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

2»

Comments

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

    Got it. Instead of "laddering" one set of ten and picking the feed for the service off one end you are taking the feed of the middle. Not an issue.

    So each set of ten Enphase units feeds to its own 15 Amp breaker and is on 8 AWG wire, correct?

    Then the two are joined and fed across 130 feet of 2 AWG aluminium wire to the disconnect and main service panel?
  • NeedMoreSolarNeedMoreSolar Registered Users Posts: 23
    Re: How big a system can you back feed to a breaker?
    Got it. Instead of "laddering" one set of ten and picking the feed for the service off one end you are taking the feed of the middle. Not an issue.

    So each set of ten Enphase units feeds to its own 15 Amp breaker and is on 8 AWG wire, correct?

    Yes but right now there are 4 sets of ten on their own 15 Amp breaker and yes they are all on 8 AWG wire. (total of 40 micro inverters)
    Then the two are joined and fed across 130 feet of 2 AWG aluminium wire to the disconnect and main service panel?

    Exactly then the four are all joined at the sub panel.

    Ok this drawing is a little crude but here it is.

    Here is the public link to the system.
    https://enlighten.enphaseenergy.com/public/systems/pAc2180504

    Here is a month/day view.
    https://enlighten.enphaseenergy.com/pv/public_systems/pAc2180504
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: How big a system can you back feed to a breaker?

    So far, so good. The sub panel would not need a main breaker because of the disconnect and breaker in the main service panel. To my mind the disconnect is redundant due to that breaker, but some local authorities require both.

    Thing is, that disconnect and breaker have to be rated for all four 15 Amp circuits to the micro inverters, irrespective of the amount of current they actually produce. So they have to be rated for (4 * 15) 60 Amp continuous service.

    This is why is behooves you to maximize each of the micro inverter circuits. Right now you have only 9 Amps each on them, and they can take more. If you can bring the disconnect and breaker up to snuff you could max out the four existing micro inverter circuits.

    Were it me I'd increase the inverter circuit breakers to 20 Amps and reduce by one circuit (60 Amp total) put in the disconnect and breaker that would handle this, and maximize each of the (now) three inverter circuits. That would be fifteen inverters per circuit for a total of 45 @ 215 Watts each totaling 9675 Watts. But that is probably more rewiring than you'd care to do.
  • NeedMoreSolarNeedMoreSolar Registered Users Posts: 23
    Re: How big a system can you back feed to a breaker?

    I already have on site 10 more micro inverters and 10 more 240 watt panels.
    What you suggest would not be hard because they all home run to the sub panel. They all go through jboxes.
    I have a 60 amp disconnect now and a 60 amp breaker at the main load center.

    but that leaves 5 micro inverters out.

    I was not planning on adding any more solar when I put this up. I was only going to put up 4.6kw when I started but plans got bigger.
    It is so easy to go from I installed solar to I want to zero out my power bill. This new 2.4Kw will come very close to zeroing out my bill.

    Thanks so much for sticking with me through this discussion.
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: How big a system can you back feed to a breaker?

    That's when you run into having five micro inverter circuits each of a 15 Amp breaker which is looked upon as 75 Amps going to the main service panel, regardless that the actual current is only 45 Amps - annoying, isn't it? The 2 AWG would still work fine but I bet they wouldn't accept the 45 Amp argument and they'd make you install 100 Amp disconnect and breaker. That's a big "whoops!" because there's no way that 100 Amp back-feed breaker could meet with the 120% rule on a 225 Amp service (you'd have to drop the main service breaker to 150 Amp).

    It's like accounting: you can make the numbers add up all sorts of different ways and argue that each is correct. :p
  • NeedMoreSolarNeedMoreSolar Registered Users Posts: 23
    Re: How big a system can you back feed to a breaker?

    Can’t I do this
    1.) 15 amp breaker has 10 micro inverters 9 amps
    2.) 15 amp breaker has 10 micro inverters 9 amps
    3.) 20 amp breaker has 15 micro inverters 13.5 amps
    4.) 20 amp breaker has 15 micro inverters 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.


    At this point I will just talk to the AHJ and go over the system. It appears my system is safe but maybe I might have to scale back to 45 micro inverters and three 20 amp breakers.
    Thanks for all your help!
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: How big a system can you back feed to a breaker?

    That's exactly how it gets tricky: actual Amps vs. total breaker rating. The code guys tend to discount actual Amps because they figure the circuit is capable of the breaker rating. (The fault lies in the idea that someone could come along later and add more inverters to the circuit, which is a bit of a silly argument if you ask me.)

    Then you have the double-derating dragon, wherein they may demand that the breaker/disconnect be derated to 80% even though the sub-breakers already are and these things are supposedly rated for continuous current.

    In other words from an engineering point of view your plan is perfectly viable but that doesn't mean it will pass inspection and ultimately that's what needs doing. Draw it up on paper with full explanation and ask the inspector about it. If there are changes he wants make sure he gives them to you in writing with a signature.

