500KCMIL and 400A breaker
- Thread starter wijeguna
- Start date
- Status
- Location
- Illinois
- Occupation
- retired electrician
Re: 500KCMIL and 400A breaker
In many cases, 240.4(B) will permit you to use the 500 kcmil on the 400 amp breaker.
Don
In many cases, 240.4(B) will permit you to use the 500 kcmil on the 400 amp breaker.
Don
- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
Re: 500KCMIL and 400A breaker
Let me expand on the question, for I had a similar discussion with my group last week. I will describe two installations. My question is this, ?Are the two installations different, in that a 400 amp breaker is permitted in the first, but not permitted in the second??
Installation #1: I want to serve an MLO sub-panel that have six disconnects serving downstream loads. I calculate the sub-panel?s total load (per NEC methods) to be 370 amps. I select a 500 MCM conductor (rated at 380 amps). I read 240.4(B) and conclude that a 400 amp breaker (i.e., the next higher standard size, but not over 800) is acceptable.
Installation #2: I install a 400 amp breaker panel. I select a 500 MCM conductor (rated at 380 amps). I read 240.4(B) and conclude that a 400 amp breaker (i.e., the next higher standard size, but not over 800) is acceptable.
Why might the two be different? Because in Installation #1, I know that the load is less than the ampacity of the 500 MCM feeder, and I know that no more disconnects can be installed. However, in Installation #2, someone could come behind me and load up that 400 amp panel to more than 380 amps.
In other words, if I know that the load can exceed the ampacity of the cable, can I still use that cable? I am considering ?load? versus ?cable ampacity,? rather than 240.4(B)?s ?cable ampacity? versus ?cable protection.?
Let me expand on the question, for I had a similar discussion with my group last week. I will describe two installations. My question is this, ?Are the two installations different, in that a 400 amp breaker is permitted in the first, but not permitted in the second??
Installation #1: I want to serve an MLO sub-panel that have six disconnects serving downstream loads. I calculate the sub-panel?s total load (per NEC methods) to be 370 amps. I select a 500 MCM conductor (rated at 380 amps). I read 240.4(B) and conclude that a 400 amp breaker (i.e., the next higher standard size, but not over 800) is acceptable.
Installation #2: I install a 400 amp breaker panel. I select a 500 MCM conductor (rated at 380 amps). I read 240.4(B) and conclude that a 400 amp breaker (i.e., the next higher standard size, but not over 800) is acceptable.
Why might the two be different? Because in Installation #1, I know that the load is less than the ampacity of the 500 MCM feeder, and I know that no more disconnects can be installed. However, in Installation #2, someone could come behind me and load up that 400 amp panel to more than 380 amps.
In other words, if I know that the load can exceed the ampacity of the cable, can I still use that cable? I am considering ?load? versus ?cable ampacity,? rather than 240.4(B)?s ?cable ampacity? versus ?cable protection.?
BAHTAH
Senior Member
- Location
- United States
Re: 500KCMIL and 400A breaker
Unless the breaker is 100% rated, the maximum load you can apply continuously is 320amps. The NEC requires the feeder conductors to have a minimum ampacity of not less than the noncontinuous load plus 125% of the continuous load. The NEC also required the overcurrent protection for a feeder to have a load rating not less than the noncontinuous load plus 125% of the continuous load. 500KCM CU is good for 380 amps, 240.4(B) would allow the 400amp breaker to protect the 500KCM, however since the breaker should not see anything over 320amps continuous there should be no problem. If you know the load will exceed the design of 320amps then you would need to design the feeder based on the higher load. Even if the load went to 400amps for a short time, the conductor insulation is designed for 430amps (THHN), so the breaker trips and you have an underdesigned system but no insulation failure.
Unless the breaker is 100% rated, the maximum load you can apply continuously is 320amps. The NEC requires the feeder conductors to have a minimum ampacity of not less than the noncontinuous load plus 125% of the continuous load. The NEC also required the overcurrent protection for a feeder to have a load rating not less than the noncontinuous load plus 125% of the continuous load. 500KCM CU is good for 380 amps, 240.4(B) would allow the 400amp breaker to protect the 500KCM, however since the breaker should not see anything over 320amps continuous there should be no problem. If you know the load will exceed the design of 320amps then you would need to design the feeder based on the higher load. Even if the load went to 400amps for a short time, the conductor insulation is designed for 430amps (THHN), so the breaker trips and you have an underdesigned system but no insulation failure.
definitive
Member
- Location
- Washington
Re: 500KCMIL and 400A breaker
Ok maybe this is a stretch, but it seems to me that #1 is a branch circuit panel, and #2 is a main service. Is this the case?
Ok maybe this is a stretch, but it seems to me that #1 is a branch circuit panel, and #2 is a main service. Is this the case?
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