Sorry, this is a long post- My posts tend to be lengthy- so if you want to get right to my questions, skip down to the ###############
The concrete for the ballast blocks is coming Monday so....
While the County inspector was here for the pre-pour setback inspection I took the opportunity to get his opinion on a few things.
I don't recall how it came up but he mentioned that my wire in conduit from the inverter to the utility panel was considered a wet location and the #10 THHN I was planning on using wasn't rated for wet locations in the 2011 code book he uses, but it was totally up to me if I wanted to use it or not.
So this got me reviewing the wire...
It appears that the Southwire brand THHN is dual rated for THWN wet locations, but at a reduced temperature (from 90C to 75C).
Further research shows that THWN-2 is rated to 90C in wet locations, but try to find it in #10...
Looking at various spec sheets many of them claim their wire is dual rated THHN and THWN-2 but reading the fine print shows that THWN-2 rating is ONLY on #8 and above. S
ince I need 80', up-sizing to #8 could add as much as $200+ bigger conduit to my project.
Initial calculations seemed to indicate that #10 THWN would work fine, although it could be borderline a couple days of the year when temps get up to the official range of 104F (40C), but I know we have seen higher on occasion, up to 111F.
Where I got concerned was when I did the the wire sizing ampacity calcs at this link....
Skipping the array side calcs and going to the inverter AC wiring...
SolarABC data for my location shows a 2% avg. of only 37C (98F -even the .40% is 39C) but like I said I know we get hotter so I used 40C and the occasional 111F would put me into the 41C group....
So, calculating the AC wire from the inverter output ...
Inverter spec output= 21A x1.25 (ampicity multiplier) = 26.25A (nominal ampicity).
The inverter is outside,on a pole but in the summer will get direct sun.
Then I did comparison calcs using both 40C and 41C as the temp, for both 75C THWN & 90C THWN-2.
NEC table 310.16 shows temp correction factor for 75C conductors of 0.88 at 40C while at 41-45C the correction is 0.82
90C conductors have a correction factor of .91 at 40c and .87 at 41C
In conduit, THWN is rated for 35A & THWN-2 is rated at 40A and my planned breaker is 30A
With only 2 'hot' conductors in the conduit it has a factor of 1 so is omitted from the formula.
Using the simplified formula:
Inverter Derated Ampacity = Nominal Ampacity / ( Temp Correction Factors)
Derated Wire Ampicity= Rated Amps x Derating factor
I come up with:
nominial @ 40C= 26.25/.88= 29.8A
nominial @ 40C= 26.25/.91=28.8A
nominial @ 41C= 26.25/.82= 32A
nominial @ 41C= 26.25/.87= 30A
Checking the #10 wire ampicity capacity in conduit and applying the derating factors:
THWN @ 40C= 35x.88= 30.8A - wire can just handle the load
THWN-2 @40C= 40/.91=36.4A - wire easily carries the load
THWN @ 41C=35x.82= 28.7A - wire cannot handle the load- will burn before the breaker trips
THWN-2 @41C= 40/.87= 34.8A - wire will work fine
So there is a potential problem during very hot days - but isn't this offset but output inefficiency of the array as temps rise?
Searching the internet I find THWN-2 doesn't exist in #10 - have to jump to #8 to get the rating...
However XHHW-2 is also rated 90C in wet conditions and does come in #10 (and 12 and other sizes) and it is priced within a couple cents of THHN, comein colors and is overall rated superior. Most places sell it in minimum 500' rolls but I did find a couple places that sell it by the foot and shipping is about $20 for 400'
So I think I have it solved but hold off ordering to double check local supply houses.
One old timer tells me that all the solar installers he knows use #8 for the hot wires and #10 for the neutral and ground. and that he sells #8 for .31/ft - so now this info is real comparative and may even beat using XHHW-2 cost-wise, plus the #8 will definitely give me a safety factor BUT I may have to return the 3/4" conduit for larger.
Finally my questions for discussion....
################
First question I have about the wire temp derating factor is that... when it is dry out and 104-110F, even though the location is considered "wet" it is in fact dry, so wouldn't the wire perform at the 90C (THHN) rating? and then during Monsoon season when it rains every day for a month, the temps are much cooler (80-90's) it would perform at the 75C THWN rating?
If this is the case then I shouldn't have any problems as the temp factors should change with the season and conditions.
