Over current protection

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Matt_hndrx

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What does it mean when it says at the termination point for sizing the breaker? Do you just look up your wire size in 310.16 and use those table's or am I missing something? and could someone maybe give be an example making up a wire size and insulation type?
 
ok say you have something like a 200 Kcmil thw conductor feeder tap conductor and you want to know what is the maximum overcurrent device you can put on it at its termination point. how would you go about that?
 
Well, to start with I don't see 200 kcmil conductors in table 310.16. But it seems that you are asking about the tap rules (240.21). Is that right?

Presuming that is your question, it all starts with the rule that every conductor must be protected against overcurrent, and most of the time the overcurrent device has to be located at the point at which the cable gets it power. That makes sense because if the conductor experienced a short circuit somewhere in the middle of the run, and if the overcurrent device was at the end of the run, the device would not be able to trip and protect the conductor.

The tap rules give us certain, specific conditions under which we can protect the conductor where it supplies its load, as opposed to where it gets its power. So to figure out what the max OCPD would be, you need to find out which of the tap rules applies to the situation.
 
ok use 300 Kcmil I would imagine if you looked up that in 310.16 found the amps there then looked up standard fuse sizes it would be 300 amp overcurrent protection device used does that make since or am I way off
 
Matt_hndrx said:
What does it mean when it says at the termination point for sizing the breaker? Do you just look up your wire size in 310.16 and use those table's or am I missing something? and could someone maybe give be an example making up a wire size and insulation type?
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Not sure what you mean but yes you would look in T310.16 in most cases. I Think the termination points means the lugs or the breakers where the wire is terminated. Those points are generally rated 75C. So if you are using 90C wire and the termination point is 75C then you must use the 75C column in that table to size your wire.
 
If I had a load that drew, let us say, 280 amps, and if I wanted to use THW-insulated copper conductors, I would look in Table 310.16, and discover that the smallest conductor that has at least that ampacity is 300 kcmil. Now I need to protect that conductor at its ampacity of 285 amps (240.4 says that), or at the next higher standard size breaker (240.4(B) says that). That would be a 300 amp breaker (240.6(A) has that list). So I agree with your answer to your own example.

Is that what you were asking?
 
In regard to Dennis' answer, look at 110.14(C). That should help also.
 
charlie b said:
If I had a load that drew, let us say, 280 amps, and if I wanted to use THW-insulated copper conductors, I would look in Table 310.16, and discover that the smallest conductor that has at least that ampacity is 300 kcmil. Now I need to protect that conductor at its ampacity of 285 amps (240.4 says that), or at the next higher standard size breaker (240.4(B) says that). That would be a 300 amp breaker (240.6(A) has that list). So I agree with your answer to your own example.

Is that what you were asking?
Click to expand...


Take a peak at 240.21(B), the last sentence... one is not permitted to use the application of 240.4(B) in conjunction with feeder tap conductors.
 
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