How big a system can you back feed to a breaker?

I upgraded to a 225 amp load center with 200 amp main when I installed solar. I have a 9.2kw Enphase system with 40 CS6P-230P Canadian Solar panels. I am upgrading by adding 10 240 watt panels 2.4kw more for a total 11.6kw. This feeds to a 100 amp sub panel with four 15 amp double pole breakers.
I have a 60 amp disconnect switch and back feeding to a 60 amp breaker into the main load center.
Main load with 200 amp main center Bus is 225 amps 120% could handle 270 amp max with solar feeding top line feeding on the bottom.
60 amp disconnect can handle 48 amp (80%)
My current setup is 36 amps and I will be adding 9 amps for a total = 45 amps. I am good so far.

It looks like I could add another 10 panels 9 more amps total 54 amps down the road. But I would have to upgrade my disconnect switch to 100 amp and my back feed breaker to 70 amp. This would be 14kw is there a limit on what you should back feed or does it just go buy the numbers?
«1

Comments

  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: How big a system can you back feed to a breaker?

    There are limits. They being what is allowed by NEC and permitted by the utility.
    NEC limits the back feed to 20% (120% current rating on the bus bars) and most utilities will only permit 10 kW. As such you get a 200 Amp service with a 40 Amp breaker handling 9600 Watt GT system. Not much advantage to having a 225 Amp service.

    You might be allowed a line side tap on your service for additional power connection, or not. At that point the size of the service feed and transformer come into play.

    Right now you've got (40 * 230) 9200 Watts. I don't think you'll be able to add 2.4 kW more without major rework of the service.
  • BB.BB. Super Moderators, Administrators Posts: 30,645 admin
    Re: How big a system can you back feed to a breaker?

    You also could (all things else being legal, to code) reduce the Main Breaker, from (for example) 225 amp to 200 amp, and that "extra" 25 amps can be assigned to your solar output.

    Note, your main panel with the reduced breaker size sill needs to be able to support your normal/maximum loads. Commercial main panels also have other code requirements.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • NeedMoreSolarNeedMoreSolar Registered Users Posts: 23
    Re: How big a system can you back feed to a breaker?

    We are allowed to have up to 20kw solar as long as we have enough demand.

    “permit residential customer to install systems with nameplate ratings up to
    the estimated maximum monthly kilowatt (KW) demand of the residence or 20KW”

    But they want us to use to back feed a breaker. Even with adding 2.4kw more we still won’t zero out so I have enough demand.
    I already upgraded to a 225 amp service. I am only using 200 main amp breaker in my 225 amp panel.
    My 225 amp main service should be able to handle 270 amp (120% rule)
    I will only be putting 45 amps into it once I add 2.4 kw more.
  • benatwhodotnetbenatwhodotnet Registered Users Posts: 16
    Re: How big a system can you back feed to a breaker?

    I happen to know about the main panel issues of backfeed. My contractor started out with a quote, using existing 200A main panel and proposing to backfeed up to 100A into it. This was immediately denied by both the municipal permit authority and the power company. The fact that a 200A main breaker was installed between the meter and the buss or feed bars where the back feed breaker would be installed, ignores the fact that back feed potentially increases the non-metered current to loads on that side of the main. The meter could still provide up to 200A into the panel, and 100A additional could be supplied by back feed. This must be added together to get the potential current on the buss bars. My panel was upgraded to 400A. The final install used a 125A back feed breaker with a 22K peak in-rush amps rating.

    http://azben.blogspot.com/2012/02/inspections-by-city-of-glendale.html
  • ggunnggunn Solar Expert Posts: 1,973 ✭✭✭
    Re: How big a system can you back feed to a breaker?
    We are allowed to have up to 20kw solar as long as we have enough demand.

    “permit residential customer to install systems with nameplate ratings up to
    the estimated maximum monthly kilowatt (KW) demand of the residence or 20KW”

    But they want us to use to back feed a breaker. Even with adding 2.4kw more we still won’t zero out so I have enough demand.
    I already upgraded to a 225 amp service.
    My 225 amp main service should be able to handle 270 amp (120% rule)
    I will only be putting 45 amps into it once I add 2.4 kw more.
    It's not the amps but the size of the OCPD's that counts toward the 20% rule. 20% of 275A is 45A, but you are feeding the bus with 60A of OCPD's. It seems to me that you already are in violation.
  • BB.BB. Super Moderators, Administrators Posts: 30,645 admin
    Re: How big a system can you back feed to a breaker?

