#10

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paul renshaw

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If it is on the supply side of the VFD the conductors and overcurrent protection are based on the VFD rated input current and the OCPD must not exceed the ratings of the conductors.
So the sections in 430 that let you size differently than what 240 states, do not apply to this situation?
 

Ingenieur

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So the sections in 430 that let you size differently than what 240 states, do not apply to this situation?

imo it would apply since it is the feeder portion of the motor circuit and governed by 430 (the VFD is the controller)
so you could size the conductor at 1.25 x FLC
and the OCP at 250%

all 2011
Table 310.15(B)(16) lists #10 as 35 A at 75C
a ** footnote says to reference 240.4(D) for OCP limitations
it says 30 A OCP unless an exception exists for specific conductors
240.4(G) ref table 240.4(G) motor and motor control circuits are excepted and governed by 430 Parts II, IV, V, VI and VII
Part II conductor sizing
Part IV branch OCP
part V feeder OCP protection

it would seem the OCP can be sized per parts IV and V (IT CB basically 250% x flc
and the conductor per part II, basically 125% of the motor or vfd flc

without knowing the flc a 40 CB and #10 would seem fine for a flc <40/2.5 = 16 A
conductor 1.25 x 16 = 20 A < 35 A #10
so if flc <+ 16 A good to go
 

luckylerado

Senior Member
So the sections in 430 that let you size differently than what 240 states, do not apply to this situation?
I do not think the exceptions apply because the limiting factor in this situation is not OCP it is the wiring method.

334.80
Ampacity
The ampacity of Types NM, NMC, and NMS cable shall be
determined in accordance with 310.15. The allowable ampacity
shall not exceed that of a 60°C (140°F) rated conductor.
 

Ingenieur

Senior Member
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I do not think the exceptions apply because the limiting factor in this situation is not OCP it is the wiring method.

334.80
Ampacity
The ampacity of Types NM, NMC, and NMS cable shall be
determined in accordance with 310.15. The allowable ampacity
shall not exceed that of a 60°C (140°F) rated conductor.

that makes sense
 
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