Inverter Overcurrent Protection Sizing Tool
Calculates the continuous protection sizing basis (125% rule) for heavy DC inverter circuits and evaluates prospective fault current interrupt ratings (AIC) across fuse families.
Enter the values above and click Calculate to view recommendations and engineering specifications.
Fuse Family Interrupt Capacity (AIC) Comparison
Calculation Assumptions
- Overcurrent protection sized to protect the supply cable while clearing sustained continuous inverter draw.
- Ampere Interrupting Capacity (AIC) evaluated against prospective battery short-circuit currents.
Engineering Limitations
- High-voltage DC systems (>48V nominal) require DC-rated fast-acting fuses (e.g. Class T); standard automotive mega fuses are unsafe above 32V DC.
A 2000W 12V inverter pulling 196A continuous through 2/0 AWG cable requires a 250A Class T fuse (20,000A AIC) placed within 7 inches of the battery terminal per ABYC E-11.
An overcurrent device has two distinct ratings: its continuous ampere rating (e.g. 300A) and its Ampere Interrupting Capacity (AIC). The AIC rating defines the maximum instantaneous fault current the fuse can extinguish without physically rupturing, exploding, or arcing over.
| Fuse Class | Typical AIC Rating | Max DC Voltage | Engineering Assessment |
|---|---|---|---|
| Class T | 20,000 A AIC | 160V DC | Ultra-fast acting; recommended for large lithium banks where short circuits exceed 10,000A. |
| MRBF | 10,000 A AIC | 58V DC | Mounts directly to battery post; suitable for compact single/dual lithium batteries or AGM banks. |
| ANL | 6,000 A AIC | 32V / 80V DC | Common in car audio and budget RV setups; acceptable for lead-acid banks with limited short-circuit current. |
| MIDI / AMI | 5,000 A AIC | 32V DC | Limited to ≤ 200A; suitable for DC-DC chargers and branch distribution. |
Frequently Asked Questions
What size fuse do I need for a 2000W or 3000W inverter?
For a 12V 2000W inverter, peak low-voltage current reaches 224A, requiring a 250A to 300A fuse. For a 12V 3000W inverter, peak current reaches 335A, requiring a 350A to 400A fuse. For 24V systems, fuse ratings are halved (150A for 2000W, 200A for 3000W), and for 48V systems, ratings are quartered (80A for 2000W, 100A for 3000W).
Why is a Class T fuse required for lithium (LiFePO4) battery banks?
Lithium batteries have extremely low internal resistance and can deliver instantaneous short-circuit fault currents exceeding 10,000 to 20,000 Amperes. Standard automotive fuses (ANL, MIDI, MEGA) have interrupt ratings (AIC) of only 2,000A to 6,000A and can violently vaporize or sustain an internal plasma arc during a dead short. Class T fuses provide a 20,000A AIC rating at up to 160V DC, extinguishing faults in milliseconds.
How close should the main inverter fuse be to the battery?
Under ABYC E-11 standards, overcurrent protection must be placed within 7 inches of the battery terminal if the conductor is un-sheathed, or within 40 inches if the conductor is fully enclosed in flame-retardant protective conduit. In campervans and off-grid power walls, placing the fuse within 7 to 12 inches minimizes the length of unprotected high-current cable.
What happens if an inverter fuse is sized too large?
The primary role of a fuse is to protect the conductor insulation from overheating, melting, and igniting during overloads or short circuits. If the fuse rating exceeds the thermal ampacity of the connected cable, the wire will melt and create a catastrophic fire hazard before the fuse element blows.