Solar Fuse Size Calculator

How to use this calculator

Choose your circuit type — the multiplier is different for PV-side vs everything else:

1. PV string / module — fuse rated at 1.56 × Isc (CSA C22.1 Rule 64-200)
2. Battery → inverter / load — fuse rated at 1.25 × continuous current (CSA Rule 64-218)
3. Inverter DC input — fuse rated at 1.25 × maximum continuous input current
4. Charge controller output — fuse rated at 1.25 × rated continuous output

Standard fuse ratings in Canada (NEC-aligned plus selected IEC sizes): 1, 2, 3, 5, 6, 8, 10, 12, 15, 16, 20, 25, 30, 32, 35, 40, 45, 50, 60, 63, 80, 100, 125, 150, 175, 200 A. Round up always.

Why 1.56× on PV strings — the Canadian Electrical Code rationale

CSA C22.1 Section 64 (Renewable Energy Systems) mirrors NEC 690 in adopting the 156% factor for PV string overcurrent sizing. The two component factors:

• 125% for continuous-duty loading. A PV array running at full output for 3+ hours is, by definition, a continuous load. CSA Rule 14-104 requires overcurrent devices on continuous loads to be rated at least 125% of the load current.
• 125% for solar irradiance enhancement. Cloud-edge focusing and cold-temperature operation can push a module’s effective Isc 20–25% above the STC nameplate. The second 1.25 multiplier provides this headroom.

Multiplied: 1.25 × 1.25 = 1.5625, which the code rounds to 156%. A module rated 11.0 A Isc needs a fuse of at least 11.0 × 1.56 = 17.2 A → 20 A standard.

Where each fuse goes — typical Canadian residential install

A grid-tied PV system in Ontario, BC, Alberta or Quebec usually has these protected circuits:

1. DC combiner string fuses — per-string gPV fuses when 3+ strings parallel. Two-string installs typically don’t need them.
2. Array DC isolator with fuses — a switch-disconnector between the array and the inverter, often integrated with combiner-box fusing.
3. Battery → inverter fuse — Class T or DC-rated MCB sized at 1.25× the inverter’s continuous DC input current, within 450 mm of the battery terminal per CSA 64-218.
4. AC side breaker — in the main panel, sized per CSA Section 4 and the inverter’s continuous AC output.

Cold weather sets the PV-side fuse voltage rating. Use Appendix B of CSA Section 64 (or the module manufacturer’s Voc-temperature coefficient) to calculate Voc at the lowest expected ambient temperature for your location — Winnipeg’s -40 °C lows push string Voc considerably above STC.

Standard fuse ratings — round up to the next size

6, 10, 15, 16, 20, 25, 30, 32, 40, 50, 60, 63, 80, 100, 125, 150, 175, 200, 250 A

If the calculation says you need 17.2 A, install a 20 A fuse. If it says 138 A, install a 150 A fuse. Never round down.

Fuse types — gPV, Class T, and the Canadian market

Three fuse families cover virtually all residential and small-commercial PV in Canada:

• gPV (CSA C22.2 No. 248.19 / UL 248-19) — purpose-built PV-string fuse. 1000–1500 V DC rating. Standard for combiner boxes. Sold by Mersen, Bussmann, Littelfuse, ETI.
• Class T (CSA C22.2 No. 248.15 / UL 248-15) — high interrupt rating (20 kA at 160 V DC), fast-acting, the standard battery-to-inverter fuse for 12/24/48 V lithium banks.
• ANL / MIDI / MEGA — slower-blow, lower interrupt rating. Acceptable for small DC loads under 32 V — common in RV solar and marine, not appropriate for large lithium banks.

The calculator gives you amperage; you select fuse class based on system DC voltage and prospective short-circuit current.

Worked examples — Canadian residential

Example 1 — 7.5 kW system, 2 strings of 12 panels. Module Isc = 11.2 A.

Per-string ≥ 11.2 × 1.56 = 17.5 A → 20 A gPV per string in the combiner. With 2 strings the per-string fuse isn’t strictly required by CSA 64-202 — but most certified installers in Canada fit them as standard practice.

Example 2 — 5 kW Schneider XW Pro hybrid, 14 kWh LFP battery, 48 V.

Continuous DC = 5000 / (48 × 0.95) = 110 A Fuse ≥ 110 × 1.25 = 137 A → 150 A Class T near the battery terminal.

Example 3 — Rogue MPT-3024 charge controller, 30 A output, 24 V LiFePO4.

Fuse ≥ 30 × 1.25 = 37.5 A → 40 A Class T or DC-rated MCB between controller and battery.

Wire sizing has to keep up

A fuse only protects a cable that can carry the rated current after de-rating. CSA C22.1 Table 2 (in conduit) and Table 4 (free air) give the ampacities for each AWG size.

For a 20 A PV string fuse, 10 AWG copper PV-USE-2 cable (carries 40 A in free air) is comfortably oversized. For a 150 A battery fuse, 2/0 AWG is the minimum on short runs, 3/0 AWG for runs over 1 m. Verify both ampacity and voltage drop using the wire size calculator.

The CSA wiring tables apply de-rating factors for high ambient temperature, conduit fill, and direct-sun rooftop runs. Rooftop conduit can hit 70 °C in summer — Tables 5C and 5D add the temperature correction.

ESA / inspector commissioning — what gets checked

Each province has its own electrical safety authority — ESA in Ontario, Technical Safety BC, Hydro Québec, etc. — but the items they inspect on a PV install are universal:

1. Fuse current rating equals or exceeds 1.56 × Isc per string.
2. Fuse voltage rating equals or exceeds the cold-temperature corrected Voc.
3. Fuse breaking capacity equals or exceeds prospective short-circuit current.
4. Manufacturer’s maximum series fuse rating (on the module sticker) is not exceeded.
5. All fuses are CSA-certified (or CSA-recognized via the cTUVus / cULus marks).

A non-compliant fuse selection means a re-inspection — and on grid-tied systems, the local distribution company will not approve connection until the inspector signs off.

Limitations and disclaimer

This calculator implements the CSA C22.1 Section 64 / NEC 690.9 sizing rules. It returns the next standard North American fuse rating. Always confirm the result against the module datasheet’s maximum series fuse rating and the inverter manufacturer’s specified DC input overcurrent protection.

In Canada, all DC overcurrent protection on grid-tied PV and battery systems must be installed by a licensed electrician under the supervision of the provincial electrical safety authority. Off-grid cabin installs in some provinces have a permit exemption — check your local AHJ before assuming DIY work is permitted.