Overcurrent Protection

[EEVA PE]

The size of overcurrent protective device required for a 645 Amp load is?

I chose 600A. The choices were 600A or 700A. The correct answer was 700 Amps. According to the NEC 240.6 Standard Ampere Rating (A) Fuses and Fixed - Fixed-Trip Circuit Breakers there was both 600 and 700 included. Why would I chose 700A overcurrent protection when the load is 645 AMP.

The Amp Load is being feed using 6 parallel Aluminum (THHN) conductors size #1 AWG. According to Table 310.16, the max current per conductor is 115 Amps, So 6 X115A = 690 Amps. This is less than 700 Amps. Not sure why the 700Amps protection is chosen instead of the 600 Amp protection. I do not see in the NEC where they allow you to insert a higher standard fuse rating or maybe I am missing something?

 

[BamaBino]

I believe this is the section of NEC that you are looking for:

240.4 Protection of Conductors

B. Devices Rated 800 Amperes or Less.

The next higher standard overcurrent device rating (above the ampacity of the conductors being protected) shall be permitted to be used, provided all of the following conditions are met:

 

[EEVA PE]

I believe this is the section of NEC that you are looking for:
240.4 Protection of Conductors

B. Devices Rated 800 Amperes or Less.

The next higher standard overcurrent device rating (above the ampacity of the conductors being protected) shall be permitted to be used, provided all of the following conditions are met:

Thanks, that answers my question.

61 days to go.

 

[BamaBino]

61 days to go.

61 for me too.

 

[EEVA PE]

Would increasing to the higher fuse size for a continuous duty motor also be correct for separate fused overload. If I had a 20HP, 208V 3 phase induction motor with a service factor of 1.0, which standard fuse is best choice? My rational according to the above posts were, looking up in NEC2008 table 430.250, I get 59.4 A. Then in 430.32 (A)(1) I look up info for service factor and get 115%. So, 59.4A X 1.15 = 68.3 A. So then I chose the answer of 70 A fuse. That answer is wrong according the the solution manual on the rational of "To avoid exceeding requirements, the 60A fuse must be selected". Is there something wrong here. Do I treat the selection of separate overload for a motor differently and chose the smaller standard fuse?

 

[sc57]

EEVA, Yes, correct answer is 60A. Refer 430.32 (A) (1) for separate overload device. "no more than the following percent of the ..."

Also for better understanding of Motor protection and sizing of conductor, overload etc. refer example D8 on page no. 1350 of NEC 2008 Handbook.

 

[EEVA PE]

EEVA, Yes, correct answer is 60A. Refer 430.32 (A) (1) for separate overload device. "no more than the following percent of the ..."Also for better understanding of Motor protection and sizing of conductor, overload etc. refer example D8 on page no. 1350 of NEC 2008 Handbook.

Thanks, sc57. Annex D examples looks very useful. I was not aware it existed. Thanks for pointing them out.

 

[sc57]

Good Luck..