Solar PV String Fuse Size Calculator
Enter the module Isc, string count, backfeed current, conductor ampacity, and fuse rating to see the minimum OCPD size and whether the selected fuse fits the PV limits.
Minimum string OCPD rating21.56
- Maximum PV current per string
- 17.25
- Potential reverse fault current
- 34.5
- Is string fusing required?
- Required: 34.5 A of reverse current exceeds the module maximum series fuse rating.
- Permitted fuse range
- 21.56 A to 30 A
- Selected fuse status
- Passes the entered current, module, and conductor limits.
- Fuse-limit headroom
- 5
Current values are in amperes.
Fuse voltage rating, interrupting rating, polarity, holder temperature rating, and equipment listing compatibility are separate checks.
How to use this calculator
- Enter the module nameplate short-circuit current and maximum series fuse rating.
- Enter the number of identical strings connected in parallel at the combiner or equipment input.
- Add any non-PV backfeed contribution from batteries, converters, or similar sources.
- Enter corrected conductor ampacity and the fuse rating you plan to use.
- Read the required range, selected-fuse status, and notes before checking voltage and interrupting ratings separately.
How the PV string fuse size is calculated
This calculator follows the common NEC Article 690 sizing path for a photovoltaic source circuit. Start with the module short-circuit current, usually labeled Isc on the module nameplate. The maximum PV current per string is the module Isc multiplied by 1.25. The minimum overcurrent protective device rating then applies another 1.25 factor, so the minimum string fuse or breaker rating is 1.5625 times Isc.
The current formulas used here are:
maximum PV current = 1.25 x Isc
minimum OCPD rating = 1.25 x maximum PV current = 1.5625 x Isc
reverse fault current = 1.25 x (parallel strings - 1) x Isc + other-source backfeed
fuse ceiling = min(module maximum series fuse rating, corrected conductor ampacity)
The selected fuse passes only when it is at least the minimum OCPD rating and no greater than the fuse ceiling. The ceiling is whichever limit is smaller: the module maximum series fuse rating or the ampacity of the string conductors after every required adjustment and correction.
When string fusing is required
Individual string protection is required when possible reverse current can exceed the module maximum series fuse rating or the corrected conductor ampacity. With one string, other PV strings contribute no reverse current, but batteries, converters, or other connected sources can still create backfeed that makes protection necessary. With two strings, many systems can meet the exception because the reverse current from the other string may stay below both limits. Three or more strings often need fuses because two or more adjacent strings can feed a faulted string.
The string count and module Isc usually move the answer most. Conductor ampacity can be just as important after rooftop temperature, conduit fill, bundling, terminal temperature, or other factors reduce the allowable ampacity. A module with a low maximum series fuse rating can also make the allowed range narrow or impossible.
What this calculator leaves out
This tool checks the current-sizing path only. It does not verify that a fuse is listed for DC photovoltaic use, that its voltage rating is high enough for the array open-circuit voltage, that its interrupting rating is adequate for the available fault current, or that the fuse holder and equipment are compatible. It also assumes the corrected conductor ampacity you enter already includes every adjustment required by the locally adopted code and the authority having jurisdiction.
Worked example
Suppose each module has an Isc of 13.8 A, the module maximum series fuse rating is 30 A, three strings are paralleled, there is no other-source backfeed, the corrected conductor ampacity is 30 A, and the selected fuse is 25 A.
The maximum PV current is 1.25 x 13.8 A = 17.25 A. The minimum string OCPD rating is 1.25 x 17.25 A = 21.56 A. Reverse fault current from the two other strings is 1.25 x 2 x 13.8 A = 34.5 A, so string fusing is required because that exceeds the 30 A module and conductor limits. The permitted fuse range is 21.56 A to 30 A, and a 25 A fuse passes with 5 A of headroom to the ceiling.
Common questions
Why is solar fuse sizing based on 1.56 times Isc?
The calculation first treats PV source current as continuous by multiplying module Isc by 1.25. It then sizes the overcurrent device at 125% of that maximum current. Multiplying 1.25 by 1.25 gives 1.5625 times Isc.
When do parallel solar strings need individual fuses?
They need individual protection when possible reverse current into a faulted string can exceed the module maximum series fuse rating or the corrected conductor ampacity. One or two strings may qualify for an exception when reverse current stays within both limits. Other-source backfeed can change that answer.
Where is the module maximum series fuse rating listed?
It is listed on the module label and in the module datasheet. Use that exact value for the module model being installed. Do not substitute the fuse rating used on a different module unless the datasheet gives the same limit.
Can I use a breaker instead of a PV fuse?
A listed DC photovoltaic breaker can be used when the equipment instructions and code rules allow it. The breaker still has to meet the same current limit, voltage rating, interrupting rating, and installation requirements. This calculator checks only the current rating window.
Why must corrected wire ampacity limit the fuse size?
The overcurrent device protects the circuit conductors as well as the module string. If temperature, grouping, rooftop exposure, terminals, or other factors reduce conductor ampacity, the fuse cannot be larger than the corrected ampacity. Enter the already-corrected value, not the base table ampacity.
How does battery backfeed change the calculation?
Battery systems, converters, and other sources can feed current into a faulted PV string even when there are few parallel strings. Add that contribution after any applicable multipliers. If the total reverse current exceeds a module or conductor limit, string protection is required.