30 amp DC switchgear with 12 gauge source wiring, OK?

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PondWater
PondWater Registered Users Posts: 9 ✭✭
in Solar Beginners Corner #1
Hi all.

I am adding 2 new solar panels to an existing off-grid system.

The existing system is 2 panels in series. The new system will be two parallel strings of 2 panels in series (4 total panels).

The existing system has a 20 amp breaker just before the MPPT and another 20 amp breaker switching between the MPPT and the Battery bank. See attached photo.


I read this: https://diysolarforum.com/threads/overcurrent-protection.46415/
which states that with less than 3 parallel strings, no OCPD is even needed. 

So is it safe to assume these breakers are just being used as switchgear and not really as OCPDs? But they ARE OCPD's in any case, and they are rated to 20 amps. With the new parallel setup, I will be running 10 gauge after the branch connectors (based on calculations specific to the panels), so I assume I need to upgrade these breakers to 30 amps?

Is there any concern with the existing source wiring (upstream of the branch connections) staying at 12 gauge?
 
The entire concept of over current protection with solar is so different from AC Mains...
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,447 admin
    #2
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    You are correct... The Solar Array Breaker here is just a "handy on/off switch".

    Need more information. What are the solar panels (link, Vmp/Imp?). How many panels?

    What is the solar charge controller (brand/model/link?). What is the battery bank voltage (12/24/48 VDC)?

    What is the (one way) length of the cable runs (Array to controller, controller to battery bank). Need to figure out safe current capacity (and breaker ratings) as AWG is both based on actual current flow and length of cables (voltage drops).

    Basically, need to figure out the working voltage and current for the Varray and Vbatt parts of your system.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • PondWater
    PondWater Registered Users Posts: 9 ✭✭
    edited April 2023 #3
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    --2 Existing panels: REC 280 TP BLK
    Peak Power (Pmpp): 280 W (STC rating) (257.1 W PTC rating)
    Rated Voltage (Vmpp): 31.9 V
    Rated Current (Impp): 8.78 A
    Open Circuit Voltage (Voc): 39.2 V
    Short Circuit Current (Isc): 9.44 A
    Max System Voltage: 1000V
    Maximum Series Fuse (DC): 20A

    --2 New panels: REC 370 AA Alpha Series (these were about as close I could find in terms of matching current rating of original panels)
    Peak Power (Pmpp): 370 W (STC rating) (351.7 W PTC rating)
    Rated Voltage (Vmpp): 38.3 V
    Rated Current (Impp): 9.66 A
    Open Circuit Voltage (Voc): 44.9 V
    Short Circuit Current (Isc): 10.21 A


    Each new panel will be put in series with an old panel since voltages are significantly different, and then these two strings will be put in parallel. 

    --Battery bank is 24V

    --MPPT is Tracer 3210AN series MPPT

    --Wiring
    Source wiring is whatever the panels came with (6-10 feet maybe?)- 12 gauge
    The run from branch couplers to MPPT is around 6 ft. at most probably-- I calculated that I need 10 gauge (see below)
    The wiring from MPPT to batteries must be around 4 feet or so.

    Imax calculation:
    Imax downstream of branch connection (parallel) = (10.21+9.44) * 1.25 *1.25 = 30.7 amps OR is current limited by smaller, making it: (9.44+9.44) *1.25*1.25 = 29.5A

    Ampacity charts seem to indicate 10 AWG is good to go for this (conduit is not above roof).

    As far as voltage drop, I did this calc, but it's probably a little rough:

    ·        Voltage drop on 12 gauge wire: (24.56* 0.00198 * 20ft.) / (38.3+31.9) = 0.9726/70.2 = 0.013 (1.3%). So 12 OK in terms of V-drop for 20 ft. but 10 gauge needed downstream of parallel branch connectors. Obviously 10 gauge would also be OK in terms of V-drop.


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  • PondWater
    PondWater Registered Users Posts: 9 ✭✭
    #4
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    I'm going to repost the Vdrop calc I did because the above one is a mess.

    The following assumes 10 feet (one way) of run at 12 gauge and operating current of 8.78A (calculation uses resistance per foot).
    And another 10 feet (one way) of run at 10 gauge and operating current of 8.78x2:



    Hopefully that makes more sense. And hopefully I am correct in my assessment that I need 10 gauge past the branch connector and a 30amp breaker?
    Does it matter that those breakers are rated for AC and (presumably?) not DC? It seems like it shouldn't matter since I don't actually need the OCP functionality. 
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  • PondWater
    PondWater Registered Users Posts: 9 ✭✭
    #5
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    It also appears that the Tracer 3210 AN has a rated current of 30 amps, and so the current 20amp breaker is actually undersized? And perhaps the wiring too if only 12 gauge...
    Or does the fact that the PV current previously (prior to upgrade) didn't go above 30A make that not the case?
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,447 admin
    #6
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    Remember that breakers and fuses are there to protect the wiring from overheating... They do not protect the attached electronics (inverter, charge controller, etc.). The "electronic" circuits in the chargers/inverters are much "faster" than the breaker(s) and are there to protect the devices.

    In general, breakers and fuses are designed to not blow below 80% of rated current and trip >100% of rated current (can be seconds/minutes/hours--And +/- a good size percentage).

    In some apartments I have taken care of, they have ~1955 breakers rated at 20 amps for the water heaters. And I measured ~19 amps. And they mostly have worked fine--50+ years later, have had a couple "fail" (basically tripping once every few weeks to every day)...

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