    I believe ggunn is correct. You cannot find 45 amp breakers.

    And, remember that you need to size the breakers/wiring to (at least) 1.25x your rated load. So, if after all the calculations (and fudge factors) show you having a 45 amp maximum solar power source, then 1.25x45a=56.25 Amp rated circuit minimum.

    The "easiest" way around this would be to reduce your main breaker to 200 amps maximum--That will give you an "extra" 25 amps of margin on your bus bar ratings--If the local code and rest of your home loads will allow this.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • benatwhodotnetbenatwhodotnet Registered Users Posts: 16
    Re: How big a system can you back feed to a breaker?
    ggunn wrote: »
    It's not the amps but the size of the OCPD's that counts toward the 20% rule. 20% of 275A is 45A, but you are feeding the bus with 60A of OCPD's. It seems to me that you already are in violation.

    I immediately "Googled" OCPD and found Obsessive Compulsive Personality Disorder and Orange County Police Department... I found a lot of references to OCPD Solar too, but cannot find anyone spelling it out. I am thinking Over Current Protection Device? Is that a breaker?
  • NeedMoreSolarNeedMoreSolar Registered Users Posts: 23
    Re: How big a system can you back feed to a breaker?
    BB. wrote: »
    I believe ggunn is correct. You cannot find 45 amp breakers.

    And, remember that you need to size the breakers/wiring to (at least) 1.25x your rated load. So, if after all the calculations (and fudge factors) show you having a 45 amp maximum solar power source, then 1.25x45a=56.25 Amp rated circuit minimum.

    The "easiest" way around this would be to reduce your main breaker to 200 amps maximum--That will give you an "extra" 25 amps of margin on your bus bar ratings--If the local code and rest of your home loads will allow this.

    -Bill
    Sorry I should have added.
    I am only using 200 amp main breaker in my 225 amp panel.
    and I will be feeding the 45 amps from the solar to the 60amp breaker in the main panel.
  • BB.BB. Super Moderators, Administrators Posts: 30,645 admin
    Re: How big a system can you back feed to a breaker?

    I think "Over Current Protective Device"... I.e., Breaker/fuse.

    And that 200 amp main breaker is a big difference between legal/safe or not. Glad to hear that you are doing things correctly.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • NeedMoreSolarNeedMoreSolar Registered Users Posts: 23
    Re: How big a system can you back feed to a breaker?

    I don’t plan on adding more after this upgrade but would it be possible to add 10 more panels in the future.
    That would put me @ 54 amps. But I would have to upgrade my disconnect switch to 100 amp and my back feed breaker to 70 amp. This would be 14kw system. (At this point I would be maxed out.)
    Thanks for the help and sorry for not adding the 200amp main at the beginning of the post.
  • BB.BB. Super Moderators, Administrators Posts: 30,645 admin
    Re: How big a system can you back feed to a breaker?

    Some codes and functional requirements (A/C, water heating, electric stove, etc.) and code may not let you go to 100 amp breaker (I have read about some cities have a requirement for minimum electrical service rating).

    The main breaker still needs to be able to support 100% of your loads if the GT system is down or the sun is not up.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • ggunnggunn Solar Expert Posts: 1,973 ✭✭✭
    Re: How big a system can you back feed to a breaker?
    I don’t plan on adding more after this upgrade but would it be possible to add 10 more panels in the future.
    That would put me @ 54 amps. But I would have to upgrade my disconnect switch to 100 amp and my back feed breaker to 70 amp. This would be 14kw system. (At this point I would be maxed out.)
    Thanks for the help and sorry for not adding the 200amp main at the beginning of the post.
    Again, it's not the amount of current that is used in the 120% rule calculation, it's the total of the ratings of your backfed OCPD's (yes, Over Current Protective Devices, i.e., fuses or breakers).

