Prius as a Generator - NEC 2011?

[guyinahouse]

Lots in the forums about charging EVs from the garage. Not so much about going the other way.

A Toyota Prius and its siblings can supply 120v 30A to a residence (akin to a portable generator) pretty much indefinitely. It's also possible to buy a Prius traction battery-to-AC inverter: http://www.converdant.biz/plug-out/ (and, no, I'm not affiliated) as a turnkey solution, or you could build your own.

Now here's the fun question: under NEC 2011, is this approach a "vehicle mounted generator" and thus a separately derived system? If it's declared separately derived under NEC 2011 250.30 through .34 (and thus requiring a grounding electrode), exactly where would grounding happen? Discretely? Not switching the ground with a transfer switch would effectively use the residence's earth ground, but then that's bonded to grounded (neutral) at the typical load center.

Strictly speaking, the Prius thing is an unbonded inverter, and not even a generator (no mechanically rotating field) at all. It's DC-to-AC, with the option of a local receptacle. It's more akin to a 4kW solar array, except for the rubber wheels of the car completely floating the AC circuit. And that, in turn, begs the question of the "bonding to the frame" answer of 250.34, which Mike Holt kinda answers at www.youtube.com/watch?v=Jj9wXUjya1g in his thoughtful video. Of course, you wouldn't bond to the frame of a Prius, like you might to the frame of a towed generator for a movie set or construction site. It's an odd duck.

So, what to do? Neutral-switching transfer switch and separate earth electrode at the separately derived entrance in the garage? Thoughts?

 

[GoldDigger]

... as a turnkey solution, or you could build your own.
...
Now here's the fun question: under NEC 2011, is this approach a "vehicle mounted generator" and thus a separately derived system?

One potential problem is that the Plug-out kits at that link do not seem to carry any sort of UL listing. At a minimum, you would not IMHO be able to interconnect it with your house wiring, even with a transfer switch. If you can demonstrate otherwise, please do.
And if you build your own, even using a UL-listed inverter between the Prius battery and your house wiring, are you still covered by that UL listing?

 

[guyinahouse]

Hmm... Nice catch, GoldDigger.

That said, I'm a little less interested in the UL listing of one vendor's box than the NEC treatment of how to connect all of 'em to a residence. People have been doing this since ~2004, well before the current NEC. See http://priups.com for lengthy engineering discussions. Hell, it was in the New York Times six years ago. It's not news anymore.

So let's stipulate that there's some combination of widgets that could be or already are UL listed on the supply side, since all the parts are individually listed. It's basically datacenter and solar gear (batteries, transfer switches, grid-tie inverters). The question is really more about the residence side. How is it treated? What about the grounding? Separately derived? I honestly think the current NEC hasn't really contemplated this, but it's kind of a no-brainer that the next version should, as more of these cars make it into people's driveways. There are 2 million on the road already, according to the government: http://www.eia.gov/tools/faqs/faq.cfm?id=93&t=4 .

IMHO, it appears that a non-separately-derived approach (i.e., treating the Prius/Nissan/Lexus like a generator) seems to be the path of least resistance. Then just use a classic manual transfer switch to switch the hot(s), as long as the aggregate load is less than the hybrid car can replenish. This is reported to be around 30A 120v AC, with ~150% locked-rotor starting loads for a Prius. I personally don't know if that's true.

But the 2011 NEC has a lot to say about generators, including vehicle-mounted ones. Yet it has nothing to say about this, despite the obvious possibility of wiring up 2% of the US housing stock right now. It's very much up for discussion, and left to the whim of the inspector. Again, it's up for discussion whether this is even a "generator" at all. Seems to me this is worth discussing.

Mike? :?

 

[GoldDigger]

Again, it's up for discussion whether this is even a "generator" at all. Seems to me this is worth discussing.
Mike? :?

One could argue that if you have both charger connected (from time to time) to the Prius battery and the inverter connected to the same battery, you have a standby UPS and would look to the rules involving them.
One factor in whether of not you want SDS status for it is definitely the grounding requirement. I imagine that in general the output neutral would not be bonded to the battery pack DC or to the car frame, but that might vary from model to model.

 

[tom baker]

I recall that chevy has truck with inverter outputs.
And the 2008 or 2005 NEC in 625 added 625.26 Interactive System to allow EVs to have bi directional power feed. Handbook states must be listed, and comply with 702 or 705. 702 has grounding requirements not sure about 705

 

[guyinahouse]

Yep, great insight. Thank you. That probably is the right section today, Tom.

...NEC in 625 added 625.26 Interactive System to allow EVs to have bi directional power feed.

That section covers: "[EV supply systems intended] to be interconnected to a vehicle and also serve as... an electric power production source..." (emphasis added).

But I'd submit that the word "also" is a big issue. For starters, adherence to 625.26 requires wiring with totally different widgets, and requires different controller logic at both the car and garage. That'd be fine if it were in pursuit of the same goal (i.e., charging the car without creating personnel risk), or if it would even work. But we're talking about one-way power transmission, using 120v or 120/240v AC from the car-based inverter to the residence. The car is never load-side, nor is the residence ever simultaneously energized from the grid. Charging the car from AC is neither necessary nor desirable. Nor - in the case of a storm or an off-grid application - is it even an option to have grid power at the residence side, for the controller. In other words, running car-to-house is much more akin to a mechanical generator or to solar PV. Though, yes, personnel protection is still necessary. :thumbsup: So GFI and grounding are very much part of my question.

The vast majority of hybrids aren't meant to be pluggable (or at least >99% don't come from the factory as pluggables), thus most are not also intended to be fed from the house to the vehicle. The key word is "also." But sending power to the house is relatively simpler, with obvious temporary use-cases, like storms.

I still think something's missing in the NEC. Despite the fact that powering a house from a Prius is a no-brainer, it will likely confuse the hell out of the AHJ. There are literally 2 million houses in the US that could be wired up like this today, were it not for its confusing treatment in NEC through 2011. This could be easily clarified in NEC 2014. It might work under 702.

Tom, thank you again for your thoughtful post. I totally missed 625.26.

 

[tom baker]

Thanks
I feel the intent of 625.26 for for future applications where the EV would backfeed the grid and or the service. It would be done via smart meters.
I have read about this application, haven't heard anything lately about it.
But I suspect its being worked on. Its the next step, for the mfgs. EVs are a done deal, its all worked out. they just need to sell more of them.