conductor derating and the 80% rule

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beechigby

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St. Croix US Virgin Islands
I have 16 215w microinverters, (.9A ea)=14.4A, 240Vac; #12 THWN-2 wire in 3/4 conduit, (2 CCC, 1N, 1EGC, and #6GEC, 5wire total) and a 240Vac 30 Disco (Term.rateing 75degC)
I have an ambient temp of 26-30C and an addr for rooftop of 33degC= 63deg C to enter T310.15 B 2 a

#12 has ampacity of 30A x .65=19.5Amps. this means I can have up to 19.5 calculated Amps running thru this wire, correct?
OR; becuause the #12 is termianted at the 75 deg switch, I must use the 75deg col. of T310.15 B 2 a, which has a factor of .47 x 30A = 14.1Amps of Ampacity, which is .3 amps less than my calculated load which means that my #12 wire is too small?
My AHJ is also saying that I must apply the 125% (80%) rule to the derated ampacity, ie 14.1 x .8= 11.28Amps is all that my #12 wire is allowed to carry?
and last, if I must upsize my 2 #12 CCC to #10, may I leave the Neutral and EGC at #12?
Thanks for any replies
 
You can use the 0.65 derating as the conductor has a temperature rating of 90deg.

As far as continuous loading, 14.4*125%=18A. Your derated #12 conductor has an ampacity of 19.5 amps, so it would be OK for the 14.4A continuous load.
 
average ambient temp has to do with exactly that. and the temp I believe is air temp measured in the shade.
and truly, as I have lived here for almost 50 years, it is rare to see a 90degf daily temp, even though the heat index says it FEELS hotter.
my question was really after appling derating, then does the 125% rule apply?
 
beechigby said:
average ambient temp has to do with exactly that. and the temp I believe is air temp measured in the shade.
and truly, as I have lived here for almost 50 years, it is rare to see a 90degf daily temp, even though the heat index says it FEELS hotter.
my question was really after appling derating, then does the 125% rule apply?
Click to expand...
Actually, no.

Here's the rule in 2011 NEC:

690.8(B) said:
(2) Conductor Ampacity. Circuit conductors shall be sized
to carry not less than the larger of 690.8(B)(2)(a) or (2)(b).
(a) One hundred and twenty-five percent of the maximum
currents calculated in 690.8(A) without any additional
correction factors for conditions of use.
(b) The maximum currents calculated in 690.8(A) after
conditions of use have been applied.
(c) The conductor selected, after application of conditions
of use, shall be protected by the overcurrent protective
device, where required.
Click to expand...
 
beechigby said:
pls bear with me as I try to grasp this.
1st I should apply the 125% rule to the Inverters AC output; 16 x .9 =14.4A x 125% = 18 amps. Correct? #12 wire
the 1st Disconnect terminal are rated for 75degF.
Click to expand...
Well... no.

What you calculated is for 690.8(B)(2)(a), i.e. 18A.

You also calculate for 690.8(B)(2)(b)... 14.4A ? .65 = 22.15A.

You don't have to derate to 80% for 2 CCC's, and complying with 110.14(C) terminal temperature limitation is a separate issue.

Per 690.8(B)(2) general statement, the larger of the two calculations is 22.15A. Your conductor must have an allowable ampacity of 22.15A or greater per Table. #14 90?C-rated copper meets that requirement.

Now for 75?C termination, the chosen size must have a 75?C column ampacity of 14.4A or greater. #14 copper meets the requirement.
 
thats a darn good explaination! Thanks, I think Im getting it
so you do the calculation both ways and choose the conductor with the higher ampacity,
but, would not the temp derate factor be used against the conductors 90deg col. max ampacity? for #12
30A x .65 = 19.5A max allowed?
also, to be clear; as far as 75 deg termnals go; I should use the 90 deg col. if I have thwn-2 wire, and just compare the derated ampacity to the 75 deg col ampacity? as long as the derated # is less than the 75 deg col, i am ok as far as the terminal limitation?
 
beechigby said:
thats a darn good explaination! Thanks, I think Im getting it
so you do the calculation both ways and choose the conductor with the higher ampacity,
but, would not the temp derate factor be used against the conductors 90deg col. max ampacity? for #12
30A x .65 = 19.5A max allowed?
also, to be clear; as far as 75 deg termnals go; I should use the 90 deg col. if I have thwn-2 wire, and just compare the derated ampacity to the 75 deg col ampacity? as long as the derated # is less than the 75 deg col, i am ok as far as the terminal limitation?
Click to expand...
690.8(B)(2)(b) calls for reverse derating. You don't calculate the derated ampacity of the wire. You calculate the minimum allowable ampacity before derating.

690.8(B)(2)(a) you calculate the minimum allowable ampacity allowing for 125% continuous load.

The wire used must have a table ampacity equal or greater than the larger of the two calculations.

For 75? terminals, pick wire size which has the lowest 75? column ampacity that's equal or greater than the calculated load current (14.4A in your case). If this wire size is less than or equal to wire size determined above, your good to go. If it is larger, then you have to upsize to this larger size.

What it amounts to is you have three separate determinations. That which requires largest wire size of the three dictates the minimum wire size allowed.
 
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