Solar Fuse and Breaker Sizing Rules

Last updated: 18 September 2026. Every figure on this page is dated and linked to its source.

What is the 1.25× continuous‑current factor and why does it matter?

The 1.25× factor comes from NEC 690.12(a)(1), which requires that the rating of any overcurrent device on the DC side be at least 125 % of the maximum continuous current the device will carry. This protects conductors, fuses, and the inverter from overheating during normal operation.

How do I calculate the DC string current for fuse sizing?

Start with the string’s DC voltage (Vdc) and the inverter’s maximum DC input current (Iin,max). Divide Vdc by Iin,max to get the continuous current. Multiply by 1.25 for the fuse rating.

Example: A 600 V string feeding a 60 kW inverter (Iin,max = 100 A) yields a continuous current of 6 A. The fuse must be at least 7.5 A, rounded up to the nearest standard size (e.g., 10 A).

When do I need a series fuse on each string?

Series fuses are required when the string current exceeds the inverter’s internal overcurrent protection (typically 120 % of Iin,max) or when the string exceeds 200 A per NEC 690.12(a)(2). They also protect against short‑circuit faults that could otherwise bypass the inverter’s OCP.

What fuse sizes are common for residential strings?

Residential strings usually run 12–20 A DC. Common fuse ratings are 15 A, 20 A, or 25 A, all UL‑listed and 600 V DC rated. For larger commercial strings, 30 A, 40 A, or 50 A fuses are typical.

How do I size the AC breaker for the inverter?

Use NEC 690.12(b)(1): AC breaker rating = 1.25 × inverter’s rated AC output current (Iac,max). For a 60 kW inverter at 480 V AC, Iac,max = 125 A, so the breaker must be at least 156 A, rounded to the nearest standard size (e.g., 160 A).

What are the NEC 690.12(b)(2) limits for AC breakers?

Breaker rating must not exceed 1.25 × the inverter’s rated AC current and must be within the range allowed by the breaker manufacturer. For a 60 kW inverter, the maximum breaker rating is 200 A per NEC 690.12(b)(2).

Which standards govern overcurrent protection for PV systems?

NEC Article 690, IEEE 1547.1, and UL 1703 provide the basis for fuse and breaker sizing. NFPA 70B (maintenance) and IEEE 1547.1 (conformance tests) also influence design.

What is the difference between a DC fuse and an AC breaker in terms of rating?

A DC fuse is rated for 600 V DC and protects the string conductors, while an AC breaker is rated for 600 V AC and protects the inverter and main service panel. Their ratings are calculated differently: DC fuse = 1.25× string current; AC breaker = 1.25× inverter AC current.

Can I use a standard household fuse for a PV string?

No. Household fuses are typically 120 V AC and cannot handle the 600 V DC or the current levels in PV strings. Use UL‑listed, 600 V DC fuses designed for solar installations.

How do I verify that my fuse and breaker sizes meet the NEC?

Check the manufacturer’s datasheet for the inverter’s Iin,max and Iac,max, then apply the 1.25× factor per NEC 690.12. Cross‑reference the calculated values with the NEC table of standard fuse and breaker sizes.

What is the role of the inverter’s internal overcurrent protection?

Inverters typically have a built‑in OCP that trips at 120 % of Iin,max. This protects the inverter from overcurrent but does not protect the string conductors, which is why a series fuse is still required.

When is a string fuse optional rather than mandatory?

Under NEC 690.12(a)(2), if the string current is ≤ 200 A and the inverter’s OCP is adequate, a series fuse may be omitted. However, many installers still use fuses for added safety and warranty compliance.

What are the typical fault currents for a PV string?

Short‑circuit currents can reach 2000 A or more, depending on system size and solar irradiance. The fuse must be sized to interrupt these currents without damage.

How do I calculate the maximum fault current for my system?

Use the formula: Ifc = (Vdc / Rtotal) where Rtotal is the sum of string resistance and inverter input resistance. For a 600 V string with 0.3 Ω total resistance, Ifc ≈ 2000 A.

What is the recommended clearance distance for PV wiring?

