Fusing Question

gkerlin
gkerlin Solar Expert Posts: 27 ✭
in Caravan, Recreational Vehicle, and Marine Power Systems #1
I have a question on series parallel fusing. I'm installing 8 Kyocerra 140w panels on a Motorhome.

Here are the specifics:

VMP 17.7v Imp 7.91a I'm wiring into 4 series strings (2 panels each) resulting in VMI 35.4v Imp 7.91

I had hoped to then bring all 8 (4 pos - 4 neg) conductors from the 4 stings below into a breaker box and thru 15a series breakers.

Installation issues however (tight spaces in the RV wall space) is preventing me from pulling all eight leads. I can fit only 4 meaning in need to partially combine on the roof.

So plan B is to still create 4 series strings of 2 panels each, but then to combine the 2 panel strings (Parallel) into 2 pair of 4 panels series/parallel strings on the roof using mc4 multi branch connectors. I will then have 2 strings of 4 panels Still 35.4v but now with imp of 15.82a.

I will bring the 4 leads (2 pos - 2 neg) down and each positive will go thru a breaker, and then be combined prior to the controller.

My question is regarding series fusing. The panels call for 15 amp but the way they will now be combined calls for 25amp. (15.82 x 1.25 x 1.25)

I know that 3 or more series strings require series fusing. However, since this is two strings, then paralleled, and fused then combined again after the fuses does that still apply.

In essence I would have 4 series strings, combined into 2 parallel/series strings fused, then combined into one array.

Is my 25a fusing proper?
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Comments

  • inetdog
    inetdog Solar Expert Posts: 3,123 ✭✭✭✭
    #2
    gkerlin wrote: »
    My question is regarding series fusing. The panels call for 15 amp but the way they will now be combined calls for 25amp. (15.82 x 1.25 x 1.25)

    I know that 3 or more series strings require series fusing. However, since this is two strings, then paralleled, and fused then combined again after the fuses does that still apply.

    In essence I would have 4 series strings, combined into 2 parallel/series strings fused, then combined into one array.

    Is my 25a fusing proper?
    You have to look at what level of fault current could flow backward into a damaged panel or damaged string.
    With only two strings the maximum fault current that one string can drive back into the other is just the Isc of the good string. The presumption is that the other string can tolerate that fault current without catching fire.
    When you have two strings, the result is that twice the nominal Isc can be driven back into a damaged string, so individual fuses are required. The label listed series fuse size is usually less than twice the panel Isc.

    In your case a damaged string can be subjected to the entire current of the string with which it is paralleled plus the total current of the other two string as passed through both of the first level series fuses. Net result is a fault current which could be three times Isc. Your fusing scheme will NOT deliver the intended protection.
    SMA SB 3000, old BP panels.
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  • gkerlin
    gkerlin Solar Expert Posts: 27 ✭
    #3
    OK. I was uneasy about how I had laid it out but couldn't wrap my head around the fusing. Your explanation made it clear. I've found some MC4 inline fuses that I can fuse each string with. I will then still use the circuit breakers below, primarily so that I can turn off the array to the controller.
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