I am looking into how one sizes the conductors from a motor controller (i.e a contactor or VFD) to a motor. I have seen this reasoning used on the MH site:
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But, I question the logic behind the note. 240.4 states that conductors "shall be protected against overcurrent in accordance with thier ampacities" and this 10 AWG would not meet this, as it has a 60A branch-circuit protector upstream. How is this not a violation of 240.4? Also where in the NEC does it ever state that an overload may be used for overcurrent protection?
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Q5. What size branch-circuit conductor and short-circuit/ground-fault protection is required for a 7 1/2-hp 230V, three-phase motor?
A5. Branch-circuit conductors are sized as 125% of the motor FLC [430.22 and Table 430.150] 22A x 1.25 = 27.5A, 10 AWG, rated 30A at 60?C [110.14(C) and Table 310.16].
The branch-circuit protection is sized in accordance with 240.6(A), 430.52(C)(1) Ex. 1, and Table 430.150. For an inverse-time breaker: 22 A x 2.5 = 55A, next size up = 60A
Note: The 10 AWG conductors are protected against overcurrent by the overload protection devices, which are sized between 115-125 percent of motor nameplate current rating [430.32].
A5. Branch-circuit conductors are sized as 125% of the motor FLC [430.22 and Table 430.150] 22A x 1.25 = 27.5A, 10 AWG, rated 30A at 60?C [110.14(C) and Table 310.16].
The branch-circuit protection is sized in accordance with 240.6(A), 430.52(C)(1) Ex. 1, and Table 430.150. For an inverse-time breaker: 22 A x 2.5 = 55A, next size up = 60A
Note: The 10 AWG conductors are protected against overcurrent by the overload protection devices, which are sized between 115-125 percent of motor nameplate current rating [430.32].
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But, I question the logic behind the note. 240.4 states that conductors "shall be protected against overcurrent in accordance with thier ampacities" and this 10 AWG would not meet this, as it has a 60A branch-circuit protector upstream. How is this not a violation of 240.4? Also where in the NEC does it ever state that an overload may be used for overcurrent protection?
