What size fuse do I need for a solar panel string?

Per NEC 690.8, PV string fuses are sized at 156% of panel Isc (short-circuit current). For a panel with 8.5A Isc: 8.5 × 1.56 = 13.3A minimum, so use a 15A DC-rated fuse. The fuse must also be ≤ the wire's ampacity.

Can I use regular AC fuses in a solar DC system?

No — AC fuses are not rated for DC circuits and are a fire hazard. DC arcs don't self-extinguish like AC arcs, so a fuse that 'works' in AC may fail to interrupt a DC fault. Use DC-rated fuses (Midnite MNEPV, Littelfuse PV series) for all solar DC circuits.

Where should the battery main fuse be placed?

The battery main fuse (or ANL fuse) must be placed within 18 inches (450mm) of the battery's positive terminal. This protects all wiring in the system from a battery short circuit — the most catastrophic type of solar system fault.

Fuse Size Calculator

Size fuses correctly for PV strings, battery banks, and inverter circuits — with NEC 690 compliant recommendations.

Circuit details

Max circuit current?

Wire gauge (AWG)?

Circuit type?

Recommended fuse

35A Fuse

Minimum fuse size31.2A

Max (wire protection)35A

Fuse typeMidnite MNEPV or Littelfuse PV fuse (DC rated)

PV string fuses must be rated for DC voltage (not AC). Use DC-rated fuses only.

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How to Use This Calculator

Enter max circuit current and wire gauge

Input the maximum continuous current in amps and the AWG size of the wire being protected. The fuse must protect the wire — it must blow at a current below the wire's ampacity. If the fuse is larger than the wire can safely carry, the wire becomes the fuse (and a fire hazard).

Select circuit type

Different solar circuits use different NEC 690 sizing rules. PV string fuses use a 156% multiplier (125% × 125% as required by NEC 690.8). Battery and load circuits use 125%. The circuit type also determines the recommended fuse style — DC-rated fuses for PV strings, ANL for battery banks.

The fuse type matters as much as the rating

A standard AC fuse in a DC circuit is a fire hazard — DC arcs don't self-extinguish the way AC arcs do. Always use DC-rated fuses and breakers in any solar DC circuit. Look for a DC voltage rating equal to or greater than your system voltage.

The Formula

PV string fuse = Panel Isc × 1.25 × 1.25 = Isc × 1.56 (NEC 690.8A: 125% overcurrent capacity × 125% continuous) Battery / load / inverter fuse = Max current × 1.25 (standard NEC continuous load rule) Constraint: Fuse rating MUST be ≤ wire ampacity (the fuse protects the wire — if fuse > wire ampacity, wire is unprotected) Select next standard fuse size above calculated minimum, but below wire ampacity

The fundamental rule: the fuse protects the wire, not the load. A fuse sized to protect a load (say, exactly the load's rated watts) may be larger than the wire can safely carry. Always size the fuse to the wire first, then verify the wire is large enough for the load.

Example

PV string fuse — 300W panel with 10 AWG wire

A 300W panel has Isc of 8.5A. It's wired with 10 AWG wire (ampacity: 35A). What fuse is needed at the combiner box?

Panel Isc8.5A

Fuse minimum (Isc × 1.56)8.5 × 1.56 = 13.3A

Wire ampacity constraint10 AWG = 35A max

Valid range13.3A – 35A

Result

Recommended fuse15A (next standard size above 13.3A)

Fuse typeMidnite MNEPV15 or Littelfuse PV (DC-rated)

A 15A DC-rated fuse is correct. An AC-rated 15A fuse in this circuit is a code violation and potentially dangerous — DC arcs at 48V+ won't extinguish in an AC fuse.

FAQ

Can I use a circuit breaker instead of a fuse? ▼

Yes, for most solar DC circuits, a DC-rated circuit breaker is a better choice than a fuse — it can be reset without replacement, serves double duty as a disconnect switch, and is generally more convenient. The same rules apply: must be DC-rated, voltage rating ≥ system voltage, current rating ≤ wire ampacity. Breakers suitable for solar DC include Midnite Solar breakers, Blue Sea Systems, and Schneider Electric. For high-current battery connections, ANL fuses or Class T fuses are often preferred over breakers due to faster trip times (important for battery short circuits).

Where exactly should I place the fuse in each circuit? ▼

Battery main fuse: within 18 inches (450mm) of the battery positive terminal — this protects all of the wire beyond that point. PV string fuse: at the combiner box or as close to the panel as practical — protects the string wires from fault current from other strings. Inverter fuse: at the battery bank, not at the inverter — the wire from battery to inverter is the high-risk section. Load circuit fuses: at the distribution bus, before the load. The general rule: fuses go at the power source end of the wire they protect.

Why do PV strings need a special 156% multiplier? ▼

NEC 690.8(A) applies two consecutive 125% factors to PV source circuits. The first 125% is the overcurrent protection rule (standard NEC requirement for continuous loads). The second 125% accounts for the fact that PV panels are a "continuous" power source — they can produce current at or near rated levels for extended periods. Multiplied together: 1.25 × 1.25 = 1.5625, rounded to 1.56×. So a panel with 8.5A Isc needs a fuse rated for at least 13.3A — even though the panel itself never produces that much current.