    Also, it's the first OCPD that encounters the backfeed which is used, so, for example, if you had a 100A breaker in the main panel being backfed from a subpanel, and the sub had 4 15A backfed breakers (no matter how much current you are feeding them) in it, the number you would have to use to calculate if you are below 120% if your main panel is 60A, not 100A.
  • solar_davesolar_dave Solar Expert Posts: 2,366 ✭✭✭✭
    Re: How big a system can you back feed to a breaker?

    If you have the panel buss bar documented at 225 amps then 270 amps total input should be allowed (225 X 120%), so a 200 amp main + a 60 amp back-feed seems to meet the NEC requirement. And your 45 amp back-feed should be run through a larger breaker so your 60 amp breaker is also in order.

    YMMV and I am not an electrical engineer.
  • benatwhodotnetbenatwhodotnet Registered Users Posts: 16
    Re: How big a system can you back feed to a breaker?

    Now I have a question along this line. My plan was originally approved for 108 panels rated 230W each (24,840W total rated). Later the fire code changed and the engineer called to say that they could not fit 108 panels with the required offsets (walking around room for firemen). We agreed to go with 99 panels rated 240W (23,760W rated). This resulted in a 5% discount in the contract price. It did not result in any engineering changes to the back feed and OCPD specifications. I have a combiner panel with 3 breakers providing 40A OCP for each of the inverters. These 3 inverters are larger SB7000US with 33 panels each instead of having 4 SB6000US with 27 panels each. The plan was updated to show 3 breakers with 40A rating, but the final back feed was not changed from 110A to 100A. Since we cannot buy a 110A breaker, a 125A was used.

    Here's the question. Should I have a 100A breaker as the final back feed after the combiner panel 3 x 40A? Does this fit better with the rules considering I never push more than 21kW (3 7kW inverters)? That's 85.7A maximum actual back feed at our normal grid 245v (I check routinely). 120% of 85.7A is 102.8A.
  • NeedMoreSolarNeedMoreSolar Registered Users Posts: 23
    Re: How big a system can you back feed to a breaker?

    I really need to just bump the 2 of the 15 amp breakers in the sub panel to 20 amp.
    Then I would have
    Two 15amp breakers =30 amps
    Two 20 amp=40 the new panels would go to these breakers.
    70 amps and back feed that to my main panel and I am maxed out. (2x15 amp breaker and 2 x 20 amp breaker= 70 amps.)


    I guess I thought the sub panel would be considered a combiner box and not my back feed breaker.
    ggunn wrote: »
    Again, it's not the amount of current that is used in the 120% rule calculation, it's the total of the ratings of your backfed OCPD's (yes, Over Current Protective Devices, i.e., fuses or breakers).

    Also, it's the first OCPD that encounters the backfeed which is used, so, for example, if you had a 100A breaker in the main panel being backfed from a subpanel, and the sub had 4 15A backfed breakers (no matter how much current you are feeding them) in it, the number you would have to use to calculate if you are below 120% if your main panel is 60A, not 100A.
  • BB.BB. Super Moderators, Administrators Posts: 30,645 admin
    Re: How big a system can you back feed to a breaker?

    This is beyond my depth... But a couple of possible issues.

    Make sure your "solar GT breakers" are on the far end of the bus from the 200 amp main breaker.

    Second, some AHJ (authorities having jurisdiction) will not want to see multiple breakers for multiple AC GT Branch circuits (grouped on the "far side" of the main bus bars). You may have to install a single (say 60 amp) branch circuit for the GT circuit to a "combiner box" where you can install your multiple strings of GT circuits.

    The combiner box should meet all of the basic requirements... GT Branch circuit breakers should not exceed the combiner box bus bar rating (i.e., maximum input breaker).

    The Breaker in the main panel shall be at least as large as the sum of the GT branch circuit breakers. I am not sure if the combiner box needs its own "sub panel main breaker").

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • NeedMoreSolarNeedMoreSolar Registered Users Posts: 23
    Re: How big a system can you back feed to a breaker?

