Electrical Codes/Safety concerns for mobile home construction

[hbiss]

Hello Forum!

I have recently started helping someone with their design for a mobile home. My client has bought a large mobile storage unit and is converting it into a mobile home. There is a trailer hitch connected to the front of it which he will eventually drive around with his mobile home touring America.

https://forums.mikeholt.com/attachment.php?attachmentid=23520&d=1565193711

That thing is not a mobile storage unit but a trailer, normally used by people like landscapers to haul their machines and contractors (like us) to bring to job sites to work out of. There are probably lots of stories out there about electrical contractors wiring these things up for mobile shops, etc. I just saw one owned by a company that re-surfaces cutting boards for super markets. It had a large surface planner and a large panel saw to cut the material if new boards were required. It was powered by an on board generator.

There is a trailer hitch connected to the front of it which he will eventually drive around with his mobile home touring America.

By the way, I hope I misunderstood the OP where he implied that this thing would be towed by a mobile home.
To tell you the truth, even towing that at highway speeds with a 1 ton pickup scares me. And touring America?

-Hal

 

[jap]

The transfer mechanism I have seen on many RVs is that you plug the shoreline into an outlet from the generator in the cable storage compartment or you unplug it from there and plug it into a park outlet. I know of no US NEC section which could be construed as requiring an Automatic Transfer Switch in a recreational vehicle. As far as I know the US NEC only requires Automatic Transfer Switches for emergency power and legally required standby power.

hornetd,,,,,,,, you make way too much sense.

You seem like my kind guy.

:thumbsup:

JAP>

 

[jap]

https://forums.mikeholt.com/attachment.php?attachmentid=23520&d=1565193711

That thing is not a mobile storage unit but a trailer, normally used by people like landscapers to haul their machines and contractors (like us) to bring to job sites to work out of. There are probably lots of stories out there about electrical contractors wiring these things up for mobile shops, etc. I just saw one owned by a company that re-surfaces cutting boards for super markets. It had a large surface planner and a large panel saw to cut the material if new boards were required. It was powered by an on board generator.



By the way, I hope I misunderstood the OP where he implied that this thing would be towed by a mobile home.
To tell you the truth, even towing that at highway speeds with a 1 ton pickup scares me. And touring America?

-Hal

You too at times Hbiss....


JAP>

 

[kwired]

Well actually the one in the photographs isn't a shipping container the way you seem to be thinking about it. That is just an enclosed utility trailer. It's the same kind of thing a contractor might use as a mobile gang box. In this case it is being converted to a Recreational Vehicle in the form of a Travel Trailer.

Utility trailers do get converted to a lot of different uses. Vendors' stands, tool cribs, communications units, quasi Manufactured Homes are all uses I've seen them put to. What has never been clear to me is do they come under any section of the NEC when they get >50 volt electrical systems built into them especially if they are not being made into a Recreational Vehicle. I made several members of an organization I belong to angry when I wanted the groups trailer to use a temporary grounding system whenever the internal 120 volt built in electrical wiring was being supplied by a generator that was not mounted on the trailer or from any other source by flexible cord. We eventually acquired and used a military surplus Grounding Kit, Surface Wire, MK-2551A/U, which goes in rather easily. The technical papers published by the army allege that those kits perform better than the 2 sectional 9 foot long copper clad steel driven rods they had used previously. I just do not know what US NEC sections would apply to a field wired trailer.

When something isn't specifically addressed in a chapter 5 article - then the chapters 1-4 general rules still apply.

 

[tom baker]

In Washington this project would be a Factory Assembled Structure. The wiring would be done under a permit and inspection.

 

[mopowr steve]

Something to consider, If an ATS is not employed one may find needing a automatic ground bonding relay (it is a listed thing I found deep in the internet a few years ago) that automatically bonds the equipment grounding conductors to neutral of on board generator when its running then isolates the equipment grounds from neutral when using shore power.​

 

[jap]

In Washington this project would be a Factory Assembled Structure. The wiring would be done under a permit and inspection.

If not a factory assembled structure would you still be required to pull a permit in your area if doing the electrical it yourself?