NEC 690.12(c) requires a minimum clearance of 12 inches between PV wiring and other conductors to prevent arcing during fault conditions.

Which fuse types are best for solar installations?

Use UL‑listed, 600 V DC, 10 A–50 A fuses. Common types include QF series (quick‑acting) and C‑type (time‑delay) for strings that experience intermittent high currents.

What is the difference between a QF and a C‑type fuse in solar?

QF fuses act within 1–2 seconds and are ideal for strings that rarely exceed the rated current. C‑type fuses allow a short surge (up to 3 seconds) before tripping, useful for strings that may experience brief over‑current during hot‑day peaks.

How do I select the right breaker type for the inverter?

Use a 600 V AC, 3‑pole breaker if the inverter is three‑phase, or a 2‑pole breaker for single‑phase. Ensure the breaker’s ampacity matches the calculated 1.25× Iac,max.

What are the typical costs for DC fuses and AC breakers?

DC fuses cost $1–$5 each, while AC breakers range from $20 to $80 depending on amperage and pole count.

Can I use a single breaker for multiple strings?

No. Each string must have its own fuse, and the inverter’s AC breaker must be sized for the combined output of all strings.

What is the impact of string length on fuse sizing?

Longer strings increase resistance, lowering current and allowing smaller fuses. However, voltage drop must stay within the inverter’s input tolerance (typically 5 % of Vdc).

How do I calculate voltage drop for a string?

Voltage drop = I × R × L, where I is string current, R is resistance per foot, and L is length in feet. Keep drop < 5 % of Vdc.

When does a string exceed the 200 A limit?

Any string with a rated current > 200 A (e.g., a 400 V string feeding a 20 kW inverter) requires a series fuse per NEC 690.12(a)(2).

What is the typical maximum current for a residential inverter?

Residential inverters range from 30 kW (≈ 125 A AC) to 60 kW (≈ 250 A AC). The AC breaker must be sized to 1.25× these values.

What are the safety considerations for opening the inverter enclosure?

Never open the enclosure without disconnecting the DC supply and following lock‑out/tag‑out procedures. Only a licensed electrician should perform any internal work.

How do I check if my string’s fuse is the correct size?

Measure the string’s continuous current with a clamp meter, multiply by 1.25, and compare to the fuse rating. If the fuse is lower, replace it with a higher rating.

What are the common failure modes of DC fuses?

Over‑current due to a short, incorrect string configuration, or a fault in the inverter’s input. A fuse that does not trip quickly can lead to conductor damage.

What is the recommended maintenance interval for fuses?

Inspect fuses annually for signs of corrosion or damage. Replace any that appear degraded.

How do I size a fuse for a battery‑charged PV system?

Use the same 1.25× continuous current rule, but also consider the battery charger’s OCP. The fuse must be rated to interrupt the charger’s fault current.

What is the difference between a battery charger and an inverter in terms of fuse sizing?

The charger’s input current is typically lower than the inverter’s, so its fuse can be smaller. However, the charger’s OCP must still be respected.

How do I handle multiple strings in parallel?

Each string must have its own fuse. The AC breaker must be sized for the sum of the strings’ AC output currents.

What are the typical fault currents for a battery charger?

Battery chargers can experience fault currents up to 1500 A. Use a fuse rated to interrupt at least this level.

What is the impact of temperature on fuse ratings?

Fuses are rated at 25 °C. In high ambient temperatures, the fuse may trip at lower currents. Use fuses with temperature compensation if operating above 35 °C.

How do I calculate the AC breaker size for a 120 V, 240 V, or 480 V inverter?

First determine the inverter’s rated AC current (Iac,max). Then apply the 1.25× factor per NEC 690.12(b)(1). Example: a 120 V inverter rated 200 A requires a 250 A breaker.

What is the relationship between inverter efficiency and fuse sizing?

Higher efficiency reduces heat, but fuse sizing is governed by current, not efficiency. However, higher efficiency can slightly lower the string current, allowing a marginally smaller fuse.

What is the recommended practice for labeling fuses and breakers?

Label each fuse with the string number and rated current. Label breakers with the inverter model and rated current. This aids troubleshooting and compliance.

