Adding 1.44kW to current 2.94kW system (Questions)

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  • solar_davesolar_dave Solar Expert Posts: 2,382 ✭✭✭✭
    Re: Adding 1.44kW to current 2.94kW system (Questions)
    Spiffster wrote: »
    I think what they didnt understand was that the feedback from the solar system puts a load on the bus bar. They couldnt get their heads around how the main breaker de-rate gives me more capacity to add solar. They are stuck on the whole, smaller breaker = less capacity logic. Im no pro so perhaps I didnt explain it well. (especially since this concept is new to me too). I think I may have found 1 electrician that is willing to work with / learn more about this solar energy stuff. He just followed up with me this morning so hopefully it works out.

    Just tell them you are sourcing power from 2 locations, the solar and the grid. And to prevent the grid + solar from overloading the bus bar, you need a derate on the grid side breaker.
  • ggunnggunn Solar Expert Posts: 1,973 ✭✭✭
    Re: Adding 1.44kW to current 2.94kW system (Questions)
    solar_dave wrote: »
    Just tell them you are sourcing power from 2 locations, the solar and the grid. And to prevent the grid + solar from overloading the bus bar, you need a derate on the grid side breaker.

    You really shouldn't have to explain this to a licensed electrician; just tell him to read NEC Article 690.64(B)(2). It's in there.
  • SpiffsterSpiffster Registered Users Posts: 23
    Re: Adding 1.44kW to current 2.94kW system (Questions)

    So this electrician seems to know what he is talking about, though his equation doesnt make a whole lot of sense to me. Here is what he said in an email:

    "You have a two inverters, one requiring a 30 amp on each buss, and one requiring a 20 amp on each buss. You do not have any other protection except the main breaker so the only rating we can use is the 150 amp main. We meet the requirements of 690.8 (PV breaker must be at opposite end of panel from main feeders) which qualifies us for the 120% allowance. Then to follow the requirements of NEC 690.64(B)(2) would look like this.

    (30 + 20 + 150) X 20% = 200 X 20% = 166.7 amps

    According to Siemens, that panel is rated at 200 (as indicated on the inside cover) so by this interpretation, you are fine.

    If the PV feeding breakers were not located at the bottom of the panel, that would put you out of compliance for 690.8 and then you would no longer qualify for the 120%. Even if that was the case, as shown above, even before taking advantage of using the bus bar rating of 120%, 200 amps does not exceed your bus bar rating."

    So the panel is 200amp buss bar with 150amp breaker already. This would concur with another electrician saying that the 150amp and 200amp Siemens both have 200amp buss bars. I guess that's why the label says "150amp (200amp continuous)"? So I should be good to go right?
  • ggunnggunn Solar Expert Posts: 1,973 ✭✭✭
    Re: Adding 1.44kW to current 2.94kW system (Questions)
    Spiffster wrote: »
    So this electrician seems to know what he is talking about, though his equation doesnt make a whole lot of sense to me. Here is what he said in an email:

    "You have a two inverters, one requiring a 30 amp on each buss, and one requiring a 20 amp on each buss. You do not have any other protection except the main breaker so the only rating we can use is the 150 amp main. We meet the requirements of 690.8 (PV breaker must be at opposite end of panel from main feeders) which qualifies us for the 120% allowance. Then to follow the requirements of NEC 690.64(B)(2) would look like this.

    (30 + 20 + 150) X 20% = 200 X 20% = 166.7 amps

    According to Siemens, that panel is rated at 200 (as indicated on the inside cover) so by this interpretation, you are fine.

    If the PV feeding breakers were not located at the bottom of the panel, that would put you out of compliance for 690.8 and then you would no longer qualify for the 120%. Even if that was the case, as shown above, even before taking advantage of using the bus bar rating of 120%, 200 amps does not exceed your bus bar rating."

