Inverter ground wire size

Maggie Belle

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I’ve looked at other discussions and can’t find a clear answer. I have. 1500 W pure sine inverter. 12v batteries. I can’t figure out what size wire to use for my ac ground to chassis.
 
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Dzl

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Hi Maggie,
Grounding/Safety, is definitely something you want to be clear about, especially on the AC side. Part of the reason you probably have not found a clear answer is because many people don't feel comfortable giving advice on specific safety questions since its a serious issue and very context specific. And because its a sometimes quite complicated topic, especially in the context of a vehicle. I am not qualified or comfortable giving this sort of advice.

I will give some general thoughts though.
1. To make sure we are on the same page can you explain exactly what you mean when you say "wire to use for my AC ground to chassis"
2. The more info you provide the better help you will get. At a minimum your inverter model (and ideally link to the manual), and a brief sketch or outline of your system.
3. There are some good resources on grounding in the resources section. Filterguy and I collaborated on (most of the work and all of the expertise was his) a few beginner resources on grounding that are a little daunting but helpful. I uploaded the grounding section of the National Electrical Code as well as the ABYC (marine code) which is a good resource for the vehicle crowd as well.
4. I would think in general, and I'm not an electrician or electrical engineer, that you would be safe using a chassis ground wire equal to or greater than the current carrying conductors in the same circuit, and greater than the rating of your fuse for that circuit. But this is something you would want to double check. In some contexts you can use a much smaller ground wire for certain purposes, but for the cost of a little extra copper, I'd just as soon keep it simple and err on the side of caution.
5. Grounding and safety is a subject you want to be clear on before you start installing things. If you aren't confident/comfortable with your understanding, it may be worth paying someone qualified to at least double check your design, or help you design or install.

I will ping @FilterGuy, he is a really helpful guy, and has more expertise and experience helping people with system design, and gives good advice/feedback
 
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Maggie Belle

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Thanks for your input. I am a newbie but am becoming more and more comfortable as I work my way through and study as much as possible. It’s been a year in the making - lots of studying and still some things to learn and safety is priority.

I have a 1500 watt GoWise pure sine inverter. My system includes two 100w lifepo batteries, 360 watts of solar and a dc to dc charger. The inverter would be connected to the positive and negative bus bar with 2/0 dc wire. Pure copper only high quality wire in my build. I’m not sure what size wire to use for the ground. Earlier I posted a manual here but it’s the wrong one. I’ll keep looking. Probably just going to call the company.
Thank you for your time. I’ll check through the resources again too.
 
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chrisski

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This seems pretty clear from your manual:

View attachment 73921
My manual says almost the same thing, except to use stranded #8 wire. That’s what I did for my 2000 watt inverter.

THere was a discussion that the Chassis wire should be at least as thick as your DC wire. That would not work with my inverter, a quality SAMLEX 2000 PST 12 volt inverter. The chassis ground lug is 8 AWG maximum, physically can’t fit a thicker wire Like the Positive and Negative DC lugs.

With lack of a better answer, common in DIY work, I’d go with the thickest wire that would fit in your ground Chassis lug, probably 6 AWG or 8 AWG.
 

smoothJoey

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I’d go with the thickest wire that would fit in your ground Chassis lug,
This is pretty much a guiding principal for me now for all device mechanical lugs whether they are current carrying or equipment ground.
 
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corn18

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My manual says almost the same thing, except to use stranded #8 wire. That’s what I did for my 2000 watt inverter.

THere was a discussion that the Chassis wire should be at least as thick as your DC wire. That would not work with my inverter, a quality SAMLEX 2000 PST 12 volt inverter. The chassis ground lug is 8 AWG maximum, physically can’t fit a thicker wire Like the Positive and Negative DC lugs.

With lack of a better answer, common in DIY work, I’d go with the thickest wire that would fit in your ground Chassis lug, probably 6 AWG or 8 AWG.
I have asked about the chassis ground wire from an inverter on several fora. There isn't a clear answer other than to go with what the manual says. Great. My Xantrex manual said 2 things: 1. 8 ga conductor 2. No smaller than one size smaller than the main DC wires, which would be 1/0 in my case. No way 1/0 would fit in the lug they provided for chassis ground. I managed to squeeze 4 ga wire into the lug and used that.

My Victron Multiplus II manual says to use 4 square mm for the chassis ground wire. That is 12 AWG. Great! But in the Victron Multiplus manual (not Multiplus II) it says the chassis ground wire should be the same size as the main DC wires. Holy Crap! That is 4/0 in my case. A 4/0 lug would not fit through the hole for the ground bolt so I used 2/0 which barely fit. Seems silly to use 2/0 for chassis ground. Especially since the manual says 12 AWG. Those recommendations for giant chassis ground wires make no sense to me. What does make sense to me is using the same gauge as the incoming AC line. So in my case, that would be 6 gauge.
 

Maggie Belle

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My manual says almost the same thing, except to use stranded #8 wire. That’s what I did for my 2000 watt inverter.

THere was a discussion that the Chassis wire should be at least as thick as your DC wire. That would not work with my inverter, a quality SAMLEX 2000 PST 12 volt inverter. The chassis ground lug is 8 AWG maximum, physically can’t fit a thicker wire Like the Positive and Negative DC lugs.

With lack of a better answer, common in DIY work, I’d go with the thickest wire that would fit in your ground Chassis lug, probably 6 AWG or 8 AWG.
Thanks. I was leaning toward 6 here as that is what will fit.
 

Maggie Belle

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I have asked about the chassis ground wire from an inverter on several fora. There isn't a clear answer other than to go with what the manual says. Great. My Xantrex manual said 2 things: 1. 8 ga conductor 2. No smaller than one size smaller than the main DC wires, which would be 1/0 in my case. No way 1/0 would fit in the lug they provided for chassis ground. I managed to squeeze 4 ga wire into the lug and used that.

