DC Overcurrent Protection & Distribution Calculators

Engineered tools for calculating continuous load protection sizing bases (125% rule), conductor protection ceilings, busbar ampacity, and fault current interrupt ratings.

Size Marine Rated Battery Fuses (30A to 300A, 10,000A AIC) mounted directly to 3/8" battery terminal posts to satisfy the ABYC 7-inch overcurrent rule.

AIC Fault Sizing

Class T vs ANL Fuse Fault Current Sizer

Calculate prospective short-circuit fault currents from LiFePO4 battery banks and determine when 20,000A AIC Class T fuses are mandatory over ANL fuses.

Busbar Hardware

Marine Busbar Terminal Stud Count Sizer

Determine required busbar stud count and spacing under the ABYC 4-lug-per-stud stacking rule to prevent terminal overheating and high resistance.

Arc Suppression

DC Disconnect Switch Arc-Rating Checker

Verify master battery switch and solar DC isolator voltage and DC interrupt ratings to prevent sustained plasma arc-over when opened under load.

Fault Energy

Class T vs ANL Fuse Blow Curve Comparator

Compare sub-2ms vs 20ms+ clearing times, let-through I²t energy, and cable adiabatic withstand limits (k²S²) under LiFePO4 dead shorts.

Solar Reverse Flow

MPPT Backfeed & PV Blocking Diode Sizer

Size blocking diodes and ideal diode controllers: breakdown voltage (VRRM), 125% continuous rating, and heatsink thermal resistance.

Vapor Safety

Marine Ignition-Protected Breaker Checker

Verify SAE J1171 / UL 1500 ignition compliance, 50°C engine room thermal derating, and AIC interrupt capacity under ABYC E-11.

Lithium BMS

Lithium BMS MOSFET vs Contactor Fault Sizer

Evaluate conduction losses, parasitic coil vampire drain, inductive flyback energy, and short-circuit silicon weld risks on lithium banks.

Ground Fault

DC Ground Fault Detector Interrupter (GFDI) Sizer

Calculate minimum insulation resistance (1,000 Ω/V), leakage current, and stray current electrolytic hull corrosion under NEC 690.41 and ABYC E-11.

Circuit Protection Engineering: Ampacity, AIC & Nuisance Prevention

In low-voltage DC power systems, overcurrent protective devices (fuses and circuit breakers) exist to protect conductor insulation from thermal destruction and catastrophic fire:

Ampere Interrupting Capacity (AIC / Breaking Capacity): A fuse's ampere rating specifies continuous carrying capacity, but its AIC rating dictates the maximum short-circuit fault current it can safely extinguish without exploding or sustaining an electrical arc. Modern LiFePO4 battery banks possess internal resistances low enough to deliver instantaneous dead-short currents exceeding 10,000A to 20,000A. Standard automotive blade fuses (1,000A AIC) or budget circuit breakers will arc-over or vaporize. High-capacity battery mains require Class T fuses (20,000A AIC) or Marine-Rated Battery Fuses (MRBF, 10,000A AIC).
The Continuous 125% Sizing Rule: Under NEC 210.20 and ABYC E-11, overcurrent protection feeding continuous loads (loads operating for 3 or more hours) must be sized at a minimum of 125% of the operating current to prevent nuisance thermal tripping caused by internal heating of the fuse element.
Physical Placement & Proximity Rules: Fuses must be positioned as close to the power source as physically possible. ABYC E-11 requires ungrounded conductors to be protected within 7 inches of the battery terminal (or up to 40 inches if the cable is enclosed in protective sheathing or conduit; 72 inches for starter feeds). Unfused battery cables represent a catastrophic fire risk if pinched or abraded.
Busbar Ampacity & Terminal Sizing: Central DC distribution busbars must be rated to carry the combined continuous load of all connected sub-circuits. Stud sizes (M6, M8, M10 / 1/4", 5/16", 3/8") must match cable lug eyelets to ensure maximum contact surface area and minimize joint resistance.