PV combiner box circuit breaker sizing?

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khotton
khotton Solar Expert Posts: 38 ✭
in Advanced Solar Electric Technical Forum #1
I am confused in how to select the proper DC circuit breaker for use in a PV combiner box. If I have a 4-panel string of 130W Kryocera panels (for a 48VDC system) what size breaker do I use? I was guessing that I would just select the closest to (but larger than Ioc) breaker, which for the Kryocera panel would suggest a 10Amp circuit-breaker (8.02Amp Ioc). Is this corrrect?

Kevin
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  • crewzer
    crewzer Registered Users, Solar Expert Posts: 1,832 ✭✭✭✭
    #2
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    Re: PV combiner box circuit breaker sizing?

    Kevin

    The breaker must be spec'd for voltage and for current. OutBack and Midnite breakers and their respective combiner enclosures are rated for 150 VDC, so that shouldn't be an issue.

    NEC 690.8(A)(1) requires a 125% multiplier for Isc to establish maximum current.

    NEC 690.8(B)(1) requires another 125% multiplier for ampacity derating unless the CB and enclosure are rated for 100% continuous duty. The OutBack and Midnite CB's and enclosures are so rated, so this second 125% multiplier isn't required.

    So, for your string of modules with an Isc of 8.02 A, you'll need a CB rated at 8.02 A x 125% = 10.025 A. Rounding up to the next standard size CB appears to be a 12 A CB from MidNite Solar.

    HTH,
    Jim / crewzer
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