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HPWC + generator load shed

Discussion in 'North America' started by sc123, Sep 27, 2016.

  1. sc123

    sc123 Member

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    Hi forum-

    I'm in the process of getting an HPWC installed, and I would like to feed it with 100A (as I have a 72A charger in my X). I also have a 20kW Generac whole-house generator with an automatic transfer switch. My electrician is looking at some options for how to do the install. I've seen the few messages at the end of the FAQ / Q&A thread which seem to ask some of these same questions, but didn't have any solutions. I'm curious if anyone has figured out a good solution here, but I'll also share my thoughts (and experience) here in case anyone else runs into this.

    The main issue is the generator can't handle the full charging load, so we need a way to kill the HPWC load when we lose power and the generator kicks in. Generac sells load shed modules for this type of application, but they only go up to 50A capacity. (basically these big fat car charging circuits are pretty new, and they haven't had to deal with big loads like this in residential settings in the past). That's obviously not ideal since it limits me to 40A charging (I believe the 50A limit would force me into the 50A circuit / 40A charging selection on the HPWC).

    A few ideas we had were:

    1) The electrician said he could install a 100A relay, and feed that relay off a separate circuit that comes off the panel, and put the load shed in the path of the relay control. When the generator kicks in, it sheds the load controlling the relay which then kills the 100A feed to the HPWC unit. The downside is this is not a typical everyday thing that's done in residential settings, and he thought it might cost in the neighborhood of $500 extra.

    2) If the HPWC had some ability to provide a low-power cutoff circuit, that could be wired into the generator transfer switch load shed unit. The generator's transfer switch provides dry contacts for this purpose that will be normally closed, and then will open up when the house load is transferred to the generator. This is how my AC condensor is wired - essentially the dry contact is put in the path of the low voltage (24VAC I believe) path to the condensor so that it never turns on when the generator is engaged. (Actually it's programmable since the load isn't absolutely too much, but may be a little taxing, but that's a different story). I called Tesla, and they don't seem to support this in the HPWC (but they were double checking - fingers crossed!). This one is too bad, it would be *much* easier (and cheaper) to eliminate the need for the high current load shed unit entirely, and just have the control circuit in the HPWC shut charging off when the disconnect relay opens. Ah well...

    3) A separate panel before the transfer switch (as suggested in the FAQ / Q&A thread) doesn't seem to make sense (i.e. it would probably cost more than #1)

    4) Any other clever ideas? I can't believe that no one has done this before. Or maybe no one has bothered to do a >50A installation with a (Generac) generator before?

    Thanks in advance.....
     
  2. miimura

    miimura Active Member

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    When I was looking at the Generac transfer switches and their load shedding logic, I was tremendously underwhelmed. The only smart thing that they would do is turn off air conditioning loads by interrupting the thermostat's low voltage signal to the compressor. Their idea of load shedding was to shut down all loads (yes, complete blackout for the house) then start them up one by one until it overloaded again, then repeat the whole process. Completely ridiculous.

    My house has the meter in a 400 amp panel where all my large loads are. Most of the household 120V loads are all on one subpanel fed by a 125 amp circuit. So, I would just forego any backup on the main panel and just put the transfer switch between the main panel and sub-panel. I would only lose my EVSE circuits, air conditioning, ovens, and electric tile floor heaters. I cannot imagine how you would put a transfer switch between the meter and the circuits in my main panel. Given that your main panel will be switched, I would think that triggering an industrial disconnect for the HPWC from the transfer switch signal would probably be the best way. Either that or just don't plug in your car when the generator is running.

    Also, your car may not like the quality of the power coming out of the generator, so it may not ever draw a significant amount of power from the generator. Now that I've thought about that issue, I would hold off spending any money on the fancy disconnect and see if the car will even charge from the generator. You could start at a low current setting on the touch screen and increase it gradually to see what happens. If it will only charge at low current settings, set it back to normal and start it charging from the grid. Then cut the main breaker and let the generator do its thing. Does the car re-start charging at all? Does it start and then stop before the generator overloads? I'm really thinking that the car will take care of itself and shut down before the generator does.
     
  3. swaltner

    swaltner Member

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    I was thinking some people here had decoded the load sharing protocol on the new version of the HPWC. That would allow you to remotely adjust the current down as needed.

