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Way to limit HPWC to ~60 amps?

Discussion in 'Model S: Battery & Charging' started by gordo, Feb 16, 2015.

  1. gordo

    gordo Member

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    #1 gordo, Feb 16, 2015
    Last edited: Feb 16, 2015
    I have my HPWC install happening in about 2 weeks. I've ordered my car with dual chargers, but until now assumed (erroneously?) that I'd only be able to make use of one when charging at home unless I got a new 100 amp line put in.

    The electrical situation in my home is that I already have a partially populated 100 amp subpanel in the garage. On a regular basis, it looks like I run around 2-3 amps at night, and 5-8 amps during the day off this panel. Reasonably, even with everything turned on, i can't imagine ever needing more than 16 amps total -- meaning that in theory, I should be able to safely run 64 amps to the car, since 64+16 = 80, which is the max rating for a 100 amp subpanel.

    Reading page 16 of the HPWC manual (https://my.teslamotors.com/sites/default/files/blog_attachments/wall_connector_install_guide_northamerica.pdf) seems to imply that if we put in an 80 amp breaker off the subpanel exclusively for the car, and set the HPWC DIP switches to 64A max, I should be able to make partial use of the second charger in my car. I assume that I'd need to run 100 amp capable wire for the car circuit though...

    Am I understanding the situation correctly?

    What if, the unthinkable happens, and while charging the car at the full 64 amps, we exceed 16 amps on the other stuff (e.g. someone plugs in a hair dryer while everything else is maxed out) and I manage to temporarily go over 80 amps total? Would I actually trip any breakers at 80 amps or would I be ok till I exceeded 100?
     
  2. pgiralt

    pgiralt Active Member

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    The wire should be based on the breaker it is attached to, so if you're going to put an 80 Amp breaker, you should put cable rated for 80 Amps (or higher) and then set the HPWC to 64 Amps. That said, if you're going to go through the trouble of running wire to the HPWC, I'd say just run 3 AWG which is rated for up to 100 Amps even if you put an 80 Amp breaker on there in case you ever upgrade the service to be able to run all the way up to 80A on the HPWC. Just make sure you set the DIP switches appropriately.

    It sounds like you don't have any high current loads on this 100 Amp panel. Technically the breaker won't trip until it's pulling 100 Amps (and even then probably for some time - it won't trip if it goes to 101 Amps for a second) but you don't want to run over 80 Amps for any lengthy period of time. I'd probably set the car to charge at 40A and only bump it up to 64A when you really need it and know that it won't be an issue.
     
  3. tom66

    tom66 Member

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    Breakers are thermal devices. A 100A breaker can operate for 3hr continuously at 100A but must be derated to 80A for continuous operation (more than 3 hrs)
     
  4. gordo

    gordo Member

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    So does that mean that technically I won't trip the breaker unless I manage to draw 36 amps on the other circuits while the car charges at max capacity (64 amps in this case) for 3 continuous hours?
     
  5. tom66

    tom66 Member

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    It shouldn't trip.

    But, it is important to check that the installation still meets your state/local county code... check with an electrician.

    I have heard stories where electricians would not sign off on systems where it was possible to go over the total breaker limit in "extraordinary use cases".
     
  6. tga

    tga Active Member

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    New HPWC's can be installed on a 15, 20, 30, 40, 50, 80, or 100 amp circuit. The car will charge at a max of 80% of the circuit rating (ie, 12/16/24/32/40/64/80A). If the car is charging at 40A or less, only the primary charger is used (secondary is idle). If charging at over 40A, both chargers split the load (not 40A on primary, remainder on secondary; ie, 64A = 32A on each charger, not 40A and 24A).

    I think you are misunderstanding the max rating for your panel. In the US, equipment is rated for intermittent loads (less than 3 hours in duration). If the load is continuous (>3hrs), you need to derate 80%. The NEC always considers car charging to be a "continuous load," even if a quick 20 minute "topping off" charge.

    A 100A panel can supply 100A intermittent, or 80A continuous.

    Assuming your 16A are classified as intermittent loads, that leaves 100A-16A, or 84A of intermittent capacity. For charging a car, you derate to 84A * 80% = 67.2A continuous.

    No, you are installing an 80A circuit, so you need 80A wire. Assuming THHN in conduit and a 75deg rating, that's #4 copper wire fed by an 80A breaker. The car will then charge at 80% of the circuit's intermittent load rating, or 64A.

    Mostly, except for the intermittent/continuous part, which I've tried to explain.

    Circuit breakers aren't precision devices. They will trip instantly for a dead short, but can run for a while at their max rating before building up enough heat to trip. As I recall, the NEC defines trip curves (must trip in x seconds for x% overload), but I don't have a copy to look it up.

