Breaker Size Calculator
// SIZE A CIRCUIT BREAKER AND MATCHING CONDUCTOR FROM LOAD, MATERIAL, TERMINAL RATING, AMBIENT TEMPERATURE, AND DERATING INPUTS //
Loads running 3 hours or more, in amps.
Intermittent or short-duration load, in amps.
Degrees C.
Next standard size at or above required load.
copper, 75C column
The selected conductor passes the calculated load and breaker screen after the entered ambient and conductor-count derating factors.
The load basis is 125% of continuous load plus 100% of non-continuous load.
Conductor ampacity uses common NEC Table 310.16 values and a simplified adjustment workflow.
Small-conductor overcurrent limits are applied for 14, 12, and 10 AWG where applicable.
Confirm terminal temperature ratings, insulation type, ambient correction, and local amendments.
Motor, HVAC, welders, transformers, and equipment-nameplate circuits may require dedicated rules.
Voltage drop is not a safety ampacity rule, but it can still require upsizing on long runs.
Use this as a first-pass breaker and conductor screen, then validate the installation details.
For feeders and branch circuits, review conduit fill, voltage drop, grounding, and equipment instructions before purchase or installation.
Check whether the selected breaker and conductor still meet voltage-drop targets.
Review detailed temperature and conductor-count derating.
Verify raceway fill after conductor size and count are known.
Run a broader cable-sizing pass for residential or commercial feeders.
Estimate resistance and power loss for the selected conductor.
Estimate material weight for feeders, long branch circuits, and reels.