Grounding and Bonding Question

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ChrisFishr

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#1. Are pull behind trailers required by NEC to be wired as Mains with Neutral bonded to ground or as a sub panel with no bonding in the trailer ?
Does NEC require any generators power a trailer in this application to be grounded ?
#2. Should a pull behind trailer be grounded for lightening protection ?
I have seen much debate about this in these forums.
 
Look at it this way.

If the trailer will be supplied by a buildings wiring system it would be against the code to re-bond that that systems neutral and ground.
 
Chris
I am not sure if I completely understand you questions. If I do, here is my response.

See 90.(B)(1)
[read this carefully]

See 250.34, possibly 550.4 or 551

Those are specific references, but there could be others as well...just cannot guess based on such vague info provided in your OP.
 
ChrisFishr said:
#1. Are pull behind trailers required by NEC to be wired as Mains with Neutral bonded to ground or as a sub panel with no bonding in the trailer ?
Does NEC require any generators power a trailer in this application to be grounded ?
#2. Should a pull behind trailer be grounded for lightening protection ?
I have seen much debate about this in these forums.
Click to expand...

presumably a pull behind trailer would be pretty difficult to ground while it is being pulled down the road. while it is parked and plugged into the grid presumably the metal frame is bonded to the EGC in the receptacle.

I have done some equipment that was built into semi-trailers. They were always done with 480/240-120 transformers that created an SDS, and the trailer frame was used as the grounding electrode. The incoming 480 had an EGC that was typically connected to the same ground bar.

IMO, it would be a bad idea to bond the EGC and the neutral in the trailer if it was fed directly from inside.

I can't recall seeing all that much debate on this subject.

Many plants felt it was required to run a bonding wire from the trailer frame to a ground rod. I never objected if they did. I did not consider it especially important either. Think about the hundreds of thousands of semi-trailers sitting in parking lot across the country, completely ungrounded.
 
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Good point, I have only seen a little bit of debate about the sub panel bonding (which mostly steemed from old versus new).

My main problem (which seems somewhat debatable) is the proper classification of my trailer with power design in it (Work benches, outlets, power equipment, lights, AC unit). In my research, it appears most RV and Camper MFG's treat the power systems in those units as sub panels and do not bond the ground and Neutral, which is CODE.
My trailer would not necessarilly plug into RV parks but could be plugged in anywhere in the USA, so that is why I am struggling with this classication of the trailer (I am not exactly sure where people would plug it in, 120V single pole or 220V double pole sources.
Could be plugged into main bonded feeds or pluged in to 110V outlets.
My designs would be typically 30A and 50A service panel installation into pull behind trailer, with Twist Lock interface, not the heavy power pin plugs. Although I am curious if the large power or service panel would change any requirements.

Mr. Petersonra - the use of transformers in your designs was for isolation and safety ? not necessarilly code ? All please feel free to add anymore information you relevant.

I am still in the process of reviewing codes and will get back to you all with more specific details about my design application.

Thanks very much gentlemen for the quick responses, I really appreciate your expertise in these areas.
 
ChrisFishr said:
Mr. Petersonra - the use of transformers in your designs was for isolation and safety ? not necessarilly code ?
Click to expand...

We needed 480V for pumps and other things, so rather than have to have two power feeds, it was convenient to add a small transformer.

IMO, if you are plugging into an existing receptacle to power the trailer, you cannot connect neutral to EGC in the trailer. And why would you want to anyway?
 
My original thoughts were that the service panel in my trailer would be treated like a service panel in my home or main panel a home.

I read somewhere in the forum that once you re-bond after a main panel in sub panel (which I now understand is a CODE violation and why, I think), you create another path for current to travel in the ground, simple Kirkoff's Current Law. Since a current will be present in the ground path, this could create a potentially dangerous situation, hence the NEC code that you cannot re-bond a sub panel. Based on I-Wires very simple statement about re-bonding and Pierre C Belarge code references, I now understand that re-bonding my trailer panel would be code violation.

The definitions of the main panel the sub panel and all the variables about where my trailer would or could be plugged into and how the trailer it's self is classified by the NEC had originally created some confusion on my end.

At this point, it seems extremely important that if I choose to power my trailer from a generator, that I must use an EGC from the generator frame to earth or if the generator is mounted on the tongue of my trailer (off the ground), I must use an EGC to earth. I would take it at this point, the generator would be treated as the main and the generator would have the ground and neutral bonded, correct ?

Also, I had planed on using a 10/4 cable to power my trailer in the 220V operation mode, based on what I have read in the forum about code. This would be a code violation, since the Neutral conductor in my cable must be the size of the two hot conductors combined correct ? The only way to safely do this would be to use a larger cable that would meet that requirement ?
 
Confirming answers with specific code references:

Confirming answers with specific code references:

#1. Re-bonded sub panel would be code violation under Codes 250.24A (1)(5)
Correct ? Thank you, Mr. Iwire

#2. At this point, it seems extremely important that if I choose to power my trailer from a generator, that I must use an EGC from the generator frame to earth or if the generator is mounted on the tongue of my trailer (off the ground), I must use an EGC to earth. I would take it at this point, the generator would be treated as the main and the generator would have the ground and neutral bonded, correct ?

Answering based on code referenced by Mr. Pierre C Belarge:
My above example applies mainly to Code 250.34 - Portable and Vehicle-Mounted Generators.

The code 250.34A says that I don't have to worry about grounding the generator with a grounding electrode as long as (1) AND (2) are met.
Correct ?

The code 250.34B says that I don't have to worry about grounding the generator with a grounding electrode as long as (1) AND (2) AND (3) are met.
Correct ?
Thank You, Mr. Pierre C. Belarge

#3. Many plants felt it was required to run a bonding wire from the trailer frame to a ground rod. I never objected if they did. I did not consider it especially important either. Think about the hundreds of thousands of semi-trailers sitting in parking lot across the country, completely ungrounded.
Thank You also Mr. Pettersonra

Chris Fishr
 
Why couldn't one use this arguement below from Mr. IWire ?

#10 06-07-2003, 04:41 PM
iwire
Moderator Join Date: Feb 2003
Location: Massachusetts
Posts: 29,947

Re: grounding a sub panel at a separate building

--------------------------------------------------------------------------------

If I was asked to do this I would not even think of not running a EGC, just out of habit from doing subpanels inside.

I do have a question for the theory guys though.

Wouldn't leaving the EGC behind and bonding at the second building be a good idea as that will create the shortest path for branch circuit fault clearing?

Say the buildings are 300' feet apart, by bonding at the second building you take 600' of conductor out of the circuit.

What is the down side of this method, other than the likelihood of a conductive path being established later?

Bob
 
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