Why does my inverter use 4AWG wire for ground plane?

[corn18]

I have a 1200W Xantrex inverter for my residential fridge. It has 4AWG wire running directly from the battery (through cutoff switch) for positive DC input. It has 4AWG wire running directly to the battery negative terminal (actually to the load side of my Victron shunt) for the negative DC input. That makes sense. It also has a 3 foot long 4AWG wire running to the chassis. This is the ground plane wire and I know what it does, but everything else I see (converter/charger, power panel) uses 12AWG bare copper wire for the ground plane connections. I read the Xantrex manual and it says to use the same wire you use for the DC inputs for the ground plane wire. While I understand why Forest River followed the manual, is that really necessary?

The reason I am asking is I am installing a LiFePO4 battery and want to make a nice, compact install of the battery, inverter and converter charger with very short wires in my basement (and ready for solar later). So I am going to move the inverter. Seems silly to run a 4AWG wire to the frame for the ground plane. I was going to replace that with the more standard 12AWG bare copper conductor to the frame.

Does this sound acceptable?

[roblombardo]

That is the code minimum. Your ground configuration is correct. It must be sufficiently sized to allow great enough fault current for over current protection to function.

[corn18]

Originally Posted by roblombardo

That is the code minimum. Your ground configuration is correct. It must be sufficiently sized to allow great enough fault current for over current protection to function.

Why does my 55A converter/charger have a 12AWG bare copper ground plane wire?

And my power panel has a 12AWG bare copper ground plane wire?

This is not the negative DC input. It grounds the case of the item to chassis ground. They are not and should not be the same, although they ultimately meet up on the chassis.

[roblombardo]

Your configuration is correct in all aspects. Grounding conductors are sized relative to the fault currents they are intended to protect against and the voltages that the systems operate at. The DC side of your inverter operating at only 12V nominal, has a very large voltage drop and virtually unlimited fault current (depending on over current protection).Therefore, it must have a much larger grounding conductor. The other two pieces of equipment you mention are very limited in their available fault current, and operate at 120V on the high side. This is a simplified explanation of the code and it’s intent. But, code minimums are not subject to dismissal based on lack of understanding.

[corn18]

Originally Posted by roblombardo

Your configuration is correct in all aspects. Grounding conductors are sized relative to the fault currents they are intended to protect against and the voltages that the systems operate at. The DC side of your inverter operating at only 12V nominal, has a very large voltage drop and virtually unlimited fault current (depending on over current protection).Therefore, it must have a much larger grounding conductor. The other two pieces of equipment you mention are very limited in their available fault current, and operate at 120V on the high side. This is a simplified explanation of the code and it’s intent. But, code minimums are not subject to dismissal based on lack of understanding.

I am trying to understand. (but appreciate your backhanded comment very much).

The WFCO converter is 55A DC. The ground plane wire is 12 ft long 12AWG bare copper conductor. That sure as heck ain't enough to handle the DC side.

Same with the power panel.

I just re-read the Xantrex manual and it recommends 8AWG for the ground plane conductor. I'm going to use that.

[roblombardo]

Originally Posted by corn18

I am trying to understand. (but appreciate your backhanded comment very much).

The WFCO converter is 55A DC. The ground plane wire is 12 ft long 12AWG bare copper conductor. That sure as heck ain't enough to handle the DC side.

Same with the power panel.

I just re-read the Xantrex manual and it recommends 8AWG for the ground plane conductor. I'm going to use that.

DON'T. Read below that. You would never downsize an already installed grounding conductor. These minimums exist to provide a minimum level of protection while mitigating installation cost. Larger sized grounding conductors above code minimum will improve overcurrent protection, and would be considered an upgrade. There is no case under any circumstance for downsizing an existing grounding conductor.

[corn18]

Originally Posted by roblombardo

DON'T. Read below that. You would never downsize an already installed grounding conductor. These minimums exist to provide a minimum level of protection while mitigating installation cost. Larger sized grounding conductors above code minimum will improve overcurrent protection, and would be considered an upgrade. There is no case under any circumstance for downsizing an existing grounding conductor.

So, I need to replace the ground plane wire on my converter/charger to 6 AWG to match the inputs?

[roblombardo]

Originally Posted by corn18

So, I need to replace the ground plane wire on my converter/charger to 6 AWG to match the inputs?

The converter requires a #8 ground minimum. The panel (30A) should be #10 unless there is an exception in article 551 which I cannot verify at the moment...

[corn18]

Originally Posted by roblombardo

The power source is #12, (20A at most), that is where the overcurrent protection is. A fault on the output side will be seen on the input side also. Therefore, #12 ground is fine. Same thinking for the panel.