    While it has a J1772 connector instead of the Tesla connector, my OpenEVSE has the newly-released Wi-Fi module on it. This has an API built-in that you can remotely adjust the current down and up as desired. Have an automated job to adjust the OpenEVSE current as needed to keep your total current below whatever limit you set.
     
  4. Lloyd

    Lloyd Active Member

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    If you increase the Gen size to 30 KW there is not an issue. I've rum mine at 80 amps with this gen set and not an issue.

     
  5. sefs

    sefs 2012 Ford Focus Electric

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    I have a 22 kW Generac, and I have a similar setup. PSP Products has a 100 amp contactor, complete with enclosure all ready to go. $109 plus shipping and you're good to go.

    Just FYI, if you're on natural gas, you're going to be lucky to get 60 amps out of a 20 kW. You're also going to lose capacity for being above sea-level. So just be aware, you're probably going to want to dial back the amps to about 40 when you're charging off generator power.

    Load Shedding Contactors
     
  6. sc123

    sc123 Member

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    Thanks for the replies. Spoke to my electrician and he said he's thinking now it might make more sense to do #3. Since the 20kW generator can't support the full load and the HPWC doesn't have an official disconnect (or, ideally, have a way to be told to limit the current when I'm on the generator), that it's just easier to bring the feed to the charger off a panel before the transfer switch.

    The problem with dialing it back manually is if power goes out and the transfer switch cuts in when the car is in the middle of high-current charging. Maybe it would be smart enough intelligently dial back the current without damaging the generator, but maybe not. I'm not that concerned about high power charging when we have an outage to take any risk like that. I can always plug into 110V for the rare case when we lose power and I can't charge somewhere else remotely. And definitely don't care enough to upgrade the generator...

    Lloyd, the link you posted gave me an error:

    Tesla Motors Club - Error
    You do not have permission to view media within this album.
     
  7. sc123

    sc123 Member

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    Yeah, it seems like this should obviously be possible, but I wouldn't consider doing anything like this unless it was officially supported by Tesla. And I certainly would want it to be some sort of dead-simple dry contact / relay type cutoff ... I'm an engineer and have enough experience with tinkering with stuff like this to know that I wouldn't want to depend on something like that to protect against an overcurrent situation ... not worth the risk in my opinion with these huge loads.

    It's too bad the HPWC doesn't have something like this. Ideally, if it had two sets of contacts (with factory jumpers installed in both), one to kill charging, and one to limit current to some low number (maybe a second dial), then that would probably solve all these issues. Ah well ....
     
  8. sc123

    sc123 Member

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    Do you have the same type setup I described in #1? So the generac load shed module is in the path of the on/off control to the PSP 100A contactor? My electrician said even if he did that, the generator could only put out about 80A max, so he'd have to wire it in as a hard shutoff (i.e. it would never be connected when the generator was on, unlike most loadsheds where they cut off for a while, then test the load and see if it doesn't drop the frequency/voltage/etc). Because of that, since the charger would never be powered from the generator, there wasn't a good reason to have the complexity of the switch - hence just do the panel before the transfer switch...
     
  9. sefs

    sefs 2012 Ford Focus Electric

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    I have it setup to be load shed from the transfer switch. I have my AC, EVSE's, garage heater, and hot tub on contactors controlled by the transfer switch. If the transfer switch senses overload via drop in frequency, it drops everything, waits 5 minutes, and brings things back on one at a time.

    Is your generator hooked up to an essentials sub panel or does it power the whole house? You said earlier it was a "whole house generator". If that's the case, there isn't an easy place to hook up the generator before the main panel.

    Personally, I would put your HPWC on the transfer switch and have the contactor be controlled. If you're out of power for a long time and it's wide spread, you're going to be really glad you can at least charge at ~40 amps. You're probably home most times you are charging, so if the power goes out, you can easily unplug your car, or go and set it down to 40 amps. It's not going to come on for 5 minutes after the generator kicks in. Worst case, you aren't home, it overloads, the transfer switch locks out that load, and you're no worse for wear. Best case, you have a prolonged outage, and you have a fully capable charging point while everyone else is searching for gas.
     
  10. gavine

    gavine Petrol Head turned EV Enthusiast

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    Why do you need to charge at 72A at home daily? Do you drive that much? Why not charge at 30A or 40A as your default and only bump up to 72A if you need to charge faster that day for some reason. No reason to charge faster than necessary at home in my opinion. It's probably more efficient charging at 30A too.

