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Great article on the 120% OR why most inverters in California are < 7600W

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  • Great article on the 120% OR why most inverters in California are < 7600W

    Hey Everybody,

    I just saw this great article from Amy at the Alt-E Store on the 120% rule for load side taps. This is topic that comes up often here (especially for folks from California) that many consumers and novice installers alike don't seem to fully understand.

    www.altestore.com/blog/2018/11/grid-tied-solar-breaker-box-120-percent-rule/

    It's very informative and clearly written. Thanks Amy!

    -Jonathan

  • #2
    FYI, Don't know if you know it or not but Alt Amy (AKA "The Solar Queen") was a regular poster here for some time. She's been mostly silent for some time now. Maybe since before you came around.

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    • #3
      Very informative. Thanks for sharing.

      I had two companies do a formal site survey on my house. One said I had to upgrade my 150A breaker box to 200A, while the other company said no breaker upgrade necessary - they're doing a line tap and won't even need to add a breaker to my breaker box.

      Are there any downsides to doing a line tap?

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      • #4
        Nice find JSchnee21 !​​​
        https://pvoutput.org/list.jsp?userid=59404

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        • #5
          Looks like i am in violation - I have a 100 amp box with a 40 amp solar breaker out at the garage.

          The failure mode seems to be if I happen to draw >120 amps - the box bars won't deliver it safely. I have the 40 amp breaker at the opposite end as suggested. Hmmm. This is a garage, not much simultaneous usage. i'm plugging my electric car into the box = 32 amps. Not much else there. Difficult to get 200 amp supply line to the garage, although I guess the 125 amp box upgrade is what I need. But wait... this current travels into the house; that is a 200 amp box though so should be good there.

          As I expected the inspection process of the county didn't pick up anything - this seems like it should have been in their wheelhouse though. It was clearly on my permit papers before I started the project.

          Thanks for posting this article !
          Last edited by QuantumSlice; 04-01-2019, 02:07 AM.

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          • #6
            Originally posted by QuantumSlice View Post
            .......I have the 40 amp breaker at the opposite end as suggested. Hmmm. This is a garage, not much simultaneous usage. i'm plugging my electric car into the box = 32 amps. Not much else there. ....
            Can you clarify how your solar breaker is positioned? I am not sure it is clear when you say "opposite end" that you mean opposite end from the supply end or from the EVSE breaker. Is your solar on a 40 Amp breaker? I assume your EVSE is on a 40 Amp breaker if it is drawing 32 Amps when continously charging.

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            • #7
              My house has a 200 amp breaker as it was built in 1997. But, my highest continuous load is only ever ~6kW. If I go crazy and run both microwaves and both bathroom heated fans while the HVAC is running, I might hit 9kW for a few minutes.

              If properly balanced this would only be:

              6000W / 240 = 25A
              9000W / 240 = 37.5A

              Even if completely unbalanced this would only be

              6000W / 120 = 50A
              9000W / 120 = 75A

              Yes, I understand starting in-rush, which is why they make starting caps and time delay breakers.

              But in reality, I have no need for 200amp service. Nor would most people I would expect. This could easily be de-rated to 150amps and I would never notice. Maybe one day, when I have two Tesla roadsters (-: Or two HVAC units and one electric car.

              From an electrical reliability standpoint, a breaker in the load center, or a proper line side tie-in using a junction box and bus bars, would be preferred to the clamp on puncture style line side tie-in I have now. But given that my inverter is 11.4kW, I don't think I could have de-rated my panel board enough to use a load side breaker as input (~60 amps). And a commercial grade line-side tie-in would have been much more expensive than what I have now.

              -Jonathan

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              • #8
                Originally posted by JSchnee21 View Post
                But in reality, I have no need for 200amp service. Nor would most people I would expect. This could easily be de-rated to 150amps and I would never notice. Maybe one day, when I have two Tesla roadsters (-: Or two HVAC units and one electric car.
                you seem to speak from confidence there that most people don't need a 200a MSP or service. There are many that have 400a.
                Most homes in the mid US have all electric with heat pumps.
                Our house has 200a MSP, and is ALL electric. :
                80a - emergency heat.
                60a - hot tub
                50a - stove,
                40a - heatpump.
                30a - water heater,
                30a - clothes dryer,

                As we have done installs all over the US, few homes could have the MSP easily derated both because of capacity and limited availability of main breakers of various sizes.



                OutBack FP1 w/ CS6P-250P http://bit.ly/1Sg5VNH

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                • #9
                  Originally posted by JSchnee21 View Post
                  Hey Everybody,
                  I just saw this great article from Amy at the Alt-E Store on the 120% rule for load side taps.
                  -Jonathan
                  Anything less than a 200A dist box can be pretty restrictive. Note, putting in a bigger box
                  with higher rated main breaker, means heavier copper feeds from the PoCo and possibly
                  even a transformer upgrade. Such things may be useful to help stabilize voltage in non
                  overload situations.

