Fusing Panels

[sprucegum]

I'm going to be wiring some new Hyundai 305W panels (9.86 Isc) in 2S2P.

I believe the fusing/wire is rated by Imax which is calculated by Isc * 1.56, so that's 15.3 per series string/panel.

These appear to have 12AWG. The spec sheet says maximum reverse current 20A.

Since two strings exceeds 20A current rating of the panel and the 12AWG I believe, am I correct that each series string should have its own 20A fuse?

 

[Brett V]

20 will be fine

 

[pvgirl]

With only 2 strings in parallel the short circuit current will self limit to the Isc value of one string, and no fuse or other over current protection is required.

 

[sprucegum]

With only 2 strings in parallel the short circuit current will self limit to the Isc value of one string, and no fuse or other over current protection is required.

This is news to me. Can you point me towards an explanation. I'm assuming this is some nuance I don't understand about the max current the panels will experience, but I'm not quite understanding what that would have to do with the limits on the cabling/connectors.

 

[sprucegum]

My understanding is that Isc will multiply in parallel, and that wiring/connections and therefore fusing should be based on Isc * 1.25 * 1.25 (or Isc * 1.56). So in this case, with two strings, that comes out to 30.76A.

That 30.76A appears to be particularly annoying. The MC-4, and the available Y's and 10AWG PV cable come in at 30A rating . I'm guessing 30.76A is close enough to fudge? And if not, a 30A fuse installed after the Y should protect the cables in the completely unlikely case that the the extra 0.76A ever occured.

 

[pvgirl]

My understanding is that Isc will multiply in parallel, and that wiring/connections and therefore fusing should be based on Isc * 1.25 * 1.25 (or Isc * 1.56). So in this case, with two strings, that comes out to 30.76A.

That 30.76A appears to be particularly annoying. The MC-4, and the available Y's and 10AWG PV cable come in at 30A rating . I'm guessing 30.76A is close enough to fudge? And if not, a 30A fuse installed after the Y should protect the cables in the completely unlikely case that the the extra 0.76A ever occured.

Yeah that's very unlucky situation as you will have to upsize to 8 AWG which you may want to do in junction box and not with MC4s, if you want to use MC4s Stäubli has genuine MC4 branch connectors and MC4 connectors for 8 AWG https://www.staubli.com/us/en/staeubli-north-america/PV-Product-Page-US-EN.html Also if your inverter as 2 inputs for the MPPT you could run each string on 12 AWG to the inverter and use the 2 connections there to parallel them, or even just run them seprately and combine to 8 AWG near the inverter.

 

[sprucegum]

@pvgirl - I'm thinking to just fuse it at 30A. What's the worst that could happen, the fuse blows? Then I'll know I need to address the problem. My suspicion is that the 0.76A will never occur sine the 1.56 represents multiple safety factors (1.25 for extreme irradiance and a 1.25 safety factor for continuous loads over 3 hours from what I've read).

I will be doing THWN in conduit to the house at 8AWG (50-60ft, so want to limit voltage drop). I'd really like to do a Y at the panels and then run a single +/- for 6-8ft down to ground level (array is going to be about 10ft high) for access/maintenance, then switch to the 8AWG in a junction box. If I wanted to be strict I guess I could just run four wires into the box instead and do all the joining in the junction box.

But like I said, really, throwing a 30A fuse on the line should satisfy even a strict safety standard and only represents a potential nuisance some day should it blow. This isn't a code install (no codes in these parts and I'm off grid), so it's just about common sense.

 

[timselectric]

Two strings = no fuse necessary.
#8 conductor is more than large enough.
No Y connectors.
You can fuse it if you want to. But, I wouldn't go with a 30a. Because of the nuisance tripping. The fuse is useless at this point. Because the array can never overpower the conductor or any other part of the system.