ECN Forum Forums General Topics of the Trade General Discussion Area Electrical systems in mobile structures.

Your guidance will come from Article 90, the introduction to the NEC.

Look at what the NEC is intended to address- and, just as important, what it does not address. You will note that vehicles, ships, aircraft, light rail, rolling stock, and a great many other things are not addressed by the NEC.

Why, you ask, might this be? Well, here are a few reasons:

First off, any 'rule book' is based on a set of basic principles. Different folks developing something, independent of each other, will often take opposing approaches. Either approach might be excellent, but the two methods are not compatable. For example, look at the gulf between "Windows" and "Apple."

Many things - ships, trains, and mines come to mind - already had their set of principles, their 'rule book,' long before the NEC came about.

Likewise, the basic assumptions might be irrellevant to another application. For example, a ground rod is a bit impractical for the Space Station. You might even create a dangerous situation - one example is when folks apply the NEC to a mine, and wind up blowing things up or electrocuting people.

That's why a designer has to have a very clear understanding of electricity, lest bad things happen. One absolutely must understand the difference between 'grounding' and 'bonding.'

That said, we are seeing far more circumstances that challenge the traditional role of the NEC, and it's not an area that's settled at all. There's more to electricity than volts and amps - and we're only beginning to learn.

To answer your basic question, though ... If the application involved people and the usual appliances at the usual voltages, MOST NEC principles will be reasonable. Otherwise, all bets are off.

Many years ago, the fire dept I was in had a pumper with a 5K winco generator on it. All the cords had 15 amp 125/250 v 3wire (non grounding)twist lock plugs and cord caps. When ever there was a fire, if they use the generator with the lights or smoke ejector, if you touched the truck you go a tingle or even a bite. Only happen at major fires where lines (water) were laid from the pumper. After this happen at a major fire I asked the chief if I could find the problem. Two of the hand lights and one fan had the ground and hot reversed on the plug. The outside of the light was hot. The neutral and ground were bonded at the generator. Since the truck was isolated (rubber tires) most of the time no one got shocked. But on a large fire, hoses were laid and all was wet, including the light, ground and the firefighter. Since the light was now grounded to the earth (hot side) The wet earth around the truck was hot in relation to the truck chassis. They were sure glad when I fixed the problem. I replaced all the twist-locks with grounded types and made sure all polarities were correct. Robert

I find it a bit odd how the NEC doesn’t seem to include mobile vehicles and trailers, like the OP has described, yet when it comes to mobile homes, 550.4,[A] also addresses mobile homes not intended as a dwelling unit and then goes on to list some examples of these other uses, like contractor onsite offices, banks, clinics, mobile stores…etc., and how they are not required to meet the provisions of this article pertaining to number or capacity of circuits required. Aren’t these mobile homes for other than dwellings also towed or transported over public roadways as well?

I think you need to go back to the definition of "mobile Home".



If this was manufactured as a utility trailer (and titled as such where they title trailers) it is not a mobile home.
The one IBM had was a garden variety semi trailer that they made pretty. It had a New York semi trailer tag on it.
The welder that fixes my boat works out of a regular box trailer that you would expect to see hauling materials to a job site but it is outfitted as a metal shop inside, running off of a big diesel generator/welder. I am not sure if he has "shore power".
When he has worked for me he just fires up the genny.


The school board also has a "bathroom" trailer and I don't think anyone ever inspected that either but school boards are pretty autonomous anyway.

Agreed. I just see it as a little strange how the NEC decided to cover interior wiring for mobile home units when used as a dwelling, even if they are transported from place to place, but when these mobile homes used as a type of commercial space, like mobile clinics, banks, etc., the usual requirements for commercial spaces don’t seem to apply. I suppose this would maybe default back to the local building code requirements for commercial spaces?

Do local building codes only apply to "permanent" structures?

To the best of my knowledge, construction & office type trailers are not under the 'local' building, elec, fire & plumbing codes for the 'unit'. They do however here in NJ require permits. Building for anchoring, stairs/ramp; electric for power hookup, and plumbing IF there are any sanitary, or domestic water connections.

The range of 'trailers' I see varies from the 'McTrailer' to ocean cargo containers, and a lot in between.

550.4(A) does give the NEC some degree of jurisdiction over a "mobile home" used as another type of occupancy. The thing that is unregulated is a utility trailer or truck body that is used as some kind of occupancy if it is not titled as an RV.
There may be some state laws that plug this hole tho, particularly if this is open to the public.

"Do local building codes only apply to "permanent" structures?"

There was a man in Reno, Nevada named Joe Comforte. He was in the business of providing "comfort," if you know what I mean.

His business establishment was made of three mobile homes, parked right at the junction of three counties. Whenever he heard of an 'inspection' by the 'AHJ,' his girls would roll the trailers a few feet, over the county line.

This charade finally ended, when the State formally admitted that there was no state rule against his business, and one of the counties licensed his operation.

It's rather impractical for any location to attempt to regulate the moveable.