Recently I was reading through the Cooper-Bussman “SPD – Selecting Protective Devices Based on the 2005 NEC”. I came upon a couple sections which through me for a loop regarding the withstand rating of conductors and the need to upsize both phase and ground conductors in order for them to carry the full rating of a possible worse case fault scenario.
It does not point out where in the NEC code that it is a requirement to do this for phase conductors but it does refer to 250.4(A)(5) and 250.4(B) (4) for grounding conductors which states that the grounding conductor be sized adequately for the short-circuit current that could be let-through. If it is stated there then why wouldn’t it be the same for the phase conductors? This whole publication does push the use of Bussman Low-Peak fuses to reduce available fault current so it does have some biases but has anyone ever applied this method of upsizing conductors based on the withstand rating. I asked one of the principals at the Electrical Engineering Consulting Firm I work at and he told me he has never applied this to any designs.
Can anyone shed some light on this or at least make it clear to me if upsizing phase and ground conductors is really spelt out anywhere in the NEC code.
Please refer to the article pages 4 & 5 of 27 (74 & 75 Page numbers) at the following link.
http://www.bussmann.com/library/docs/spd02/SPDSection10.pdf
It does not point out where in the NEC code that it is a requirement to do this for phase conductors but it does refer to 250.4(A)(5) and 250.4(B) (4) for grounding conductors which states that the grounding conductor be sized adequately for the short-circuit current that could be let-through. If it is stated there then why wouldn’t it be the same for the phase conductors? This whole publication does push the use of Bussman Low-Peak fuses to reduce available fault current so it does have some biases but has anyone ever applied this method of upsizing conductors based on the withstand rating. I asked one of the principals at the Electrical Engineering Consulting Firm I work at and he told me he has never applied this to any designs.
Can anyone shed some light on this or at least make it clear to me if upsizing phase and ground conductors is really spelt out anywhere in the NEC code.
Please refer to the article pages 4 & 5 of 27 (74 & 75 Page numbers) at the following link.
http://www.bussmann.com/library/docs/spd02/SPDSection10.pdf
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