Sizeing PV (DC) breakers

[schmism]

When considering using a combiner box at your PV array (such as MidNite solar PV combiner)

How does one size the breakers/fuses?

Hypothetically a 500w panel outputs 10.29A @48v

If you ran this panel into the above combiner box, what would you install for a breaker? 10A and not pull the theoretical max watt off the panel - which I think my brain says you could "regularly" trip this as the pannel is capable of higher output and a mpp SCC would be be looking to draw those watts.

Is there an effective use of these breakers to provide over current protection to the panels? Or are they mostly seen as a disconnect for the panels at the panels?

So in the above case what do you use? 10A 15A?

If building an array, what do you spec? Pannel spec plus some overhead? (10%?)

To be clear I see this as a different use case than the fairly clear cut use of DC breaker as over current protection on something like a battery bank.

 

[Craig]

I would spec panel plus 25% or the nearest one above that value. In your case the 15 amp breakers would be fine. The only thing that would trip these is a lightning strike or something. So in effect there most important use is as a switch to turn panels off if needed.

 

[gnubie]

When it comes to protecting the panels, unless you have more than two equal power strings (or single panels) in series there isn't a breaker / fuse that will offer any protection so in that case the breaker is just a convenient way to disconnect the string for maintenance etc. If you look at the label on the back of the panel you will usually see a maximum series fuse specification. You would use a fuse somewhere between Isc (with thermal derate taken into consideration) and that maximum value.

In series the lowest current rating panel applies, but typically all the panels are the same.

Why won't a fuse blow in a 2 equal power strings scenario? By design you want to be able to pull as much current out of the array as possible, so your fuse / breaker must be rated to carry that. If one string develops a short and the other dumps all its capability into it, that will never exceed the rating of the fuse. Series (blocking) diodes will prevent that from happening and allow the working string to provide power to your charger / inverter.

 

[GXMnow]

On a DC solar setup, the panels should be fused/breakered above their max short circuit current. An MPPT charge controller or inverter will vary it's loading to find the peak power, and that can cause it to pull the voltage low to near the short circuit current. "Shade Tolerant" MPPT systems can pull quite low to check if more power can come from non shaded panels when some are shaded in a series string.

Unlike a battery or power supply, a solar panel running into a short will just sit at the maximum current for the amount of light hitting it. It will not cause it to fail or pull extreme damaging currents. The breakers are there in case something bad happens. A lightning strike as mentioned, or a panel getting smashed and shorted out. Power from other panels in parallel could try to push damaging current. Just make sure the wiring is all rated to handle at least 20% more current than the breakers.

 

[schmism]

Thanks guys!

 

[PHoganDive]

On a DC solar setup, the panels should be fused/breakered above their max short circuit current. An MPPT charge controller or inverter will vary it's loading to find the peak power, and that can cause it to pull the voltage low to near the short circuit current. "Shade Tolerant" MPPT systems can pull quite low to check if more power can come from non shaded panels when some are shaded in a series string.

Unlike a battery or power supply, a solar panel running into a short will just sit at the maximum current for the amount of light hitting it. It will not cause it to fail or pull extreme damaging currents. The breakers are there in case something bad happens. A lightning strike as mentioned, or a panel getting smashed and shorted out. Power from other panels in parallel could try to push damaging current. Just make sure the wiring is all rated to handle at least 20% more current than the breakers.

This is the main point of using a combiner box with multiple strings of panels, with each string individually fused or breakered.

 

[SolarQueen]

NEC requires the Isc of the string x 1.56 (continuous use (over 3 hours) 1.25 and over irradiance (sun being brighter than standard test condition) 1.25). Once you are at the output of electronics, either charge controller or inverter, you only need the one continuous use x1.25, not over irradiance.

For example, three strings of panels with Isc of 10A, each fuse or breaker in the combiner box must be 10A x 1.56 = 15.6A round up to 20A. If it is going into a 60A charge controller, the breaker on the output of the controller would be 60A x 1.25 = 75A, round up to 80A breaker.

 

[tictag]

All excellent advice.

The way I think of this is that strictly speaking you don't need breakers at all, so long as the PV cable is rated for Isc (plus multipliers). The panel/array can never produce more current than this i.e. it's not a battery. So breakers are really only there to protect from misahaps e.g. accidental shorts, squirrels eating through insulation etc. That's why you should bond all frames to a common return and earth, if required (so a 2 x Isc short to the frame will trip the breaker, for example). As has been said, the minimum value should be above Isc (plus multipliers) to avoid nuisance tripping.

Personally, I wouldn't bother protecting small PV arrays e.g. <1,000W, just an isolation switch will do. Other opinions are available!

 

[SolarQueen]

All excellent advice.

The way I think of this is that strictly speaking you don't need breakers at all, so long as the PV cable is rated for Isc (plus multipliers). The panel/array can never produce more current than this i.e. it's not a battery. So breakers are really only there to protect from misahaps e.g. accidental shorts, squirrels eating through insulation etc. That's why you should bond all frames to a common return and earth, if required (so a 2 x Isc short to the frame will trip the breaker, for example). As has been said, the minimum value should be above Isc (plus multipliers) to avoid nuisance tripping.

Personally, I wouldn't bother protecting small PV arrays e.g. <1,000W, just an isolation switch will do. Other opinions are available!

The reason I used 3 strings in my example is that NEC requires over current protection (OCP) if the sum of the strings is greater than the Max Series Fuse Rating of the panel. Let's use a 305W panel as an example. It has an Isc of 9.76A and max fuse size of 20A. So if you have 2 strings, 9.76A x 2 = 19.52, no OCP needed. If you have 3 parallel strings, 9.76 x 3 = 29.28. You need OCP.

Edit: my dog hit post before I was ready.

 

[tictag]

It has an Isc of 9.76A and max fuse size of 20A. So if you have 2 strings, 9.76A x 2 = 19.52, no OCP needed. If you have 3 parallel strings, 9.76 x 3 = 29.28. You need OCP.

Very interesting. Do you know what determines this "Max fuse size" specification? I mean, a PV cell/panel can only generate Isc so I guess it must be either ampacity limitations in the diode block or maybe the as-fitted cables?

 

[Robin Gudgel]

Look at the label on the back of the PV module. All UL listed modules have a max series fuse rating. That is the size fuse or breaker to use. The rating means UL tested the internal wiring of the panel to not burn up with that much current going through it. The rating has absolutely nothing to do with the output rating of the panel.

 

[SolarQueen]

Hi @Robin Gudgel , great to see you here!
Amy @ altE