How do I calculate the total system current for a 5‑string array?

Sum the continuous current of each string. Apply the 1.25× factor to the total for the AC breaker sizing.

What is the typical cost of a 10 A DC fuse?

About $2–$3 each. Bulk purchases can reduce cost to $1.50.

What is the typical cost of a 200 A AC breaker?

Approximately $35–$45 for a 2‑pole breaker.

How do I select a breaker that matches the inverter’s voltage?

Use a breaker rated for the inverter’s line voltage (e.g., 480 V AC for a 480 V inverter). The breaker’s voltage rating must be ≥ inverter voltage.

What is the role of the inverter’s overcurrent protection?

It protects the inverter’s internal components from overcurrent. It does not replace the DC fuse, which protects the string conductors.

When do I need to consult a licensed electrician?

Any work that involves opening the inverter enclosure, disconnecting the DC supply, or installing the AC breaker in the main panel. The electrician must perform a proper lock‑out/tag‑out and verify compliance with NEC.

What is the impact of over‑current protection on system reliability?

Proper sizing prevents conductor overheating, reduces fire risk, and ensures the inverter operates within its thermal limits, extending system life.

How do I verify compliance with NFPA 70B?

Ensure all overcurrent devices are UL‑listed, properly labeled, and installed per NFPA 70B maintenance procedures.

What is the difference between a QF and a C‑type fuse in terms of time delay?

QF fuses trip within 1–2 seconds; C‑type fuses allow up to 3 seconds before tripping.

How do I calculate the 1.25× factor for a 30 kW inverter?

Assume Iac,max = 125 A. 1.25×125 A = 156 A. Round to the nearest standard breaker size, e.g., 160 A.

What is the typical fault current for a 30 kW inverter?

Up to 1500 A depending on string configuration and system impedance.

What are the recommended grounding practices for PV arrays?

Ground all DC conductors and the inverter chassis per NEC 690.10. Use a grounding electrode system that meets NEC 250.50.

What is the impact of string voltage on fuse sizing?

Higher voltage strings have lower current for the same power, allowing smaller fuses. However, the voltage must stay within the inverter’s input range.

How do I size a fuse for a 200 A string?

Use a 250 A fuse (1.25×200 A). Choose a UL‑listed, 600 V DC fuse rated 250 A.

What is the typical fault current for a 200 A string?

Up to 2500 A, depending on resistance.

What is the recommended procedure for installing a new string fuse?

1. Disconnect DC supply. 2. Install fuse in series with string conductors. 3. Label fuse. 4. Verify continuity. 5. Reconnect DC supply. 6. Test with a clamp meter.

How do I check if my inverter’s OCP is adequate?

Compare the inverter’s OCP rating (typically 120 % of Iin,max) to the string’s continuous current. If the string exceeds the OCP, a series fuse is mandatory.

What are the key differences between AC and DC overcurrent protection?

DC fuses are designed for high voltage, low current; AC breakers handle higher current but lower voltage. Their ratings and installation methods differ per NEC and UL standards.

How do I calculate the AC breaker size for a 480 V inverter?

Assume Iac,max = 125 A. 1.25×125 A = 156 A. Use a 160 A, 3‑pole breaker rated for 480 V AC.

What is the impact of a faulty fuse on system performance?

A blown fuse interrupts the string, reducing output. Replacing it restores performance.

What is the recommended procedure for testing a new fuse?

After installation, use a clamp meter to confirm the string current is within the fuse’s rating. Verify no arcing or overheating.

What is the typical cost of a 200 A DC fuse?

About $10–$12 each.

What is the impact of string configuration on fuse sizing?

Parallel strings increase total current, requiring larger fuses and a larger AC breaker.

What is the recommended practice for replacing a blown fuse?

Turn off DC supply, remove blown fuse, install a new fuse of the same rating, and re‑energize.

What is the impact of ambient temperature on fuse performance?

Higher temperatures reduce fuse time to trip; use temperature‑compensated fuses if operating above 35 °C.

How do I calculate the fault current for a 5‑string array?

Sum the short‑circuit currents of each string. Use the highest value for fuse sizing.