    In the event of a power outage, I would think charging slower is better than spending all of this time, effort and money to make it able to charge at 72A on generator. I'm failing to see the logic.
     
  11. sc123

    sc123 Member

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    For most people it would probably not matter, but for us the biggest reason is my wife's a realtor, and can easily drive >200 miles in a day showing properties, and having the option to stop home for a quick charge may come in handy occasionally (and time matters in that case). Doesn't happen every day, but it does happen. We got an inventory car that had the 72A charging option already, so I figure I might as well have the ability there.

    The transfer switch is currently inline before the main 200A panel. We're going to go with the "new panel before the transfer switch" option - he'll just put it outside between the meter and the transfer switch - space isn't an issue. I basically decided that I'd rather have the 72A charging option with no generator backup vs the lower current charging option with backup. In the unlikely event of a long outage I can always charge on 110 on a generator switched plug, charge somewhere else, or worst case scramble to throw in a NEMA 15-40 outlet on the switched panel if it becomes an issue later.

    Hopefully installed this weekend, I'll try to take some pictures and post them here if I'm around when he does the work....
     
  12. Saghost

    Saghost Active Member

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    Oh, now that's an interesting idea.

    Set the HPWC as slave, and spoof the master signaling to change how much power is available to it. In principle this can give you easy options to turn the power down or off with just some simple circuitry.

    I don't think I've seen anything on the HPWC sharing interface communications here so far, so it'd probably be a bit of an experimental adventure to figure it out.
     
  13. sefs

    sefs 2012 Ford Focus Electric

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    I wouldn't decrease setting on the HPWC; the generator is overload protected by it's load shedding capability with the 100A contactor. But it's up to you, best of luck.
     
  14. sc123

    sc123 Member

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    I get what you're saying, but my electrician wasn't comfortable with throwing a big load on a load shedding-controlled contactor like that. He is concerned about the scenario where the car is charging at 72A when an outage happens (and no one is there to manually dial the current back), and then the load shedding module keeps trying to switch in the ~72A load on the generator (which can only supply ~80A total). I appreciate his concerns, and I'd rather play it safe as far as protecting the generator, and just live without the HPWC during an outage in order to get the full 72A charging capability. There are enough Plan Bs (and Cs) to get a charge in that case where it's not the end of the world...
     
  15. sefs

    sefs 2012 Ford Focus Electric

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    It wouldn't harm the generator at all (that's the purpose of load shedding), but what you are going with works just as well.
     
  16. sc123

    sc123 Member

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    I hear you, but my electrician disagrees with that assertion, and I have to go with his opinion here ... :)
     
  17. Lloyd

    Lloyd Active Member

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    I don't know how to grant you permission. Anyone?
     
  18. hcsharp

    hcsharp Active Member

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    You can do the same thing with the HPWC. Take the pilot wire which is at +-12v or less while charging and run it to the NC switch on the generator control. As soon as the switch opens, your car will stop pulling any current.
     
  19. sc123

    sc123 Member

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    That's interesting. Can you elaborate a little more on specifically which connections inside the HWPC would be used, and how they work? Mine is still sitting in a box. I'm already going down the path of a separate circuit tapped off before the transfer switch, but others might find it useful.

    I called Tesla's charging support line, and spoke to someone who said the HPWC doesn't have that capability. He actually went off an investigated to be sure and got back to me a few days later. So it doesn't sound like this is an "official" capability, though it obviously wouldn't surprise me that there was some way to make it work - it's essentially not too far off what the multi-HPWC configuration does. I guess the question would be whether you'd want to count on something that's not officially supported to protect against a potential generator overload (assuming your generator is borderline / not capable of supporting the HPWC load, like my case) - if anything went wrong you're probably on your own as far as dealing with the repercussions if anything goes wrong...
     
  20. hcsharp

    hcsharp Active Member

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    It's not officially supported and may void your warranty but it's easy to do and there's no danger of hurting anything. Well, no danger as long as the pilot wire is not routed or connected in such a way that it could be shorted to high voltage (120/240v). I believe it's the blue or purple wire inside the HPWC that feeds into the cable to the connector that plugs into your car. It's easy to tell. If the HPWC is powered up but not plugged into your car, the pilot wire will read about 12v. The other small wire (orange?) will have about 3.3v. I might have the colors reversed but you can tell the pilot wire b/c it will be at 12v.

    I'm not necessarily recommending this hack. I'm just explaining that it's easy to do.
     

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