How to Fuse a Solar Panel Array
Built by Jeremy Panasuk. Published October 3, 2026.
The fuse rule on this page is the manufacturer's, from Canadian Solar's installation manual. A licensed electrician applies the adopted National Electrical Code, including Article 690, to the array you actually build. This page does not quote that code.
The module used for the arithmetic
Annex C of the Installation Manual of Standard Solar Modules (EN-Rev IM/GN-AM-EN/2.2, July 2020) lists the CS6A-220M at Standard Test Conditions. The manual defines those conditions as 1,000 W/m², an AM1.5 spectrum, and a cell temperature of 25°C. For that model the table gives Pmax 220 W, Vmp 24.8 V, Imp 8.87 A, Voc 30.6 V, Isc 9.31 A, and a maximum series fuse rating of 15.00 A. The manual says the electrical characteristics are within ±10% or [0, +5 W] of the indicated values of Isc, Voc, and Pmax.
It also says a module may produce more current or voltage than at STC. For component ratings and capacities, short-circuit current at STC should be multiplied by 1.25, and a temperature correction from the manual's Table 1 should be applied to open-circuit voltage. Depending on local regulations, an additional 1.25 multiplier on short-circuit current may apply when sizing conductors and fuses. The manual states that extra step as a total multiplier of 1.56.
| Check | Arithmetic | Result |
|---|---|---|
| One string, Isc × 1.25 | 9.31 × 1.25 | 11.64 A |
| One string, if the 1.56 multiplier applies | 9.31 × 1.56 | 14.52 A |
| Maximum series fuse on this model | Annex C | 15.00 A |
| Reverse current from two other strings | 9.31 × 2 | 18.62 A |
11.64 A and 14.52 A both sit under the 15.00 A fuse rating. 18.62 A does not. That is the three-string case: if one string is faulted, the other two can feed their short-circuit current back into it, and 18.62 A is above the module maximum fuse rating the manual says the reverse current must stay under. Two strings are the manual's limit for a parallel connection with no series OCPD in each string. The third string is where the manual requires a certified overcurrent device in series in each string.
Four modules are not three strings
Run Battle Born's 2,400 Wh through sizeSystem at 4 sun hours: 2,400 ÷ 4 ÷ 0.75 = 800 W. Divide that array by this module's 220 W nameplate: 800 ÷ 220 = 3.64, so the count rounds up to 4 modules, 880 W of nameplate. Four modules wired as one series string are one string. The manual's OCPD sentence is about how many strings you parallel, not about how many modules sit inside a string. A single string does not trigger the "three or more strings" line.
The charge-controller size is a different current. sizeController divides array watts by battery volts by 0.85. For the 880 W nameplate at 24 V that is 880 ÷ 24 ÷ 0.85 = 43.1 A, and the listed controller size is 60 A. That 60 A is the calculator's controller pick. It is not the 15.00 A maximum series fuse, and it is not a substitute for the overcurrent device the manual requires once three strings share a parallel connection.
The manual also says only modules with similar electrical outputs should be connected in the same string, and that series connections must stay at or below the maximum permitted system voltage on the nameplate, including the cold open-circuit voltage from Table 1. This page does not copy a maximum system voltage off a different model.
One common mistake
Buying one fuse because the array has several modules, or skipping the fuse because each module is under 15 A. The manual's test is the number of parallel strings and the reverse current those other strings can push. Two strings of this module can source 18.62 A into a third. That is the figure to put next to the 15.00 A maximum series fuse rating, and it is why the manual draws the line at two strings without a series OCPD.
Limits
Canadian Solar marks the manual for professional use only. It says that if the disconnects and overcurrent devices cannot be opened or the inverter cannot be powered down, cover the fronts of the modules with an opaque material before working on a module or the wiring. Do not connect or disconnect modules when current is present. A licensed electrician applies the electrical code adopted in your jurisdiction, including NEC Article 690 where that article is what the authority having jurisdiction enforces. Fuse ratings on a different module's label replace the 15.00 A used here. The controller math for a battery bank is on panels to charge a battery, and the module count is on how many solar panels.
FAQ
When does the Canadian Solar manual require a series overcurrent device in each string?
Two strings may be paralleled without an overcurrent protection device in series in each string. Three or more strings require an appropriate, certified overcurrent protection device in series within each string, and the reverse current of any string has to stay below the module maximum fuse rating.
Why is 18.62 A the number next to the 15 A fuse?
The CS6A-220M lists Isc 9.31 A and a maximum series fuse rating of 15.00 A. Two other strings at that Isc are 9.31 × 2 = 18.62 A, which is above 15.00 A. One string at Isc × 1.25 is 11.64 A, and at the manual's 1.56 multiplier it is 14.52 A.
Does a 60 A charge controller replace the series fuse?
No. Four CS6A-220M modules are 880 W. At 24 V the calculator's controller current is 880 ÷ 24 ÷ 0.85 = 43.1 A, so it lists a 60 A controller. That is sizeController. The 15.00 A figure is the module's maximum series fuse rating.
Compiled with AI assistance from the sources below. Spot an error? Email [email protected].
Sources
- Canadian Solar, Installation Manual of Standard Solar Modules, EN-Rev IM/GN-AM-EN/2.2, July 2020 — accessed Oct 3, 2026.
- Battle Born Batteries, How to Size a Deep Cycle Battery Bank, May 22, 2026 — accessed Oct 3, 2026.
- SolarSizer calculator, assets/app.js (sizeSystem and sizeController) — accessed Oct 3, 2026.