How breaker trip

[elevate]

240-21 (5) Outside feeders taps do not need OCP as long as the conductors terminate in a correctly sized breaker no matter how long.
What will cause the breaker at the end of the tap wire to trip if the short happens before the breaker? Can the breaker installed in the middle trip protecting wire ahead of it and behind? will it apply on overhead spans of feeder also?

 

[templdl]

240-21 (5) Outside feeders taps do not need OCP as long as the conductors terminate in a correctly sized breaker no matter how long.
What will cause the breaker at the end of the tap wire to trip if the short happens before the breaker? Can the breaker installed in the middle trip protecting wire ahead of it and behind? will it apply on overhead spans of feeder also?

Are you sure that the feeder is not feed by a fuse? If so the fuse provides short circuit protection and the breaker provdes overcurrent protection because any current as seen by the feeder will be seen by the breaker. The breaker wouldn't see a fault should it occure in the feeder however which would be cleared by a properly sized fuse.

 

[iwire]

What will cause the breaker at the end of the tap wire to trip if the short happens before the breaker?

Nothing, it will not trip.

 

[kwired]

Feeder taps are a different animal than branch circuits or feeders when it comes to overcurrent protection. The load end overcurrent device still protects all feeder taps from overloading as current in = current out. Short circuits and ground faults that happen ahead of the load end are covered by the line side overcurrent device. There may be more conductor heating than if the protection was at the ampacity of the conductor though, but shorts and ground faults are going to be very high current and very quick response time in general. If there is no line side overcurrent device you have service conductors and not a feeder tap, or some really bad installation. Tap rules limit length and size of the tap conductor to prevent excess heating during fault conditions, but outside feeder taps are allowed unlimited length and size. Outside feeder taps are not as likely to burn down a building during a fault condition is one reason why this is allowed IMO.

 

[elevate]

A friend was asking me some questions about an installation and I was not clear on what to tell him. This is about doing it safe for the least amount of money and if the code allows for it it must be ok. Thanks.