My Victron Multiplus II manual says to use 4 square mm for the chassis ground wire. That is 12 AWG. Great! But in the Victron Multiplus manual (not Multiplus II) it says the chassis ground wire should be the same size as the main DC wires. Holy Crap! That is 4/0 in my case. A 4/0 lug would not fit through the hole for the ground bolt so I used 2/0 which barely fit. Seems silly to use 2/0 for chassis ground. Especially since the manual says 12 AWG. Those recommendations for giant chassis ground wires make no sense to me. What does make sense to me is using the same gauge as the incoming AC line. So in my case, that would be 6 gauge.
This was my confusion in a nutshell.
 

Dzl

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Those recommendations for giant chassis ground wires make no sense to me.
What does make sense to me is using the same gauge as the incoming AC line.
Can you explain your thinking, particulary with respect ot sizing relative to the much smaller AC wiring?

The logic of using the same size wire for your EGC (safety ground/equipment ground) as the current carrying conductors (on the DC side), is that the purpose (well, one purpose) of an EGC is to provide a low current path back to the source that will cause the overcurrent protection (OCP) to trip. Therefore if your OCP is say 100A and your primary wires are rated for say 150A, you'd want your ground wire to be able to carry at least enough current to safely and expeditiously trigger the OCP. If you use a tiny wire for the EGC, there is a possibility of a short with enough current to exceed the rating of the EGC but not enough current to trip the OCP.

Possibly sizing it equal to the current carrying conductors is overkill, since an EGC is not expected to continuously carry current, it just needs to do so for long enough to trip the OCP, but I would think you would still want it near the same size.

I'm not an EE or an Electrician, there are undoubtedly factors I'm overlooking (and also specifics that will depend on use case, system design, the various specific (often overlapping) reasons for grounding. A small wire is adequate to just equalize potential/bleed off any buildup of charge, but it seems a big wire capable of carrying the max current of the circuit would be needed for catastrophic protection.
 

corn18

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The chassis ground on an inverter is for the AC side. Nothing in my camper that is DC only has a dedicated chassis ground. That’s why I think the chassis ground for my inverter should be the same size as the AC feeding it.
 

rmaddy

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The chassis ground on an inverter is for the AC side. Nothing in my camper that is DC only has a dedicated chassis ground. That’s why I think the chassis ground for my inverter should be the same size as the AC feeding it.

I have chassis ground for my metal cased inverter (VE MultiPlus), metal cased charge controller (VE SmartSolar), and my metal PV breaker disconnect box. Only one of those three deals with AC.

My understanding is that the chassis ground is for the metal case so you don't get zapped touching the metal case if something inside the metal case has gone wrong such that there is now current going to the metal case.
 

corn18

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I have chassis ground for my metal cased inverter (VE MultiPlus), metal cased charge controller (VE SmartSolar), and my metal PV breaker disconnect box. Only one of those three deals with AC.

My understanding is that the chassis ground is for the metal case so you don't get zapped touching the metal case if something inside the metal case has gone wrong such that there is now current going to the metal case.
I can understand a safety ground for anything with PV voltages. Those can get pretty high with high amps. But anything running 12V doesn't need a case ground for electrical shock safety. IMHO. And never saw a code that requires case ground for a strictly 12V gadget. But I am not an electrician so it is quite possible I am full of it.

BTW, what size wire did you use to ground your VE SCC? That tiny little screw certainly won't hold a very big wire.
 
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Dzl

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The chassis ground on an inverter is for the AC side.
I am almost certain this statement is true in a generalizable sense. Maybe true in your specific case, I dont know, but I dont think so. I am far from an expert on equipment and system grounding, but I think you need to revisit your understanding.

One major purpose of an EGC is to give fault current a path back to the source, the source being the (DC) battery bank in an off grid system. Its as important to the DC side as the AC side, maybe more so, at least that is my understanding.

I can point two at least two reputable inverter manufacturers (Samlex pg 55, Outback pg 17) that refer to the inverter ground lug as DC side grounding, or DC ground. Screenshots of the relevant sections are below.

Samlex Evo Inverter/Charger
1638316284800.png


Outback FX Inverter/Charger
1638316225067.png


Nothing in my camper that is DC only has a dedicated chassis ground.
Reputable (Victron, Magnum, etc) solar charge controllers often do. DC-DC chargers sometimes do. I think you are right that proper grounding on the AC sdie is much more common and much more non-negotiable than the DC side, but some of the same principles that makes it important for AC make it important for DC, particularly at high currents or voltages. I think that one reason you see less grounding on the DC side is that usually stuff is pretty low voltage/low power.
 

offgriddave

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As a bench setup, I have a 700w inverter hooked up via lamp wire crossed together to a copper plated 8ft ground rod. 2x 16awg, the brown lamp wire stuff from the 80s you can get a roll of it for $35 at a plumbing store. Not the best solution... This is just a temporary setup, it was some wire I had on the bench. But just to give you an idea how little it takes. It shows you how easy it could be. You could get some bear copper wire from stripping 10awg or 8awg and use that.
 

Dzl

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Also note this language from the Samlex Evo manual:
As per American Boat and Yacht Council (ABYC) Standard E-11 for AC and DC Electrical Systems on Boats, the size of DC side grounding wire shall not be smaller than one size under that required for current carrying conductors supplying the device. Hence, for application on EVO on boat / yacht, the size of the DC side grounding conductor should be of the same or one size smaller than the size of battery cable
This comes from from the ABYC (Marine) standard. I don't know if the NEC has similar language or not. I haven't found the NEC to be very clear when it comes to DC in general.
 
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