We get this question quite often:
"My solar module is 12A. Should I use a 15A or 20A fuse?"
There is no good answer from that number alone.
When we select a PV fuse, we normally start with the module datasheet. Then we look at the string configuration and the conditions where the fuse will actually operate.
For a typical project, the numbers we want to see are Isc, Imp, the maximum series fuse rating, system voltage and the number of parallel strings.
Start With Isc
Let's take a simple example.
A module has:
Imp: 11A
Isc: 12A
Maximum series fuse: 20A
The fuse should not simply be selected as a 12A fuse because the module current is 12A.
The fuse has to carry the normal operating current without nuisance operation, while still providing protection when abnormal current flows through the string.
That's why Isc is normally one of the first numbers we check.
Is 1.25 × Isc the Fuse Size?
Not necessarily.
A value of 1.25 × Isc is commonly used as part of PV current calculations.
With an Isc of 12A:
12 × 1.25 = 15A
So 15A gives us a reasonable starting point.
But we would not simply take that result and place a 15A fuse into every 12A string.
The module manufacturer's maximum series fuse rating still has to be checked, along with the system configuration and the applicable requirements.
For example, if the module allows a maximum series fuse of 20A, that puts an upper limit on the fuse selection.
If the calculation gives a value above that limit, choosing a larger fuse is not an option just because it is available.
This is one of the reasons we always ask for the module datasheet.
What Happens When Strings Are Connected in Parallel?
This is where PV fuse sizing becomes more interesting.
Consider a combiner box with ten identical strings.
Under normal conditions, each string supplies its own current.
Now imagine one string has a fault.
The other nine strings are still connected to the system, so they can feed current toward the faulted string.
That current does not come from the faulted module alone.
This reverse-current condition is one of the things the string fuse is there to deal with.
The number of parallel strings therefore matters when checking the protection requirement.
It is also why looking only at the module's normal operating current can give you the wrong answer.
Don't Forget the Module's Maximum Series Fuse
This value is normally printed clearly on the module datasheet, but it is easy to overlook.
Suppose the module says:
Maximum series fuse: 20A
If your calculation points to 15A, there is no problem with the fuse being below that limit, assuming the rest of the design is correct.
But if the calculation suggests 25A, you cannot simply choose a 25A fuse.
The module manufacturer's maximum series fuse rating has to be respected.
A larger fuse is not automatically better protection.
In fact, an oversized fuse may not provide the protection required by the circuit.
A Quick Example
Suppose:
Isc = 12A
Using 1.25 × Isc:
12 × 1.25 = 15A
A 15A PV fuse would therefore be one of the first options we would look at.
Now change the module:
Isc = 15A
Then:
15 × 1.25 = 18.75A
If the available fuse ratings include 20A, that may be the next option to check.
But we still need to compare it with the module's maximum series fuse rating and the actual system conditions.
That's the important part.
The calculation gives you a starting point. It does not make the final decision by itself.
What About a 10A, 15A or 20A Fuse?
This is probably the most useful way to look at it.
If the module Isc is around 8A, you may be looking at a smaller fuse.
If Isc is around 12A, 15A may be a starting point.
If Isc is around 15A, 20A may come into consideration.
But these are examples, not a sizing table.
The actual selection still depends on the module data and the system.
We would not recommend choosing a PV fuse from a chart that says:
12A panel = 15A fuse
without checking anything else.
Voltage Has to Match Too
A fuse with the correct current rating can still be the wrong fuse if its voltage rating is too low.
For example:
1000V DC / 20A
and
1500V DC / 20A
are different requirements.
The current is the same, but the DC voltage is not.
For a 1500V PV system, the fuse needs to be suitable for the system voltage and the required DC interruption conditions.
This is particularly important because a DC arc does not have the natural current zero that occurs in an AC circuit.
So when we receive an inquiry for a "20A PV fuse," one of our first questions is:
20A at what voltage?
Breaking Capacity Is Another Check
The fuse also needs to be able to interrupt the fault current that it may actually see.
This is where rated breaking capacity comes in.
In a PV array with multiple parallel strings, other strings can contribute current to a fault.
The fuse must be capable of interrupting that current safely at the specified DC voltage.
So we don't normally confirm a fuse from the current rating alone.
We check the voltage and breaking capacity as well.
Temperature Can Make a Difference
Most PV engineers already know that outdoor equipment can get hot.
What matters for the fuse is not just the weather report.
The temperature inside a closed combiner box can be considerably higher than the ambient temperature, especially when the box is exposed to direct sunlight.
Then you have several fuses installed close together, the fuse holder, cable connections and continuous current.
All of these affect the thermal conditions.
For a project operating in a hot environment, we would rather know the expected temperature before confirming the final fuse size.
A fuse that looks fine at room temperature may have a different thermal situation when it is working continuously inside a hot enclosure.
Don't Select the Fuse Holder Later
The fuse and fuse holder should normally be considered together.
The holder needs to match the fuse dimensions and electrical rating.
It also has to fit the available space in the combiner box.
This sounds obvious, but we have seen projects where the fuse was selected first and the holder was dealt with later.
By then, the enclosure layout may already be finished.
That can create unnecessary problems with cable routing, terminal connections and heat dissipation.
For a new project, it is easier to confirm both at the beginning.
What We Usually Ask for at Chifeng
If someone sends us:
"1500V, 20A PV fuse. Please quote."
we can quote it.
But if they ask:
"Is 20A the correct fuse for my PV string?"
then we need more information.
Normally we ask for:
Module Isc
Module Imp
Maximum series fuse rating
System voltage
Number of parallel strings
Operating current
Installation location
Ambient temperature
Fuse size
Fuse holder
Required certification
If the customer has the module datasheet, we prefer to see it.
If there is a combiner box drawing or single-line diagram, even better.
It takes less time to check the original information than to correct a fuse selection after the equipment has already been built.
Why This Matters to Us as a Fuse Manufacturer
Chifeng has been manufacturing fuses since 1988.
We have worked with circuit protection for much longer than the current PV market has existed, and our products now cover applications including photovoltaic, energy storage and EV systems.
For PV projects, we supply fuse solutions for different current and voltage requirements, including 1000V and 1500V DC applications.
Depending on the project, customers may also require TÜV, UL, CE or other certification.
We check those requirements against the actual product model rather than treating certification as a general label for the whole product range.
For an OEM customer, this is important.
The fuse has to fit the circuit, but it also has to fit the equipment, the documentation and the customer's specification.
So, What Size Fuse Should You Use?
If you have a module with:
Isc = 12A
then:
1.25 × Isc = 15A
So 15A is a sensible place to start the evaluation.
But don't order the fuse yet.
Check the module's maximum series fuse rating.
Check the system voltage.
Check the number of parallel strings.
Check the installation temperature.
Check the applicable standard and the fuse's breaking capacity.
Then confirm the actual fuse model.
That's the approach we use at Chifeng.
If you're deciding between 10A, 15A, 20A or another PV fuse size, send us the module datasheet and the basic system information.
We can check the numbers against the actual circuit rather than guessing from the module current.
The right PV fuse is not the next standard ampere rating above the panel current. It is the fuse that matches the circuit it is protecting.