A possible NEC conflict?

sub3marathonman Solar Expert Posts: 300 ✭✭✭
I followed this website http://solar.wiseowlnetworks.us/Solar-Math.html in calculating the breakers for the combiner box. I am using Sharp 224U1F panels, three panels in series / string, 8.33A Isc. So following their math, and based on the temperature here in Bartow FL, and apparently incorrectly using NEC Table 690.7 Voltage Correction Factors as the website stated, I calculated 11.87A. So I purchased the 12A breakers for the combiner box.

Now, I've reread the NEC 690.8 (A) (1), and it only says multiply by 1.25 times the Isc. By following the website, I multiplied by 1.14. And both the NEC and the website stated to multiply by 1.25 to comply with 690.8 (B) (1). So following the NEC I come up with 13.0A, which would then indicate a 15A breaker.

I have not derated things for heat though, I don't know if that is required for the 690.8 calculation. I have figured that the Isc would increase about 2.5% if the panels were operating at 157°F, but I don't know if that temperature is a reasonable expectation or not.

So now I'm wondering are 12A breakers NEC compliant or not? I could definitely understand if I used too large a breaker, such as 20A, but I don't see that using too small a breaker, such as 12A instead of 15A, should pose a safety hazard. I can see that maybe, in extreme heat, it would trip, but that is only a slight inconvenience in my opinion. I would like to be able to use the 12A breakers now that I have them if possible and if it is compliant with the NEC and the inspectors here if Florida.


  • BB.
    BB. Super Moderators, Administrators Posts: 33,439 admin
    Re: A possible NEC conflict?

    Several things going on here...

    1. Figure out how much current does your device use/generate (nominal maximum--instantaneous/peak" loads--such as a 10 amp motor taking 40 amps to start are not included).

    2. Take the Amperage from #1, and multiply it by 1.25 to get the minimum required circuit current.

    3. Look at the NEC Wire chart for minimum wire awg and insulation requirements based on voltage/rated circuit current above--and the wiring conditions (ambient temperature, conduit or not, conduit fill, moisture, oil/etc. exposure, UV exposure, etc.).

    4. Pick your breaker/fuse based on a) maximum circuit voltage, b) AC vs DC, c) trip type fast/slow, etc. d) maximum rated circuit current, such that the breaker does not exceed the circuit rated current from #3.

    Next, you have to look for voltage drop... Low voltage DC does not allow for much drop before it causes problems (i.e., 120 volts dropping 10 volts is 110 Volts--not a big issue--although, one should keep the total drop to 3% or less; 12 volts dropping 10 volts is 2 volts--nothing is going to work correctly in this case). You use the larger gauge wire (if needed) and you can either up size the breakers/fuses, or just leave them sized as original (sizing breakers/fuses as originally defined is probably the better choice in most situations).

    If you go by the NEC and Solar Interpretations, there are multiple 1.25x assumptions (maximum current, maximum sun, maximum x 1.25, etc.)...

    I don't agree with many of those requirements (Wiles' Appendix I), but your local building inspector may... In the end, the NEC is designed to protect wiring and for normal applications, the circuits shall not carry more than 80% of their designed/rated load (hence the 1/0.80 = 1.25x of your rated load).

    And in your case, the rated load of the circuits will be a combination of Isc (maximum short circuit current from solar panels without a circuit breaker/fuse, or the Imp + a bit) for the input, and if you have a MPPT controller, the wiring/breaker should be either the maximum of the solar panel output current x 1.25 (remember, if MPPT controller then there is a conversion of Vpanel*Ipanel=Vbatt*Ibatt) and the controller current (either actual panel wattage or controller maximum allowed)--which ever is lower (if you have few panels and a large controller, you don't need large wiring... If you have lots of extra panels that exceed the controller's rating capabilities--then the MPPT controller will limit its output current to its operational maximum).

    If you need to follow NEC and the solar requirements to the letter--Jim/Crewzer has gone through a couple examples in detail here before... It is not a trivial task.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • sub3marathonman
    sub3marathonman Solar Expert Posts: 300 ✭✭✭
    Re: A possible NEC conflict?

    Thank you, thank you, thank you Bill!

    Although actually I guess I should be saying Thank you, thank you, thank you Crewzer too.

    I didn't know these circuit breakers from Midnite were rated for continuous duty, and therefore didn't need the second 1.25 safety factor as stated in 690.8 (B) (1).

    I guess that means I can stop worrying, well at least about the circuit breakers.
  • crewzer
    crewzer Registered Users, Solar Expert Posts: 1,832 ✭✭✭✭
    Re: A possible NEC conflict?


    Jim / crewzer