Grounding electrode & series string fuse

electro7

Senior Member
Hi,

We are building a carport structure for solar panels. I understand this falls under a "separate structure" for NEC grounding. I am thinking to use a 8ftx5/8" ground rod. My question is, do I need 2? Not following code on that easily.

Next we are getting called by plan check for the listing of our panels calls out "series fuse rating 20A". As of right now we have SMA string inverters that do not have fuses for strings. Plan checker says it will violate warranty on panels if series string fuses are not installed. Any thoughts on this? He needs a letter from the panel manufacturer saying they are not needed and he has never been provided that.

Thanks again!
 

jaggedben

Senior Member
I think technically, unless you can prove that one rod has less than 25ohms resistance to earth, the code requires a second. A lot places I work don't enforce that, FWIW. I wouldn't bother with 5/8 as opposed to 1/2 myself. Another option would probably be to bury one of the steel structural members deep enough to qualify as an electrode.

The plan checker is confused. See NEC 690.8 for whether any fusing is required (see exceptions). The series rating only applies to a fuse that's otherwise required. I'd ask him to show me the documentation that says otherwise.
 

electro7

Senior Member
Okay, so I was reading the electrode section correctly with the 25 ohms to ground. Thanks.

He is going off the module spec sheet in the electrical characteristics that has a line that says "series fuse rating 20A". Because it says that, he thinks its required to meet the listing of the module. And if we do not install one then it breaks the UL listing of the module and warranty of the module. Never heard that before and made me wonder. Is he still mistaken? I seem to think he is.
 

jaggedben

Senior Member
Okay, so I was reading the electrode section correctly with the 25 ohms to ground. Thanks.

He is going off the module spec sheet in the electrical characteristics that has a line that says "series fuse rating 20A". Because it says that, he thinks its required to meet the listing of the module. And if we do not install one then it breaks the UL listing of the module and warranty of the module. Never heard that before and made me wonder. Is he still mistaken? I seem to think he is.
Find the module installation manual and show him (presumably) that it doesn't require fuses in your situation. Here's an example I found for a well known manufacturer, hopefully yours says something similar. I would hope something along these lines would end the discussion.

A maximum of two strings can be connected in
parallel without the need to incorporate an overcurrent
protection device (fuses, etc.) in series
within each string. Three or more strings can be
connected in parallel if an appropriate, certified
over-current protection device is installed in series
within each string.
 

ggunn

PE (Electrical), NABCEP certified
Okay, so I was reading the electrode section correctly with the 25 ohms to ground. Thanks.

He is going off the module spec sheet in the electrical characteristics that has a line that says "series fuse rating 20A". Because it says that, he thinks its required to meet the listing of the module. And if we do not install one then it breaks the UL listing of the module and warranty of the module. Never heard that before and made me wonder. Is he still mistaken? I seem to think he is.
Yes, he is mistaken as long as you are not combining more than two strings without fusing. Actually, the rule is a little different but it nearly always matches up to more than two strings needing fusing.

It is a common misconception among inspectors who don't understand how solar works that conductors need to be protected from solar production, both on the AC and DC sides. Backfeeding, either from the service or from parallel wired strings is the only reason for OCPD. Solar components are current limited devices.
 

electro7

Senior Member
Thanks!

I am not finding any similar verbiage for Sunpower modules. Probably will need to add fuses. Going to try tech support though first.

Thanks again!
 

ggunn

PE (Electrical), NABCEP certified
Thanks!

I am not finding any similar verbiage for Sunpower modules. Probably will need to add fuses. Going to try tech support though first.

Thanks again!
SunPower DC modules are just like any other DC module in that respect; two strings max in parallel if there are no string fuses. SunPower AC modules are, well, AC, so this doesn't apply. SunPower AC modules are parallel AC with 12 modules max to a branch circuit.
 

electro7

Senior Member
This plan checker is stuck on the Sunpower (DC module) spec sheet that says in the electrical characteristics "series fuse rating 20A". It doesn't even say required. He thinks that not having a fuse on each string will break the UL listing of the module and the warranty.

How do I argue his point? I want to because this guy is pulling out all the stops and it is extra work for me to install separate fuses.
 

ggunn

PE (Electrical), NABCEP certified
This plan checker is stuck on the Sunpower (DC module) spec sheet that says in the electrical characteristics "series fuse rating 20A". It doesn't even say required. He thinks that not having a fuse on each string will break the UL listing of the module and the warranty.

How do I argue his point? I want to because this guy is pulling out all the stops and it is extra work for me to install separate fuses.
The guy is just plain wrong. Petty bureaucrats' only power is to say "no" and when they don't know their job it's frustrating. I feel your pain.

I design a LOT of SunPower systems, both with AC and DC modules, and I have never run into anyone as ignorant as this guy is. Close, though. Does he have a supervisor who has a clue?

I'd call or drop an email to SunPower tech support and get it in writing that their modules don't require a string fuse to keep their listing.
 

electro7

Senior Member
Thanks man. He kinda feels like one of those guys who likes to use his authority and knows just a little bit about the code to make him dangerous, lol. I'm going to touch base with Sunpower as you mentioned. Thanks again!
 

electro7

Senior Member
So want to make sure I am understanding 690.8 & .9 correctly.

I have a Fronius Primo 15.0 inverter which I believe has 8 total inputs (4 on MPPT 1 and 4 on MPPT 2). There are 4 integrated fuse holders on MPPT 1 (for both positive and negative) and no integrated fuse holders on MPPT 2. I am planning on bringing 6 strings into the inverter.

If I bring 4 strings into MPPT 1, which would all be fused, and 2 strings into MPPT 2, which would not have fuses, does this comply with NEC? Or would I need to install external fuses on the 2 strings going into MPPT 2?
 
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