                  I read that whenever 2 power sources are available, they should feed opposite ends of
                  the load distribution box. So source power will flow toward load breakers in the middle
                  of the breaker bus bars. With current flowing toward center loads, no part of the bus bar
                  will carry the sum of the source currents. If you could find a plug in breaker larger than
                  the original main breaker, in theory it would be possible to overload the single tab feeding
                  it. An unlikely arrangement.

                  BUT with both power sources at one end, currents will add and possibly could equal the
                  sum of both source breakers. To limit max busbar current in this case, the original
                  main breaker could be downsized, assuming it to still be at least as large as the new
                  source breaker. Downsizing the main breaker AND placing the second power source
                  at the opposite end of the box will give belt and suspenders protection.

                  Here in the Midwest (where it can really get cold) I have seen heating evolve from coal, to
                  oil, to natural gas. Some off the gas lines use propane. All electric is the rare, rather
                  expensive exception. This house has gone another step to cold weather heat pumps powered
                  by solar net metering. Bruce Roe
                  Last edited by bcroe; 04-01-2019, 11:17 AM.

                  Comment


                  • #10
                    JSchnee21 my house was built in 2005 and it only came with a 150A breaker. The builder wanted like $750 extra for a 200A upgrade IIRC and I refused to pay it. They still gotta nickel and dime people buying a house on something so minor. How much more could it have been for the upgraded parts? It's the same labor involved.

                    Interesting point you brought up about having an electric car and dual zone AC (our heat runs on natural gas). I bought a Model 3 in November. On the NEMA14-50 outlet, the car charges at 40amp on 240V. Now I wonder if the 150A breaker can handle both AC units running while the car is charging..
                    Last edited by sunpoweredev; 04-01-2019, 03:32 PM.

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                    • #11
                      Originally posted by sunpoweredev View Post
                      ...........

                      Interesting point you brought up about having an electric car and dual zone AC (our heat runs on natural gas). I bought a Model 3 in November. On the NEMA14-50 outlet, the car charges at 40amp on 240V. Now I wonder if the 150A breaker can handle both AC units running while the car is charging..
                      Your Model 3 may tell you if there is voltage sag before your main breaker goes. Of course you can always drop the charging rate from your phone app. I rarely charge my Tesla at more than 32 Amps. Before I got my Permission to Operate on my solar I was charging during the day at 16 Amps to soak up the solar generation that I was not getting credit for. I dont have a long commute.

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                      • #12
                        Thanks everyone for keeping me honest and giving a broader perspective.

                        Yes, I am fortunate to use natural gas for most of my appliances and heat. But clearly as Butch points out, many homes which don't have access to natural gas or oil heat, or whom live in a more temperate climate where heat pumps are more typical, may definitely need 200 amp electrical services. Especially in larger homes with multiple dryers, hot water heaters, and HVAC units.

                        When I was a volunteer FF we saw our share of houses with 400amp services as well in PA (or two separate 200 amp services). But these were all >5000 sq feet so i wouldn't have considered them to be "typical."

                        As EV's begin to catch on and move more mainstream, this is potentially going to be a real issue. Especially with Tesla leading the way for faster and faster charge rates. At the upper echelons of L2, the 17,200W charging rate is ~72 amps @ 240V. So that would be a 90-100 amp breaker. Even Tesla's standard L2 charge rate is 12,500W (~52 amps @ 240V).

                        Of course you can always charge slower (-: But who wants that.

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                        • #13
                          Originally posted by JSchnee21 View Post
                          Even Tesla's standard L2 charge rate is 12,500W (~52 amps @ 240V).
                          Older Teslas are 10kW; newer ones are 11.5kW. Fortunately you can choose when you charge them; charging from 2-6am puts less stress on your system (and the grid as a whole.)
                          Of course you can always charge slower (-: But who wants that.
                          Most people won't care if their charge takes from 2am to 4am vs 2am to 6am.

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                          • #14
                            Originally posted by sunpoweredev View Post
                            Very informative. Thanks for sharing.

                            I had two companies do a formal site survey on my house. One said I had to upgrade my 150A breaker box to 200A, while the other company said no breaker upgrade necessary - they're doing a line tap and won't even need to add a breaker to my breaker box.

                            Are there any downsides to doing a line tap?
                            The only real downsides to a line (supply) side are that some inspectors don't understand them and some POCOs or AHJs do not allow them (with no apparent justification.)
                            SunnyBoy 3000 US, 18 BP Solar 175B panels.

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                            • #15
                              Originally posted by JSchnee21 View Post
                              .......

                              Of course you can always charge slower (-: But who wants that.
                              Well, it all depends on where you are standing. I am currently on a 5,000 mile road trip in a Tesla. I am 75 years old and have to pee every 100 miles. Superchargers are conveniently located every 100 or so miles on the Interstate. It only takes 20 minutes to get another 100 miles of range at a slow Supercharger and that is about how long it takes this 75 year old guy to pee. LOL

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