My volunteer emergency response team has a 20-foot enclosed trailer to haul our equipment, with a small office area for radio communications. Being "that electrical guy," I've been asked to review the existing 120/240 volt installation for adequacy and code compliance. I'm using 2007 California Electrical Code, which is based on 2005 NEC.
It seems to me that this most closely meets the definition of a recreational vehicle, so it should be evaluated according to article 551, with the exception granted by 551.4(A) because it isn't used as living quarters.
My problem is that I can't understand what exemption is being offered by 551.4(A), which says that the trailer "shall not be required to meet the provisions of Part I pertaining to the number or capacity of circuits required." Part I has no such provisions. The branch circuit requirements are stated in 551.42, but that's in Part IV.
Ok, so if I ignore the branch circuit requirements in 551.42, then how can I verify that the power supply assembly meets the requirements of 551.44, or the plug requirement of 551.46(C), when those articles refer only to the configurations permitted by 551.42?
Your scenario does not fall under Recreational Vehicle, based on defs in Article 551
Also, keep in mind that the NEC is not a design tool.
Are you planning to run the rig from an onboard genset ? If so, you (or someone) has to determine the anticipated connected load of the equipment that you intend to use, then determine the gen size, and a panel size. If you plan on using the rig at the 'barn', then a xfr switch and a shore power inlet should be added.
Again, IMHO, follow basic NEC sections, and good workmanlike practices.
PS: I went thru similar situation with an Emergency Rig (Rescue/EMT) years back for a trauma ctr. It was a purchased rig that required a lot of modifications.
My thinking was that any vehicle having a permanently-installed electrical system that's cord-connected to primary power should conform to some code or another. The 551 definition for "travel trailer" seemed to be a pretty good match except for the term "living quarters," which is what caused me to look at 551.4(A). If you're correct that 551 doesn't apply, then that makes things simpler.
The electrical was installed about ten years ago, pretty decently. There are two cords attached to the panelboard: a 10/4 with L14-30P for the portable (external) generator, and a 12/3 with a 5-15P for shore power. A 2-pole 30 A switch wired into the panel selects the source. I'm not happy with this arrangement, partly because seeing even a dead cord with a male plug on the end of it sticking out of a live panel gives me the screaming heebee-jeebies, but also because we can only safely utilize 15 amps on shore power.
What's got me looking more closely is that at the station the 12/3 shore power cord is being plugged into a 120-volt, 30-amp RV receptacle with an adaptor - not a safe situation! It's also unsafe because there are some multiwire circuits, and the neutrals aren't protected when that source is switched onto both phases at the panel.
The attached cords are definitely going to go. The trick will be how to fully utilize either the 120/240-volt, 30A generator power -or- the 120-volt, 30A RV power.
I install and service these type of units all the time. The common method transfer power is to use a 2 pole double throw contactor with the normally open contacts on the shore power side. Just connect the coil to the shore power side. All input wiring should match input source current ( 10 ga for 30 a ) etc. Best wiring method is stranded wire in a raceway. If you must use Romex, used marine AC cable. It is like romex but with a heavy jacket and stranded conductors. Do not use solid conductors as the constant flex can break at joints and has a greater chance of insulation rub-thru. One more thing, make sure the neutral is NOT bonded to the ground anyware. If the generator is internally bonded, if the unit has a 120/240 feed it will require a 3 pole contactor or transfer switch.
Thanks for the comments, and I'm sorry it took me so long to reply to your post!
You probably missed it buried in all the other verbage above, but the generator set isn't permanently installed. It's on wheels, and we unload it and set it up 25 feet away. There's no need for automatic transfer. If it were built in, I would probably do as you suggest.
All of the existing wiring is in FMC with solid branch-circuit conductors, so it didn't surprise me to find a grounding conductor broken off at a crimp ferrule. I've only seen one area where rub-through might occur, but that was due to over-fill of a box. It's already been strongly suggested by another team member that we replace the solid conductors with stranded, and I'll count yours as another vote in favor!
The neutral bar is definitely isolated from ground, no worries.
What's been bugging me is that the trailer is set up as 120/240, but our shore power is a TT-30R (120V 30A). I don't like the idea of the shore power getting switched onto both main breaker phases because the 30A cord set isn't protected by the 60A sum of the two poles. I'd require some kind of "extra" 30-amp fuse or breaker in series with the cord set.
Instead, my inclination is to set everything up as 120V only, with a recessed L5-30P to attach ONE of two 10/3 cord sets: one having an L5-30P cap to the genset, or a shore-power cord with a TT-30P molded plug. This arrangement doesn't deliver as much potential total power as the 120/240 arrangement when running from the generator, but it does deliver the same power in both conditions. We don't have any 240-volt appliances, tools or equipment.
The generator has a "120-240" switch that places the windings in parallel for 120V-only or in series for 120/240. I like the idea of switching it to the 120V-only configuration, which gives me a big "pool" of 120V power for the trailer and other equipment, with the advantage of eliminating the concern for phase balancing.
Is there another approach I should consider? Thanks again.
Lots of these are set up for 120v only. Even larger campers that have 120/240v cords have all 120v equipment just split between poles. If you just need 30 amps, and everything will run with 30 amps, just bridge both poles together and run your 30a cord to the box thru your 30a inlet. As long as the cord is connected to a 30A or less ckt, ether at the shore power or a 30 a 120v plug on the generator, your covered. If you wish to retain the 120/240 system, you will need a 4 pole inlet and a 4wire cord. You can ether buy (at a camper supply store) a 4wire to 3 wire adapter, or make one. to allow you to connect the 120/240 cord to a 120v supply. As long as the cable and connectors are sized for the largest amps the supply (generator or shore ckt)you will have no problems. The size of the main breaker in the unit is not a big factor (but must be at least as large as the supply.)as the building breaker (shore power) or breaker on the generator will protect the supply cord. I assume the vehicle has a breaker panel that brakes the 30a power cord down to 15 or 20 amp ckts. Robert
I'm amazed at what's being sold at RV supply stores. Like the adaptor that lets you plug any old extension cord into a 30-amp receptacle. Or that 4-wire to 3-wire adaptor you mentioned... I hope none of those 120/240-wired RVs have multiwire branch circuits with common neutrals!
Yes, there is currently a distribution panel with 15 and 20 amp branch circuit breakers and a 40-amp 2-pole breaker as a backfed main (408.36(F) violation already noted). Since the supplies are only 30 amps, I assume the main was intended as a disconnect only.
My concern about matching the main breaker rating to the power cord comes from 551.45(C): "A main overcurrent protective device not exceeding the power-supply assembly rating shall be provided where more than two branch circuits are employed." ("Power-supply assembly" is CEC-speak for "power cord".) Even if the main breaker was dual 30A, I just can't see how this requirement is met when using an adaptor to power both phases from a 120V 30A cord set. If I just ignore this article and depend on the breakers at the genset and at the shore-power outlet to protect the cord, life is certainly easier.
I should probably stop worrying about it and just do what's reasonable. I don't even know who the AHJ would be for something like this, anyway.