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💡 Interpretation
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🏆 Design Challenge
Size the conductor ampacity basis (430.24) and the maximum feeder SC/GF device (430.62) for the scenario currently configured in the sidebar. Grading uses the same engine as the Results tab.
Current Scenario
Add valid motors in the sidebar to generate a design challenge.
In scope
- Conductor ampacity basis for grouped Article 430 motors plus optional non-motor load, per NEC 430.24.
- Maximum feeder short-circuit / ground-fault protective device per NEC 430.62, using the 430.52 branch maximums.
- Table 310.16 conductor selection with 110.14(C) terminal limiting and optional 310.15 ambient / conductor-count adjustment.
⚠ Not supported / not a substitute
Two Separate Calculations
Motor feeders require two distinct calculations that answer different questions. The conductor ampacity basis (430.24) is 125% of the highest motor FLC + 100% of the other motor FLCs + the non-motor load. The feeder overcurrent device is a separate short-circuit / ground-fault calculation (430.62) that is normally much larger, because it must let motors start without opening.
Why Only One 25% Adder?
Per NEC 430.24, only the single highest-rated motor (by FLC, not by horsepower) receives the 125% factor; every other motor is added at 100%. On multi-motor feeders you do not multiply every motor by 125% — that would double-count starting demand that never happens simultaneously in the code's model. When two motors tie for largest, exactly one of them gets the adder.
Conductor Ampacity: Three Constraints, Not One
- Table 310.16 base ampacity from the insulation column you select (60/75/90°C).
- Corrections and adjustments — ambient temperature (Table 310.15(B)(1)) and more than three current-carrying conductors (Table 310.15(C)(1)) multiply that base value.
- Termination temperature limit (110.14(C)) — the final answer can never exceed the un-derated ampacity of the terminal's rated column. This is why the 90°C column is a derating basis only.
If the ambient exceeds the insulation's rating entirely, there is no permitted ampacity at all for that column — parallel conductors do not fix that; a higher-rated insulation or a lower ambient does.
Branch Maximums Feed the Feeder Maximum
NEC 430.62(A) sets the feeder SC/GF device at no more than the largest branch-circuit SC/GF device of any one motor in the group, plus the sum of the full-load currents of the other motors. The "largest branch device" is that motor's maximum permitted device per Table 430.52 including the 430.52(C)(1) Exception No. 1 next-size-up allowance. The feeder device itself gets no next-size-up: you take the largest 240.6(A) standard rating that does not exceed the calculated maximum.
Instantaneous-Trip Breakers Are Branch-Only
An instantaneous-trip (magnetic-only) breaker provides no overload-region protection and is permitted only as part of a listed combination motor controller on an individual motor branch circuit (430.52(C)(3)). It is not a feeder protective device, which is why this tool does not offer it as a feeder device type. Where the branch devices are instantaneous-trip breakers, 430.62(A) Exception No. 1 requires the feeder calculation to assume each such device is rated at the maximum percentage of Table 430.52 for the type of device actually used on the feeder.
Maximum, Not Minimum
430.62 gives a ceiling. It says nothing about whether a device is large enough to carry the load or to survive starting current. A 15 A breaker is technically "within" a 175 A maximum and is also completely unusable. The minimum side comes from 215.3 and the 430.63 combined-load rules, plus real starting behavior — and this tool does not evaluate it.
Common Mistakes
- Sizing the OCPD to the conductor load instead of using the 430.62 motor short-circuit method.
- Applying 125% to every motor — only the largest-FLC motor gets it.
- Using nameplate amps instead of Table 430.248/430.250 FLC (NEC 430.6(A)). Nameplate current is for overload sizing, not conductor and SC/GF sizing.
- Applying the adder to the largest horsepower instead of the largest FLC — different voltages make these different motors.
- Reading the 90°C column as the answer and ignoring the 110.14(C) terminal limit.
- Summing currents across different voltages or phases without a per-phase VA method. This tool blocks that outright.
- Using the breaker standard-size list for fuses, which overstates small-motor fuse maximums several-fold.