The "Next Size Up" rule for overcurrent protection of conductors

[duck_huntr85]

Hey all,
Studying for my masters and came up with a question. So for circuits rated less than 800 amps, you are allowed to use the next size up rule to size your overcurrent protection of the circuits conductors. so lets say i have a 500 amp non-continuous load with 75 degree terminals and i want to feed it with parallel conductors. 4/0 is rated at 230 amps and a pair of them would be rated at 460 amps. Would i be allowed to apply the "next size up" rule since there isnt a standard fuse or breaker that corresponds to this amperage? (which would allow me to run paralleled 4/0 conductors to feed the 500 amp)

 

[Smart $]

Hey all,
Studying for my masters and came up with a question. So for circuits rated less than 800 amps, you are allowed to use the next size up rule to size your overcurrent protection of the circuits conductors. so lets say i have a 500 amp non-continuous load with 75 degree terminals and i want to feed it with parallel conductors. 4/0 is rated at 230 amps and a pair of them would be rated at 460 amps. Would i be allowed to apply the "next size up" rule since there isnt a standard fuse or breaker that corresponds to this amperage? (which would allow me to run paralleled 4/0 conductors to feed the 500 amp)

If your load is 500A non-continuous... NO.

All the ratings associated with a load must be equal or greater than the load: 460A < 500A.

 

[Smart $]

Keep in mind that when you apply 110.14(C) for terminal temperature limitation, you use the calculated load (converted to amperes)... period. Whether the load is continuous or non-continuous is irrelevant. You do not add an extra 25% for continuous loads.

 

[duck_huntr85]

i posted this question and layed down for bed. and thought about it and ofcoaurse the conductors have to be sized to atleast handle the amount of current the circuit has so that question didnt really work out. but i still think that 2 paralleled 4/0's can be protected be a 500 amp breaker under a different scenario

 

[Cow]

i posted this question and layed down for bed. and thought about it and ofcoaurse the conductors have to be sized to atleast handle the amount of current the circuit has so that question didnt really work out. but i still think that 2 paralleled 4/0's can be protected be a 500 amp breaker under a different scenario

Sure, if you had a load 460 amps or less they could be.

 

[charlie b]

Let me clarify (or cloud, if you prefer) the issue a bit. Regardless of the calculated load, be it below or above 460 amps, you can protect a pair of 4/0 with a 500 amp breaker. :happyyes: It is a different matter as to whether you are allowed to use a pair of 4/0 conductors to serve the load. In other words, it is one question as to whether a breaker is sufficient to protect a conductor, and another question as to whether a conductor is sufficient to serve a load.