Jap>

 

[hornetd]

Field construction of utility trailer conversions happen everywhere

Field construction of utility trailer conversions happen everywhere

In Washington this project would be a Factory Assembled Structure. The wiring would be done under a permit and inspection.

I cannot believe that no one in Washington has ever done a field conversion of a utility trailer to another use. There are a lot of businesses that do conversions or axle up builds but there is nothing to prevent individuals from performing these conversions. The challenge here is that no one will inspect it so there is no second look at the basics of safety which the electrical code would provide.

I wonder if someone asked the Authority Having Jurisdiction to inspect such a conversion if they would even be willing to do it. I cannot say anything about Washington in particular but in most places only a random sample of the factory built units are actually inspected. That is the same approach that Underwriters Laboratories uses for manufactured electrical devices.

 

[hornetd]

Why tranfer the Equipment Grounding Conductors?

Why tranfer the Equipment Grounding Conductors?

Something to consider, If an ATS is not employed one may find needing a automatic ground bonding relay (it is a listed thing I found deep in the internet a few years ago) that automatically bonds the equipment grounding conductors to neutral of on board generator when its running then isolates the equipment grounds from neutral when using shore power.​

Why would the condition of the Equipment Grounding Conductors need to change at all? I understand the need to prevent the neutral conductor from being bonded to the frame of the generator and thereby the the frame of the trailer. It is important to keep neutral currents from flowing on the metal components so that the voltage drop will not raise the touch potential of exposed conductive surfaces to a dangerous level. When I was doing power production work in the Air Force we always did one of two things. We either isolated the neutral from ground at the generator or we installed the extra pole in the transfer switch to transfer the main bonding point between the building's Service Equipment and the generator.

In a completely off the subject side note has anyone had to convince a project manager that when it says optional in the catalog it means One or the Other rather than not needed. It is like pulling teeth as the old saying goes.

 

[mopowr steve]

It was actually for a concession trailer where-as the onboard generator has the neutral and equipment ground tied together (as should be) but when it came to plugging in to shore power the equipment ground and neutral need to be isolated (otherwise if plugged into GFCI circuit it would trip, because of the neutral/equip gnd tied together at generator.)

 

[hornetd]

It was actually for a concession trailer where-as the onboard generator has the neutral and equipment ground tied together (as should be) but when it came to plugging in to shore power the equipment ground and neutral need to be isolated (otherwise if plugged into GFCI circuit it would trip, because of the neutral/equip gnd tied together at generator.)

That I can understand. Can you explain how the "automatic ground bonding relay" works and what makes it different from a transfer switch.

--
Tom Horne

 

[mopowr steve]

It only breaks the bond between neutral and equipment ground when the generator is not running, there-fore making the panel “ready for shore power connection”. This is utilized when there is a manual transfer interlock used instead of an ATS.

 

[romex jockey]

well this is interesting...so where would the MBJ exist? On the camper, or within the external supply to it? ~RJ~

 

[hornetd]

well this is interesting...so where would the MBJ exist? On the camper, or within the external supply to it? ~RJ~

A basic principle of Safety Engineering is that it should take 2 failures of the system that is being designed to subject people to the risk of injury.

If I am understanding what mopowr steve is saying this bonding relay is the answer to having a Main Bonding Jumper in 2 different and mutually incompatible places. There is a Main Bonding Jumper at the park Service Equipment Enclosure. There is also a Main Bonding Jumper between the Neutral Point of the RV's Generator windings and the frame of the Generator. Since that frame is Bonded to the frame of the vehicle, when the generator is mounted on the vehicle, there is a deliberate connection between the generator winding's neutral point and the conductive parts of the vehicle frame. That connection must be opened when the RV is operating on park power because any connection between the Grounded Current Carrying Conductor ("NEUTRAL") and ground on the load side of the parks wiring system Service Disconnecting Means can provide a leakage pathway for the neutral current to flow on the exposed conductive parts of the parks wiring system and, more importantly for your purposes, on the exposed conductive parts of the RV. If the second failure were also present and the neutral conductor were to have an open or high impedance connection most of the neutral current would travel on those surfaces to return to the park's Service Equipment and thus to the power utility's transformer secondary from whence it came. Since every foot of conductor takes some voltage to move the current through it the total voltage used up in that way is called voltage drop. With the neutral intact and insulated from ground the voltage drop on the neutral conductor does not have any deleterious effect. But if the neutral current were to flow over the unintended pathways of the metal parts of the electrical system the resultant voltage difference between the frame of the RV and the earth beneath it will create what is called a touch potential that could give you a shock when you went to open the door from the outside of the RV. The relay that mopowr steve is talking about brakes the connection between the generator winding and the generator frame whenever the generator stops running.