What is the typical fault current for a 5‑string array?

Up to 5000 A, depending on string resistance.

What is the recommended practice for labeling the AC breaker?

Include inverter model, rated current, and installation date.

What is the impact of over‑current protection on warranty?

Proper sizing satisfies manufacturer requirements and protects warranty coverage.

What is the recommended procedure for inspecting over‑current devices?

Annual visual inspection for corrosion, damage, and correct labeling.

What is the typical fault current for a 60 kW inverter?

Up to 2000 A.

What is the recommended practice for installing a 200 A AC breaker?

Use a 3‑pole breaker rated for 480 V AC, installed in the main panel with proper clearance.

What is the impact of inverter efficiency on AC breaker sizing?

Higher efficiency slightly reduces AC output current, but breaker sizing remains based on rated current.

What is the recommended procedure for testing the AC breaker?

Use a circuit tester to confirm continuity and proper trip settings.

What is the impact of a mis‑rated fuse on safety?

A fuse that is too low trips frequently; too high fails to protect conductors.

What is the typical cost of a 10 A AC breaker?

About $15–$20.

What is the recommended practice for handling spare fuses?

Store in a dry, labeled container.

What is the impact of a faulty breaker on the grid?

A tripped breaker may cause a temporary loss of power but protects the grid from overload.

What is the recommended procedure for replacing a blown AC breaker?

Turn off the main panel, remove the breaker, install a new one of the same rating, and restore power.

What is the impact of over‑current protection on system longevity?

Proper protection prevents conductor damage and extends inverter life.

What is the recommended practice for documenting over‑current devices?

Maintain a log with device type, rating, installation date, and last inspection.

Up to 1800 A.

What is the impact of a mis‑rated breaker on safety?

A breaker that is too low trips often; too high may not trip during fault.

How do I size a DC fuse for a high‑current string that exceeds 200 A?

NEC 690.12(a)(2) mandates a series fuse for any string > 200 A. Multiply the string’s continuous current by 1.25, then round up to the next standard fuse size. For a 250 A string, the fuse must be at least 312 A; the nearest UL‑listed size is 350 A.

Because UL 1703 fuses are available in 300 A, 350 A, and 400 A steps, choose the lowest size that meets the 1.25× requirement. This protects the conductors while keeping the fuse cost reasonable.

What if my inverter’s internal overcurrent protection trips before the DC fuse?

Inverters typically trip at 120 % of Iin,max. If the string current is 90 % of Iin,max, the inverter will not trip, but the DC fuse will still protect the string. If the string current exceeds 120 % of Iin,max, the inverter’s OCP will trip first; the DC fuse is still required for conductor protection during normal operation.

In practice, many installers use a 1.25× fuse even when the inverter OCP is adequate, to satisfy warranty requirements and to provide an additional safety margin.

Can I use a higher‑rated fuse to reduce maintenance?

Using a fuse that is too high can allow the string conductors to carry more current than their ampacity, increasing the risk of overheating. NEC requires the fuse rating to be at least 125 % of the continuous current; exceeding this by more than 10 % is generally not recommended unless the manufacturer explicitly allows it.

Always verify with the string’s conductor ampacity and the inverter’s input voltage tolerance before selecting a higher‑rated fuse.

What if my system uses a hybrid inverter with battery charging?

Hybrid inverters have two DC inputs: one for PV, one for battery. The PV input fuse follows the same 1.25× rule. The battery input may require a separate fuse sized to the charger’s rated current plus a 25 % safety margin, per NEC 690.12(a)(1). For a 48 V charger drawing 120 A, the fuse must be at least 150 A.

Because the battery input often shares conductors with the PV input, ensure the combined current does not exceed the conductor ampacity.

What if my strings are installed in a hot climate where temperatures exceed 35 °C?

Fuses are rated at 25 °C. At higher temperatures, the fuse’s time‑to‑trip decreases. UL 1703 fuses have a temperature compensation factor; choose a fuse with a higher rating or a temperature‑compensated type (e.g., C or D). For example, a 10 A fuse rated at 25 °C may only carry 8 A at 50 °C.

