Author Topic: Calculating Conductor Ampacity  (Read 9429 times)

Tony Henderson

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Calculating Conductor Ampacity
« on: March 06, 2012, 08:50:20 AM »
I'm studying the new NABCEP installer resource guide by Brooks and Dunlop, and I have a question on page 65, section 2.72, calculating required ampacity of conductors.

On Step 5, why does 13.93 amps fail because of 690.9(c)?

Tony H.

Arvore

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Re: Calculating Conductor Ampacity
« Reply #1 on: March 17, 2012, 02:11:55 PM »
Tony,

This does indeed look confusing at first glance. However, my thinking is that it failed because they opted for a 15A fuse instead of 14A (OCPD calculation resulted in 13.1A which should lead to a 14A fuse). According to NEC 690.9(C), second paragraph, OCPD <15A go in 1A increments. So, technically a 14A fuse should have been used and would have satisfied NEC 690.9(C). But because they went with the 15A fuse it seems to violate NEC 240.4(B), which states,"The next higher standard overcurrent device rating (above the ampacity of the conductors being protected) shall be permitted to be used..."

Victor

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Re: Calculating Conductor Ampacity
« Reply #2 on: April 02, 2014, 08:34:15 AM »
I think the reason that 10 AWG is needed is because the 14 A fuse could allow a 14A fault current into this circuit, but the 12 AWG wire selected in Step 4 only has a conditions of use adjusted ampacity of 13.92 A.