Receptacle Roundup III: NEMA TT-30

Funny, a plug designed especially for vehicles has existed for many decades… kinda.  The NEMA TT-30 standard (sometimes called RV 30) was made for smaller RVs: conventional trailers (“Airstreams”), pop-ups, and fifth-wheels, camper vans, and the small coachbuilts.  Hence the TT (“Travel Trailer”); the -30 indicates the current rating in amperes.  Most RV parks have electricity somewhere, and that electricity usually includes at least one TT-30 receptacle.  Do you see where I’m going?  There are over 10,000 RV parks, and they are usually off a highway.

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The power standard was designed for modern comforts at RV sites.  As devices were pretty much 120V AC, 60 Hz, so is a TT-30.  However, an RV might hold a few devices, which might be running at the same time.  So the park side must yield more power than a standard 3-prong (NEMA 5-15).  At 30 amps, you can consider the TT-30 to be like smooshing a small power strip into one, mighty socket.  As usual, the nameplate “30 amp” is the peak load; sustained current should be downrated by a 20% safety margin, to 24 amps.  That’s still double a standard 3-prong, maxed out to its sustained load rating (12A, down from 15A).

At 120V, 24A (~2.9 kW), fully charging from a TT-30 would take from under two hours for an “empty” Toyota Prius Plug-In, to under ten for an all-electric Nissan Leaf, with the Chevrolet Volt in between.  Together with other factors, RV sites then serve as important staging points for pretty much every long-distance and cross-country attempt in an EV, since the original EV1 in the Nineties.  But don’t take my word for it:

John Glenney took a Tesla Roadster across the country and more.  He used only “unintentional” charging facilities, mostly RV parks plus two random outlets.

-Geoffrey Kinsey also went cross-country in a Tesla Roadster, using a combination of hotel outlets and Tesla locations in major cities, and RV parks and the occasional friend’s house in more sparse areas.

-Jim and his wife took their electric BMW Mini on an East Coast vacation (New Jersey to the Carolinas) via RV parks.  Lots of juicy details and pics.

-Steven and Jesse took their new Tesla Model S on a cross-country trip, depending on RV parks in the West and some open stretches in the East.

Tom Saxton and his wife used RV parks to take their Tesla Roadster down the Pacific coast.

-Tony Williams relied on RV parks to take a Nissan Leaf (!) from Baja to Canada- his “BC2BC” run.  Even though there are numerous EVSE locations along the West Coast, the parks were necessary for rural legs where no “modern” EVSEs (J1772 or CHAdeMo) were installed.

Peter and company also took their new Model S from the factory in California to the East Coast via RV parks.

And yes, owners of these sites generally don’t discriminate against passenger cars versus true RVs.  However, some are independently owned and/or operated, and the average desk clerk could just be marking time.  So calling ahead is a good thing.

Worst case, the TT-30 pinout is electrically the same as 5-15, just with a larger layout.  So you can plug in a standard 5-15 device with a simple mechanical adapter (though your charger likely won’t take advantage of the higher current).

Even if you never intend to stop by an RV park, might the standard be of use to you?  Sure- as I said, it’s compatible with household current.  The TT-30 receptacle is a drop-in replacement for household 5-15 units; RV owners sometimes install a TT so their trailer can be used in their driveway.  Switching a receptacle is well within the ability of a competent homeowner, as long as you feel good with a screwdriver and take all necessary and reasonable precautions (yes, you too).

The only issue is the higher current.  Most modern houses have electrical circuits built to 15 amps, with some 20A.  There may likely be a dedicated circuit with one dryer or range outlet at higher current, to say nothing of a central AC unit.  I’ll leave it up to you to decide what to do with any electric ranges and stoves, but I doubt you’ll ditch air conditioning.  If you can’t drop the electric white goods, you will likely have to install a 30A circuit breaker and pull heavier-gage wire (maybe 10-12 AWG, depending on the length of the run) through the walls.  Obviously, this is easier if your electric vehicle and its plug will be closer to the breaker panel.  Yet another option is to have the electric company hook up a second meter, then run a heavy circuit from that.

So, with all these advantages, why aren’t we seeing more TT-30 installations?  Why aren’t manufacturers including TT support in new vehicles (well, besides actual Travel Trailers)?

The TT-30, while more powerful than standard 5-15s, is still skimpy- really, what isn’t more powerful than the household outlet?  It’s not faster than baseline J1772, and certainly not the average J1772, or a maxed-out unit.  And this is for an “intact” TT-30.  Many RV parks have outlets worn from years of use.  When a circuit breaker trips often (such as when entire families try a strange outlet), it trips earlier next time… which makes it trip earlier next time, etc.  After years, a “30 amp” outlet might only deliver half that, without tripping the breaker.  Your advantage over NEMA 5-15 is then gone.

Despite being just a bit more power than 5-15, the TT-30 plug is now noticeably bigger.  Car owners can simply throw in another adapter in the trunk… or backseat, or whatever.  I’m a bit pressed for space on the bike.

More ominously, a TT-30 looks like another common receptacle, the NEMA 10-30.  The 10-30 was used for dryers before about 1996, and is thus found in millions of homes.  At 240V, its electricity will fry a 120V computer.  Since the TT-30 plug can just barely be forced into a 10-30 socket, BZZZZZT.

Less dangerous but possibly more insidious are TT-30s which have not been installed properly.  Again, the TT looks like the common 10-30, but should be wired like the 5-15.  Miswiring the TT won’t fry a heater or other crude, analog device, so the error might go unnoticed for a while.  But like 240V above, it’s a threat to more advanced electronics, like a charge controller.

And, of course, the 120V versus 240V thing is still out there.  I don’t care, with my moderate operating potential.  But every EV car today operates above 120V, and has to upconvert, incurring some conversion cost.

For all these strengths and weaknesses, the TT-30 must compete with similar “medium” standards.  The NEMA L5-30 connectors work the same, but the plug locks into the receptacle by twisting.  Not that much of a killer app for a vehicle, but that security is demanded by some industrial users, so L5-30 has gotten some traction.  And then there’s the rare, non-locking NEMA 5-30.  As far as an electron is concerned, a 5-30 is also no different from a TT.  But it looks nothing like the dryer plug, more like a 5-15, so odds are it’s running right.

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NEMA TT-30 Plus

-Standardized like NEMA 5-15, and grandfathered in by electrical codes
-Cheap, already deployed near highways for travelers
-Directly compatible with most home wiring (though not necessarily at full power)
-Double the power of 5-15, without Y-cords’ hassles
-Can power regular 5-15 devices with a simple adapter

Minus

-Public installs often degraded, sometimes miswired and dangerous
-Looks like NEMA 10-30: can be force-mated, causing fried electronics
-Still low-powered; great overnight and at work, but slow in other cases
-Still just 120V- efficiency losses when charging most cars
-Plugs getting a bit large (more an issue when on two wheels)

Neutral

-Direct safety, weatherproofing, and vandalproofing same as 5-15
-Competing interfaces are as good, physically and power-wise

NEMA TT-30 Ground

The Travel Trailer plug has advantages and disadvantages, but is undeniably available by many highways… with disadvantages.  If your plans will sometimes go down those highways, adding connectors for your home (overnight) charging could be handy.  Others might want to look into a TT-30 adapter, to keep around in case of rural stays.

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9 thoughts on “Receptacle Roundup III: NEMA TT-30

  1. Holy curramp- DON’T use 12 ga. wiring if installing a 30A receptacle. I just checked, and my memory was off a size. 10 ga. copper is the minimum thickness of wire for 30A, and possibly more for long runs. Sorry for the slip, don’t violate code or even start a fire on my behest.

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