    Why yes; I have had the occasional problem with inspectors that don't know their brass from their oboe. How did you guess? :p
  • WayneTPVSWayneTPVS Registered Users Posts: 9
    Re: How big a system can you back feed to a breaker?

    Needmoresolar.
    A. 225 busbar with 200A main is allowed 70a OCPD by the 120% rule, however you may not feed more than 80% continous duty = 56a total 2nd feed.
    B. Do not change 15a breakers for 20a unless Enphase says you can. Typical MAX OCPD for enphase branch circuit is 15A or 15 inv @.8A = 12a = 80% of a 15a OCPD
    C. If your subpanel is a dedicated landing place for solar with NO LOADS and MARKED as such, the 120% may be exempted because no loads can be feed from both source.
    Theoretically you can have 6 15a breakers without a main ( 6 handle rule) backfeed to 70a OCPD in the main as long as you dont exceed 56a total inverter(s) output.
    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.
  • WayneTPVSWayneTPVS Registered Users Posts: 9
    Re: How big a system can you back feed to a breaker?
    Thing is, that disconnect and breaker have to be rated for all four 15 Amp circuits to the micro inverters, irrespective of the amount of current they actually produce. So they have to be rated for (4 * 15) 60 Amp continuous service.

    Why are you counting downstream OCPD as a requirement for upstream OCPD ? 9 amps on a source circuit needs a 15a OCPD to meet 80% rule, but that doesn't mean x 4 needs 60 OCPD... 9a x 4 = 36a x 1.25 = 45a min.. a 50a breaker would be allowed. And as long as the wiring is properly rated for the design OCPD.
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: How big a system can you back feed to a breaker?
    WayneTPVS wrote: »
    Why are you counting downstream OCPD as a requirement for upstream OCPD ? 9 amps on a source circuit needs a 15a OCPD to meet 80% rule, but that doesn't mean x 4 needs 60 OCPD... 9a x 4 = 36a x 1.25 = 45a min.. a 50a breaker would be allowed. And as long as the wiring is properly rated for the design OCPD.

    The difference is between what engineers know will work and what unqualified electrical inspectors will understand as an interpretation of the NEC. If you'd read the whole thread you'd see that is the issue all along, and I have already told him to draw up his plans and see if they'll pass local inspection before proceeding. I specifically mentioned the problem of the double-derating dragon. All it takes is one dim-witted inspector and the whole thing is a mess.

    Try to pay attention. I do know what I'm talking about.
  • NeedMoreSolarNeedMoreSolar Registered Users Posts: 23
    Re: How big a system can you back feed to a breaker?
    WayneTPVS wrote: »
    Needmoresolar.
    A. 225 busbar with 200A main is allowed 70a OCPD by the 120% rule, however you may not feed more than 80% continous duty = 56a total 2nd feed.
    B. Do not change 15a breakers for 20a unless Enphase says you can. Typical MAX OCPD for enphase branch circuit is 15A or 15 inv @.8A = 12a = 80% of a 15a OCPD
    C. If your subpanel is a dedicated landing place for solar with NO LOADS and MARKED as such, the 120% may be exempted because no loads can be feed from both source.
    Theoretically you can have 6 15a breakers without a main ( 6 handle rule) backfeed to 70a OCPD in the main as long as you dont exceed 56a total inverter(s) output.
    Enphase allows 17 M215 micro inverters to a 20 Amp breaker.
    Thanks for the reply.
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: How big a system can you back feed to a breaker?
    Enphase allows 17 M215 micro inverters to a 20 Amp breaker.
    Thanks for the reply.

    Yup, 'cause 17 * 0.9 Amps is 15.3 so a 15 Amp breaker would be tripping repeatedly (daily) and would not have the NEC derating.

    Frankly your plan of having five groups of ten inverters each on its own 15 Amp breaker is fine. Technically it would only achieve 45 Amps maximum, and so all you'd need is a 50 Amp breaker (no separate disconnect) at the mains. BUT as I mentioned before it's not about what works it's about what will pass.

    Look on the bright side: maybe you'll get an intelligent inspector who understands. Or are they an extinct species now? :p
  • solar_davesolar_dave Solar Expert Posts: 2,366 ✭✭✭✭
    Re: How big a system can you back feed to a breaker?
    Look on the bright side: maybe you'll get an intelligent inspector who understands. Or are they an extinct species now? :p

    Now there is an Oxymoron! I am almost certain they don't exist.
  • WayneTPVSWayneTPVS Registered Users Posts: 9
    Re: How big a system can you back feed to a breaker?

    Hopefully you do have an inspector that knows what their doing.. It needs tweeking but give this a try to help lay it out.
    http://www.turbopvsolar.com/NeedMoreSolar11.5kw.pdf
    Enphase allows 17 M215 micro inverters to a 20 Amp breaker.
    Thanks for the reply.
  • NeedMoreSolarNeedMoreSolar Registered Users Posts: 23
    Re: How big a system can you back feed to a breaker?

