Overcurrent protection for a rooftop conductor after deration

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Scratchbuilt

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Can anyone provide some clarification on this? I think am pretty sure I know the answer but I would like some other opinions.

I have a 3-wire feeder (1-phase + nuetral) that leaves a 100A disconnect switch at the main service. The 100A disconnect is tapped off a gutter fed by an 800A main disconnect.

The engineer is specify # 3 THHN/THWN be installed a cross a roof to refeed an existing 100A panel located in an existing unit. Since these conductors will run across the roof, I have performed the required ampacity adjustments based on 310.15 (B)(3)(c). I am coming up with a corrected ampacity of 94.3 amps for these conductors (# 3 THHN = 115A x .82 adjustment factor per Table 310.15 (B)(2)(a)).

I can still install 100A fuses in the service disconnect to protect these conductors based on 240.4(B) correct? Or since the conductors have had an ampacity adjustment, they can now not exceed 100A? So I would need to install 90A fuses instead. I pretty sure that I can still install 100A fuses for protection, right?
 
Scratchbuilt said:
Can anyone provide some clarification on this? I think am pretty sure I know the answer but I would like some other opinions.

I have a 3-wire feeder (1-phase + nuetral) that leaves a 100A disconnect switch at the main service. The 100A disconnect is tapped off a gutter fed by an 800A main disconnect.

The engineer is specify # 3 THHN/THWN be installed a cross a roof to refeed an existing 100A panel located in an existing unit. Since these conductors will run across the roof, I have performed the required ampacity adjustments based on 310.15 (B)(3)(c). I am coming up with a corrected ampacity of 94.3 amps for these conductors (# 3 THHN = 115A x .82 adjustment factor per Table 310.15 (B)(2)(a)).

I can still install 100A fuses in the service disconnect to protect these conductors based on 240.4(B) correct? Or since the conductors have had an ampacity adjustment, they can now not exceed 100A? So I would need to install 90A fuses instead. I pretty sure that I can still install 100A fuses for protection, right?
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Yes, you can protect the conductors with an ampacity of 94.3 with a 100A OCPD, per 240.4(B)...(the load must be 94.3A or less.)

But no, you cannot use the 90deg starting ampacity for the derating for THHN/THWN installed in a wet location. If the conductor is THHN/THWN-2, then you can start at 115A.
 
Scratchbuilt said:
Can anyone provide some clarification on this? I think am pretty sure I know the answer but I would like some other opinions.

I have a 3-wire feeder (1-phase + nuetral) that leaves a 100A disconnect switch at the main service. The 100A disconnect is tapped off a gutter fed by an 800A main disconnect.

The engineer is specify # 3 THHN/THWN be installed a cross a roof to refeed an existing 100A panel located in an existing unit. Since these conductors will run across the roof, I have performed the required ampacity adjustments based on 310.15 (B)(3)(c). I am coming up with a corrected ampacity of 94.3 amps for these conductors (# 3 THHN = 115A x .82 adjustment factor per Table 310.15 (B)(2)(a)).

I can still install 100A fuses in the service disconnect to protect these conductors based on 240.4(B) correct? Or since the conductors have had an ampacity adjustment, they can now not exceed 100A? So I would need to install 90A fuses instead. I pretty sure that I can still install 100A fuses for protection, right?
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Note how the nec doesn't use the term "derate" but rather calls it "ampacity adjustments. Your adjusted result is nothing more than an alternate value to 310.15(16) values based on conditions of the application. Overcurrent protection can still be next size up - up to 800 amps but applied load can not be more than the adjusted value.
 
Thanks guys! Just what I thought but wanted reassurance. Yes, the wire is rated thwn-2, I just always assume that seeing as my suppliers don't stock anything but that.

Derating vs. adjustment factor; I guess I need to get that terminology stuck in my head as it was changed in the 2011 code cycle if I remember correctly.
 
Scratchbuilt said:
Thanks guys! Just what I thought but wanted reassurance. Yes, the wire is rated thwn-2, I just always assume that seeing as my suppliers don't stock anything but that.

Derating vs. adjustment factor; I guess I need to get that terminology stuck in my head as it was changed in the 2011 code cycle if I remember correctly.
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adjustment factor or correction factor have been used in NEC for a very long time.

What has changed more recently is that they recognized the harsh conditions commonly found at rooftop areas and have mandated we treat them as a higher ambient temp then most other outdoor applications.
 
kwired said:
adjustment factor or correction factor have been used in NEC for a very long time.

What has changed more recently is that they recognized the harsh conditions commonly found at rooftop areas and have mandated we treat them as a higher ambient temp then most other outdoor applications.
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Wires have been in those harsh conditions a long time. Where was any real damage found?
 
ActionDave said:
Wires have been in those harsh conditions a long time. Where was any real damage found?
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I can not answer that, and don't know if any real world data contributed to the changes. If there was no real failures out there to help get this change then it doesn't make any sense to have the change.

At the same time I think most of us can agree the temp in those locations is generally higher then other locations, those that don't agree can maybe spend some time on some rooftops in mid summer afternoons and then report back their findings:happyyes:

If the conductors in those locations in the past have no increased degrading then IMO we maybe have been applying adjustments wrong in the past or are just over conservative to be on the safe side. Now the 2014 has made changes again involving the use of XHHW, can't remember exactly what the change was and don't have a 2014 to look it up. But my thoughts when I first saw it was, why doesn't XHHW have higher than 90C rating if it can take more heat? Of course we would need another temp column to ampacity tables as well - maybe they just didn't want to take on the task of developing another column to the tables just yet:roll:
 
ActionDave said:
Real world data to be used as a basis for a code change. ??.interesting idea. I wonder if it will ever catch on?
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I think it had caught on for the first 60 - 80 years of NEC existence but has since been lost and marketing has been driving code changes more than anything else.
 
kwired said:
I think it had caught on for the first 60 - 80 years of NEC existence but has since been lost and marketing has been driving code changes more than anything else.
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I think there is an equal number of sincere, well meaning, folks on the panels and submitting proposals.

But at some point in the process a bunch of busy-body, do-gooder stuff gets passed that is a solution in search of a problem. Not seeing a whole lot of wiring failures on rooftops on my fair city. Not the conductors anyway. Conduits are another matter, but I have strayed OT enough already.
 
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