    In this specific example, you are OK. You are running 64A of continuous load, plus 16A of (presumably) intermittent load. Converted to the system's intermittent rating, that's 64A*125%+16A = 96A intermittent load. Although why you would go out to your garage to dry your hair is beyond me. :wink:

    If you really want to go all out and run your HPWC at full capacity (80A), you are probably better off running a dedicated line back to your main panel. Reducing the number of connections on a high current circuit will eliminate potential points of failure, voltage drop under load, etc.

    Personally, I'd install an 80A circuit on the subpanel with #4 copper. I'd start the car charging at 64A, then make sure there is minimal voltage drop or heating at any of the connections. I'd measure voltage under load at multiple points in the circuit, looking for suspect connections. I'd pull of the covers on my main and sub panels and take pictures with a FLIR to verify none of the connections get hot after the car has been charging for a while. Of course, that's all somewhat dangerous, and shouldn't be done unless you know what you are doing.
     
  7. FlasherZ

    FlasherZ Sig Model S + Sig Model X + Model 3 Resv

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    With the new wall connector, in a 100-amp rated panel with some intermittent loads as you mention, you have room for a 64A continuous load, which means minimum you need an 80A breaker and 80A conductor for the branch circuit from subpanel to HPWC (#4 wire). If you want to future-proof for a future panel upgrade (to, say a 125A panel), then put in #3 or #2 to the HPWC branch circuit with the 80A breaker and 64A continuous load. If there's a subpanel in the garage, it's likely to be a short run and won't cost all that much more. If you can find an older HPWC that allowed the 90A setting, you have room for it (72A charging load) - you'll need to size conductors and breakers appropriately.

    To be honest, many inspectors would likely pass the inspection with the full 100A breaker on the car plus the handful of circuits for intermittent loads. This is because feeders are generally given a bit more latitude; aggregation rarely fills a feeder and an assumption is made most loads are intermittent. But to be fully code-compliant, you'd have to limit yourself to 64A charging load (80A breaker setting).

    You only have to size for "reasonably expected" loads, not the maximum with everything on. So if you assume up to 10A of intermittent loads (you said 5-8A), you're fine with a 10A headroom. Also, if this is a subpanel, remember that 120V loads properly balanced across the legs in a panel get halved when it comes to feeder capacity. A 120V/4A load on L1 and a 120V/4A load on L2 count as 4A on the 240V feeder (the total is 4A across L1+L2 with neutral carrying only the difference between the two legs). So if you're adding a bunch of 120V appliances up that are balanced across multiple circuits, you can take this into account.

    The "derate" word keeps getting thrown around... circuit ratings and OCPD's are not "derated" for continuous load operation. Rather, the circuit size must be engineered for (continuous_loads * 1.25) + intermittent_loads. It's a point of technicality which becomes important when you start needing to account for multiple conditions that cause true derating - current-carrying conductor rules, temperature rules, etc.). All EV charging loads are considered continuous, regardless of expected charging time.
     
  8. gordo

    gordo Member

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    Thanks, this is all very helpful. Sounds like I should be ok to charge up to 64 amps... Glad I'm having a professional do the install.
     
  9. ArtInCT

    ArtInCT Always Learning

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    This thread is very informative as I will be running a new service (200 Amp) run into my barn & garage.

    Planning on the HPWC in the garage and a 50A receptical as well to act as a backup should thge HPWC ever have a problem.

    Going this route as the barn/garage are 60 feet from the house but 10 feet from a utility pole.
    It will actually be less expensive to add a second service vs trenching and running close to 120 feet of #2 from the existing house panel the location in the barn where a new panel will have to be installed.

    Do you all think the back up receptical is a wise idea?

    Electrican said it is minor to add it while he is doing the HPWC and the panel and meter.

    Your THoughts are Welcome.
     
  10. SW2Fiddler

    SW2Fiddler Bannd Member

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    I do!
     
  11. andrewket

    andrewket 2014 S P85DL, 2016 X P90DL (soon 100)

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    I agree. My electrician wouldn't do it though because it would cause my sub panel to be oversubscribed. The fact that the backup 14-50 and the HPWC would never be used at the same time didn't matter.
     
  12. FlasherZ

    FlasherZ Sig Model S + Sig Model X + Model 3 Resv

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    Yes, a backup receptacle is indeed a very good idea.

    - - - Updated - - -

    A bit surprised... if it's a subpanel, it's protected with an upstream breaker and that's sufficient to protect the feeder.
     
  13. breser

    breser AutoPilot Nostradamus

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    I put a 14-50 and an HPWC in but my HPWC isn't on a subpanel and neither is the 14-50. The 14-50 is in a completely different bay in the garage, but my intention was only to have it as a backup. Inspector was confused a bit about it but I explained my intentions and he was fine with it.
     
  14. ArtInCT

    ArtInCT Always Learning

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    Ok, so that is the way I will go with the HPWC and a backup 240 Volt recepticle as well. I will probably have a 110 15 amp outlet added to each wall of the two car garage from the 200AMP panel as well. Thanks much to all who have shared their thoughts. Best Art
     

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