PV fusing

[Shale MacGregor]

Panel short circuit current rating is 3.84A
Panel series fuse rating 15A
Combiner using 15A fuses with 7AWG
Panels hooked up with 10AWG Wire

Can I combine multiple strings in parallel into a single fuse, and if so how many? My logic is that each string can't produce more than 3.84A, so even if all 3 strings went into full short, no more than 11.4A would ever cross the wire. This would allow me to take my 6 string combiner and instead combine 18 strings instead.

My issue is that so many MPPT are 150VDC or 200 max, and my panels are high voltage low amperage so don't work well without running a lot of parallel (it would take 10 strings in parallel just to max out a 40amp MPPT)

Thanks!

 

[MisterSandals]

Can I combine multiple strings in parallel into a single fuse,

If you have 3 or more strings, each string will each need a fuse.

The rest of your logic to keep volts within your SCC limit is sound.

 

[Shale MacGregor]

If you have 3 or more strings, each string will each need a fuse.

The rest of your logic to keep volts within your SCC limit is sound.

Is the 3 or more just a code rule, or is there a safety reason involving amps and protecting wires that I am not grasping? I am aware that this is an odd situation as the amps are quite low on these panels compared to most others.

 

[MisterSandals]

Is the 3 or more just a code rule, or is there a safety reason involving amps and protecting wires that I am not grasping?

It is probably code but not sure. The reasoning is that if there are any string issues (like a short) you can backfeed too much power into the other strings connected in parallel.

Maybe someone can provide a more technical answer than my “I know it’s standard practice”.

 

[Shale MacGregor]

It is probably code but not sure. The reasoning is that if there are any string issues (like a short) you can backfeed too much power into the other strings connected in parallel.

Maybe someone can provide a more technical answer than my “I know it’s standard practice”.

Roger, I get that. I was just putting 2 and 7 together and trying to make sure I was supposed to add and not take the square root of the product

My thought is that fusing is to protect the wiring, and since the max amperage a panel can generate in a dead short is a given, that there would be no way to super-short or produce more than the sum of the panels in parallel could produce.

In most cases the ampacity rating of the wiring is such that 1 and 2 panels shorting is below the safety rating, but 3 or more is above. Obviously one could come up with a panel that has higher amp rating than would be safe to wire in parallel for a given wiring, but that seems less likely.

I appreciate the feedback and help thinking this through!

 

[meetyg]

I found this guy's explanation very helpful:

How/When to Fuse a Solar Panel Array

This blog post is going to teach you how to determine when and if you need to add in-line fuses when designing a camper solar array. Sometimes you need to fuse your solar array, and

www.explorist.life

 

[Hedges]

3.84A x 1.56 = 6.00A
(1.25 extra for higher illumination x 1.25 for safety margin = 1.56 multiple for design)

Seems to me with 3 panels in parallel and no fuses, no panel can ever get backfed more than 12A, less than 15A spec.
So I don't see a real need for fuses in that case.

With more than 3 strings of panels connected in parallel, consider 2 panels per fuse.
15A limit - 6A from other string = 9A max fuse. (so all sources together can't exceed 15A into shorted string.)
Two strings in parallel need minimum 2 x 6A = 12A minimum fuse.
No fuse value carries output of two strings with required margin, but also limits backfeed current to required level.