OCPD Calculations for Array

[jahob000]

Hi, So I'm trying to learn this calculation and I understood most of it but I ran into a problem.

I have a Canadian Solar CS5P 250W Module. The Isc is 5.49A and the maximum fuse rating is 10A. Here's the specs here http://www.enfsolar.com/pv/panel-dat...rystalline/382

So here's the first and second part of the calculation.

Imax = Isc x Number of strings x 125% x 125% = A
I derate = Imax / Correction Factor Temperature x Correction Factor Conduit = B

Imax = 5.49A x 3 x 1.25
I derate = 25.74A / .71 x .80

Even though I completed the calculations, something is off. In the first calculation, my answer is greater than the Max Fuse Rating. Is each string supposed to be less than that fuse? Or am I supposed to have multiple fuses?

Thank you for your help.

[mapmaker]

Or am I supposed to have multiple fuses?

When you have 3 or more strings of panels, each string should have its own OCP. If one string shorts out, the other two (or more) strings can push too much current into the faulted panel string.

--Mapmaker

[jahob000]

When you have 3 or more strings of panels, each string should have its own OCP. If one string shorts out, the other two (or more) strings can push too much current into the faulted panel string.

--Mapmaker

So then does that mean I would just do 5.49A x 1.25 x 1.25 and not multiply the number of strings since it's not going to be one fuse?

[mapmaker]

So then does that mean I would just do 5.49A x 1.25 x 1.25 and not multiply the number of strings since it's not going to be one fuse?

Correct. Most panels have a "series fuse rating". If your panels have such a specification, use it rather than using your calculation.

Do you have a combiner box? If not, take a look at the Midnite Solar MNPV3 combiner. A combiner makes the parallel connections, holds your circuit breakers, and is a good place to put a lightning arrester. I strongly suggest circuit breakers, rather than fuses.

--mapmaker

[jahob000]

Correct. Most panels have a "series fuse rating". If your panels have such a specification, use it rather than using your calculation.

Do you have a combiner box? If not, take a look at the Midnite Solar MNPV3 combiner. A combiner makes the parallel connections, holds your circuit breakers, and is a good place to put a lightning arrester. I strongly suggest circuit breakers, rather than fuses.

--mapmaker

I don't physically have the equipment. I'm practicing these calculations for testing and what not. So I can't switch to circuits. Also I forgot that I needed a combiner since the inverter I'm using only room for 2 circuits. Thanks for the reminder.

I'm still a little confused though. So for my panels, the max fuse size I can have is 10. If I just give each string a 10A fuse, then what happens if 2 of the strings send back feed? After redoing my calcs by not having one fuse for 3 strings, each string is a potential of about 9A. So that's 18A that can gang up on the remaining string.

This is where I get stuck. Am I missing something obvious? I'm still learning.

Thanks for all your help so far.

[mapmaker]

I'm still a little confused though. So for my panels, the max fuse size I can have is 10. If I just give each string a 10A fuse, then what happens if 2 of the strings send back feed? After redoing my calcs by not having one fuse for 3 strings, each string is a potential of about 9A. So that's 18A that can gang up on the remaining string.

Actually, your panels will rarely, if ever, put out more than Isc per string.

If two strings "gang up" on the other string, they will be pushing up to 2 X Isc amps through the faulty string and will trip the 10 amp breaker that protects that faulty string. No single string will be able to trip its own 10 amp breaker.

--mapmaker

[jahob000]

Actually, your panels will rarely, if ever, put out more than Isc per string.

If two strings "gang up" on the other string, they will be pushing up to 2 X Isc amps through the faulty string and will trip the 10 amp breaker that protects that faulty string. No single string will be able to trip its own 10 amp breaker.

--mapmaker

I know it would rarely happen. But I have to calculate for a worse case scenario.

Wow you're right! I can't believe I missed the obvious. The whole point of the fuse is to keep it from passing. I guess maybe the problem I had was that I was picturing things wrong. I was trying to get a visual in my head of which way things would flow.

I forgot that obviously if there was any kind of group fault the source would be a module. The fault would work it's way down the string, then down the wire towards the combiner box and hit the fuse. If two strings weren't sized right it would travel to the remaining string and do some damage to that one remaining string. Is that right?

I want to make sure I have the right visualization of this.

Again thank you for everything so far.

Edit: Also these are my new calcs without multiplying the strings

A
Imax = 5.49A x 1.25 = 6.86A
Imax = 6.86A x 1.25 = 8.58A (10A Fuse for each string)

B
I derate = 8.58A/ .71 x .80
I derate = 8.58A/ .57 = 15.05A

When I was choosing my wire size, it turns out I can use a 16 gauge wire but I've never heard of an array have a wire smaller than a 10 gauge. So is something still off?

By the way, those calcs are part of my testing so I can't choose not to do them.