Solar panel fuse question

[Kebast]

Here are the specs for a panel I'm looking at:

Nominal Maximum Power at STC (Pmax): 240 Watts
Optimum Operating Voltage (Vmp): 29.9 Volts
Optimum Operating Current (Imp): 8.03 Amps
Open Circuit Voltage (Voc): 37.0 Volts
Short Circuit Current (Isc): 8.59 Amps
Maximum System Voltage: 600V (UL) / 1000V (IEC)
Maximum Series Fuse Rating: 15 amp
Power Tolerance: +5 Watts
Temperature Coefficient (Pmax): -0.43%/°C
Temperature Coefficient (Voc): -0.34%/°C
Temperature Coefficient (Isc): 0.065%/°C
Temperature Coefficient NOCT: 45°C

What does the Maximum Series Fuse Rating mean? Can I put multiple of these panels in series?

[Naptown]

yes you can put them in series
the limiting factors of how many you can put depends on the controller or inverter you plan to tie them into But the US maximum is 600V
When you put them in series multiply the VOC of the panel x1.25 to get the maximum voltage when it is very cold (Probably than it will ever get in KY)
Then you can series as many of these up to the limit of voltage off the controller or inverter.
A group of panels in series is called a string.
You can connect two strings together without fusing them.
If there are three or more strings then each string must be fused.
I generally use fuses rated at 125% of the Isc rating of the panel

[Kebast]

slightly confused

I'm probably going with the Morningstar TS-MPPT-45 (Link), so I should be able to put 3 of the panels in series (at 24 volts output). I'm still slightly confused about the "Maximum Series Fuse Rating: 15 amp" term. Is that just stating that the manufacturer recommends no greater than a 15 amp fuse per panel? Two of these panels (or series of panels) in parallel would be greater than 15 amps. It might just be terminology that is tripping me up, but I can't quite figure out what the spec means.
When you say "If there are three or more strings then each string must be fused", is that because the output would be greater than 15 amps? If so, wouldn't any two strings also warrant a fuse?

[inetdog]

I'm probably going with the Morningstar TS-MPPT-45 (Link), so I should be able to put 3 of the panels in series (at 24 volts output). I'm still slightly confused about the "Maximum Series Fuse Rating: 15 amp" term. Is that just stating that the manufacturer recommends no greater than a 15 amp fuse per panel? Two of these panels (or series of panels) in parallel would be greater than 15 amps. It might just be terminology that is tripping me up, but I can't quite figure out what the spec means.
When you say "If there are three or more strings then each string must be fused", is that because the output would be greater than 15 amps? If so, wouldn't any two strings also warrant a fuse?

The assumption being made is that one panel or string should be capable, in a fault situation, of tolerating an amount of energy equal to what it produces itself when working properly. But it may not be able to tolerate the output of two or more other comparable systems "ganging up on it."
The fuse it there to protect the panel it is in series with, not to protect anything else.

[Kebast]

Thanks!

Makes sense now, thanks for the info.

[davidwilliamson82]

I have to clarify that you only multiply Voc by 1.25 if you have no weather data, as per NEC 690.7. In order to find your maximum expected output voltage, you subtract 25 from your average low temparature (in Celsius) and multiply this by the Voc temparature coefficient. This product can then be added to the Voc of a module to get the module's corrected Voc. multiply a mod's corrected Voc by the number of mods in a string to determine the max array output voltage.

Multiply Isc by 1.25 (this accounts for unusually high irradiance.) to find the PV source circuit's maximum expected current. Multiply the irradiance-corrected Isc by 1.25 to size your source circuit conductors and OCPDs. Any time you combine strings, you need to multiply the corrected current by the number of strings.

Be safe.

[Naptown]

Multiply source and output circuits by 1.56 per the NEC
Fuses I generally use Isc x 1.25 and round up to nearest.