Hello all,

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 opinions and advice are much appreciated.

Regards,
John

John:

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.

Hi John,

Thank you for your comments.

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.

Thanks again,

John

John:
You may want to consider flanged 'inlets' for conncetions on the exterior for shore & gen. They are available with spring loaded flap covers.

A manual transfer switch @ 30/60 amps would be a better solution for selecting source, and would eliminate the 'hot' inlet.

Beefing up the cabling from the inlets to the xfr switch and to the panel to 30 amp (or larger) would also be suggested.

As to my comment re: Art 551, being the non-applicable article...you should/must still consider the other articles within the NEC for wiring methods (wire/cable/conduit sizes, etc).

Agreed -- I've already identified a few other problems (like 10 conductors AND a 3-way switch in a handy-box) that will get fixed in this refresh.

Regards,
John

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.

Hello W.U.,

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.

Regards,
John

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

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.

Thanks again...

Regards,
John

I was thinking if this was in Manitoba I would probably talk to Manitoba Dept of Labour - Mechanical and Engineering Branch and see if this falls in there mandate to inspect it.

Their electrical inspectors have the power to certify installations for one off things like your emergency trailer.

In California there must be an equivalent state department that would be able to do this.

I got in rather heated conversations with ambulance manufactures, one in Ark and one here in Houston. They would install a small panel in the unit served by a shore power cord. They would hook the neutrals and grounds on the same buss. The panel would serve several chargers and on later units, onboard AC unit. The first complaint I got was when the EMT (Firefighter) unplugged the cord he would get a shock. When the cord was pulled, the neutral would break first making the chassis hot for a second. I isolated the neutral and solved the problem. Told the manufactures and they both said the NEC does not apply to them and that the way they wired the panels was correct. I sent a certified letter to both, telling them they would be libel if someone got killed. Some smaller fire dept. have substandard wiring. If you were to plug the unit into a ungrounded ckt with reverse polarity, the whole chassis would be hot. Speaking of campers, lots of campers were wired like this with the neutrals and grounds bonded together. They trip a GFCI real quick. Most have a 30a cord that is connected to a non GFCI ckt at the camp site. Trouble comes when someone uses a 30a to 15a adapter in a GFCI outlet. I agree camper supply co have some not so great adapters, a cord to plug a 30a in a 50a plug and such. In your case, I have less of a problem plugging a 30a-50a plug in a 15,20,or 30 ckt even if the single ckt supplies both poles than a 40-50a ckt suppling a 30a or less load. As long as the wire size matches the source breaker size. Robert

Over here in the sticks,
You are not allowed to have a cord or anything protrude from a caravan/trailer.

It must be connected to the trailer with an "appliance inlet" that has male pins and is permanently installed on the side of the "vehicle".

You connect your live source with a female (weather-proof) cord connector, RCD/GFCI protection comes from the source of supply.

For those that have heard of it, the connectors must be rated to IP 56 (dust/moisture proofing).

One thing I would stipulate though, with regard to the flexible cords, is get the heaviest insulation possible, for the size of wires, these cords will get knocked around, run over and generally abused.
I'm involved with Civil Defence over here and our gear gets a really hard time.

Robert,

I can understand the ambulance manufacturers asserting that the NEC doesn't apply to them, but connecting the neutral and ground together strikes me as both pointless and dangerous. I can only imagine how thrilled the operator would be the first time he plugged it into an outlet with reversed hot and neutral. Ka-boom.

Mike,

The currently-installed cords are only a few feet long, and they tuck into a storage space so they don't protrude for travel. Nevertheless, I hate them, and they're going to be replaced with a recessed inlet as you described. I rather like the marine-grade ones with the locking ring on the cord cap, which I believe are IP65.

Regards,
John