What Is Overcurrent Protection (OCPD)?

Key takeaways

• Mersen and Eaton call for string fuses at 3 or more strings. A Utah checklist asks for them above two. Wiles says calculate.
• A string cannot overload itself, but parallel strings can push current into a faulted one.
• Invented example: a 10.4 A Isc module with a 20 A fuse limit needs no string fuse at 2 strings (13.0 A). It does at 3 (26.0 A).
• The fuse must sit inside a window. The floor is 1.56 x Isc / Kf. The ceiling is the lower of the module’s maximum series fuse and the conductor limit.
• Fuses and breakers must be DC-rated and listed for PV. AC breakers on the inverter output are a separate check.

To draw OCPD ratings on a single line diagram, see the best solar SLD software. Only Aurora’s SLD editor shows a fuse rating field in our table below.

Overcurrent protection at a glance

OCPD covers string fuses, combiner fuses, DC breakers and the AC breaker at the inverter output. Each one needs a current rating and a DC or AC voltage rating.

Why does PV need overcurrent protection?

A module is a current-limited source, so one string cannot overload its own wiring. Other sources can: parallel strings, inverter failure and batteries.

EC&M (Mike Holt, 17 July 2024) says "OCPDs are required for PV system circuit conductors connected at one end to a current-limited supply". Under 690.9(A)(1), as printed, they are not required where "The PV system DC circuit conductors have an ampacity of at least the DC current per Sec. 690.8(B)." or where "the currents from all PV sources do not exceed the OCPD rating specified by the manufacturer". The article does not name an edition.

Wiles (IAEI Magazine, posted 16 May 2008) is older background and was written for the NEC of that era. It gives the three-string case. If one string faults, the other two can each deliver up to 1.25 x Isc into it. Eaton’s guide adds that unpredicted fault currents may arise from inverter failure or batteries connected to the strings.

This "backfeed" is current from other strings. A back-fed breaker on a main panel is a different issue. See the busbar 120% rule and labels and placards pages.

When is OCPD required for PV strings?

The answer turns on how many strings share a circuit and on two limits: the module’s maximum series fuse rating and the conductor. The documents below agree on three or more strings and differ on two. The newest is listed first, and the Wiles article is kept as history.

The Utah checklist asks for fuses with "more than two" strings. Mersen’s two-string exception reads "or", while Wiles compares with both limits. This page flags a string if backfeed goes over either one.

The check follows Wiles. It is a minimum test, not a way around the 3-string rule. It runs in this order.

1. Backfeed into one string = (N - 1) x 1.25 x Isc, where N is the strings in parallel.
2. Limit = the lower of the module’s maximum series fuse rating and the conductor limit.
3. If backfeed is above the limit, fuse each string. Eaton recommends fuses in both positive and negative cables.
4. If backfeed is within the limit, no string fuse follows from this check. Check the combiner output and any battery or inverter source separately.

Under the Wiles-style check, the most strings with no string fuse is 1 + the whole number in limit / (1.25 x Isc).

Figure 1. With example inputs, backfeed passes the 20 A limit at 3 strings, and a Kf of 0.80 leaves no standard fuse in the window.

How do you size a string fuse? A worked check

Find the minimum from Isc, round up to a standard size, then check the module and conductor limits. Mersen’s note and Eaton’s guide both work this way.

1. Minimum rating = 1.56 x Isc / Kf. Mersen’s note cites two 125% rules (690.8(A)(1) and 690.9(B)(1)) and uses 1.56. The two factors multiply to 1.5625 (see Voc and Isc).
2. Round up to the next standard size. EC&M says the next standard size up is permitted.
3. Check the fuse is no larger than the module’s maximum series fuse rating and the conductor limit.
4. Check the voltage. Eaton’s guide uses at least 1.2 x Voc x modules in series. The Utah checklist asks for devices designed for the maximum DC voltage.

Kf is a temperature factor. Mersen applies it for sustained ambients above 40 °C and reads 0.90 from its chart at 50 °C ambient. The cases below use example values. Cases A to F change one input at a time. Fits means the next standard size sits inside the window, not that the design is signed off.

Cases G and H recompute the two published examples. Not stated means the source or page we read does not give that figure or guidance.

Case D is the conflict Wiles flags: some module markings fall below 1.56 x Isc. In A to F the voltage minimum is 972 V (1.2 x 45.0 V x 18, example).

The fuse must also be DC-rated. Penn State’s AE 868 lesson (undated) says OCPD must be listed and rated for DC. It says automotive fuses may not be used. Eaton’s October 2018 catalog prints 600, 1,000 and 1,500 V DC families, and 50 kA ratings for its PV 10 x 38 mm fuses.

String fuse calculator

Where does OCPD appear in the plan set?

On the single line diagram as ratings, and in the equipment schedule and labels. The Utah checklist asks for these items.

• The ratings of all breakers or fuses, DC and AC (checklist item 20).
• Fuse ratings for each string where strings are combined (item 27).
• The DC fuse ratings in the inverter specification, if any (item 49).
• A DC combiner listed to UL 1741. Over 30 A output needs a disconnect (items 65 and 66, see DC disconnect).
• A sign at any breaker back-fed by the PV system (item 79).

See also three-line diagram, permit plan set and combiner box.

How solar software handles overcurrent protection

Only Aurora’s SLD editor, for Europe, documents a fuse rating field. Solargraf and Aurora’s Instant Plan Sets cover breakers in part, and no page documents a sizing check for string fuses.

Partly means the page covers breakers or wire sizes but not string fuses. What each page says, and its limits:

• Solargraf: permit settings help article, "Published on 19 Mar 2026": lets users exclude breaker sizes when generating single line diagrams. If a 35A breaker is unavailable, "Solargraf will automatically use the next available breaker size (e.g., 40A)", recalculating wire sizes. Fuses, string fuses and OCPD are not mentioned.
• Aurora Solar (Instant Plan Sets): help page (no date): "OCPD" appears as a column in the circuit table, with no explanation. A dropdown selects the breaker rating "from a list of compatible ratings" for the communication gateway. Fuses are not mentioned.
• Aurora Solar (SLD editor, Europe only): help article (no date). A disconnect can be fused ("providing overcurrent protection in addition to the disconnecting function") with a "Fuse rating" field in amperes. It gives no guidance on choosing the rating.

OpenSolar’s stringing article (updated 24 July 2025) and HelioScope’s (updated 9 July 2025) do not mention fuses or breakers in their text. For tools that draw OCPD on the single line diagram, see the best solar SLD software. For plan-set checks, see the best solar permitting software.

Common mistakes with overcurrent protection

FAQs

Related terms

• Voc and Isc: the module values behind the 1.25 and 1.56 factors.
• Combiner box: where string fuses often sit.
• DC disconnect: the switch that sits beside the fuse.
• Grounding and earthing: joining exposed metal to earth.

Sources

Dates are as printed in each document. All opened October 2026.

Last updated 30 October 2026. Next scheduled review: 30 January 2027.