OCPD Sizing for 840 Ampacity?

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Philip N

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Hello everyone, I'm reviewing a one-line diagram and I am a little confused on what size OCPD might be necessary for a conductor of 840 ampacity. I'm looking at two sets of 3#600mcm running from a chosen 800A fuse to a new switchboard. Referring to NEC 2008, I am stuck in a gray area between 240.4(B) and 240.4(C). A 1000A fuse would not be acceptable because that would go against 240(C), but we can't use the chosen 800A fuse because that would go against the guidelines of 240.4(B)(3). Would the only solution be resizing the conductors? Thank you.
 
Philip N said:
Hello everyone, I'm reviewing a one-line diagram and I am a little confused on what size OCPD might be necessary for a conductor of 840 ampacity. I'm looking at two sets of 3#600mcm running from a chosen 800A fuse to a new switchboard. Referring to NEC 2008, I am stuck in a gray area between 240.4(B) and 240.4(C). A 1000A fuse would not be acceptable because that would go against 240(C), but we can't use the chosen 800A fuse because that would go against the guidelines of 240.4(B)(3). Would the only solution be resizing the conductors? Thank you.
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800A fuse is correct. Neither 240.4(B) nor 240.4(C) come into play.
 
Is your load actually 840 amps, or do you just have 840 amps of conductor? As long as the load does not exceed 800 amps, you can use and 800 amp OCPD for this circuit. If the OCPD needs to be 840 amps you can use a breaker that complies with 240.6(C) and set to 840 amps
 
don_resqcapt19 said:
Is your load actually 840 amps, or do you just have 840 amps of conductor? As long as the load does not exceed 800 amps, you can use and 800 amp OCPD for this circuit. If the OCPD needs to be 840 amps you can use a breaker that complies with 240.6(C) and set to 840 amps
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Thanks! Up to the second level of OCPD is visible to me, so I don't know what the load actually is. Just 840 amps of conductor.
 
Philip N said:
Thanks! Up to the second level of OCPD is visible to me, so I don't know what the load actually is. Just 840 amps of conductor.
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Considering the new switchboard amperage is 800A, it turns out the 800A fuse should be enough no matter what. I don't know why I realized this sooner, thank you all for the input.
 
Philip N said:
we can't use the chosen 800A fuse because that would go against the guidelines of 240.4(B)(3).
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Just a style note: 240.4(B) use the phrase "shall be permitted". That means it imposes no requirements, but offers a permission for something that might otherwise be prohibited (in this case, prohibited by the opening sentence of 240.4). So it's not possible to violate 240.4(B), you just might not earn the permission granted by 240.4(B) (as you would not in your example).

Cheers, Wayne
 
wwhitney said:
Just a style note: 240.4(B) use the phrase "shall be permitted". That means it imposes no requirements, but offers a permission for something that might otherwise be prohibited (in this case, prohibited by the opening sentence of 240.4). So it's not possible to violate 240.4(B), you just might not earn the permission granted by 240.4(B) (as you would not in your example).

Cheers, Wayne
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Ah, I didn't quite catch that until you mentioned it. Thanks a lot.
 
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