CI Practice Test 1 Problem 26

[power62]

What is largest allowable rating of an overcurrent device (to protect 300 Kcmil, TW, 60 deg, AL conductor in a steel conduit feeding 277V load)? 2008 NEC.

Ampacity calculaled is 190 amps.

Their answer is 180A.

Should it not be 200 A (next higher Standard rating of device).

Thanks

 

[iahim]

I just took this test and came up with the same answer (200A). Anybody disagrees?

Thanks!

 

[Redskinsdb21]

YOUR THINKING OF FUSE SIZE...you round those up...this is overload protection...round down

 

[iahim]

Redskins,

Thanks for your response! I'm traveling today and don't have the CI book with me, but if I remember correctly, the problem asks for the overcurrent protective device to protect a feeder at 277V. That would be a circuit breaker or fuse, in my opinion. How did you deduct that they ask for overload protection? I don't think they mentioned any motor in the problem.

 

[daw4888]

You don't want to pick a overload protection device with a rating above the max rating of the conductor. Why would you want to pick a device that would allow 199 amps to flow through a conductor that is only rated for 190 amps? See the logic?

 

[daw4888]

You might be getting confused when picking a fuse compared to the size of your load. If you had a customer that has a max load(with emergency limits built in) of 190 amps of Load, and the conductor allowed, then you would up size the fuse to at least 200. When sizing to serve load you up size, when sizing for protection you downsize your number.

 

[iahim]

I think the trick is that the conductors being protected could be part of a branch circuit with more than one receptacle, for cord and plug connected portable loads (how likely is that!). That will prohibit the use of the next higher standard OCPD per 240.4(B)(1). So you have to protect it with 190A or less. But I'm still not happy with their answer. 180A is not a standard fuse or circuit breaker rating. 150A and 175A are standard ratings. I guess they are thinking that you use a 200A electronic trip breaker with adjustable LTPU set at 180A, but then why not set it to 190A? I know, that's not a choice...

 

[user 26087]

What is largest allowable rating of an overcurrent device (to protect 300 Kcmil, TW, 60 deg, AL conductor in a steel conduit feeding 277V load)? 2008 NEC.

Ampacity calculaled is 190 amps.

Their answer is 180A.

Should it not be 200 A (next higher Standard rating of device).

Thanks

If I'm reading your question right, you are to determine what size breaker to protect the conductors. The tables show the cable is rated for 190A and if you went with a 200A breaker or fuse you now wouldn't be protecting the conductors. So if you have a situation you could burn up your conductors before your overcurrent protection would ever trip. Therefore you want something that is less than what your cables are rated for, i.e. 180A

Does this help?

 

[jcbabb]

I think the trick is that the conductors being protected could be part of a branch circuit with more than one receptacle, for cord and plug connected portable loads (how likely is that!). That will prohibit the use of the next higher standard OCPD per 240.4(B)(1). So you have to protect it with 190A or less. But I'm still not happy with their answer. 180A is not a standard fuse or circuit breaker rating. 150A and 175A are standard ratings. I guess they are thinking that you use a 200A electronic trip breaker with adjustable LTPU set at 180A, but then why not set it to 190A? I know, that's not a choice...

iahim,

Your final response is exactly the case. None of the previous responses were correct at all.

Per the NEC 240.4(B), you are allowed to select the "next size up" when certain qualifications are true. Since you are not given details on these qualifications, you can not assume that they are true, and therefore must choose the next smaller size.

 

[iahim]

iahim,

Your final response is exactly the case. None of the previous responses were correct at all.

Per the NEC 240.4(B), you are allowed to select the "next size up" when certain qualifications are true. Since you are not given details on these qualifications, you can not assume that they are true, and therefore must choose the next smaller size.

Thanks jcbabb!