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Old 09-03-2021, 10:02 AM   #1
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Why does my inverter use 4AWG wire for ground plane?

I have a 1200W Xantrex PSW 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?
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Old 09-03-2021, 10:24 AM   #2
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Did you check the manual? Both manuals I checked, for the Freedom and the Freedom X specify 8 AWG for the ground, either bare or green. I wouldn't use anything smaller.
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Old 09-03-2021, 10:44 AM   #3
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Quote:
Originally Posted by HarryStone View Post
Did you check the manual? Both manuals I checked, for the Freedom and the Freedom X specify 8 AWG for the ground, either bare or green. I wouldn't use anything smaller.
Well, I went back and read the manual and now I am more cornfused.

page 17 table 3 says 8 AWG but right under that is a note that says

"In general, the equipment ground cable size must not be smaller than one AWG size than the supply cable."

Since they recommend 2AWG for the DC inputs, then that would imply 3 AWG for the DC ground wire. Ouch.

Then on page 30 it says:

"Recreational Vehicle Use 8AWG copper wire and connect it between the Chassis Ground lug and the vehicle’s DC grounding point (usually the vehicle chassis or a dedicated DC ground bus)."

I'm going to go with 8AWG bare copper and call it a day.
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Old 09-03-2021, 12:14 PM   #4
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Use the larger wire specified.

Positive, negative, and system ground wires are all protected by circuit breakers or fuses. The circuit breaker or fuse is in the positive wire near the power source, but also protects the other wires.

A system ground may be a little smaller than the positive and negative. One reason is the system ground is frequently bare, and dissipates heat better.

Another reason is the system ground does not have to conduct over current for long. It usually trips a circuit breaker or fuse quickly.

A third reason, is the voltage drop from end to end is not a concern as it is for the positive and negative cable. Its main purpose is to trip circuit breaker or fuse when a malfunction occurres.

The difference in whether to use 8 or 4 gauge for system ground depends on the fuse or circuit breaker in the positive wire from the power source.

The purpose of the 4 gauge wire from battery to circuit breaker may or may not be to reduce end to end voltage drop. I would certainly not decrease that size in order to run for a longer distance.

The key is what is the size of the circuit breaker protecting that circuit. Follow the instructions for the installation for the inverter. When other system instructions specify something less, use the larger wire specified.

I wish you good luck and happy trails ahead!
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Old 09-03-2021, 01:08 PM   #5
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Quote:
Originally Posted by Persistent View Post
Positive, negative, and system ground wires are all protected by circuit breakers or fuses. The circuit breaker or fuse is in the positive wire near the power source, but also protects the other wires.

A system ground may be a little smaller than the positive and negative. One reason is the system ground is frequently bare, and dissipates heat better.

Another reason is the system ground does not have to conduct over current for long. It usually trips a circuit breaker or fuse quickly.

A third reason, is the voltage drop from end to end is not a concern as it is for the positive and negative cable. Its main purpose is to trip circuit breaker or fuse when a malfunction occurres.

The difference in whether to use 8 or 4 gauge for system ground depends on the fuse or circuit breaker in the positive wire from the power source.

The purpose of the 4 gauge wire from battery to circuit breaker may or may not be to reduce end to end voltage drop. I would certainly not decrease that size in order to run for a longer distance.

The key is what is the size of the circuit breaker protecting that circuit. Follow the instructions for the installation for the inverter. When other system instructions specify something less, use the larger wire specified.

I wish you good luck and happy trails ahead!
Awesome sauce! The CB protecting the DC side is 100A. Which makes sense because it is a 1200W inverter. The CB on the AC side is 15A, which also makes sense I guess because it is @ 120V vs 12V. So the current is a lot less.

If the negative DC input is coming directly from the chassis (which it is in my case) then why can't I just connect the ground plane wire to the incoming wire? Seems silly to have two 4 ft/4 awg wires going to the exact same spot.
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Old 09-03-2021, 01:59 PM   #6
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The purpose of the ground wire size is; if the inverter shorts internally to ground, you want that energy going somewhere, and if you use a wire that is too short, then you have essentially a long resistor, which will burn up with sparks and possible flames, instead of taking that current to ground.
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Old 09-03-2021, 02:37 PM   #7
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I saw this on a different inverter and was similarly puzzled.

One can say it's to "safely" direct fault currents to ground but this is 12V DC. I'm not aware of why you would need a safety ground for 12V. If it's to provide a safety ground for the 120V output you most certainly wouldn't need a 4 ga wire for a 15 or 20A circuit. Maybe there's some other technical reason why you would need such a heavy conductor to connect to chassis ground but it's not terribly obvious. I get that not all installations are negative ground and maybe that's part of it, but for a negative ground installation it does seem kind of redundant.