    Thanks for taking the time to do this doc. I modify it some.
    I changed the numbers around just a little for ease of wiring. Each set of M215’s you have to terminate.
    With special Enphase termination caps this will keep me to only have two more termination caps.

    But other than that this is what I would like to do.

    This doc even shows the array weight nice..

    Here is a picture of the current install.
    The lean-to will be on the bottom right that is where the 10 new panels/micro inverters will go.
  • WayneTPVSWayneTPVS Registered Users Posts: 9
    Re: How big a system can you back feed to a breaker?
    Thanks for taking the time to do this doc. I modify it some.
    I changed the numbers around just a little for ease of wiring. Each set of M215’s you have to terminate.
    With special Enphase termination caps this will keep me to only have two more termination caps.

    But other than that this is what I would like to do.

    This doc even shows the array weight nice..

    Here is a picture of the current install.
    The lean-to will be on the bottom right that is where the 10 new panels/micro inverters will go.


    Ya NP, the app I developed produces this in about 5-10 min.. Made for this very scenario where your trying to show your concept design.
    A slightly older version is available here http://www.turbopvsolar.com/TPVS_Dwg.html
    Glad you were able to use this to fit the need, and good luck with the expansion.
  • ggunnggunn Solar Expert Posts: 1,973 ✭✭✭
    Re: How big a system can you back feed to a breaker?

    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.

    That may be true but the inspector would be easily shown to be in error. The definition of the maximum output current for an inverter is spelled out clearly in the code and is one of the more easily understood parts of the code if one can read plain English.

    690.8(A)(3)
  • ggunnggunn Solar Expert Posts: 1,973 ✭✭✭
    Re: How big a system can you back feed to a breaker?
    The difference is between what engineers know will work and what unqualified electrical inspectors will understand as an interpretation of the NEC. If you'd read the whole thread you'd see that is the issue all along, and I have already told him to draw up his plans and see if they'll pass local inspection before proceeding. I specifically mentioned the problem of the double-derating dragon. All it takes is one dim-witted inspector and the whole thing is a mess.

    Try to pay attention. I do know what I'm talking about.
    C'mon, man. The condescending attitude is not necessary.

    That the sum of the first breakers that are backfed is the the number to be used in the calculation of the 120% rule, not the breaker that connects the main panel to the subpanel is straightforwardly described in the code. That the rating of the breaker in the main is calculated as 125% of the sum of the maximum currents from inverters, not the sum of the backfed breakers in the sub, is also there. It's not a question of what engineers think will work.
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: How big a system can you back feed to a breaker?

    ggunn, I know you understand how to do it as do the other installers who are members of this forum.

    Unfortunately I've had a lot of bad experience with inspectors over the years, and not just electrical. In Canada at least they just hire any bozo who applies it seems. The likelihood of getting someone who does not understand it is very high, and proving them to be wrong is impossible. Once I handed an engineering report to one who said "I don't understand this!" and tossed it on the floor. No joke. I admit to being terribly cynical where inspections are concerned.

    Anyway we all agree that electrically it will work without problem and meets with NEC requirements. But he may be in for some difficulty when it comes to proving it. Were it a commercial install (done by a contractor) it would likely be less of an issue because often inspectors assume they know what they are doing. This is not always true either.
    That the sum of the first breakers that are backfed is the the number to be used in the calculation of the 120% rule, not the breaker that connects the main panel to the subpanel is straightforwardly described in the code. That the rating of the breaker in the main is calculated as 125% of the sum of the maximum currents from inverters, not the sum of the backfed breakers in the sub, is also there. It's not a question of what engineers think will work.

    This is also 100% accurate, but does not address the confusion that can arise when you have the sub-panel of breakers joining the micro-inverters plus the disconnect plus the main breaker in the box. If the inspector approaches the matter "from both ends" as they often do he may think the disconnect and main needs to be the sum of the individual breakers, which is wrong in this case. Solar installs being relatively new, not all of the AHJ's understand how it is done.

    Yes, I am jaded. Half a century of fixing other people's mistakes for a living will do that to you.
  • NeedMoreSolarNeedMoreSolar Registered Users Posts: 23

     I got some advice on this post a couple years ago. Thanks guys!

    I just wanted to follow up we just went over 50 MWh solar power produced.

    See what good advice can do :)

  • BB.BB. Super Moderators, Administrators Posts: 30,645 admin
    Congratulations--You did all the hard work.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Ethan BrushEthan Brush Solar Expert Posts: 231 ✭✭
    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.


    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.
    RE breaker size vs maximum current, that has changed so we need to know what code cycle he is on.  2014 lets you use inverter output current where previously (2008 or 2011, I don't recall) you had to use the breaker size. Also FYI, 45 is a standard size breaker.
Sign In or Register to comment.