When to fuse several series strings

gkerlin · July 2015

I think I know the answer to this question but wanted to make sure.

I'm going to wire 8 140w kyocerra panels in 4 strings (2 panels each in series) 4 strings combined parallel.

It is my understanding that with 3 or more strings - each string needs to be fused prior to them being combined into the full array.

Is that correct?

And - if so... I've calculated the fuse size for each string as being 15amp. (7.91imp x 1.56 rounded up to the next higher size fuse)

Do the experts agree?

Thanks

Greg

vtmaps · July 2015

I think you have it figured out correctly.... However, you can save yourself the trouble of calculating the fuse size by just looking at the panel specifications. Most panels have a series fuse size specification.

One thing I would do differently is to use circuit breakers rather than fuses. It is a wonderful thing to be able to easily switch a string on or off.

--vtMaps

gkerlin · July 2015

vtmaps wrote: »

I think you have it figured out correctly.... However, you can save yourself the trouble of calculating the fuse size by just looking at the panel specifications. Most panels have a series fuse size specification.

One thing I would do differently is to use circuit breakers rather than fuses. It is a wonderful thing to be able to easily switch a string on or off.

--vtMaps

Thanks. fuse was a generic term. My last had 3 strings and 3 circuit breakers in the 5th wheel basement. Here I am going to combine up on the roof however so I'm not certain if I'll put in fuses or breakers. I'm going to make another post on here looking for suggestions on a combiner box (wired or unwired) that would be suitable for a roof top location.

BB. · July 2015

If you have an easily accessible breaker/per string switch--It is really nice to debug if you have a bad panel/string of panels (switch on 1 string at a time and read your charge controller's input/output current--if you have a meter on the controller).

And/or have a place where you can access each string from the array (i.e., positive wiring) with a DC Current Clamp DMM--Again, easy to check that everything is working well.

-Bill

inetdog · July 2015

BB. wrote: »

If you have an easily accessible breaker/per string switch--It is really nice to debug if you have a bad panel/string of panels (switch on 1 string at a time and read your charge controller's input/output current--if you have a meter on the controller).

And/or have a place where you can access each string from the array (i.e., positive wiring) with a DC Current Clamp DMM--Again, easy to check that everything is working well.

-Bill

That link is the exact Sears product that I have and recommend. But be careful not to get the slightly lower priced unit that measures AC and DC volts but only AC amps. Some online store descriptions make it easy to confuse the two.

pebbledropper · July 2015

Greg,

I don't understand. Are you sure you're referencing Vmp. For 140W panels tha should be around 16-17. The Imp for 140W kinda fits your 7.91 number. The reason I ask is you are hooking each pair in series. That will double your V but keep your Imp the same. What I'm trying to say is have you got 7.91 Amps coming to the combiner per pair of panels?

Jerry

inetdog · July 2015

pebbledropper wrote: »

Greg,

I don't understand. Are you sure you're referencing Vmp. For 140W panels tha should be around 16-17. The Imp for 140W kinda fits your 7.91 number. The reason I ask is you are hooking each pair in series. That will double your V but keep your Imp the same. What I'm trying to say is have you got 7.91 Amps coming to the combiner per pair of panels?

Jerry

It was a typo. He multiplied that value by a fudge factor to get current, so he had to mean Imp not Vmp.
And for 140W and Imp = 7.91, the Vmp should be about 17.7.

vtmaps · July 2015

inetdog wrote: »

It was a typo. He multiplied that value by a fudge factor to get current, so he had to mean Imp not Vmp.
And for 140W and Imp = 7.91, the Vmp should be about 17.7.

Yes, the 7.91 is Imp, not Vmp, but the 1.56 was not a fudge factor. Fuse size should be 125% X 125% = 156% of the Imp. I believe that with some circuit breakers that are rated for continuous current, only 125% of Imp is necessary.

--vtMaps

gkerlin · July 2015

inetdog wrote: »

It was a typo. He multiplied that value by a fudge factor to get current, so he had to mean Imp not Vmp.
And for 140W and Imp = 7.91, the Vmp should be about 17.7.

Correct - I made a typo. I corrected the original post.

inetdog · July 2015

vtmaps wrote: »

Yes, the 7.91 is Imp, not Vmp, but the 1.56 was not a fudge factor. Fuse size should be 125% X 125% = 156% of the Imp. I believe that with some circuit breakers that are rated for continuous current, only 125% of Imp is necessary.

--vtMaps

The presence of the double 125% factor will vary with NEC version.
But the simplest thing is always to look at the Maximum Series Fuse value listed on the panels and not worry what the Minimum value might be.

pebbledropper · July 2015

Q 1:
I know one of the 25% bumps is for super cold, snow reflection, and just because. What's the other 25% for?

Thinking ahead some, if it's for the MPPT boost, that hasn't happened yet at the combiner. You only care about MPPT sized wiring and protection from the controller on. Yes?

Q 2:
So, if I series 2 160W 12V panels, each an Ioc of 9.61, can I use a 15A breaker in that Pos conductor? And if I have 8 panels paired in 4 24V series, can I use 4 15A breakers at the combiner point where the 4 series Pos conductors go parallel? If so, using some terminal blocks and switchable breakers, I intend to make my own RV combiner box where I can get to it standing on the ground. I'll run Pos and Neg conductors from there to the controller using whatever AWG the tables call for. The Amp load for that wire would be 9.61 x 4 x 1.25 (or 1.53?) = 48.05A (or 58.8A), yes??

Jerry

gkerlin · July 2015

If you sized the fuse at 100% of expected maximum current it would blow. So the 125% is to allow it to not blow but still be suitable to protect the wire.

The breaker or fuses would be before you combine the strings and in the positive conductor. If you have 4 strings then 4 fuses. Then after the breaker you can combine them and run on to the controller. As someone mentioned, if you don't want to do the calcs each panels spec shows a series fuse rating.

vtmaps · July 2015

pebbledropper wrote: »

Q 1:
I know one of the 25% bumps is for super cold, snow reflection, and just because. What's the other 25% for?

The other 25% is for fuses that may blow if they carry their rated current. There are some circuit breakers that can carry their rated current indefinitely, and those breakers do not need the additional 25%.

--vtMaps

When to fuse several series strings

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