250
%
35
A
35
A
17.5
A
16.1
A
250
%
35
A
35
A
17.5
A
16.1
A
The motor circuit protection calculator determines the maximum overcurrent protective device (OCPD) rating for motor branch circuits per NEC Article 430. Motor circuit protection must satisfy two competing requirements: it must be large enough to allow motor starting (LRC = 5-8× FLC) without nuisance tripping, yet small enough to provide short-circuit and ground fault protection for the motor branch circuit conductors.
NEC Table 430.52 specifies maximum OCPD ratings as a percentage of FLC for different protection device types. For AC motors with inverse time circuit breakers: 250% of FLC. For dual-element (time-delay) fuses: 175%. For non-time-delay (fast-acting) fuses: 300%. For instantaneous trip (motor circuit protector) breakers: 800-1300% (special listed devices).
The logic behind these high percentages is that motors draw 5-8× FLC during starting (locked rotor current) for several seconds. Standard inverse time breakers at 125% would trip immediately on motor start. At 250% of FLC, the breaker allows the starting current to flow for the short starting period (typically 2-15 seconds) before thermal memory causes a trip, while still protecting against sustained overcurrents from short circuits.
Dual-element fuses (250% maximum instead of 175% — wait, 175% is actually lower than breakers) are more sensitive to overloads while still allowing starting. Their two-element design provides both fast-blow short circuit protection and time-delay overload capability. They are the most selective protection option for motor circuits, often used in industrial applications where nuisance tripping must be minimized.
If the calculated maximum OCPD doesn't allow the motor to start (particularly for high-inertia loads), NEC 430.52(C)(1) Exception No. 2 allows increasing the rating up to 400% of FLC for dual-element fuses and 600% for circuit breakers, provided the motor is not damaged thereby and the branch circuit conductors are protected by the rating of the feeder overcurrent device or a separate conductor protection device.
The disconnecting means (motor disconnect switch) must be rated at least 115% of FLC per NEC 430.110 for AC motors, and conductors at least 125% of FLC per NEC 430.22. This calculator provides all four values: OCPD type and maximum rating, standard OCPD size, minimum conductor ampacity, and minimum disconnect rating.
Max OCPD = FLC × table percentage. NEC Table 430.52 percentages: inverse time CB = 250%, dual-element fuse = 175%, non-time-delay fuse = 300%. Standard OCPD size = next standard fuse/breaker size at or below max OCPD. Conductor ampacity = FLC × 1.25. Disconnect rating = FLC × 1.15.
If the motor fails to start with the calculated OCPD rating, NEC permits higher ratings (up to 400% dual-element fuse, 600% inverse time CB) when properly documented and approved. Overload relay settings (typically 115% of nameplate FLA) are separate from the OCPD and provide running overload protection that the OCPD does not.
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Results
25 HP/460V: FLC = 34A (NEC Table 430.250). Max CB = 85A → use 80A standard. Conductors ≥ 42.5A → AWG 8. Disconnect ≥ 39.1A → 60A minimum rated.
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Results
Same 25 HP motor with dual-element fuses: max fuse = 59.5A → use 50A dual-element fuse. Lower maximum than CB provides better overload coordination.
OCPD (breaker/fuse) protects conductors from short circuits — it is rated high (175-300% FLC) to allow motor starting. Overload relay protects the motor from prolonged overloads — set at 115-125% of nameplate FLA. Both protections are required per NEC 430.
NEC Table 430.52 specifies maximum OCPD ratings as a percentage of motor FLC for different device types and motor types: AC motors with inverse time CB = 250%, dual-element fuse = 175%, non-time-delay fuse = 300%, instant-trip (MCP) = 800%.
HACR (Heating, Air Conditioning, Refrigeration) breakers are inverse time circuit breakers specifically listed for motor circuits. They have the same 250% maximum rating as standard inverse time breakers and are marked as suitable for continuous motor loads.
An MCP is an instantaneous-trip-only circuit breaker specifically listed for motor branch circuit short-circuit protection. It provides no overload protection. MCPs are used with separate overload relays and can be set as high as 800-1300% of FLC. They are common in MCC (Motor Control Center) designs.
Per NEC 430.52(C)(1) Exception 2, if the maximum OCPD is insufficient to allow the motor to start, you may increase to: 400% for dual-element fuses, 400% for non-time-delay fuses (above 100A), 600% for inverse time breakers. Document the exception in the design.
No — the OCPD (breaker or fuse) sized per NEC 430.52 provides short-circuit and ground fault protection only, not running overload protection. Separate thermal overload relays in the motor starter provide running overload protection per NEC 430.32.
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