Sharing common (ground) questions

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Sharing common (ground) questions

Postby Vspec » Thu Jun 08, 2017 8:31 am

Bit of background before I start.

My Neo NAMR, build thread here was ordered with a 30amp package (AC) and a fan, LED strips inside, loading light side door and ramp door(DC). The AC has the earth (not ground) tie into the frame of the trailer. The DC, which is currently feed by the trailer (7pin) wiring as the ground (common) also connected to the frame. All the none trailer related DC share a single feed (black wire) and all DC (trailer and trailer lights (stop, turn,etc)) use the frame as a common.

NOTE: from this point on, I will only use common referring to DC and earth referring to AC.

I am puzzled as to why the AC earth and the DC common are shared. Would that not pose an issue if both AC and DC were energized at the same? I don't plan for this condition to happen often if I run the generator while the trailer is still hooked up to the TV.

The DC electrical will happen in two phases.

Phase 1: Leave everything the way it is until we have a better understanding of our desired layout but gain the ability to run the DC when not connected to the trailer (black wire is not energized unless engine is running) via either a battery or a converter hooked up the AC when shore power (or generator) is available.

Phase 2: Will probably included a PD4045 (need provision for my GFI outlet), radio, USB for charging, 12v socket, extra lights and maybe solar and maybe maybe maybe a DC fridge.

For phase 1, I see a couple of options:

Option A: Leave everything as is, do not isolate DC trailer lights and DC trailer. Connect 12v + source by tapping into feed (black) and connecting 12v - to the common (frame) would throw a fuse for good measure. This has me worried that if trailer was connected to TV and 12v source was energized at the same time bad things would happen.

Option B: Do not isolate DC trailer lights and DC trailer. Cut feed from trailer harness. Feed DC trailer from 12v source like option A. I have the same concerns as option B where the common is shared.

Options C: Isolate DC trailer from DC trailer lights by running separate common and cutting feed (black) from trailer harness. Feed 12v same way as Option A.

Option D: Same as option C but use fused distribution block and run dedicated feed for all appliances (LED strips, fan, flood lights). Run dedicated but shared common. This looks like the safest option.

Option E: Same as option D but run dedicated common wire. Note sure if necessary, doubles the work and supplies.

Any disadvantages to running a single common?

Do I plan to have LED strips, fan, side and rear floods at the same time? Not really but it could possibly happen. Do I know how the amp draw when everything is on? Not yet, I'd have to disconnect the DC ground from the trailer harness and hook it up to my multimeter while connected to the TV with it's engine running.

Thanks for your thoughts!
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Re: Sharing common (ground) questions

Postby H.A. » Thu Jun 08, 2017 3:26 pm

The AC has the earth (not ground) tie into the frame of the trailer.
Terminology of AC power, "earth" and "ground" are the same thing. (USA folks mostly use "ground". 'Brits & many others, use "earth")
I suspect you are confusing "ground" and "neutral". Neutral is a grounded conductor but ground and neutral are not interchangeable as words or electrical connections.
I am puzzled as to why the AC earth and the DC common are shared. Would that not pose an issue if both AC and DC were energized at the same?
AC earth will never be energised unless there is a problem with the AC wiring. Further, If connected to a working GFCI circuit, AC power is instantly switched off were a problem occur.

"Common" or "ground" on your DC system is simply using metallic structure as a negative conductor of a DC circuit.
DC positive is electrically isolated from either conductor of your AC power. DC negative may share the same metallic structure as an earth connection.
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Re: Sharing common (ground) questions

Postby Vspec » Thu Jun 08, 2017 6:28 pm

Thanks for the answer. On the AC side what I meant by earth is the naked copper conductor that is usually burried into the ground, err soil.
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Re: Sharing common (ground) questions

Postby low277 » Thu Jun 08, 2017 9:37 pm

Vspec wrote:Thanks for the answer. On the AC side what I meant by earth is the naked copper conductor that is usually burried into the ground, err soil.

In the U.S. the naked, bare, green or green with yellow stripe is the grounding conductor. It is not normally a current carrying conductor. It's purpose is to provide a low resistance path for any fault current to return to its source and open the over current protection device ie. Fuse or breaker.

Back in my electrical school days they taught us in AC theory that there are only 3 kinds of conductors in a circuit.
Ungrounded, Grounded, and Grounding. Grounding can be further divided into Grounding or Bonding. There are whole books written on Grounding/Bonding. If your into that kind of stuff!!! :)
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Re: Sharing common (ground) questions

Postby Dale M. » Fri Jun 09, 2017 7:35 am

The ground or earth (bare copper -green or green/yellow) is termed as "Safety Ground" and should always be at same potential as earth we stand on ( usually bonded to chassis and power supplied through campground electrical) ... AC neutral is a load carrying "return" for electron flow and strangely enough same potential as Ground... General in residential wiring the neutral and the ground are tied Common at entry point (main meter panel).... Having DC ground or "negative" bonded to "safety ground" is pretty normal as in DC circuits it is also the "return"... Dont worry the electrons know where to go at all times UNLESS you manage to put the "hot" side of a DC circuit to the "hot" side of the AC circuits then they get confused and cause "smoke"...

