Wiring for new solar array

garyreedgaryreed Registered Users, Users Awaiting Email Confirmation Posts: 4
Thanks in advance for any thoughts.
I am installing an array of 62 panels with micro inverters on my barn. It is  grid tie system. The barn is about 125 feet from the house where the meter and main breaker panels are located. The barn currently has power and a new 100amp breaker panel. The wiring schematics for the solar array call for a new line capable of carrying 75 amps from the barn to the meter at the house.

My question, can I connect the 75 amps from the array to the line already going from the meter to the barn and save running 125 feet of new line?

Thanks Gary  


  • BB.BB. Super Moderators, Administrators Posts: 32,015 admin
    Welcome to the forum Gary,

    That is a tough question... I don't know what the present NEC requires for GT solar connections--You are in Colorado? Have you asked your building department/code inspectors?

    From a "functional" point of view, is your present barn panel a 100 Amp or 200+ amp panel? Since there are "two sources" of energy in the barn (100 amps from main panel, 75 amps from solar), the box would need to be rated >=175 Amps (technically, a 150 Amp rated box plus 20% "extra" allowed for GT solar circuits) would be needed.

    Have you talked with your Utility? Most (just a swag) utilities only need one bi-directional meter in the main socket. However, there are some utilities that require a separate meter just for the GT solar (for billing).

    What is the rating of your home main panel... A 200 Amp panel + 20% solar = 240 Amps total sported (200a main breaker + 40a solar branch breaker), or if you had, for example, a 150 amp breaker and (240-75=) and a 90 amp (maximum) solar branch circuit breaker.

    Many codes do not allow a mix and max of GT + loads in a GT panel... And depending on how your micro array is configured... For example, 4x 20 Amp 240 VAC circuits to a 100 amp sub panel, then wire that panel to an 80 Amp solar branch circuit in the barn (200 amp panel with 100 amp breaker back to main panel) sub panel, then back to the main panel--Seems like it should be OK (not sure if you need both a 100 amp breaker in barn panel and main panel or no "main breaker" in the sub panel, and a single 100 amp branch circuit breaker to barn). (not sure how you get a single 75 amp circuit--The typical micro GT circuits are 20 amp each).

    There are some other issues that people have run into... In remote areas, the AC mains voltage is highly variable. And for folks that run >250-255 VAC at times, adding energy through a long cable run from the barn to the main breaker panel to the street/pole transformer can increase the voltage enough to cause the GT solar inverters to shutdown (temporarily for 5 minutes) due to high line voltage. (150 feet of 2 awg with 75 Amps is ~3.5 volt increase in voltage--aka voltage drop "add" from barn to main panel).

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • garyreedgaryreed Registered Users, Users Awaiting Email Confirmation Posts: 4
    Thanks Bill,
    Yes we are in Colorado and are currently without a state inspector for our area. I have put in a couple of calls to see if the next one over can answer some questions but so far no luck. As far as the utility is concerned, their engineer said I can tie into the main line between the meter and the 200amp house panel by using the pass through junction box which is were the barn service comes off and another out building service comes off. When asked about using the barn service, his suggestion was keeping the solar power out of the panel by using another pass through above that panel. That is where I hit the wall with the electricians I have talked to. They all say no but not for the reasons that you mentioned, but that the power can only go one way in the line. 

    With that said I also ran into my second issue, and that is that I have 5 runs of solar with 20amps each and an Enphase combiner box with Envoy that can only except 4 20amp   the I have the biggest combiner box available  
  • BB.BB. Super Moderators, Administrators Posts: 32,015 admin
    edited January 2020 #4

    AC electrical math can be very complex... But to get a quick understanding, pedaling a tricycle. You push down on the forward pedal, and you move/continue forward. Push down on the back pedal, and you slow down or go backwards.... More or less, the "phase" of the current vs the voltage is sort of like pedaling. The current (and voltage) is always going "back and forth" (AC power = Alternating voltage). It is how the current is "moving" with respect the voltage that controls the direction of energy flow, and a form of "current efficiency" (power factor, real vs apparent power, etc.).

