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.
Heavy Protection
Inverter Overcurrent Protection Sizing Tool
Calculates continuous sizing basis and AIC fault ratings for heavy DC inverters, comparing Class T, MRBF, and ANL fuse families.
Hardware
DC Busbar Ampacity & Sizing Tool
Determines continuous ampacity ratings, stud thread sizes (M6, M8, M10), and torque specifications for central DC distribution blocks.
Conductor Protection
DC Fuse Ampacity Rating Sizer
Sizes branch circuit fuses to protect wire insulation from thermal damage while avoiding nuisance tripping under continuous loads.
Disconnect
Main Battery Disconnect Switch Sizer
Evaluates continuous and cranking amp requirements for master battery cutoff switches and dual-bank selector switches.
Monitoring
DC Shunt Sizing & Wiring Tool
Sizes battery monitor shunts for full-scale current rating, signal millivolts (50mV/75mV), and proper negative busbar placement.
Solar Protection
Solar Combiner Box & String Fuse Sizer
Applies NEC 690.9 rules to determine when series fuses are required for multi-string parallel solar arrays.
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.