Branch conductor sizing clarified

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cajun62234

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Correct sizing of branch circuit conductor:
Example: 20hp/480v/3ph motor = 27A.
There is a dispute of sizing: a EE wants the OCP to be sized at 175%, but the conductors sized at 125%; stating that the starter OLs protect the motor, and therefore the conductors need only be sized at 125%. My interpetation of the code is that the conductors need to be rated equal to the rating of the OCP, otherwise the conductors are technically a 'fuse', if rated less. The fact that the OLs protect the motor doesn't have anything to do with the OCP not being able to protect the conductors.
Your opinions please.
 
The EE is correct. This is a motor circuit and the overload protection of the conductors is provided by the motor overload protective device. The branch circuit OCPD provides the short circuit and ground fault protection. In this application the code would permit the branch circuit breaker to be rated at 70 amps with #10 copper conductors. If the protection is a fuse it could be rated at 50 amps with #10 conductors. See Part IV of Article 430.
 
The EE is correct provided that the motor is the only load connected to the circuit. The "OCP" for a motor branch circuit is for short circuit and ground fault protection only. The OLs provide overcurrent protection for the motor as well as the motor branch circuit. The motor is the only thing that can overload the circuit.
 
cajun62234 said:
The fact that the OLs protect the motor doesn't have anything to do with the OCP not being able to protect the conductors.
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Actually it has everything to do with it.

As the motor is the only load on the circuit and it is protected by the OLs it is imposable to overload the circuit conductors other then during short circuits or groundfaults.

The 'over-sized' breaker can in fact protect the conductors from overload during short circuits or ground faults and the motor Ols prevent a long term overcurrent condition.

The EE is within the NEC rules and it is also safe.
 
O.K.... I can accept it.... but why is it necessasary to size the OCP at 175% if the conductors are o.k. at 125%??.. why not size the OCP at 125% also?
 
cajun62234 said:
O.K.... I can accept it.... but why is it necessasary to size the OCP at 175% if the conductors are o.k. at 125%??.. why not size the OCP at 125% also?
Click to expand...
The short answer is, the OCP is sized to let the motor start.

Roger
 
cajun62234 said:
O.K.... I can accept it.... but why is it necessary to size the OCP at 175% if the conductors are o.k. at 125%??.. why not size the OCP at 125% also?
Click to expand...


If you look at`table 430.52 you'll see that there are instances where you could use an instantaneous trip breaker and set it at 800% of the FLC of the motor.
 
Cajun, to follow the code on this, start with Table 310.16's asterisked note to see 240.4(D) (which applies even without 310.16), which takes you to Table 240.4(G), which in turn directs you to see 430, parts III thru VII.
 
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