Now I think I am understanding. The difference between the inverter and the converter is the input amperage. 100A in means 100A ground plane wire. 20A in means 20A ground plane wire. Does it really matter if it's AC or DC WRT the ground plane wire? Just wondering.

Another question then. The 12VDC negative input comes directly from the chassis (actually, the load side of the Victron shunt which is attached directly to the chassis). So I have a 4awg wire running from the chassis to the DC input of the inverter. Now I have to run another 4awg wire from the ground plane lug to the chassis, correct? One wire can't do both?

[jbflag21ds]

4AWG seems large buts it is what it is.

My 3000W inverter charger designed for 30A service only calls for an 8AWG ground cable to the frame.

[corn18]

Originally Posted by jbflag21ds

4AWG seems large buts it is what it is.

My 3000W inverter charger designed for 30A service only calls for an 8AWG ground cable to the frame.

Well, we both drive 2015 Chevy 2500HD trucks, so we should be able to figure this out

[roblombardo]

Originally Posted by jbflag21ds

4AWG seems large buts it is what it is.

My 3000W inverter charger designed for 30A service only calls for an 8AWG ground cable to the frame.

It is to be sized 1 size smaller (minimum) than the input conductor. There is no maximum size.

[roblombardo]

Originally Posted by corn18

Now I think I am understanding. The difference between the inverter and the converter is the input amperage. 100A in means 100A ground plane wire. 20A in means 20A ground plane wire. Does it really matter if it's AC or DC WRT the ground plane wire? Just wondering.

Another question then. The 12VDC negative input comes directly from the chassis (actually, the load side of the Victron shunt which is attached directly to the chassis). So I have a 4awg wire running from the chassis to the DC input of the inverter. Now I have to run another 4awg wire from the ground plane lug to the chassis, correct? One wire can't do both?

#1 CORRECTION: from a code compliance standpoint the converter needs a #8 ground minimum. I suspect it would offer adequate protection as stated above, but if your looking for a conductor to change, there it is. I'm rusty on article 551 as its not in the my daily perview and I don't have my code book with me at the moment...



#2 The battery side shunt should go directly to the battery. The load side of the shunt should have all other negative connections including the chassis connection. This way, no current can circumvent the shunt's measurement.

[corn18]

Originally Posted by roblombardo

#1 CORRECTION: from a code compliance standpoint the converter needs a #8 ground minimum. I suspect it would offer adequate protection as stated above, but if your looking for a conductor to change, there it is. I'm rusty on article 551 as its not in the my daily perview and I don't have my code book with me at the moment...



#2 The battery side shunt should go directly to the battery. The load side of the shunt should have all other negative connections including the chassis connection. This way, no current can circumvent the shunt's measurement.

Thanks for taking the time to walk me through this. Really helps so I can lay out a good setup. The factory install seems to meet "code" but is a royal rat's nest. They use a 6 ft cable to go 2 ft. I guess they just get handed standard cables and hook it all up.

[Sarahcat]

Originally Posted by corn18

I have a 1200W Xantrex inverter for my residential fridge. It has 4AWG wire running directly from the battery (through cutoff switch) for positive DC input. It has 4AWG wire running directly to the battery negative terminal (actually to the load side of my Victron shunt) for the negative DC input. That makes sense. It also has a 3 foot long 4AWG wire running to the chassis. This is the ground plane wire and I know what it does, but everything else I see (converter/charger, power panel) uses 12AWG bare copper wire for the ground plane connections. I read the Xantrex manual and it says to use the same wire you use for the DC inputs for the ground plane wire. While I understand why Forest River followed the manual, is that really necessary?

The reason I am asking is I am installing a LiFePO4 battery and want to make a nice, compact install of the battery, inverter and converter charger with very short wires in my basement (and ready for solar later). So I am going to move the inverter. Seems silly to run a 4AWG wire to the frame for the ground plane. I was going to replace that with the more standard 12AWG bare copper conductor to the frame.

Does this sound acceptable?

Just for the sake of clarity, the term for this connection is “ground”, not ground plane. With regard to a bond to the trailer frame, it’s not unreasonable to have a much lighter gauge wire to connect the negative terminals from one component to another, since those bonding connections are sized for the capacity of circulating current in the ground system. The 4 AWG cables from the battery to the inverter are sized for the maximum input current for the rated output current of the inverter, which requires over 80+ amperes for a 1000 watt A.C. load. (1000 watts/12 volts = 83.33 amps, assuming 100% conversion efficiency, which is not realistic). 83.33 watts or more can flow between the battery and the inverter at maximum load, but that current does not flow through the grounding bond. To understand this, the circuit can be modeled, as is done in circuit analysis, but that is usually not done in practice, due to the expertise required.