IF your trailer/caravan is factory made chances are that its 99% sure to be wired correctly as most manufacturers know proper way to wire its individual systems... The problem lies where people who are not really up in electron flow theory try to second guess how its done and think about it to deeply... IF its not a problem, don't fix it...

As it is everything in the world eventually connects to a common earth (and I mean the big dirt ball) and it has not exploded yet...

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Any statement made by me are strictly my own opinion.
You are free to ignore anything I say if you do not agree.

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Re: Sharing common (ground) questions

Postby Vspec » Mon Jun 12, 2017 6:29 am

What about option A to E?
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Re: Sharing common (ground) questions

Postby steve cowan » Mon Jun 12, 2017 7:02 am

The "earth" ground you are referring to on the AC circuits purpose is if for some reason the trailer frame became energized it would complete a path to "earth" instead of a body
touching frame and completing path.For the DC side I always run a "common" wire to each individual item cause wire is cheaper than the PITA that will eventually show up in a
poor connection to the "common" frame.
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Re: Sharing common (ground) questions

Postby low277 » Mon Jun 12, 2017 5:14 pm

steve cowan wrote:The "earth" ground you are referring to on the AC circuits purpose is if for some reason the trailer frame became energized it would complete a path to "earth" instead of a body
touching frame and completing path.For the DC side I always run a "common" wire to each individual item cause wire is cheaper than the PITA that will eventually show up in a
poor connection to the "common" frame.


:thumbsup: :thumbsup: I also like to run the common wire in DC circuits!
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Re: Sharing common (ground) questions

Postby Vspec » Mon Jun 12, 2017 11:13 pm

A single (wire) common? Or a (common) wire for each item?
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Re: Sharing common (ground) questions

Postby steve cowan » Tue Jun 13, 2017 4:03 am

Yes, 2 wires to each DC item. A positive + and a negative - .The negatives are all connected together on a "common" point or buss/terminal. I'm not saying your factory built
trailer should be rewired but if adding on other components 2 wires are a better choice.
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Re: Sharing common (ground) questions

Postby Vspec » Tue Jun 13, 2017 7:57 am

At this point I have 100ft of 18g white and 10ft 18g black wire (I also have the same in 16g but since the rest of the trailer is 18g I may stick with that) so running both black and white wires is not terribly more work. And it actually much simpler to run wires then try to tie them into a single one.

I already have this for the feed: https://www.princessauto.com/en/detail/fuse-block-w-led-warning-light/A-p8684631e

I'll just need to find a bus bar for the common.
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Re: Sharing common (ground) questions

Postby bobhenry » Tue Jun 13, 2017 1:10 pm

I was a bit anal when wiring my 1st teardrop and ran each circuit with a dedicated ground. 90 percent of problems on DC equipment (especially running lights) are ground (bad ground) related. My little primitive electrical box was all fused and each ground was led back to a common grounding (buss) bar.

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Re: Sharing common (ground) questions

Postby Vspec » Mon Jun 19, 2017 7:52 am

I decided to splurge for Blue Sea Systems ST Blade Fuse Block - 12 Circuits with Negative Bus, should simplify wiring and future proof for additional circuits to come.

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I have to decide if I want to retain ability to power from the TV. A battery switch could do the trick but once I figure out my power needs this should no longer be an issue.

Which brings me to find a good battery monitoring, amp monitoring and power consumption monitoring tool. Would something like this do the trick?

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Re: Sharing common (ground) questions

Postby kludge » Mon Jun 19, 2017 5:18 pm

Without talking about each option independently, let me see if I can give you a few pointers.

1) My advice would be to leave the trailer DC wiring alone (brake light and running lights and stuff) if at all possible. Leave those DC current return paths connected to the trailer frame.

2) Regarding the stuff you want to add: Don't run a bunch of DC positive wires to a bunch of stuff and then try to run everything back to the DC source on the negative side all through one single common. a) if you have a problem in the return wire, everything goes out, or only a few things go out, and it's hard to trouble shoot. Every DC positive run should have it's own return (negative) run back to the DC source (battery, truck via the 7-pin, DC converter, whatever). b) without doing "a" you can easily run too much current through a "common" DC return and heat up/burn up the wire. When using the frame as a DC return (with short pigtails for the lights, or screws that directly connect to the chassis), the frame is big enough to handle the current all added together. If you're using the truck wiring, via the 7-pin, make sure THAT wire is big enough to handle everything too.