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Old 09-03-2021, 02:39 PM   #8
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So, you have 4 gauge to inverter, 4 gauge to vehicle frame ground, and 4 gauge to battery negative.

You also may have other connections like chassis 12 volt wire and other things that may be smaller wire.

Frame ground should be fine connected to either battery or inverter.

You could even connect inverter negative to frame and battery negative to frame with no other wire in between. Actually I have not seen specifications that include this configuration. I am not sure what frame ampacity and voltage loss would be.

PS: A ground plane usually refers to a radio frequency transmission from transmitter through antennae into air or vacuum. Ground plane is the earth side of the transmission.
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Old 09-04-2021, 07:23 AM   #9
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Bonding neutral and ground

After rereading the post, it occurred to me there is a second system ground from an inverter. The 120 volt neutral wire must be bonded to (connected to) system ground wire.

For 12 amps AC you could use 14 gauge wire. The thing is, it may need to be removed when you connect to shore power.

If you have a transfer switch, it may disconnect the bond when it connects to shore power. If you don't disconnect it, it will trip shore power GFI's.

My system is a 1000 watt inverter. The 120 volt output is completely separate from shore power. The inverter bond connection does not trip GFI's. If your refrigerator is the only appliance on the inverter and it never switches to shore power then a permanent bond connection is OK.
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Old 09-05-2021, 08:05 AM   #10
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Here's the inverter diagram I'm wondering about, and accompanying text. I can't figure out what the purpose of this heavy duty chassis ground is for.

Wire Earth Ground as shown in Figure 5 using UL Listed 4 AWG (25
mm2) or larger green wire. The Earth Ground conductor must be larger
than the battery power conductors.

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Old 09-05-2021, 08:58 AM   #11
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Quote:
Originally Posted by Mark_K5LXP View Post
Here's the inverter diagram I'm wondering about, and accompanying text. I can't figure out what the purpose of this heavy duty chassis ground is for.

Wire Earth Ground as shown in Figure 5 using UL Listed 4 AWG (25
mm2) or larger green wire. The Earth Ground conductor must be larger
than the battery power conductors.
The instructions and the picture do not appear to go together. The picture says 300 watts Pure Sine Wave. You should not need 4 awg wire for any connections.

14 gauge wire is more than enough for 300 watts, 2.5 amps, 120 volts. 10 gauge is more than enough for 25 amps 12 volts DC.

The "earth ground" may be needed in some stationary installations although that seems unlikely for a 300 watt system, but is typically not needed in mobile applications.

A 14 gauge system ground wire is needed on the 120 volt side (I did not see where to connect it).

10 gauge frame ground is needed on the 12 volt side. However, there is probably a larger, maybe 4 awg ground on the battery negative for other 12 volt devices.

So, frame ground the battery negative with 4 gauge wire.

Use 10 gauge for the inverter "earth ground", negative, and positive wires. The 10 gauge positive should have a 30 amp fuse or circuit breaker.

You may need larger 12 volt DC wire for longer runs. See installation instructions for size - distance. Or, just use 8 gauge to be sure.

14 gauge will be more than enough for the 120 volt side at any likely length.
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Old 09-05-2021, 11:34 AM   #12
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The diagram illustrates the DC input so heavier wire gauge is expected. The description does go with this diagram, it's on the same page and references that very figure. I'm trying to picture a scenario with a ground fault on either the DC input or AC output would result in currents that require a wire gauge greater than the primary cables. Maybe it's a case of liability overkill but I can't picture a technical reason why this would be necessary.

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Old 09-06-2021, 08:13 AM   #13
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Inverter Wire Size

I see the Morning Star Sure Sine 300 manual says what you say it does.

https://www.morningstarcorp.com/wp-c...uresine-en.pdf

However, the 4 gauge wire and 100 amp fuse are not consistent with the loads the wire will carry.

The 300 watt inverter will likely never draw much more than 23 amps DC. 10 gauge wire with a 30 amp fuse is likely enough. 8 gauge may produce a little better performance when the battery bank is drawn down low.

The inverter has a 200% surge capacity. That means the inverter could briefly draw 46 amps. 8 gauge wire would be sufficient. Possibly 6 gauge would work better and prevent low voltage shut down during motor starting. Long distance wiring may require a larger wire for motor starting. Now we are up to 4 gauge.

My inverter wire gauge chart shows 4 gauge wire is good for 50 amps at 33 feet. 8 gauge wire is good for 50 amps up to 16 feet. 10 gauge wire is good for 50 amps up to 4 feet.

10 gauge wire and connectors may get hot if run at 50 amps for more than 30 seconds. I would not choose it for 50 amps, or even motor starting.
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Old 09-06-2021, 08:26 AM   #14
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There are many different opinions above. Contact the inverter and RV manufacturers to ask the questions to get their reasoning.

Without doing so, larger the wire greater the safety.
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