    So the panel is 200amp buss bar with 150amp breaker already. This would concur with another electrician saying that the 150amp and 200amp Siemens both have 200amp buss bars. I guess that's why the label says "150amp (200amp continuous)"? So I should be good to go right?
    Correct on both counts. His equation doesn't make sense (he is dividing 200 by 1.2 instead of multiplying, which is incorrect), but if the busbar is 200A and the main breaker is 150A, then you have (1.2)(200)-150=90A headroom in the panel. In some jurisdictions the inspector won't allow more than two breakers to feed a busbar in a service panel and they have to be at opposite ends no matter what the sizes are; in that case you'll have to install a subpanel to combine the outputs of the inverters and have no loads in that sub.
  • BB.BB. Super Moderators, Administrators Posts: 31,875 admin
    Re: Adding 1.44kW to current 2.94kW system (Questions)

    I am not sure I agree with his math:
    • 150 Amp Panel * 20% = 30 amp maximum solar circuits (150 Amp Panel Rating)
    • 200 amp panel rating * 20% = 40 amp maximum additional source current (solar)
    • 240 amp panel rating - 150 amp main breaker = 90 Amp maximum additional source current circuits (solar)
    Your electrician's math:
    • (30 + 20 + 150) X 20% = 200 X 20% = 166.7 amps
    Looks more like a maximum circuit current derating... You are only able to run 80% of a circuit's rated current... The breaker and wiring must be 1/0.80 or 1.25x larger than the continuous load. So, a "200 Amp" panel and breakers would be rated for:
    • 200 amps * 0.80 = 160 amps maximum continuous loads.
    But that is neither here nor there useful for this discussion. I would assume your Solar Breakers/Wiring are already sized per the NEC... For example a 17 Amp maximum output inverter would need:
    • 17 amps * 1.25 NEC factor = 21.25 Amp Circuit
    So, that would be rounded up to 25 amp circuit wiring and breaker.

    Back to your issues--Assuming the panel is rated for 200 Amps (bus bars and enclosure), it does sound like you are ready to go (and the solar circuits are "at the bottom" of the bus bar, opposite end from the main breaker).

    I would take a photograph of the name plate (internal, external, etc.) of your main service and take it to your local building department (assuming you have one) and see if they agree with the 90 amp solar circuits with 150 amp main breaker.

    If they agree--then you should be safe to proceed.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • SpiffsterSpiffster Registered Users Posts: 23
    Re: Adding 1.44kW to current 2.94kW system (Questions)
    BB. wrote: »
    I am not sure I agree with his math:
    • 150 Amp Panel * 20% = 30 amp maximum solar circuits (150 Amp Panel Rating)
    • 200 amp panel rating * 20% = 40 amp maximum additional source current (solar)
    • 240 amp panel rating - 150 amp main breaker = 90 Amp maximum additional source current circuits (solar)
    Your electrician's math:
    • (30 + 20 + 150) X 20% = 200 X 20% = 166.7 amps
    Looks more like a maximum circuit current derating... You are only able to run 80% of a circuit's rated current... The breaker and wiring must be 1/0.80 or 1.25x larger than the continuous load. So, a "200 Amp" panel and breakers would be rated for:
    • 200 amps * 0.80 = 160 amps maximum continuous loads.
    But that is neither here nor there useful for this discussion. I would assume your Solar Breakers/Wiring are already sized per the NEC... For example a 17 Amp maximum output inverter would need:
    • 17 amps * 1.25 NEC factor = 21.25 Amp Circuit
    So, that would be rounded up to 25 amp circuit wiring and breaker.

    Back to your issues--Assuming the panel is rated for 200 Amps (bus bars and enclosure), it does sound like you are ready to go (and the solar circuits are "at the bottom" of the bus bar, opposite end from the main breaker).

    I would take a photograph of the name plate (internal, external, etc.) of your main service and take it to your local building department (assuming you have one) and see if they agree with the 90 amp solar circuits with 150 amp main breaker.

    If they agree--then you should be safe to proceed.

    -Bill

    By "Building department" do you mean the city or county? I do know I will need to get a permit.
  • BB.BB. Super Moderators, Administrators Posts: 31,875 admin
    Re: Adding 1.44kW to current 2.94kW system (Questions)

    Whatever your local authority having jurisdiction is (AHJ?). You probably should go over your previous paper work with the AHJ and the Utility--and use it as a template for your install/upgrade.

    In many areas, a total system rating of 10kW (CEC--i.e., derated total system, not panel STC marketing ratings) or less is usually pretty easy. Over 10kW, your utility may begin to make things more difficult.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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