{"id":20076,"date":"2025-11-29T01:45:00","date_gmt":"2025-11-28T17:45:00","guid":{"rendered":"https:\/\/viox.com\/?p=20076"},"modified":"2025-11-15T15:45:18","modified_gmt":"2025-11-15T07:45:18","slug":"solar-pv-fuse-requirements-nec-690-9-parallel-strings","status":"publish","type":"post","link":"https:\/\/test.viox.com\/ru\/solar-pv-fuse-requirements-nec-690-9-parallel-strings\/","title":{"rendered":"\u201c\u041f\u0430\u0440\u0430\u0434\u043e\u043a\u0441 \u043f\u0440\u0435\u0434\u043e\u0445\u0440\u0430\u043d\u0438\u0442\u0435\u043b\u044f\u201d: \u043f\u043e\u0447\u0435\u043c\u0443 NEC 690.9 \u0443\u0442\u0432\u0435\u0440\u0436\u0434\u0430\u0435\u0442, \u0447\u0442\u043e \u043f\u0440\u0435\u0434\u043e\u0445\u0440\u0430\u043d\u0438\u0442\u0435\u043b\u044c \u043d\u0435 \u043d\u0443\u0436\u0435\u043d (\u043d\u043e \u043e\u043d \u0432\u0441\u0435 \u0440\u0430\u0432\u043d\u043e \u043c\u043e\u0436\u0435\u0442 \u043f\u0440\u0438\u0433\u043e\u0434\u0438\u0442\u044c\u0441\u044f)"},"content":{"rendered":"
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If you are designing a solar array, you know the golden rule of electrical engineering: Wires must be protected by fuses or \u0432\u044b\u043a\u043b\u044e\u0447\u0430\u0442\u0435\u043b\u0438<\/a>.<\/strong><\/p>\n

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<\/strong> need to install fuses.<\/p>\n

This feels wrong. It feels like a typo. How can a power source not require protection?<\/p>\n

Welcome to the \u201cFuse Paradox.\u201d<\/strong><\/p>\n

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 \u201ccode-minimum\u201d installer from a true solar engineer.<\/p>\n

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The Physics: \u201cWho Is Attacking Whom?\u201d<\/h2>\n

\"The<\/p>\n

To understand why you don\u2019t need a fuse, you first have to understand what a fuse is actually protecting against in a PV system.<\/p>\n

In a standard house circuit, the fuse protects the wire from the source<\/em> (the grid). If you short a wire, the grid dumps infinite current until the wire melts.<\/p>\n

Solar panels are different.<\/strong> They are \u201cCurrent Limited Sources.\u201d<\/p>\n

If you have a 10A solar panel and you short-circuit its positive and negative cables together, it won\u2019t explode. It will just produce\u2026 about 10-11 Amps (its Short Circuit Current, or Isc). It physically cannot produce more current than the sun allows.<\/p>\n

Therefore, a solar panel cannot burn itself out<\/strong>.<\/p>\n

So, why do we fuse PV arrays at all? We aren\u2019t protecting the panel from itself. We are protecting it from its neighbors.<\/strong><\/p>\n

The danger is Back-Feed Current<\/strong>. If one string fails (e.g., a short inside a module), the \u0414\u0440\u0443\u0433\u043e\u0435<\/em> parallel strings will dump their power backwards<\/em> into the broken string.<\/p>\n

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The Math: The \u201c2-String\u201d Exemption<\/h2>\n

\"The<\/p>\n

Let\u2019s run the math that allows the \u201cFuse Paradox\u201d to exist.<\/p>\n

The Variables:<\/strong><\/p>\n