    Anyway... There are few utility systems out there that actually measure/fault on the "wrong direction" of energy flow (the only one I know if locally is in San Francisco when they have something like 4x major sources of energy coming downtown. The system will "fault" if the energy flows "backwards" up one of the power feeds--And could/does make solar energy in Downtown not feasible).

    For the typical home, the utility meters do have (for the most part) of "understanding/measuring" the direction and amount of energy flow--According to how the meters are programmed (and they can report backfeed of energy, or "charge for backfeeding", etc. depending on how the meter is programmed/what the utility allows).

    Anyway... Assuming normal utility and home hardware, the big thing is to ensure (to the best of code) that you cannot feed too much current into wiring and cause a meltdown/fire. For example, if you have (rated) 200 Amps from the street to the home main panel, and inset the "barn feed" into the meter to main panel connection, at that point, the input to the "node" is now 200 amps main and 100 amps solar, or 300 Amps available on the wiring. If the wiring from the street is only rated at 200 to the meter, there is (potentially) 300 amps available (to the wire input to the main panel)... And could overheat / exceed the ratings for the wiring/panel/etc...

    If you put the 100 amps into the with a 140 amp derated main breaker (240 Amps available on main AC Panel Bus for breakers), then there is nowhere in the system where you can have excessive current flow/overheating (the main AC panel bus is allowed 20% over rating, really because you have 140 Amps coming "down" from the main breaker, and 100 amps coming "up" from the solar--So there is 240 Amps available at the breaker plugs/jaws to busbar, but never exceed 200 Amps on the bus bar in normal operation).

    I am probably both getting out of my depth (I am not an NEC guru) and I am explaining how I understand the issues (which may or may not be "accurate"). And there are sometimes other methods of AC mains connections available, sometimes allowed for commercial but not for residential, etc.--AKA "Line Side Tap"--As I recall).

    In most places, "residential" GT Solar is assumed to be 10 kWatts or less. For yours, you are looking at something like 15-18 kWatts(?). And the larger power system runs into what is generally assumed to be a Home Sized utility drop vs a commercial drop.

    You may, for example, run into the utility wanting to run your GT Solar (plus home) energy requirements (main breaker ratings, number of kWatts of solar, etc.) past their network engineer (can the local transformer/distribution network "handle" the amount of loads and generation provided by your home, and any neighbors in the area).

    It is possible that you would need a licenced electrical engineer and "wet stamp" for your plans--At least for your peace of mind (and if there is every a fire/insurance issues), and for the utility (if they care about the drop to your home).

    In California, it has been assumed that your local solar harvest should (over one year) "zero out" the energy consumed by your home... Larger than that, may fall into "local green generation" rules (and billing/payment plans).

    Before spending the money, make sure that your utility is onboard, and you understand what rate plan (or rate plans) your home/solar will fall under. 10+ years ago, such plans favored the GT Solar homeowner. Today, many are trying to put a "brake" on GT Solar installations... Rate plans that are not cost effective (for the customer), down to flat out refusing any new connections...

    For some areas, like Hawaii, Nevada, and such (exact states??), they flat out refuse... More or less, it is/was assumed that if 5% of generation power was solar, then there would little "effect" on the utility distribution network. More than 5-10% of network capacity was solar, then it can become both a big issue for energy management and a possible hit on their profits.

    Do not buy anything/do any work for GT solar until you get the OK from your utility (signed letter). Up in Quebec, as I recall, there were folks that installed larger commercial grade GT solar and they were delayed/refused final connection to the network.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • garyreedgaryreed Registered Users, Users Awaiting Email Confirmation Posts: 4
    Thanks again Bill,
    I now have a better understanding of electrical system workings in understandable terms for the non-professional. In dealing with the engineers, and others like yourself, have also realized there is a difference between legal or bley   also 
  • garyreedgaryreed Registered Users, Users Awaiting Email Confirmation Posts: 4
    Hi Bill,
    Sorry, I apparently have trouble typing.

    It seems as though there is a difference between legal, or barely legal, and best practices. The line side tap is legal, but not considered best practice. I have an engineer coming next week.
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