Two requirements, not one
An overcurrent device has to satisfy two separate things at once, and most sizing guides only cover the first.
It must be large enough to carry the circuit. NEC 690.9(B) for PV circuits and NEC 210.20(A) for branch circuits both require at least 125 % of the maximum or continuous current. A device sized exactly to the load nuisance-trips.
It must be small enough to protect the conductor. NEC 240.4 requires the conductor to be protected in accordance with its ampacity. Two qualifications matter:
- 240.4(B) permits rounding up to the next standard rating where the conductor’s ampacity does not itself land on one, for devices up to 800 A.
- 240.4(D) caps the device on 14, 12 and 10 AWG regardless — 15 A, 20 A and 30 A for copper.
Those two requirements can conflict. A 20 A continuous load needs a 25 A device, and 12 AWG copper carries 25 A at 75 °C — but 240.4(D) caps its device at 20 A, so that pairing is illegal and you need 10 AWG. The calculator refuses the combination rather than quietly returning a number.
The module series fuse rating
This is the check that most solar fuse calculators leave out entirely, and it can only bite you in one direction: too large.
Every module datasheet carries a maximum series fuse rating, typically between 15 and 30 A. It exists because the module’s own conductors and bypass diodes can only survive so much fault current back-fed from the other parallel strings. NEC 690.9(B) tells you the smallest device permitted; the module tells you the largest. If the two ranges do not overlap, the array needs rewiring — fewer strings per fuse, or a fuse per string — not a bigger fuse.
Enter it above whenever you are sizing a PV source circuit. The calculator will confirm the device fits inside it, or refuse and explain why.
Standard device ratings
NEC 240.6(A) lists the standard ampere ratings. You round up to one of these, never down and never to an arbitrary value:
15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200, 225, 250, 300, 350, 400, 450, 500, 600 A — plus the additional standard fuse ratings of 1, 3, 6, 10 and 601 A.
Fuse types used in solar
| Type | Where | Note |
|---|---|---|
| Class T | Battery to inverter | High interrupting rating, typically 20 kA. The usual choice on lithium banks, which can deliver enormous fault current. |
| ANL / MEGA | Battery and high-current DC | Common in RV and marine work. Lower interrupting rating than Class T — check it against your bank. |
| MidiVal / MIDI | Branch circuits up to about 200 A | Compact, widely available. |
| Inline blade | Small DC branch circuits | Fine for lighting and pumps at modest current. |
| PV string fuse (gPV) | Combiner boxes | Listed specifically for PV, rated for DC and for the string voltage. |
| DC-rated breaker | Anywhere a switch is useful | Must be listed for DC at your circuit voltage. Doubles as a disconnect. |
Interrupting rating matters as much as the ampere rating. A device has to be able to break the fault current available at that point without failing. A lithium bank can deliver several thousand amps into a dead short, which is beyond what many inexpensive fuses can safely interrupt. That figure is on the device datasheet, and this calculator does not compute available fault current — it is a system-specific number that depends on your battery and your cable.
Where devices go
At the source of fault current, as close as practical. On a battery circuit that is the battery positive terminal. On a PV circuit it is the combiner box. The point is that the device protects everything downstream of it, so anything upstream of the device is unprotected cable.
A common and dangerous shortcut is fusing at the inverter or the load end. That leaves the entire run exposed to a short to the chassis or the enclosure, which in a metal-bodied vehicle or a metal enclosure is the most likely fault of all.