible with the steel and tin.
If you tie to the frame then both banks have a direct connection to the ground... you will need a like separate connection to the positive side to keep the connections balanced.

I don't think the frame negative power connection really helps you.

Do you have 12v or 6v batteries?
MEXICOWANDERER
Explorer
Dec 10, 2018
All cross pieces running from side to side on the chassis are RIVETED to the frame rails and they are not welded::The easiest and most secure way to BRIDGE across the frame rails is at the engine -- the common denominator for a buss:: short lengths of 0 single aught wire can easily pass 500 amps without excessive voltage drop::I have installed systems up to 4,000 watts without the need to run a negative wire forward to the house batteries::A hole clearanced for a 3/8" bolt is fine:: Shine the steel frame rail bright:: Bolt the cable lug to the frame rail and DOUBLE NUT lock it::I use black RUSToLEUM bed liner paint over the completed work and it should last for thirty years or so even with road splash:: Spray the bedliner a few inches up the cable -- covering heat shrink tubing etc.

If you are afraid of corrosion of the fasteners, choose Silicon Bronze which will not corrode ever. It even beats 316 stainless steel.
BFL13
Explorer II
Dec 10, 2018
I think you are right from reading the manual where they use "chassis". It is true they all use the frame but separately, sort of.

See page 12 diagram. There is a "system" ground to the frame AND the neg from the battery to the inverter neg intake. Also there is an inverter chassis ground (equipment ground) to the frame.

Also something in there about the accessories like the monitor ending up as the neg path if you are not careful.

http://www.magnum-dimensions.com/sites/default/files/MagDocs/64-0089%20A%20Owner%20Manual%2C%20MSH3012RV%20Series.pdf

My Vector inverter manual has a special warning about the neg intake from battery and the inverter's chassis ground too.

Do not connect this (inverter's) chassis ground wire to the negative DC input terminal--it goes to a "solid electrical ground" ie the RV frame
2oldman
Explorer II
Dec 10, 2018
BFL13 wrote:
IMO this refers to the chassis of the inverter and not to the frame of the RV
Imo it refers to the chassis of the coach. Would you try to use the same grounding path for the inverter negative as the small grounding cable from its chassis? I don't think so.

I don't know why this is even an issue. Are we trying to save a few bucks on cable?
BFL13
Explorer II
Dec 10, 2018
2oldman wrote:
jharrell wrote:
My Magnum inverter ...manual (snip) specifically says not to use the chassis as DC negative, in their words:Do not attempt to use the chassis in place of the battery negative connection for grounding.
The inverter requires a reliable return path directly to the battery."
Ahh.. why believe the manual? :)

IMO this refers to the chassis of the inverter and not to the frame of the RV
2oldman
Explorer II
Dec 10, 2018
jharrell wrote:
My Magnum inverter ...manual (snip) specifically says not to use the chassis as DC negative, in their words:Do not attempt to use the chassis in place of the battery negative connection for grounding.
The inverter requires a reliable return path directly to the battery."
Ahh.. why believe the manual? :)
MEXICOWANDERER
Explorer
Dec 10, 2018
No OEM frame laterals I know of are not riveted. Keep it in mind in salt ice areas.