Proper Fusing Of Three Parallel Panels
Hi all, I need some help with fuse size of my little array. I've searched this forum and others and answers seem to range from 'fuse to the wire size being used', 'go 25% above Isc of panel' or 'use a...
Forum Discussion
Trying to make sense of panel fusing.
A fuse is a current protection device. A fuse at the output of a panel is to protect the wires from the fuse to whatever it is connected to, mainly the controller.
An array is the parallel connection of two or more panels (strings).
A string is the series connection of two or more cells. A panel can be say 30 cells or 90 cells. A string can be one panel of say 90 cells or three panels of 30 cells in series, same exact thing!
Say a panel is rated at 10A short circuit current and the fuses are 15A.
So what happens if there is a short directly at a panel before the fuse?
If there is only one panel the panel output goes to zero and the controller gets no input. The fuse will not blow, no current through it.
If there are two panels in parallel and one shorts directly at a panel the output the voltage goes to zero and the controller gets no input. The short circuit current supplied by the other panel, 10A, now goes through both fuses, the one on the good panel and the one on the shorted panel. Neither fuse blows because they are rated to handle the current from one panel. The wires are safe because they are rated to handle at least the short circuit current from one panel.
If there are three panels in parallel and one shorts directly at a panel the output voltage goes to zero and the controller gets no input. The short circuit current supplied by the other two panels is added, now 20A total, and flows through each of the good panel’s fuses, 10A each, and through the shorted panel fuse. This fuse now gets two times its normal current, 20A, and should blow from over current. When the fuse blows the voltage goes up at the controller and will still charge the battery from the two good panels.
If the wire is rated to handle the current from the two good panels no fuse would be required. This would be the case in most of the installations talked about on the forum; most use much larger wire than needed to keep voltage drop at a minimum.
This brings us to a reason not to use fuses at the panel, voltage drop!
In order for a fuse to blow it has to dissipate some power to heat it to the melting point. This means that the fuse must have resistance. A 7A fuse resistance is about .013 Ohms and a 10A fuse resistance is about .008 Ohms. A 10-gauge wire has a resistance of about .001 Ohm per foot. So adding a 7A fuse is like adding about 13 feet of 10-gauge wire from the panel and a 10A fuse is like adding about 8 feet of 10-gauge wire. Plus there are all the connections that add resistance and could become loose or not a make good connection.
Bottom line is if the wires from each panel can handle the total current from all the panels, most likely they can, there is no reason to use fuses and they introduce voltage drop plus added connections.
A fuse is a current protection device. A fuse at the output of a panel is to protect the wires from the fuse to whatever it is connected to, mainly the controller.
An array is the parallel connection of two or more panels (strings).
A string is the series connection of two or more cells. A panel can be say 30 cells or 90 cells. A string can be one panel of say 90 cells or three panels of 30 cells in series, same exact thing!
Say a panel is rated at 10A short circuit current and the fuses are 15A.
So what happens if there is a short directly at a panel before the fuse?
If there is only one panel the panel output goes to zero and the controller gets no input. The fuse will not blow, no current through it.
If there are two panels in parallel and one shorts directly at a panel the output the voltage goes to zero and the controller gets no input. The short circuit current supplied by the other panel, 10A, now goes through both fuses, the one on the good panel and the one on the shorted panel. Neither fuse blows because they are rated to handle the current from one panel. The wires are safe because they are rated to handle at least the short circuit current from one panel.
If there are three panels in parallel and one shorts directly at a panel the output voltage goes to zero and the controller gets no input. The short circuit current supplied by the other two panels is added, now 20A total, and flows through each of the good panel’s fuses, 10A each, and through the shorted panel fuse. This fuse now gets two times its normal current, 20A, and should blow from over current. When the fuse blows the voltage goes up at the controller and will still charge the battery from the two good panels.
If the wire is rated to handle the current from the two good panels no fuse would be required. This would be the case in most of the installations talked about on the forum; most use much larger wire than needed to keep voltage drop at a minimum.
This brings us to a reason not to use fuses at the panel, voltage drop!
In order for a fuse to blow it has to dissipate some power to heat it to the melting point. This means that the fuse must have resistance. A 7A fuse resistance is about .013 Ohms and a 10A fuse resistance is about .008 Ohms. A 10-gauge wire has a resistance of about .001 Ohm per foot. So adding a 7A fuse is like adding about 13 feet of 10-gauge wire from the panel and a 10A fuse is like adding about 8 feet of 10-gauge wire. Plus there are all the connections that add resistance and could become loose or not a make good connection.
Bottom line is if the wires from each panel can handle the total current from all the panels, most likely they can, there is no reason to use fuses and they introduce voltage drop plus added connections.
