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Old 05-18-2021, 01:20 PM   #1
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Denver , Colorado
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Full Monty - Ground Wire Question

All -

I am in the process of doing a full monty on a 1974 Ambassador. I am finally getting to the point of systems and I am starting to design all the wiring schematics for the power system.

I have already ordered the majority of equipment from AM Solar, but the question I am not finding a lot of info on is ground wiring. Keep in mind this entire discussion is for the 12V system NOT 120V.

So with all my history of remodeling cars I have always relied on frame/body as the 12V ground for all relatively low AMP draws. Things like LED lights, fans etc.

On the Airstream I am planning on running a ground wire to the frame from the grounding block/battery. I have read different opinions on using the shell/ribs as grounding sources for lower amp items such as lights and fans. Here are the options the way I see them and would love your feedback.

1) Run ground wires (same gauge as power wire) back to grounding block. This is definitely the most expensive option due to the extra wire, but also simply a lot of wire.

2) Run a 6 or 8 gauge wire from the grounding block down the center line of the inside shell and install a 2 or 3 grounding blocks to bring ground wires to. (Accomplishes same as #1, but seems cleaner in my head).

3) Ground all low amp draw items (think LED lights & fans) to the closest shell/rib.

Let me know your thoughts!
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Old 05-18-2021, 01:35 PM   #2
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Full Monty - Ground Wire Question

I’d vote for option 1: run both + and - to all the DC loads. Small gauge wire is cheap. I’d use 2-conductor. You can daisy-chain LEDs to save wire. Your AC system is also “grounded” (bonded) to the frame and you don’t want current flowing through your AC grounds. Your car doesn’t have an AC system.
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Old 05-18-2021, 02:56 PM   #3
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Quote:
Originally Posted by AKNate View Post
I’d vote for option 1: run both + and - to all the DC loads. Small gauge wire is cheap. I’d use 2-conductor. You can daisy-chain LEDs to save wire. Your AC system is also “grounded” (bonded) to the frame and you don’t want current flowing through your AC grounds. Your car doesn’t have an AC system.


Good point on the car not having an AC system.
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Old 05-18-2021, 06:09 PM   #4
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I used dual stranded runs for the 12v, like option 1, and daisy chained the lights as mentioned, but the panel box ground bar and batteries are grounded to the frame as is the 120v ground.
As far as I know, you just want to make sure not to bond the neutral and ground in your a/c system. I will be watching to see what others have to say.
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Old 05-18-2021, 06:27 PM   #5
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I did some rewiring on my 2009.

In the course of that I found each major component of the trailer body--the two end caps, the two sides, and the roof, had a ground post to which the white (ground) was run. The frame also has a ground post.

All 12V loads were 'home runned'--that is they had a dedicated return run of white wire back to the 12V load center. No loads were return circuited thru the frame or body.

Now why Airstream does that I can only speculate, but I chose to do what they did.

Burnside
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Old 05-18-2021, 06:28 PM   #6
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Due to the inherent rust on the frames of an AS, I'd agree that running a pos. & neg. wire to each device is important. Seems like 90+ % of electrical issues I've seen on motor homes and TTs can be traced to a faulty ground, so you might as well give yourself as good a start as possible.

Quote:
Originally Posted by AKNate View Post
I’d vote for option 1: run both + and - to all the DC loads. Small gauge wire is cheap. I’d use 2-conductor. You can daisy-chain LEDs to save wire. Your AC system is also “grounded” (bonded) to the frame and you don’t want current flowing through your AC grounds. Your car doesn’t have an AC system.
Most motor homes use the chassis for both the 120v ground AND the 12v system. Nearly every vehicle I've seen has the 12v DC system negative side tied to the chassis, and nearly every 120v appliance mounted on a motor home will also be grounded to the chassis.
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Old 05-19-2021, 10:12 AM   #7
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Hi

If you look at a modern AS, they do indeed run ground to the *frame* of the trailer. They seem to avoid running ground to the shell. Simply put, they run ground cables to steel, but not to aluminum.

There are a lot of reasons this is a good idea. Getting a solid electrical connection to the shell is tough. Keeping everything well connected without welding is difficult. That's just the start of the list.

So: Running wires back to a common point is fine. Running ground wires to multiple ground blocks hooked by heavy wire is fine. Tying those ground blocks to the steel frame should make the voltage drop lower.

12V wiring is a double whammy sort of thing. You have a wire rating to keep it from catching fire. You also have a wire rating to keep the voltage drop from causing problems. Once you get past a few feet, the voltage drop part tends to be the bigger issue.

The good news is that we live in the era of LED lighting. The good old days of 5A here and 5A there for this or that light are long gone. You can put a lot more lights on a piece of number 18 wire today than you used to be able to.

Fun !!!

Bob
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Old 05-20-2021, 08:25 AM   #8
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Quote:
Originally Posted by uncle_bob View Post
Hi

If you look at a modern AS, they do indeed run ground to the *frame* of the trailer. They seem to avoid running ground to the shell. Simply put, they run ground cables to steel, but not to aluminum.

Bob

Er, I don't think so. But I agree with the rest of your post about 12V wiring.

Sincerely,

Burnside
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Old 05-22-2021, 04:58 PM   #9
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I think all the clearance lights and backup light LEDs are grounded to the shell at their fixtures. My trailer's DC is now grounded to the frame in the front of the trailer and the left rear frame. The AC is ground to the frame in the rear. All the bucked rivets and elevator bolts conduct ground. For safety's sake the shell and frame as well as any generator should be grounded to an earth. That would eliminate the possibility of potential voltage between the trailer and earth rather than through a human standing and touching the skin. If you rely on a campground pedestal to be wired correctly, you may be in for a surprise. That is why surge protectors are such a good idea.
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Old 05-22-2021, 11:56 PM   #10
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Full Monty - Ground Wire Question

Quote:
Originally Posted by guskmg View Post
For safety's sake the shell and frame as well as any generator should be grounded to an earth. That would eliminate the possibility of potential voltage between the trailer and earth rather than through a human standing and touching the skin.

Portable generators should not be grounded to earth and doing so actually creates the hazard you are talking about. If your generator is not referenced to earth, you could connect the hot wire straight to the skin of the trailer, walk up and touch it and be fine because there is no path back through you to the generator. If however your generator is earth grounded, you now have the potential voltage between that hot wire and the earth you’re standing on.
I agree with you that a good EMS is important and improves safety. Campground power IS referenced to earth and a faulty ground connection when connecting to that power can be very unsafe.
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Old 05-23-2021, 05:31 AM   #11
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Agree - for hot skin safety what's important is that the shore power cord makes a good connection on the ground conductor. Lots of things can interfere with the ground connection - on both the cord side and the trailer side.
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