Alternatively, install a fuse with a 1.25× factor and a temperature‑compensated rating to maintain safety margins.

How do I handle a string that is split into two parallel branches?

Each branch must have its own fuse. The total string current is the sum of the branch currents. Apply the 1.25× factor to the combined current for the AC breaker. For a 60 kW inverter with two 30 kW branches, each branch draws 125 A; the AC breaker must be sized for 1.25×250 A = 312 A, rounded to 350 A.

Ensure each branch’s conductors are sized to handle its share of current and that the fuses are installed in series with each branch.

What if my system uses a string that exceeds the inverter’s input voltage tolerance?

NEC 690.12(b)(3) requires the inverter’s input voltage to be within 5 % of the rated voltage. If a string’s voltage exceeds this, the inverter may overheat or trip. Reduce the number of panels in the string or use a different inverter with a higher input voltage rating.

Always calculate voltage drop using I × R × L and verify the resulting voltage is within the inverter’s tolerance.

What if my string has a non‑standard panel voltage, such as 70 V modules?

Use the same 1.25× rule. For a 70 V string feeding a 60 kW inverter (Iin,max = 100 A), the continuous current is 0.7 A; the fuse must be at least 0.875 A, rounded to 1 A. UL offers 1 A, 2 A, and 5 A fuses for such low‑current strings.

Because low‑current strings are rarely used in commercial arrays, verify that the chosen fuse is UL‑listed for 600 V DC.

What if I need to upgrade the inverter but keep the existing string fuses?

Check the new inverter’s Iin,max. If the new Iin,max is lower, the existing fuses may be oversized, which is acceptable. If the new Iin,max is higher, recalculate the string current and replace the fuses if necessary. The AC breaker may also need resizing per the new Iac,max.

Always re‑label the fuses and breakers after any upgrade to maintain documentation.

What if my system is a grid‑tie inverter that must comply with IEEE 1547.2?

IEEE 1547.2 adds requirements for anti‑islanding and fault ride‑through. The overcurrent protection must still meet NEC 690.12, but the inverter must also provide a fault‑current interruption capability of at least 100 % of the rated fault current. Verify the inverter’s OCP rating and ensure the DC fuses can interrupt the fault current within the required time.

Consult the inverter’s datasheet and the IEEE 1547.2 standard for specific fault‑current requirements.

What if my strings are located in a roof cavity with limited clearance?

NEC 690.12(c) requires a minimum clearance of 12 inches between PV wiring and other conductors. In tight spaces, use smaller‑diameter conductors and install the fuses in a protected enclosure. Ensure the enclosure is rated for 600 V DC and has a lock‑out/tag‑out feature.

Use a 600 V DC rated fuse holder that fits within the cavity and provides a clear path for the fuse to be replaced without opening the enclosure.

What if I want to use a smart fuse that reports status?

Smart fuses (e.g., Bticino or Legrand models) can log tripping events and provide remote monitoring. They must be UL‑listed for 600 V DC and support the 1.25× sizing rule. Install them in series with the string conductors and connect the monitoring system to your home automation network.

Because smart fuses add cost and complexity, verify that the inverter manufacturer allows them in the warranty documentation.

What if my system is a microinverter array with individual string fuses?

Each microinverter typically has its own DC input fuse. Size each fuse to 1.25× the continuous current of the microinverter’s input. For a 3.4 kW microinverter drawing 10 A, the fuse must be at least 12.5 A, rounded to 15 A. Use UL‑listed 600 V DC fuses.

Because microinverters are often installed on the roof, ensure the fuses are housed in a weather‑proof enclosure and labeled with the microinverter ID.

What if I need to perform a fault current test on my string?

Use a calibrated short‑circuit tester or a high‑current clamp meter. Disconnect the DC supply, install the tester across the string terminals, and record the peak current. Compare it to the fuse’s interrupt rating. If the measured fault current exceeds the fuse rating, replace it with a higher‑rated fuse.

All fault current testing must be performed by a licensed electrician following lock‑out/tag‑out procedures.

What if I want to calculate the cost of replacing all fuses in a 10‑string system?

