Grounding Portable Trailer Transformer

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SMKDEB1

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Terre Haute, IN
I have a portable safety shower trailer that has a 8 kw generator for emergency use and a 15KVA, 480V-120/240V, 1-PHASE transformer mounted on the trailer for normal use. The 15KVA transformer is fed from a remote 480V panel via a SO cord. I am installing a manual transfer switch on the trailer that will isolate the two sources from one another.
All of the load from the generator and the transfomer is contained to the portable trailer. The trailer is to be used as a temporary safety shower.
Do I need to ground the transformer neutral?
Do I need to ground the trailer to a ground rod?
 
Vechicle are not in the scope of the NEC.
This is an interesing question as you have a generator and a transformer, two Seperatly Derived Systems.
A transformer always requires a system bonding jumper and a grounding electrode.
A generator sometimes requires a system bonding jumper and a grounding electrode.
 
hmmm

hmmm

Okay, well per Art. 250.34 (B) I don't have to connect the frame of the vehicle to the GEC for the generator. Does the the secondary of the transformer fall under the same rules as long as the transformer is bonded to the frame of the trailer? Is the transformer like the generator in this case?

If i'm understanding this correctly, as long as I bond the transformer to the trailer and the neutral to the transformer case, then I don't need to connect the transformer to a ground rod.

Does this sound right?
 
more info

more info

Let me add some more information. The generator and the transformer will be routed to the transfer switch, the transfer switch will power a distribution panel on the trailer. The panel will power lights, a immersion heater and a submersible pump in a 500 gallon tank. We will specify the breakers to be
GFCI.
 
Transformer on Trailer

Transformer on Trailer

Trailer electrical system would fall under NEC 550.4(A) and/or 551.4(A).

The generator and transformer secondary neutrals can be permanently connected to each other and to equipment ground and the grounding stake at the transfer switch using NEC 250.24(A)(3) to pretend that the transfer switch is a double ended switchboard.

Or, bond the neutral to ground at both the transformer secondary and the generator and use a transfer switch that switches the neutral.

The trailer frame has to be bonded to both the equipment ground for the 480 volts and the grounding electrode and equipment ground for the 120/240 volts.

The trailer needs a ground rod that can be shared by both the gnerator and the transformer. Even if the supply system were 120/240 volts and there is no generator a local ground rod provides a place for low energy lightning to go. About 97% of lightning damage in a telephone cable plant comes from silent lightning or invisible lightning. That is, a little bit of lightning does a lot of damage even though telephone cables have to be rated 300 volts to carry ringing current and so that the protector blocks have something that is strong enough to be protectable.

Your water supply and drain pipes should also be bonded to each other and the electrical ground using #8 to #6 solid copper wire. If there is plastic pipe, there should a be a 12 inch section of copper pipe for bonding which is also a Duquesne Light Company requirement for electric water heaters that have plastic piping.

Mike Cole
 
Trailer

Trailer

The trailer would have to be built as either a mobile home ( Article 550 ) or an industrialized unit a.k.a. manufactured building ( also Article 550 ) or as a recreational vehicle ( Article 551 ).

Since this is a business application the most logical category would be and industrialized unit also known as a manufactured building. Article 550 covers both dwelling unit and commercial manufactured buildings as well as temporary commercial trailers.

Mike Cole
 
It does not have to be either a manufactured building or a mobile home.

IMO to make either of those you would need to be a factory and I imagine some sort of approval process before you could mark your product as a mobile home or manufactured building.

It can be just a trailer with some wiring systems in it.

I would wire it like it was any separate structure.
 
It is not a mobile home or a manufactured building. It is a pull behind trailer 18' long and 96" wide with the following mounted on it:generator, safety shower/eye wash station, 500 gallon tank, distribution panel, transformer, transfer switch and some lights.
Thanks
 
In Washington State that type of trailer would fall under the Factory Assembled Structures rules, IE the building department for non site built structures.
It would require plan review and inspections, but its a different AHJ than the NEC folks.
 
Transformer on Trailer

Transformer on Trailer

The problem is that NEC tries to shove both commercial and residential trailers and manufactured buildings into 1 article on the basis that these things are so similar. I guess that there is some logic to that but it confuses a lot of people.

Ohio Department of Commerce has a similar department for inspection of manufactured buildings, trailers, and mobile homes, what ODC likes to call an "industrialized unit". This AHJ is rather a matter of who is doing the inspections much like how UDSA inspects meat plants and somebody else inspects cereal plants. Cities and counties here are responsible for stick built buildings. I am not sure who inspects roof and floor truss assemblies at the factory that makes them.

As long as somebody is doing quality control.

Mike Cole
 
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