Fuses less than Maximum Series Fuse rating OK?

[chrisski]

Wondering if I need to go from my 10 amp circuit breakers to 15 amp to match the Maximum Fuse Series Rating of 15 amps?

I will have a set of three 100 watt panels in series with a circuit breaker. Optimum Amps is 5.7 optimum volts is 21.6 and shot circuit amps is 6.3 and optimum volts are 17.9. Maximum Fuse Series Ratings is 15.

So, since three panels will be in series, the amps will be close to 5.7, And volts close to 54. Its a DC circuit and rated volts are much higher.

My fuse is a little more than 1.5 times the short circuit amp rating, which is the rule of thumb I had used prior to finding the Maximum Series Fuse Rating.

EDIT: I think the intent of Max Fuse Rating is that I can Parallel two of these panels without a breaker. THe manufacturer, Renogy, does not have a recommended fuse size for a single panel. I will have two strings of these three panels and hope to add a third string.

Second Edit: My gut tells me sizing at 10 amps, around 1.5 times max amps is the right thing to do. I’m waiting from the experts for a reply.

[bob-n]

It is always "safe" to use a lower current fuse. It may pop as a nuisance, but it won't cause a fire. Your calculations show that it won't pop as a nuisance, so you're fine with a 10A fuse.

Note that the fuse needs to be rated for at least the maximum open-circuit voltage of the string plus some safety margin. There are some 32V fuses out there, but yours is probably high-voltage rated. I'm just adding this out of caution.

[chrisski]

It is always "safe" to use a lower current fuse. It may pop as a nuisance, but it won't cause a fire. Your calculations show that it won't pop as a nuisance, so you're fine with a 10A fuse.

Note that the fuse needs to be rated for at least the maximum open-circuit voltage of the string plus some safety margin. There are some 32V fuses out there, but yours is probably high-voltage rated. I'm just adding this out of caution.

Thanks. I got PV fuses rated at 150 volts. I looked for a 20 amp fuse locally for between the combiner and the charge controller and it was just like you said, 32 volt or 50 volts, which is way too low for my PV usage. I may even insert a third fuse location between the Charge controller and battery busbar at 60 amps, that particular fuse is rated at 24 volts DC and 60 amps, but with my 12 volt system, I won't reach 24 volts at that location.

Told today that by code the fuse rating should be 1.56 times the Open amp rating, and that's what my 10 amp fuse is by coincidence. I would like to take a look at this code someday. That would probably be a good reference for the rules of thumb I'm picking up.

[bob-n]

The National Electric Code (NEC) is a copyright document (book). You can't legally download it from the web for free, although you might be able to find sections of it for download, that may or may not be legal. Even better, you can find short articles explaining parts of the code. This one may be very helpful to you:

https://ep-us.mersen.com/sites/mersen_us/files/2018-11/TT-PVPN5-Sizing-Fuses-of-Photovoltaic-Systems-per-NEC-Tech-Topic.pdf

[J.P.M.]

The National Electric Code (NEC) is a copyright document (book). You can't legally download it from the web for free, although you might be able to find sections of it for download, that may or may not be legal. Even better, you can find short articles explaining parts of the code. This one may be very helpful to you:

https://ep-us.mersen.com/sites/mersen_us/files/2018-11/TT-PVPN5-Sizing-Fuses-of-Photovoltaic-Systems-per-NEC-Tech-Topic.pdf

Many libraries in decent sized cities as well as many university libraries have copies of national standards.

[foo1bar]

The National Electric Code (NEC) is a copyright document (book). You can't legally download it from the web for free,

The NEC may be copyrighted - but what is adapted as the law in a location may or may not be copyrightable. Last I checked there were conflicting court rulings on copyright of things like the NEC once they've been incorporated into law for a place.
So - last I knew it was still an open question whether you could or could not legally download a copy of the California Electrical Code (or Ohio, or Florida or whatever) which included NEC.

The NFPA (the organization that publishes the NEC) does have the NEC available online for free. (even can access different year versions of it so you can get the right one for your locale)
It's a little annoying to navigate - and not a simple text document or PDF that you can browse offline.
But it is available - at least sort-of.

Frankly, *IF* you find a source for the NEC to download it, I wouldn't worry about them coming after you if you download a copy just for yourself.
But if you are the one putting it up on a website, the NFPA probably will sue you.

[nwdiver]

Wondering if I need to go from my 10 amp circuit breakers to 15 amp to match the Maximum Fuse Series Rating of 15 amps?

I will have a set of three 100 watt panels in series with a circuit breaker. Optimum Amps is 5.7 optimum volts is 21.6 and shot circuit amps is 6.3 and optimum volts are 17.9. Maximum Fuse Series Ratings is 15.

So, since three panels will be in series, the amps will be close to 5.7, And volts close to 54. Its a DC circuit and rated volts are much higher.

My fuse is a little more than 1.5 times the short circuit amp rating, which is the rule of thumb I had used prior to finding the Maximum Series Fuse Rating.

EDIT: I think the intent of Max Fuse Rating is that I can Parallel two of these panels without a breaker. THe manufacturer, Renogy, does not have a recommended fuse size for a single panel. I will have two strings of these three panels and hope to add a third string.

Second Edit: My gut tells me sizing at 10 amps, around 1.5 times max amps is the right thing to do. I’m waiting from the experts for a reply.

If you only have 1 string of panels then you don't need a fuse. The panels themselves are current limiting. Fuses are only required in PV circuits if you have >2 strings in parallel.

[chrisski]

If you only have 1 string of panels then you don't need a fuse. The panels themselves are current limiting. Fuses are only required in PV circuits if you have >2 strings in parallel.

Part of the circuit breakers are for ease in trouble shooting if that comes up, and also to be in place if I add more strings. Way its looking now because of available roof spacing is I will add a second charge controller before adding a third string.

[Sunking]

You have no use for fuses and a waste of money other than a means to disconnect. Solar panels are not voltage sources like a battery. They are current sources. Look at the spec called Isc or current short circuit. On a 100 amp panel the most current the panel can deliver is 6 or 7 amps. Over current protection on panels is only required if you have 3 or more parallel strings of panels. Systems with only 1 or 2 strings do not require any fuses because it is impossible to burn anything up wiht proper sized wiring.

Keep in mind fuses and breakers do not prevent shock or electrocution. A fuse or breaker only protects the wire connected to the load side connection and not capable of doing anything else. In your application you could use very small wire of 16 AWG and 8 amps fault current will not even make the wire warm enough to feel. . .

[bcroe]

In some situations a disconnect device may aid trouble shooting. I was able to isolate an
underground cable at both ends and apply a cable locating signal to it, with perhaps more
breakers than were absolutely required. Walked it with a detection unit and left flags before
the next installation. In other cases integrity may be ohmed out. Bruce Roe

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