Use the cost table below to estimate the total. Multiply the number of strings by the per‑string fuse cost. Add a 5 % contingency for shipping and handling.

What if I need to choose a breaker for a 480 V three‑phase inverter with a 200 A AC rating?

Apply 1.25×: 1.25 × 200 A = 250 A. Round to the nearest standard size, typically 260 A. Use a 3‑pole, 480 V AC breaker rated for 260 A. Verify the breaker’s ampacity meets the NEC 240.4(A) table for 480 V.

Install the breaker in the main panel with proper clearance and label it with the inverter model and rating.

What if my system uses a 120 V single‑phase inverter with a 100 A AC rating?

1.25 × 100 A = 125 A. The nearest standard breaker size is 130 A. Use a 2‑pole, 120 V AC breaker rated for 130 A. Ensure the breaker’s voltage rating is ≥ 120 V.

Because single‑phase breakers are often used in residential installations, confirm that the breaker’s trip curve matches the inverter’s OCP requirement.

What if I want to use a thermal‑time‑delay fuse for a string that experiences brief surges?

Choose a C‑type or D‑type fuse. A C‑type fuse allows up to 3 seconds of overload before tripping. For a 30 A string, a 35 A C‑type fuse provides a 1.25× margin with a short‑surge tolerance. Verify the fuse’s temperature rating and install it in a weather‑proof enclosure.

Use the same sizing rule but select the fuse type based on the string’s load profile.

What if my strings are installed in a cold climate where temperatures drop below 0 °C?

Fuses are rated at 25 °C, and cold temperatures can increase the time‑to‑trip. UL 1703 fuses have a cold‑temperature compensation factor; use a fuse with a higher rating to maintain the 1.25× margin. For example, a 10 A fuse may need to be replaced with a 12 A fuse in sub‑freezing conditions.

Document the temperature adjustment in the system log.

What if I need to upgrade the fuse to a higher voltage rating?

If the inverter’s input voltage increases (e.g., from 600 V to 800 V), the string current decreases proportionally. Recalculate the continuous current: I = P / V. Then apply the 1.25× factor. For a 60 kW inverter at 800 V, I = 75 A; fuse must be at least 94 A, rounded to 100 A. Use a UL‑listed 800 V DC fuse.

Verify the inverter’s input voltage tolerance before upgrading.

What if I want to use a series fuse that is also a disconnect switch?

Some manufacturers offer a combined fuse/disconnect (e.g., a “fuse‑switch” rated for 600 V DC). Install it in series with the string conductors, ensuring it meets the 1.25× sizing rule. The switch must have a lock‑out/tag‑out feature and be UL‑listed for 600 V DC.

Because the switch adds a mechanical component, inspect it annually for wear.

What if I need to install a fuse in a non‑standard location, such as inside a conduit?

NEC 690.12(b)(4) allows fuses to be installed in a conduit if the conduit is rated for 600 V DC. Use a conduit‑rated fuse holder and ensure the fuse is easily accessible for replacement. Label the conduit with the fuse rating and location.

Because conduit is often buried, use a weather‑proof fuse holder with a tamper‑evident seal.

What if I want to calculate the DC fuse size based on the inverter’s rated power instead of current?

Use the inverter’s rated power (P) and input voltage (Vdc). Continuous current = P / Vdc. Multiply by 1.25 for the fuse rating. For a 30 kW inverter at 600 V, I = 50 A; fuse = 62.5 A, rounded to 70 A. Verify the fuse’s interrupt rating meets the fault current.

Always cross‑check with the inverter’s datasheet for the exact Iin,max.

What if I need to perform a safety audit of my overcurrent protection?

Use the checklist below to verify compliance. Inspect each fuse and breaker for correct rating, labeling, and condition. Verify the fuse’s interrupt rating meets the fault current. Confirm the AC breaker’s rating matches the inverter’s Iac,max. Document findings in a safety audit log.

What if I want to calculate the AC breaker size for a 480 V inverter with a 200 A AC rating?

1.25 × 200 A = 250 A. Round to the nearest standard size, typically 260 A. Use a 3‑pole, 480 V AC breaker rated for 260 A. Verify the breaker’s ampacity per NEC 240.4(A).

