Solar Fuse Size Selector

Calculate NEC 690.8 compliant DC fuse ratings for PV string circuits.

PV Array Inputs

Recommended Fuse

Fuse Rating — A
Temperature-corrected Isc— A
Minimum Fuse (Isc × SF)— A
Backfeed Minimum (N-1 × Isc × SF)— A
Maximum Allowed— A
Fuse Required on Each String?—

How it's calculated

  1. Isc_adj = Isc × (1 + coeff × (T_min − 25))
  2. Min fuse = Isc_adj × safety factor (NEC 1.56)
  3. Backfeed min = (N_parallel − 1) × Isc_adj × safety factor
  4. Take the larger of the two minimums
  5. Round up to the next standard fuse size ≤ Max OCPD

Frequently Asked Questions

How do I calculate the correct fuse size for a solar PV string?

Multiply the panel's Isc by 1.56 per NEC 690.8(B) for continuous loads, apply cold-temperature correction, then round up to the next standard fuse rating.

When do I need fuses on parallel solar strings?

NEC 690.9 requires fuses on each string when three or more strings are in parallel. With two strings, fuses are required only if backfeed current exceeds the panel's max series OCPD rating.

What is the maximum fuse rating allowed for a solar panel?

The panel nameplate specifies the 'Max Series Overcurrent Device Rating' — typically 15A–20A for residential modules, 30A–60A for commercial ones. Never exceed this value.