    The system the way it is right now 9.2 kw and it has been inspected and running for 2 ½ months.
    225 amp main load center with 200 amp main breaker.
    100 amp sub panel in my out building the PV system feeds 240 AC (Enphase Micro inverters)
    Into four 15 amp breakers (36 amps coming from PV)
    This feeds to a 60 amp disconnect switch. (By meter)
    This feeds to a back feed breaker 60 amp in the main load center.

    I am adding 2.4kw 10 240 watt panels with Enphase micro inverters.
    To do this I am going to just change out two 15 amp breakers for 20 amp breakers tie in 5 new panels at the jboxes down to the each new breaker. That will be 13.5 amps feeding to each new 20 amp breaker. This will feed to the disconnect switch and the main load center. Like before but 45 amps instead of 36 amp will be coming from PV system.
  • solar_davesolar_dave Solar Expert Posts: 2,366 ✭✭✭✭
    Re: How big a system can you back feed to a breaker?

    As long as the 100amp sub panel is dedicated as a AC combiner with no loads you should be OK.
  • ggunnggunn Solar Expert Posts: 1,973 ✭✭✭
    Re: How big a system can you back feed to a breaker?
    BB. wrote: »
    The Breaker in the main panel shall be at least as large as the sum of the GT branch circuit breakers. I am not sure if the combiner box needs its own "sub panel main breaker").

    That's not necessarily true. You figure your individual backfed breakers as the next standard size up from 125% of the maximum output current of a single inverter and the breaker in the main as the next standard size up from 125% of the summed maximum output current of all the inverters. Where those calculated maximum maximum currents fall in the array of standard sizes may or may not make the breaker in the main larger than the sum of the breakers in the sub.

    The sub does not need a main breaker.
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: How big a system can you back feed to a breaker?

    Ultimately whatever plan you come up with has to pass local inspection, so the AHJ is the final word on whether or not you can do this. Sometimes they do not agree with NEC and sometimes engineers do not agree with NEC.

    So we can tell you how it should be from an engineering and code POV, but ... we don't any of us get to stamp "APPROVED" on the plan. :roll:
  • NeedMoreSolarNeedMoreSolar Registered Users Posts: 23
    Re: How big a system can you back feed to a breaker?
    ggunn wrote: »
    That's not necessarily true. You figure your individual backfed breakers as the next standard size up from 125% of the maximum output current of a single inverter and the breaker in the main as the next standard size up from 125% of the summed maximum output current of all the inverters. Where those calculated maximum maximum currents fall in the array of standard sizes may or may not make the breaker in the main larger than the sum of the breakers in the sub.

    The sub does not need a main breaker.

    Each breaker in the upgraded system (sub panel)
    1.) 15 amp breaker has 10 panels 9 amps
    2.) 15 amp breaker has 10 panels 9 amps
    3.) 20 amp breaker has 15 panels 13.5 amps
    4.) 20 amp breaker has 15 panels 13.5 amps
    This would come to 70 amps adding all the breaker but the actual amps it will be handling are 45 amps.
    1.25 X 45= 56.25
    I would need a 70 amp back feed breaker in the main service load center.
  • NeedMoreSolarNeedMoreSolar Registered Users Posts: 23
    Re: How big a system can you back feed to a breaker?
    Ultimately whatever plan you come up with has to pass local inspection, so the AHJ is the final word on whether or not you can do this. Sometimes they do not agree with NEC and sometimes engineers do not agree with NEC.

    So we can tell you how it should be from an engineering and code POV, but ... we don't any of us get to stamp "APPROVED" on the plan. :roll:

    Thanks for all the input! I have read lot of code and NEC requirements but I am a home owner so getting advice from people have more experience is very valuable.

    We live in a very small place and they want to make sure it is safe and not just dot I’s and cross the T’s.
    I just want it to be safe also.
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: How big a system can you back feed to a breaker?

    Safe is easy. Code compliant is difficult. :p

    If the conductors are sized to handle the maximum current and the OCPD sized according to the wire, it's safe.

    But you have multiple GTI's tied in on separate breakers, which means the current becomes cumulative and has to be accounted for at every step. Normally one GTI will have one breaker on it and that suffices for OCP and disconnect. Here you've got multiple breakers which code will no doubt demand a separate disconnect acting on all.