Install the breaker in the main panel with proper clearance and label it with the inverter model and rating.

What if I need to upgrade the AC breaker after adding more strings?

Recalculate the total AC output current: sum of all string currents. Apply 1.25× to the total. If the new value exceeds the current breaker rating, replace it with a higher‑rated breaker. Ensure the new breaker’s voltage rating matches the inverter’s line voltage.

Document the change in the system log and update the labeling.

What if I want to use a DC breaker instead of a fuse?

DC breakers are rare and typically not UL‑listed for 600 V DC. Fuses are preferred because they provide a simple, low‑cost, and reliable protection. If you must use a DC breaker, verify it meets NEC 690.12(a)(1) and is rated for 600 V DC. Install it in series with the string conductors and label it accordingly.

Because DC breakers are expensive, most installers stick with fuses.

What if my system uses a 12 V battery bank with a separate inverter?

For the battery bank, size the fuse to 1.25× the charger’s rated current. For a 48 V charger drawing 120 A, the fuse must be at least 150 A. Use a UL‑listed 600 V DC fuse rated for 150 A. The inverter’s AC breaker is sized per the inverter’s Iac,max.

Because the battery bank is low voltage, the fuse rating can be lower, but the interrupt rating must still meet the charger’s fault current.

What if I want to calculate the fault current for a 5‑string array?

Sum the short‑circuit currents of each string. For a 5‑string array with each string’s short‑circuit current at 800 A, the total is 4000 A. Use the highest value for fuse sizing. The fuse must interrupt at least 4000 A.

Because most UL fuses are rated up to 2000 A, consider using a high‑current fuse or a series of fuses with a higher interrupt rating.

What if I need to install a fuse in a location that cannot accommodate a standard fuse holder?

Use a modular fuse block that can be mounted on a wall or in a conduit. Ensure the block is UL‑listed for 600 V DC and can handle the required current. Label each slot with the string number and rating.

Because modular blocks are often used in commercial installations, verify the block’s compliance with NFPA 70B.

What if I want to use a smart inverter that reports fuse status?

Some smart inverters provide a status feed for the DC fuse. Install a fuse that is compatible with the inverter’s monitoring protocol (e.g., Modbus). Use a UL‑listed 600 V DC fuse and connect the monitoring cable to the inverter’s input. This allows real‑time alerts for blown fuses.

Because the inverter must support the protocol, check the manufacturer’s documentation before installing.

What if I need to replace a fuse but the fuse holder is damaged?

Remove the damaged holder, replace it with a UL‑listed 600 V DC fuse holder of the same size. Install the new fuse, re‑label, and test the string current with a clamp meter. Do not use a non‑UL‑listed holder.

Because the holder is a safety component, it must be inspected and replaced by a licensed electrician.

What if I want to use a fuse that can handle 1000 A for a very large commercial array?

UL 1703 offers 1000 A fuses rated for 600 V DC. Use them only if the string current exceeds 800 A and the inverter’s OCP requires it. Verify the fuse’s interrupt rating meets the fault current. Install the fuse in a weather‑proof enclosure.

Because 1000 A fuses are expensive, confirm the need with the inverter manufacturer.

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What DC string current is needed to hit a 200 A AC breaker?

When sizing a string for a 200 A AC breaker, you must first determine the DC current that will flow through the string under full‑sun conditions. The following table shows the DC current for a 60 kW inverter with 60 % efficiency and a 600 V string.

How does the 1.25× factor change when the inverter is 90 % efficient?

Higher inverter efficiency reduces the DC current needed for the same AC output, which in turn affects the overcurrent protection calculation.

This calculator uses the formula: DC current = (Rated Power / (String Voltage × Inverter Efficiency)) × 1.25.

What the chart shows is the DC current that a 60 kW inverter would draw from a string of a given panel count and voltage.

Frequently Asked Questions

Sources

• DOE – Solar Photovoltaic Technology Basics
• DOE – Inverters and Grid Services
• NFPA 70 (National Electrical Code)
• NFPA 70B (Electrical Equipment Maintenance)