I have some reservations about using a separate device to do this because it would need to be carefully interlocked to the transfer mechanism so that the shore power will not connect if it does not break the connection. Best practice is to have that disconnection made by the same device which connects the shore power.

--
Tom Horne

 

[PaulMmn]

...In a completely off the subject side note has anyone had to convince a project manager that when it says optional in the catalog it means One or the Other rather than not needed. It is like pulling teeth as the old saying goes.

Doesn't "optional" mean Not Required At All, but you may include this if you really want to?? Oh, and if you really want to, here are the things you can select. Do you want the standard cloth seats, or one of these 'optional' seat covers: Leather, Dragon Hide, Ostrich Hide? Otherwise shown as "available leather seats" in the ads.

 

[hornetd]

Doesn't "optional" mean Not Required At All, but you may include this if you really want to?? Oh, and if you really want to, here are the things you can select. Do you want the standard cloth seats, or one of these 'optional' seat covers: Leather, Dragon Hide, Ostrich Hide? Otherwise shown as "available leather seats" in the ads.

You should begin your new career as a project manager at once. Your a natural. In electrical work an option is a choice as in you have 2 options here. An extra is something you can take or leave.

--
Tom Horne

 

[PaulMmn]

You should begin your new career as a project manager at once. Your a natural. In electrical work an option is a choice as in you have 2 options here. An extra is something you can take or leave.
--
Tom Horne

Ah! It's not "optional," it's "Options." A difference in meaning with a change of a few letters!

Or, the catalog lists the standard features, then says, "Optional-- fuse box instead of switch box." Or does the catalog assume you know what the optional item will be instead of?

 

[hornetd]

Ah! It's not "optional," it's "Options." A difference in meaning with a change of a few letters!

Or, the catalog lists the standard features, then says, "Optional-- fuse box instead of switch box." Or does the catalog assume you know what the optional item will be instead of?

The people who write the catalog language are under the mistaken belief that the catalog will be used by someone who knows which end of the windings gets connected to what part of the building wiring. They seem to have failed to anticipate that the catalog would be used by people who were trained in the Andrew Carnegie school of business. "If you take care of costs profit takes care of itself." That is why so many of the conversations with project "managers" end with the installation foreman screaming "Have you ever done electrical work! What was it like!"

--
Tom Horne

 

[iceworm]

... To tell you the truth, even towing that at highway speeds with a 1 ton pickup scares me. And touring America? ...

I'm not getting your meaning. Scared of what? That cargo hauler was designed to load up to maybe 12,000 lbs, connect to a suitable towing capacity vehicle, using a suitable rated hitch, and drive anywhere the pickup can drag it.

Got to go - I'm dragging my 7500 lb toy hauler, using my pussycat F150, about 1000 Miles across CONUS interior this next week.

the worm

 

[jap]

It was actually for a concession trailer where-as the onboard generator has the neutral and equipment ground tied together (as should be) but when it came to plugging in to shore power the equipment ground and neutral need to be isolated (otherwise if plugged into GFCI circuit it would trip, because of the neutral/equip gnd tied together at generator.)

Most standard shore power receptacles for RV"s are 30 amp 125v or 120/240v 50 amp non-gfi protected outlets, unless your under the new code requirements.

If an ATS was not in play, and you simply unplugged one of these cords (That fed the panel in the concession stand) from the generator and plugged it into one of these standard non-gfi protected receptacles, they would not trip.

Since your indicating that when plugging into shore power it would "Trip the GFI", then, I'm assuming your talking about plugging the feeder cable to the concession stand into a "Shore Power" 120v GFI protected outlet, or, using some type of 30a 125v or 50a 125/240v adapter to be able do so.



JAP>