3) Here is a great wire size chart and voltage drop calculator --> https://www.powerstream.com/Wire_Size.htm

Use it to see what size wire is appropriate for your appliances/loads and then check the chart to see if the wire is big enough to carry the load.

4) Fuse EVERY DC run on the positive side as close as possible to the source. The value of the fuse must NEVER exceed the current rating of the wire. Also fuse the positive wire going from the DC source to the fuse block. (Most of the RV/Marine fuse blocks have places for all these things already, but you can "easily" make your own stuff.)

5) Split up the loads on different circuits and mix it up a little. Example: put the left side interior lights and charging outlets (cigarette lighter ports) on one circuit and the right side lights and charging ports on another circuit. This way if you pop fuse or break a wire, you're not in the dark and can still charge stuff until you can get somewhere to replace the fuse or troubleshoot the wiring.

6) Do I ground (earth) the chassis or do GCFI's for the AC? Yes. Do both. I had a lightning strike take out ALL the GFCI's in my house once. Also I will never put all my faith in breaker on on the pole will work, or that it's GCFI circuit will work, or that it's properly earth grounded. If one of those things is bad, I need a working one on my trailer.

7) Can I tie my DC returns (negative side) to the trailer chassis/truck wiring? Yes. You should absolutely do this. Bring all the DC negatives back to the fuse block and tie them to a negative bus and then one big fat wire from the negative bus to the chassis, or T it into the 7-pin wiring. Perhaps the best place to screw it down is the same place on the chassis that the 7-pin harness it screwed down.
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Re: Sharing common (ground) questions

Postby Vspec » Tue Jun 20, 2017 7:25 am

kludge wrote:Without talking about each option independently, let me see if I can give you a few pointers.

1) My advice would be to leave the trailer DC wiring alone (brake light and running lights and stuff) if at all possible. Leave those DC current return paths connected to the trailer frame.

That was my plan.

2) Regarding the stuff you want to add: Don't run a bunch of DC positive wires to a bunch of stuff and then try to run everything back to the DC source on the negative side all through one single common. a) if you have a problem in the return wire, everything goes out, or only a few things go out, and it's hard to trouble shoot. Every DC positive run should have it's own return (negative) run back to the DC source (battery, truck via the 7-pin, DC converter, whatever). b) without doing "a" you can easily run too much current through a "common" DC return and heat up/burn up the wire. When using the frame as a DC return (with short pigtails for the lights, or screws that directly connect to the chassis), the frame is big enough to handle the current all added together. If you're using the truck wiring, via the 7-pin, make sure THAT wire is big enough to handle everything too.

With the 12 circuits BSS fused block, each circuit will be fused and each will have it's own return (-). Current appliances include: 4 LEDs strip (which will be split front and rear), 1 LED side door flood/loading light, 1 electric fan and 2 LED ramp flood/loading lights. That will make 5 circuits. Currently everything is wired with 18awg. My plan is to extend (solder + heat shrink) the existing wires back to the distribution block keeping the same gauge.

3) Here is a great wire size chart and voltage drop calculator --> https://www.powerstream.com/Wire_Size.htm

Use it to see what size wire is appropriate for your appliances/loads and then check the chart to see if the wire is big enough to carry the load.I need to do some investigation
as I don't have any of the specs on the appliances. I am hoping 18 awg will be sufficient.


4) Fuse EVERY DC run on the positive side as close as possible to the source. The value of the fuse must NEVER exceed the current rating of the wire. Also fuse the positive wire going from the DC source to the fuse block. (Most of the RV/Marine fuse blocks have places for all these things already, but you can "easily" make your own stuff.)

BSS fused block should take care of that. I do need a fuse for the source.

5) Split up the loads on different circuits and mix it up a little. Example: put the left side interior lights and charging outlets (cigarette lighter ports) on one circuit and the right side lights and charging ports on another circuit. This way if you pop fuse or break a wire, you're not in the dark and can still charge stuff until you can get somewhere to replace the fuse or troubleshoot the wiring.

BSS fused block should take care of that. 12 circuits is more than I need.

6) Do I ground (earth) the chassis or do GCFI's for the AC? Yes. Do both. I had a lightning strike take out ALL the GFCI's in my house once. Also I will never put all my faith in breaker on on the pole will work, or that it's GCFI circuit will work, or that it's properly earth grounded. If one of those things is bad, I need a working one on my trailer.

AC is not being touched. The only time they will get near one another is when I will be charging the battery or providing 12v DC source.

7) Can I tie my DC returns (negative side) to the trailer chassis/truck wiring? Yes. You should absolutely do this. Bring all the DC negatives back to the fuse block and tie them to a negative bus and then one big fat wire from the negative bus to the chassis, or T it into the 7-pin wiring. Perhaps the best place to screw it down is the same place on the chassis that the 7-pin harness it screwed down.

Would that not cause an issue if both the 7-pin wiring and the DC source have their (-) tied together and both are providing power at the same time?

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