Newb question on AC Ground. Be Gentle

[stu_stumble]

First off. I would like to say that it is amazing at the wealth of information that everyone on this forum has to offer about tiny travel trailers and there construction. So just let me say thank you up front for all of this wonderful information and for any else that I will receive.

I have been reading alot of posts on electrical and my head is still swimming.

Ok. So I am going to be build a mini hopper style trailer as I just love the design for some reason. For electric I want to install a small 110 AC system for when I go to a camp ground. I want to simply just install a small breaker box with one 20 amp breaker feeding 2 outlets in the interior of my TTT. I will have the first outlet in the series of the two be a GFCI outlet so the second one is also protected. My question is do I or do I not ground the AC system to my trailer frame. I have read many posts but it still does not seem clear to me. I just am not sure if I am to run a ground out of the breaker box from the ground bar to the metal on my trailer frame so the system is grounded. Trailer body itself will just be plywood and then epoxyed and painted. Only metal would be screws and actual trailer frame. Any help with this would be really appreciated. If this is not the way to do this then I would like to know. I dont want to get cooked by touching the trailer or have anyone else get hurt either. Any help wold be appreciated. If you have any pictures of how you installed a system like this especially how you wired it in the breaker box and then how you ran your ground that would be awesome. Thanks fro the help.

[starleen2]

First the disclaimer – By far I am not a professional electrician. I have rudimentary knowledge of electricity and above average wiring skills – enough to get me in trouble sometimes. However I strive to do things right and safety is a must. If electrical wiring is beyond your skill level, get someone who is both knowledgeable and professional! I make no claims to the reliability of the above other than it is my own design. You have been warned.

Note The Grounding Buss - this is for the ground leg of the 30 amp hookup at campground. This IS NOT the same as the neutral leg of the 110v circuit. Most electrical boxes do not have them - but can be added as an accessory. In this application - It is needed! I in no way would advocate 110V grounding to the frame in the event that you make the connection to the earth and frame


Modified

[stu_stumble]

Thanks Starleen for the information and pic. I really appreciate it. I just have a few more questions if that's ok.

I see in your pic how the 30 am wire coming in is hooked up to your panel but I dont see where the ground wire (Green Wire) in this cable is hooked up to. Is there a small ground bus behind the wire I cant see. Also, I cant quite tell from the picture but how would you hook up the Grounding Buss - the one for the ground leg of the 30 amp hookup at campground that is an add on. I see the copper wire running from the ground bus but can not tell where this leads to.

Thanks again.

[CarlLaFong]

I'm no electrician either, but when I get around to building mine, I will wire it as if it is a big extension cord. Everything will ground through the third prong on the shore power cord. People don't understand, sometimes, the difference between AC ground and DC ground. The ground in a DC system completes the circuit and is necessary for the DC appliances to work. It is, technically, not a ground, but a return. AC ground, through the third prong, is a safety item. Many old homes had two prong plugs and no ground

[Shadow Catcher]

You do NOT want to "ground" anything to the frame particularly the AC. This was done to Compass Rose and it fried the converter and left the frame HOT electrically.
The analogy of treating the electrical as an extension cord is a good one, it all goes to the pedestal outlet. And by the way check the pedestal before you plug in.

[stu_stumble]

Thanks for the help CarlLa Fong and Shadow Master. I understand what you are saying about the grounding thing and the difference between AC and DC grounds. I got it now. I appreciate it. I think I am going to just wire a simple 110 system as seen in :

http://tnttt.com/viewtopic.php?t=37732.

I like this idea and it seems just perfect for what I will need. I do really appreciate everyone's help with this issue. Thanks so much. You are all such wonderfully nice people.

[eamarquardt]

The subject of grounding has been discussed a lot. Some of the thoughts expressed make sense and some don't.

Here is what the California electrical code says about grounding in motor homes: 551-56 A) REQUIRED BONDING All exposed metal non current carrying parts that may become energized shall be effectively bonded to the grounding terminal or enclosure of the distribution panelboard. B) BONDING CHASSIS A bonding conductor shall be connected between any distribution panelboard and and accessable terminal on the chassis.

In short: In my understanding, the 120 volt neutral neutral is not connected to the ground plane, at any point, in an RV. This prevents a reversal of hot and neutral resulting in a hot chassis. The chassis and as much of the metal of the RV as practical should be connected to the ground connection point of the distribution panel and then, via the cable connecting the RV to the campground power supply, to the campground electrical ground.

The California Electrical Code is available here:

http://www.bsc.ca.gov/default.htm

I see no reason to ground the negative side of the battery to the chassis as it is better to use dedicated wire for the return of the current from the positive post to the negative post (although the electron flow is from negative to positive) for 12 volt circuits. I see it as a plus to keep the 120 volt a/c and 12 volt dc systems completely isolated from one another. I have seen problems when the ground lead of an inverter is electrically connected to the negative terminal on a battery. It seems tough to keep all the terminology straight.

Please correct me if I'm mistaken.

Cheers,

Gus

[stu_stumble]

Thanks Gus for the information. I appreciate it. I know to never connect the negative line to the chassis as this would make the chassis hot . I just was not sure if you were supposed to ground the trailer chassis to the actual ground lug in the electrical service panel.

