Capacity questions


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Old 08-02-09, 01:36 PM
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Capacity questions

I have been referencing UGLY's Electrical References. My question is about for motors to start the motor and motor circuit conductors are allowed to be protected by circuit breakers at values that are higher than the actual motor and conductor ampere ratings. What protects the conductor if the current exceeds the conductors capacity?

Is it O.K to reduce the wire size of a circuit at a junction box? The reason being I will have 10 conductors in a conduit and the capacity of the wire has to be reduced by 50 percent. After the junction box I will split up the circuits so I won't need that big of wire.
#10 THWN-2 is rated for 40 amps, but it says unless specifically permitted in 240.4(E) or 240.4(G) the overcurrent protection shall not exceed 30 amps for #10. What does that mean, what is in 240.4?
 
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Old 08-02-09, 02:25 PM
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I am not that good with motor circuits but I will give it a shot.

Originally Posted by scotty18
I have been referencing UGLY's Electrical References. My question is about for motors to start the motor and motor circuit conductors are allowed to be protected by circuit breakers at values that are higher than the actual motor and conductor ampere ratings. What protects the conductor if the current exceeds the conductors capacity?
Really nothing other than the overload on the motor. There is still protection of short circuit. The reason is the motor draws so much more current at startup that a "standard" breaker would trip. You can go up to 250% of the full load current (not the overcurrent rating of the circuit) of the motor.


Originally Posted by scotty18
Is it O.K to reduce the wire size of a circuit at a junction box? The reason being I will have 10 conductors in a conduit and the capacity of the wire has to be reduced by 50 percent. After the junction box I will split up the circuits so I won't need that big of wire.
#10 THWN-2 is rated for 40 amps, but it says unless specifically permitted in 240.4(E) or 240.4(G) the overcurrent protection shall not exceed 30 amps for #10. What does that mean, what is in 240.4?
240.4 is the Protection of Conductors section. In subsection (D) stated that you may not go over 30 amps for #10 wire. There is also an exception there for sections 240.4(E)(G) Subsection (E) and (G) basically negates subsection (D) for special purposes like single motor taps (E) and specific conductor applications (G)
 
 

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