If you are designing a solar array, you know the golden rule of electrical engineering: Wires must be protected by fuses or شکن ها.
So, when you dig into the National Electrical Code (NEC) Section 690.9, you might run into a clause that stops you in your tracks. It suggests that if you have two strings of solar panels wired in parallel, you do not need to install fuses.
This feels wrong. It feels like a typo. How can a power source not require protection?
Welcome to the “Fuse Paradox.”
This rule exists not because the NEC is lax on safety, but because the physics of a solar panel are fundamentally different from a battery or the grid. Understanding this paradox is the key to distinguishing a “code-minimum” installer from a true solar engineer.
The Physics: “Who Is Attacking Whom?”
To understand why you don’t need a fuse, you first have to understand what a fuse is actually protecting against in a PV system.
In a standard house circuit, the fuse protects the wire from the source (the grid). If you short a wire, the grid dumps infinite current until the wire melts.
Solar panels are different. They are “Current Limited Sources.”
If you have a 10A solar panel and you short-circuit its positive and negative cables together, it won’t explode. It will just produce… about 10-11 Amps (its Short Circuit Current, or Isc). It physically cannot produce more current than the sun allows.
Therefore, a solar panel cannot burn itself out.
So, why do we fuse PV arrays at all? We aren’t protecting the panel from itself. We are protecting it from its neighbors.
The danger is Back-Feed Current. If one string fails (e.g., a short inside a module), the other parallel strings will dump their power backwards into the broken string.
The Math: The “2-String” Exemption
Let’s run the math that allows the “Fuse Paradox” to exist.
The Variables:
- Panel Isc (Short Circuit Current): 10 Amps (The attack power).
- Max Series Fuse Rating: 15 Amps (The defense power). This is printed on the back of every solar panel. It tells you how much reverse current the panel can handle before it catches fire.
Scenario A: 2 Strings in Parallel (The “Stalemate”)
You have String 1 and String 2 connected in parallel.
- The Fault: String 1 develops a short circuit.
- The Attack: String 2 flows backwards into String 1.
- Current Calculation: The only source of current is String 2. So, the attack force is 10 Amps.
- The Result: The 10A attack is less than the 15A limit (Max Series Fuse Rating) of String 1.
The panel can handle the heat. The wire can handle the heat. No fire occurs.
Verdict: NEC 690.9 says No Fuse Required.
Scenario B: 3 Strings in Parallel (The “Ambush”)
Now, you add a third string.
- The Fault: String 1 develops a short circuit.
- The Attack: String 2 (10A) AND String 3 (10A) both flow backwards into String 1.
- Current Calculation: 10A + 10A = ۲۰ آمپر.
- The Result: The 20A attack is greater than the 15A limit of String 1.
String 1 is now overwhelmed. It overheats. It melts. The roof catches fire.
Verdict: NEC 690.9 says Fuses are MANDATORY on every string.
Beyond the Code: Why You Should Fuse Anyway
So, if you have a 2-string system, the code says you can save money and skip the combiner box fusing.
But ask any veteran service technician, and they will tell you: “Install the fuses anyway.”
Here is why the “Code Minimum” isn’t the “Best Practice.”
1. The “Hot Disconnect” Nightmare
Code compliance is great until you have to fix something.
If you skip the fuse holder (which usually acts as a disconnect switch), your strings are hard-wired together (often with Y-connectors).
If you need to troubleshoot String 1, you have to unplug the MC4 connectors. If the sun is shining, that circuit is under load. Pulling an MC4 connector under load creates a DC Arc—a plasma flame that can melt the connector contacts and burn your hands.
الف VIOX PV Fuse Holder gives you a safe, touch-proof way to “switch off” a string before you work on it. You flip the carrier open, the circuit breaks inside a contained chamber, and you are safe.
2. The “Future-Proofing” Factor
Solar systems grow. You might start with 2 strings today. Three years later, you might want to add a 3rd string to charge an EV.
If you hard-wired your 2-string system without fuses (because “Code said I didn’t need them”), adding that 3rd string suddenly makes your entire existing array illegal and dangerous. You have to rip out the wiring and start over.
If you installed a جعبه ترکیبی VIOX with fuse holders from day one, adding a string is plug-and-play. You are already protected.
Summary: Don’t Outsmart Your Safety
The “Fuse Paradox” is a fun bit of engineering trivia, but it’s a bad design philosophy.
While the physics of a 2-string system might save you from a fire, omitting the fuses robs you of control, safety, and expandability.
- N ≤ 2 Strings: Fuses are optional (but recommended for isolation).
- N ≥ 3 Strings: Fuses are mandatory (for fire protection).
Don’t just build for the inspection; build for the 20 years of operation that follows.
Technical Accuracy Note
- استانداردها:
- NEC 690.9 (Overcurrent Protection): Defines the requirements for protecting PV source circuits.
- Exception: Fuses are not required if the short-circuit currents from all other sources do not exceed the ampacity of the conductors or the maximum series fuse rating of the module.
- فرمول: The general rule for requiring fuses is:
(N - 1) x Isc > Max Series Fuse Rating.- Where ن is the number of parallel strings.
- Isc is the Short Circuit Current of the panel (adjusted for temperature/irradiance safety factors, usually 1.25).
- VIOX Solutions: VIOX manufactures 10x38mm gPV fuses specifically designed to interrupt the low-overcurrent faults common in DC solar arrays, which standard AC fuses may fail to clear.
