Let's break down the requirements in NEC 690.9(B)(1) and address your questions about fuse sizing in PV systems

Understanding 690.9(B)(1)

This section mandates that the overcurrent protective device (OCPD), like a fuse or circuit breaker, be sized to handle at least 125% of the maximum current calculated according to 690.8(A). 690.8(A) basically states that the maximum current in a PV source circuit (a string) is 125% of the module's rated short-circuit current (Isc) under Standard Test Conditions (STC).Therefore: 125% (690.8(A)) 125% (690.9(B)(1)) = 156.25% of Isc. This is often rounded to 156% for practical calculations.

Why a Minimum Fuse Size?

The reason for this minimum sizing is not primarily about overcurrent protection in the traditional sense (protecting conductors from overheating in a fault scenario). While that is a consideration, the main focus is on: Preventing Nuisance Tripping

PV modules can, under certain conditions, produce currents exceeding their STC Isc. These conditions can include:

High Irradiance

Irradiance levels higher than STC (1000 W/m²) are possible in real-world installations, especially on clear days with cloud edge enhancement.

Low Cell Temperature

Lower cell temperatures can also result in higher currents.

Reflections

Reflections from snow or other surfaces can increase irradiance on the modules.If the fuse were sized too close to the STC Isc, these perfectly normal operating conditions could cause it to blow unnecessarily, leading to system downtime and maintenance costs.

DC Fuse Characteristics

DC fuses behave differently from AC fuses. DC current can sustain an arc across a blown fuse more easily than AC current (which crosses zero volts 120 times per second in a 60Hz system). DC fuses need to be specifically designed to interrupt DC currents. Sizing the fuse properly ensures it can reliably interrupt a fault condition when needed.

Think of it this way

The 156% Isc minimum is, in part, a safety margin to accommodate the real-world operating characteristics of PV systems, not just a worst-case fault scenario. It's more akin to a "service factor" built into the OCPD rating.

Is there a maximum fuse size requirement?

YES! You're right, blindly using a huge fuse would defeat the purpose of overcurrent protection. The code addresses maximum fuse size through several mechanisms:1. 690.9(B)

While this section establishes a minimum, it also implies a maximum based on the ampacity of the conductors. The fuse must be sized to protect the conductors it is connected to.2.

690.8(B)

This section covers conductor ampacity calculation. The key is to calculate the adjusted ampacity of the conductors based on temperature correction and ambient temperature. The conductor's ampacity, after all adjustments, MUST exceed the current calculated in 690.8(A), which is 125% of Isc. The OCPD MUST NOT EXCEED the ampacity of the conductors they are protecting.
3.

690.9(C) (Equipment)

The overcurrent protection is not permitted to exceed the manufacturer's equipment listing limits. This is crucial for things like inverters and charge controllers. Their input and output terminals are often limited in the maximum overcurrent protection allowed.4.

NEC Article 310 (Conductors for General Wiring)

This is the overarching section that defines conductor ampacity. You have to consider the type of conductor, insulation rating, and installation method to determine its ampacity. Derating factors (for temperature, number of conductors in a conduit, etc.) must be applied.

Example Scenario

Let's say you have a PV module with an Isc of 10A.1.

Minimum Fuse Size (690.9(B)(1))

10A 1.5625 = 15.625A. You would likely use a 16A fuse (standard size).
2.

Conductor Sizing (690.8(B))

The conductor ampacity must be at least 10A 1.25 = 12.5A. Let's say you choose 12 AWG THWN copper wire installed in conduit on a rooftop in a hot climate. You have to derate the conductor ampacity based on ambient temperature (Table 310.16, 310.17). After all derating, the ampacity MUST be at least 12.5A.
3.

Maximum Fuse Size

The fuse cannot be larger than the conductor's adjusted ampacity. If, after all derating, the 12 AWG wire has an ampacity of 15A, you cannot use a 16A fuse. You would have to use a 15A fuse or consider increasing the conductor size.

In summary: The 156% rule is a minimum to prevent nuisance tripping and ensure reliable DC fault interruption. The maximum fuse size is limited by: The ampacity of the conductors (after derating). The equipment's listing requirements (e.g., inverter input current limits).Careful conductor ampacity calculations and consideration of equipment ratings are essential for safe and code-compliant PV system design. Always consult the latest edition of the NEC and local amendments. Flag for review