The string box: what is inside and when you need one

September 29, 2026 · 5 min read · Danilo Nascimento

See what a string box contains, when each string needs a fuse and how to check it with the module current, in a calculator with a complete example.

What the string box is

The string box (string combiner box) is the enclosure, near the panels or the inverter, where the DC strings meet before going on to the inverter. It gathers protections and the disconnection point of the DC side. It does not always join the strings in parallel: it can also keep separate circuits, one per inverter input.

Stringsin parallel
DC
String boxfuses, isolator and SPD
Inverter
Board

The string box sits between the strings and the inverter.

What is inside

Fuses per string

They protect each string against reverse current coming from the other strings in parallel. They must be fuses made for photovoltaic DC.

DC isolator

Safely disconnects the DC source from the inverter for maintenance. Many inverters already include one.

DC SPD

surges

Diverts overvoltages, such as those from nearby lightning, to earth, protecting the inverter.

Busbars and terminals

connection

They gather the positive and negative conductors firmly, without depending on improvised splices.

The enclosure

physical protection

It must have an ingress protection rating (IP) against water and dust, and sun (UV) resistance suited to the site.

When each string needs a fuse

The logic is that of reverse current. If one string fails in a short circuit, the others, in parallel, push current into it. If that current can exceed what the module withstands, the string needs a fuse. Choose the number of strings and the module data:

Interactive

When does a string need a fuse?

Choose how many strings are in parallel and the module data. The calculator shows the reverse current a string can receive and the usual fuse range.

Strings in parallel3Module IscAModule maximum series protectionA

28 Apossible reverse current

Neededprotection per string (illustrative)

21–25 Ausual fuse range

Calculation rule: reverse current = (n − 1) × Isc, compared with the module maximum; usual range from 1.5 × Isc up to the maximum. No extra factors: some standards apply them. Check the local standard and the datasheet. Illustrative check: it does not allow you to skip protection; cables, design current and installation also count.

The check, step by step

1
1. Read the datasheet
Note the short-circuit current (Isc) and the maximum series fuse rating given by the module manufacturer. Here: 14 A and 25 A.
2
2. Count the strings in parallel
With n strings, each one can receive current from the other n − 1.
3
3. Work out the reverse current
(n − 1) × Isc. With 3 strings: 2 × 14 = 28 A, above the module’s 25 A: a fuse is needed.
4
4. Choose the fuse
The usual range runs from 1.5 × Isc (21 A) up to the module maximum (25 A), with a voltage rating for photovoltaic DC above the system’s maximum voltage.

One, two or three strings

	1 string	2 strings	3 or more strings
Reverse current (Isc of 14 A)	0 A	14 A	28 A or more
Fuse per string (maximum of 25 A)	Not required in the simple calculation	Not required in the simple calculation	Needed
DC isolator	In the inverter or the box	In the inverter or the box	In the box, usually
String box	Optional	Depends on the inverter	Common

✓ not required in the simple calculation · ◐ depends on the case · ✗ needed

Connectors: use pairs from the same manufacturer and model, fully seated
Enclosure: ingress protection suited to the site and protection from the sun
Labelling: label each string and warn of the DC voltage hazard even with the inverter switched off
Earth: connect the enclosure and the SPDs to earthing, as in the article on grounding
Design: see also the protection of the PV system

Draw it in Editor Solar

This is Editor Solar itself, already loaded with the diagram from this article. Move the equipment, edit labels, connect new parts and export to PNG, SVG or PDF. It is free and runs right in your browser.

Three strings in parallel reach the string box, which gathers fuses, isolator and SPD ahead of the inverter.

Try it: Add a fourth string to the diagram and redo the reverse current. Does the fuse per string stay in the same range?

Ready-made templates: see all diagram templates

Open this diagram in the full editor ↗

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