BTW, a 3000 watt inverter at maximum load would increase 12 volt load 3x to at least 250 amperes, however, the circulating current in the ground bond would change little, if at all.

[Larry-NC]

Originally Posted by Sarahcat

Just for the sake of clarity, the term for this connection is “ground”, not ground plane.

Right. The term "ground plane" is used in conjunction with radio frequency (RF), any frequency which could conveniently be used for transmission of radio waves.
EDIT: It case it's not obvious, DC and 60 Hz do not qualify as radio frequency (unless you happen to have a 60 Hz conductor that's 6/10 of a mile long on your RV).

For example, if you had an monopole antenna (AM, FM, CB, etc.) on the metal roof/hood of your car, that metal serves as the ground plane to the antenna. If you wanted to calculate the transmission (or receive) pattern, you would do so with respect to the ground plane metal.

In multilayer printed circuit cards for today's high-speed electronics, one plane (layer) of metal is at ground, often an outside layer. This helps to reduce undesired electromagnetic radiation which interferes with other electronics. Definitely used in computers/tablets/phones, and the dozens of computers in your car/truck/motorhome.

This is always an area-based conductor. It doesn't make sense to call a cable a ground plane as Sarah has stated.

[roblombardo]

Originally Posted by Sarahcat

Just for the sake of clarity, the term for this connection is “ground”, not ground plane. With regard to a bond to the trailer frame, it’s not unreasonable to have a much lighter gauge wire to connect the negative terminals from one component to another, since those bonding connections are sized for the capacity of circulating current in the ground system. The 4 AWG cables from the battery to the inverter are sized for the maximum input current for the rated output current of the inverter, which requires over 80+ amperes for a 1000 watt A.C. load. (1000 watts/12 volts = 83.33 amps, assuming 100% conversion efficiency, which is not realistic). 83.33 watts or more can flow between the battery and the inverter at maximum load, but that current does not flow through the grounding bond. To understand this, the circuit can be modeled, as is done in circuit analysis, but that is usually not done in practice, due to the expertise required.

BTW, a 3000 watt inverter at maximum load would increase 12 volt load 3x to at least 250 amperes, however, the circulating current in the ground bond would change little, if at all.

The last part of this is incorrect. There should never be any significant current flowing on the grounding conductor under normal operation. Let me try to explain this again, suppose we have a line to inverter chassis fault where in the inverter chassis becomes energized. There must be a ground path of sufficient size to pass sufficient current for the overcurrent protection on the feed side of the inverter to function. If the ground is too small, insufficient current will flow and the overcurrent protection device will not open, thus allowing the chassis of the inverter to remain energized and remain a hazard. Ground conductors are always sized relative to the feed. It’s not a one to one relationship nor does it scale proportionally, but it is relative. Larger feed circuits will always have larger ground conductors.

[Larry-NC]

Originally Posted by roblombardo

The last part of this is incorrect. There should never be any significant current flowing on the grounding conductor under normal operation. Let me try to explain this again, suppose we have a line to inverter chassis fault where in the inverter chassis becomes energized. There must be a ground path of sufficient size to pass sufficient current for the overcurrent protection on the feed side of the inverter to function. If the ground is too small, insufficient current will flow and the overcurrent protection device will not open, thus allowing the chassis of the inverter to remain energized and remain a hazard. Ground conductors are always sized relative to the feed. It’s not a one to one relationship nor does it scale proportionally, but it is relative. Larger feed circuits will always have larger ground conductors.

Rob, Rob, slow down.

Your knowledge of the electrical code is impressive, but we're talking about the DC side here. Since it's below 42 volts, the IEC stuff doesn't apply. Anything below 42 volts is considered inherently safe.

When Sarah is talking about 80+ amps at 12 VDC, that's clearly on the DC side.

As such, it is very common in trailers (and older motorized vehicles) to use the metallic chassis as the power supply return. Ground and "negative" are completely blurred. Situations like an AWG 4 or 6 wire to a starter motor with +12 VDC where the negative battery cable terminates someplace on the engine block. Or tail lights with one hot wire, shell mounted on the body sheet metal.

In my 2008 trailer, the converter negative goes to the fuse panel. From there an AWG 4 cable goes to...the frame. And three feet further down the frame is where the battery cable terminates.

[roblombardo]

If you reread my last post I was only taking issue with the equipment grounding conductor sizing, which is the whole point of this thread. While some articles of the NEC do not apply to systems operating under 50V, many do. In fact NFPA section 551 defines requirements for RV installations, operating above and below 50V.

[corn18]

I have never seen a separate equipment grounding wire on my truck. Only on my 5er and only on things that have 120V somewhere in them.