Cable size from bus to line side of motor breaker

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mull982

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I know that when sizing cable for a particular motor circuit it needs to be 125% of the FLA of the motor.

My question is what happens when you are tapping off of a bus to feed the line side of a breaker in a combination motor starter? Does this tap also only need to be 125% of the motor FLA or should the tap be sized per some other breaker (upsteram?) rating? What section of the code references this?

My second question is can an instantaneous motor breaker be used to feed multiple motor overloads to in turn feed several motors?


Lastly, I seen before where feeders have hit the line side of a breaker and then were jumpered to the line side of another breaker and so on to basically supply incoming power to the line side of several breakers. How many times can these jumpers be jumped to connect various breakers? Is this basically like a bus or is it considered to be a varation of the tap rule? I'm guessing it is considered a bus and needs to adhere to the rules for sizing bus.
 

don_resqcapt19

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mull982 said:
I know that when sizing cable for a particular motor circuit it needs to be 125% of the FLA of the motor.

My question is what happens when you are tapping off of a bus to feed the line side of a breaker in a combination motor starter? Does this tap also only need to be 125% of the motor FLA or should the tap be sized per some other breaker (upsteram?) rating? What section of the code references this?
I am not sure but it is my opinion that these conductors would have to be protected per the rules in 240.21(B).

My second question is can an instantaneous motor breaker be used to feed multiple motor overloads to in turn feed several motors?
Instantaneous trip breakers can only be used when they are part of a listed combination starter. 430.52(C)(3).
Lastly, I seen before where feeders have hit the line side of a breaker and then were jumpered to the line side of another breaker and so on to basically supply incoming power to the line side of several breakers. How many times can these jumpers be jumped to connect various breakers? Is this basically like a bus or is it considered to be a varation of the tap rule? I'm guessing it is considered a bus and needs to adhere to the rules for sizing bus.
The feeders would have to be protected at their ampacity. The taps would have to be protected per the tap rules.
 
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rcwilson

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Redmond, WA
See 430.53 (d) for taps to single motors and motor contactors, Also see 430.28 for the tap rules for feeders to motors. I believe the 450.28 rules apply to the daisy-chained wires in a panel. Many different scenarios with different rules.

Note that the line side lugs of the daisy-chained breakers need to be listed for two conductors per lug.
 

augie47

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taps

taps

In reagrd to the second part of your question, I may be proven wrong, but I beleive all of the pertinent tap rules require the tap terminate at a single circuit breaker or set of fuses
 

mull982

Senior Member
This is my interpretation of the code regarding this issue.

Starting from a transformer as a source. On the secondary of the transformer I have an OCD and then leaving the OCD I have cables or bus. These cables or bus are considered a feeder.

(1) This so called feeder can go directly to several motor starter breakers (by daisy chaining to the line side of each breaker) but is considered a feeder throughout and must be sized at total of all motors plus 25% of largest motor.

(2) At any point we tap off this feeder this is considered a feeder tap and must be applied per the feeder tap rules.

(3) If a feeder tap is made and terminated into an OCD then evertying after this is considered a branch circuit and must apply to branch circuit rules.

In my example in the first paragraph, how do you distinguish between a feeder and feeder tap when jumpering between multiple breakers? How do you determine between a feeder and feeder tap in general?
 

mull982

Senior Member
We had another incident last night where one of our size 5 starters was damaged. The cables that connected the MCC bus to the line side of the breaker burnt up. This is the second time this has happened in recent months.

I was suprised to find that the wires that were connecting the vertical MCC bus to the line side of the starter breaker were only a parallel #4. With #4 being good for roughly 85A, this would be a total of 170A in parallel. This 170A is much less than the 270A rated current on the size 5 contactor, and much less than any 200hp+ motor FLA that would be on this starter.

I am thinking that these cables are burning up because they are undersized for the rated starter load. Is this a code violation? Why would an MCC manufacturer put underrated cables in this application?
 

don_resqcapt19

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I am thinking that these cables are burning up because they are undersized for the rated starter load. Is this a code violation? Why would an MCC manufacturer put underrated cables in this application?
It is not a code violation as the NEC does not apply to the internal wiring of a starter bucket.
 
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