Cable sizing with feeder breaker

[mull982]

I have a load (480V) that after derating and such comes to about 185A. I am using a 200A feeder breaker to serve this load. When seleting feeder cable do I size the feeders to the 185A load or do I have to size them to the 200A breaker rating? Another words can the ampacity of your cables be less then the rating of the breaker, or does the breaker always have to be equal to or less than the cable ampacity to protect the cables?

Just for example sake, if I had the 185A load and I selected cables with an ampacity of 185A could I use a feeder breaker of 185A if one was avaliable hypothetically speaking?

Thanks

Mull982

 

[Dennis Alwon]

Read article 240.4(B). This may help you understand the situation.

 

[Dennis Alwon]

It appears that 185 amp load will require a conductor with an ampacity of 200 amps. Actually you could pull a 4/0 tw and put it on a 200 amp breaker

 

[Dennis Alwon]

If your load was 180 amps you could pull a 4/0 alum with a 75C insulation and put a 200 amp breaker on it.

 

[infinity]

I have a load (480V) that after derating and such comes to about 185A.
Mull982

What do you mean by derating the load?

 

[mull982]

By derrating the load, I mean derrating for contiuuous duty (25%) or derrating for motor (25%). Or whatever other derrating factors our outlined by the NEC.

 

[charlie b]

By derrating the load, I mean derrating for contiuuous duty (25%) or derrating for motor (25%).

That is not “derating.” That is determining the “calculated load.” Once you have the calculated load, you pick a conductor that has at least that ampacity. If the calculated load is 185, you could use a conductor with an ampacity of 185 (if there were such a thing). You could also protect that 185 amp rated cable with a breaker rated at 200 amps (the next higher standard size).

Suppose, however, that the area in which these conductors is to be run has an ambient temperature of 87 degrees F (31 C). That requires you to calculate a new value for the ampacity of the conductor. If for example you started with 3/0 THHW copper, with a 75C tabulated value of 200 amps, you would multiply by the correction factor of 0.94 (see the table underneath table 310.16), and get an ampacity of 188. That is the process of “derating.” You are reducing the rating of the conductor. By the way, since the calculated ampacity (188 amps) is higher than the calculate load (185 amps), you can use this conductor, and you can still protect it with a 200 amp breaker.