    The panels do not actually relate to the current issue. A panel can be 25 Watts @ 12 Volts or 250 Watts @ 30 Volts. It is the AC output from the GTI's that is at issue. If you have a 5 kW inverter with only 1 kW of panels on it the output wiring and breaker has to be sized according to the inverter's 5kW rating, regardless of how much power it actually produces.
  • NeedMoreSolarNeedMoreSolar Registered Users Posts: 23
    Re: How big a system can you back feed to a breaker?

    Enphase micro inverters are .9 amps for each micro inverter. That is where I have been saying panels really I should say micro inverter.
    Yes this all feeds back to a disconnect switch then goes to my main panel.
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: How big a system can you back feed to a breaker?

    Enphase inverters get connected in branch circuits. The total Wattage of these branch circuits is the number used in determining output. So if you have ten 215 Watt Enphase inverters on one circuit it is treated as 2,150 Watts or 9 Amps. It is important to use the right terminology on these things even though the end result may be the same: inspectors are easily confused. :p

    That same branch circuit can have 17 Enphase units on it. At that point you should be wiring and protecting for 20 Amps, not 15.

    Another issue that is the difference between "right" and "AHJ" is that you could have a 5kW central inverter with 10kW of panel on it. It could only produce 5kW (albeit for more hours of the day) but the AHJ may not see it that way and expect the output of the inverter to handle 2X what it is actually capable of producing.
  • NeedMoreSolarNeedMoreSolar Registered Users Posts: 23
    Re: How big a system can you back feed to a breaker?

    Right now I have I have 10 micro inverters M215 connected to a 15 amp breaker. I center feed these to a Junction box 5 M215 then the other 5 M215 then these go from the Jbox and home run to the subpanel 15 amp breaker. This keeps the voltage rise down.
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: How big a system can you back feed to a breaker?
    Right now I have I have 10 micro inverters M215 connected to a 15 amp breaker. I center feed these to a Junction box 5 M215 then the other 5 M215 then these go from the Jbox and home run to the subpanel 15 amp breaker. This keeps the voltage rise down.

    Okay, having ten on one circuit makes perfect sense to me. But why did you divide up the other ten into two fives? Long wire distances? Voltage rise is not a problem if the wiring is sized correctly (remember it can be larger than what is needed to carry the maximum expected continuous current, just not smaller).

    I do not fully understand your wiring. Were it me I would have ten units on each of two 15 Amp circuits (with breakers) then, if necessary, tied together in a sub panel to create a 30 Amp circuit (with 8 AWG wire) making the final run to the main service panel and connecting to it through a 50 Amp breaker.

    And that is where the issues of the main service's capacity come into play as the 120% rule would apply in respect to that breaker size, not the 30 Amps actual maximum current (yes, it is overkill and you can argue against it from an engineering POV but you probably wouldn't get anywhere with the NEC or the AHJ). This is back to the "there's no such thing as a 45 Amp breaker" problem and the ratings of the main service panel.
  • NeedMoreSolarNeedMoreSolar Registered Users Posts: 23
    Re: How big a system can you back feed to a breaker?

    The building is 130 feed from my house and it had #2 aluminum wire from my main load center to the sub panel.
    I wanted to use that wire so I went with # 8 copper to all jboxes to micro inverters to sub panel.
    #2 aluminum from the sub panel to the disconnect switch then number #6 copper from disconnect switch to main load center.
    The distance and using #2 aluminum made me do the center feeds to the micro inverters.
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: How big a system can you back feed to a breaker?

    Well 130 feet of 2 AWG aluminium ought to handle 30 Amps @ 240 Volts all right: looks like 1% V-drop across that.

    I do not understand what you mean by "center feeds to the micro inverters". Could you clarify that?
  • NeedMoreSolarNeedMoreSolar Registered Users Posts: 23
    Re: How big a system can you back feed to a breaker?

    Center feed micro inverters is just breaking the 10 micro inverters into 5 and 5 to keep the voltage rise down. There trunk cable is only # 12.
    http://enphase.com/wp-uploads/enphase.com/2011/12/EnphaseTechBrief_Vdrop_M215.pdf
    I used this document to calculate the voltage rise.

    This looks like my system
    The voltage rise chart tells the story.
Sign In or Register to comment.