Q
FLC vs. FLA -- the distinction that drives every motor calculation
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A
FLC (Full-Load Current, from NEC Tables 430.247-430.250) is used for ALL circuit/conductor/protection sizing calculations. FLA (nameplate Full-Load Amperes) is used ONLY for sizing overload protection.
NEC 430.6(A)
Q
The four layers of motor circuit protection
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A
1) Overload protection (protects the motor from damage), 2) Branch-circuit short-circuit/ground-fault protection, 3) Motor-circuit conductors, 4) Motor controller and disconnecting means -- each sized by a different rule.
NEC Art. 430, Parts III-IX
Q
Motor branch-circuit short-circuit/ground-fault protection sizing
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A
A percentage of FLC from Table 430.52, varying by device type -- e.g. up to 250% for an inverse-time circuit breaker on a general-purpose motor (with permitted increases if the initial value doesn't allow starting).
NEC 430.52, Table 430.52
Q
Disconnect location relative to the motor
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A
Within sight from the motor and driven machinery (unless the disconnect is capable of being locked in the open position and specific conditions are met).
NEC 430.102(B)
Q
Motor control circuit conductor protection
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A
Motor control circuit conductors (e.g. to a start/stop station) require their own overcurrent protection, sized per 430.72, separate from the motor's power circuit protection.
NEC 430.72
Q
Locked-rotor current vs. full-load current
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A
Locked-rotor current (LRC) is the current drawn the instant a motor starts, before it reaches speed -- much higher than FLC, and it's what horsepower-rated disconnects and short-circuit protection must safely handle at startup.
NEC 430.7, 430.110
Q
Motor nameplate -- required marking
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A
Must include (among other items) the manufacturer's name, rated voltage/full-load current, rated frequency and number of phases, rated full-load speed, and rated temperature rise or insulation class.
NEC 430.7(A)
Q
Service factor and its effect on overload sizing
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A
A higher marked service factor (1.15 or greater) permits overload protection sized at a slightly higher percentage of FLA (115% vs. 125%), since the motor is designed to handle modest overload conditions without damage.
NEC 430.32(A)(1)
Q
Motor short-circuit protection -- when the standard percentage won't allow starting
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A
The next higher standard protective device size (or a specific increased percentage) is permitted if the value from Table 430.52 isn't sufficient for the motor to start.
NEC 430.52(C)(1), Exception 1-2