Thanks again.

[starleen2]

Thanks Starleen for the information and pic. I really appreciate it. I just have a few more questions if that's ok.

I see in your pic how the 30 am wire coming in is hooked up to your panel but I dont see where the ground wire (Green Wire) in this cable is hooked up to. Is there a small ground bus behind the wire I cant see. Also, I cant quite tell from the picture but how would you hook up the Grounding Buss - the one for the ground leg of the 30 amp hookup at campground that is an add on. I see the copper wire running from the ground bus but can not tell where this leads to.

Thanks again.

The green wire is hard to see, but it does go to the grounding Buss - It's just behind the 30 amp cable. The Buss is screwed to the back of the box and all of the ground wires hook to it (bare copper or green wires). In theory any grounding will go through the cable to the campground hook up. One this example I used solid wire - but on on newer versions I used stranded.

[eamarquardt]

If one looks closely, the neutral buss of Starleen's panel appears/is insulated from the metal distribution box thus keeping it isolated from "ground".

Cheers,

Gus

[stu_stumble]

Thanks Starleen and Gus. Appreciate the help. You guys are awesome. It all makes sense. Thanks so much.

[Engineer Guy]

What Gus said twice...

This topic goes round and round. The high points here to me are:

1. An RV Chassis often floats from Earth Ground on Tires; a Jack Pad of Wood; and Stabilizers that do not ground well, if at all. Weird situations can energize the Chassis. There's no downside to protecting against those situations, so grounding the Chassis well to Green Wire AC Earth Ground is all good. You want any voltage inadvertently on the RV Chassis to flow to Earth Ground on a properly-grounded Green Wire; not flow through your Body instead.

2. RV Park Pedestals are notoriously mis-wired and/or not maintained. I advocate carrying and using a Voltmeter or a Receptacle Tester. A 15 Amp -> 30 Amp Plug Adapter should enable using such a Tester on 30 Amp 115 VAC Receptacles. It's smart, 100% of the time, to first check for reversed Neutrals or missing Earth Ground on RV Park Pedestals.

Relative to the RV Park Pedestal as the voltage source, Park Pedestal or RV-mounted GFIs compare outgoing 115 VAC current [on the Black Wire] to return current on the Neutral lead [White Wire]. If the assumption of Neutral being wired correctly isn't true [because it's reversed], it throws up into the air the presumption that GFIs will work when needed, and save a Behind.

In the days before Double Insulated Tools - the reason there's now 2 Prong 115 VAC Tools even available - all Tools had 3 prong AC Plugs in order to ground the Drill or Circular Saw metal outer shell. Think of any RV - Homebuilt or not - like an older Drill, since RVs aren't Double Insulated. The Metal Chassis and any outer Metal covering needs to be connected via Green 3rd Wire Earth Ground TO Earth Ground if you want to protect against unforeseen situations. Some of us on this Forum advocate wearing a Belt AND Suspenders; some don't.

The Fine Print, as mentioned below, is that double [redundant] grounding points have to be avoided. This means the RV Chassis ideally should not be grounded also through Stabilizer Jacks or the Tongue Crank, etc., while also properly grounded back through the RV Park Pedestal. All these conditions are met automatically in a immobile House, wired once to known-good AC Power, but are not necessarily met in an RV.

I only scanned these Links very quickly for accuracy/content, since I don't have a Dog in this proverbial fight.

RV Wiring Discussion

Electrical Safety For Campers

[86bigred]

yes you do want to ground the chassis to ground. all new rv's are.with the frame grounded it could become energized hurting you or loved ones.i spent the first 9 years working as an electrician. you always ground all the metal on a trailer.if it is not if there was a short and the frame became energized the frame will be hot,and if you touch the frame you could get shocked.with the frame grounded if the hot wire touches the frame it will trip the breaker. read your electrical codes on this,it thee law for your protection and others.

the main panel at the camp ground or your home the neutral is bonded to the ground.

if you want a second opinion contact an electrician.

[low277]

The national electric code (NEC) article 551.56(B) states that the chassis must be bonded to the distribution panelboard. (c) states that it must be 8 awg copper minimum.

Remember that in a AC system neutral and ground are two different things.
The neutral is the return path for a 120 volt circuit. The ground is the fault path if something goes wrong, it will bring the current back to the source to trip the breaker.

Also if the GFCI in the supply cord or the rv pedestal is working correctly it will allow no more than about 5 milliamps of differential between the power on the hot/phase conductor and the neutral. If there is more differential than this the GFCI device assumes that the power is going to ground and trips.

If you do not bond the frame and the gfci isn't working/missing the frame could become energized and you could get a shock/electrocution hazard if some is in contact with the frame and a earth ground or some or part of the vehicle that is in contact with the neutral.

[afreegreek]

the electrician at the shop I work at says that you MUST ground the frame on the AC system.. if the frame is somehow energized and it's not grounded you can be electrocuted..

this electrician is in charge of the entire electrical system on 20+ million dollar yachts that have multiple power sources and voltages in both AC and DC.. I'd take his advice if I were you