2. Which option would a better choice-
a. using XHHW-2 #10 for all 4 wires,
b. XHHW-2 for only the two Hot wires and THHN for the ground and neutral or
c. Two #8 for the hot and THHN for the other two
d. Just use common THHN/THWN for all the wires as it won't be a problem?
Thanks
Al
The concrete for the ballast blocks is coming Monday so....
While the County inspector was here for the pre-pour setback inspection I took the opportunity to get his opinion on a few things.
I don't recall how it came up but he mentioned that my wire in conduit from the inverter to the utility panel was considered a wet location and the #10 THHN I was planning on using wasn't rated for wet locations in the 2011 code book he uses, but it was totally up to me if I wanted to use it or not.
So this got me reviewing the wire...
It appears that the Southwire brand THHN is dual rated for THWN wet locations, but at a reduced temperature (from 90C to 75C).
Further research shows that THWN-2 is rated to 90C in wet locations, but try to find it in #10...
Looking at various spec sheets many of them claim their wire is dual rated THHN and THWN-2 but reading the fine print shows that THWN-2 rating is ONLY on #8 and above. S
ince I need 80', up-sizing to #8 could add as much as $200+ bigger conduit to my project.
Initial calculations seemed to indicate that #10 THWN would work fine, although it could be borderline a couple days of the year when temps get up to the official range of 104F (40C), but I know we have seen higher on occasion, up to 111F.
Where I got concerned was when I did the the wire sizing ampacity calcs at this link....
Skipping the array side calcs and going to the inverter AC wiring...
SolarABC data for my location shows a 2% avg. of only 37C (98F -even the .40% is 39C) but like I said I know we get hotter so I used 40C and the occasional 111F would put me into the 41C group....
So, calculating the AC wire from the inverter output ...
Inverter spec output= 21A x1.25 (ampicity multiplier) = 26.25A (nominal ampicity).
The inverter is outside,on a pole but in the summer will get direct sun.
Then I did comparison calcs using both 40C and 41C as the temp, for both 75C THWN & 90C THWN-2.
NEC table 310.16 shows temp correction factor for 75C conductors of 0.88 at 40C while at 41-45C the correction is 0.82
90C conductors have a correction factor of .91 at 40c and .87 at 41C
In conduit, THWN is rated for 35A & THWN-2 is rated at 40A and my planned breaker is 30A
With only 2 'hot' conductors in the conduit it has a factor of 1 so is omitted from the formula.
Using the simplified formula:
Inverter Derated Ampacity = Nominal Ampacity / ( Temp Correction Factors)
Derated Wire Ampicity= Rated Amps x Derating factor
I come up with:
nominial @ 40C= 26.25/.88= 29.8A
nominial @ 40C= 26.25/.91=28.8A
nominial @ 41C= 26.25/.82= 32A
nominial @ 41C= 26.25/.87= 30A
Checking the #10 wire ampicity capacity in conduit and applying the derating factors:
THWN @ 40C= 35x.88= 30.8A - wire can just handle the load
THWN-2 @40C= 40/.91=36.4A - wire easily carries the load
THWN @ 41C=35x.82= 28.7A - wire cannot handle the load- will burn before the breaker trips
THWN-2 @41C= 40/.87= 34.8A - wire will work fine
So there is a potential problem during very hot days - but isn't this offset but output inefficiency of the array as temps rise?
Searching the internet I find THWN-2 doesn't exist in #10 - have to jump to #8 to get the rating...
However XHHW-2 is also rated 90C in wet conditions and does come in #10 (and 12 and other sizes) and it is priced within a couple cents of THHN, comein colors and is overall rated superior. Most places sell it in minimum 500' rolls but I did find a couple places that sell it by the foot and shipping is about $20 for 400'
So I think I have it solved but hold off ordering to double check local supply houses.
One old timer tells me that all the solar installers he knows use #8 for the hot wires and #10 for the neutral and ground. and that he sells #8 for .31/ft - so now this info is real comparative and may even beat using XHHW-2 cost-wise, plus the #8 will definitely give me a safety factor BUT I may have to return the 3/4" conduit for larger.
Finally my questions for discussion....
################
First question I have about the wire temp derating factor is that... when it is dry out and 104-110F, even though the location is considered "wet" it is in fact dry, so wouldn't the wire perform at the 90C (THHN) rating? and then during Monsoon season when it rains every day for a month, the temps are much cooler (80-90's) it would perform at the 75C THWN rating?
If this is the case then I shouldn't have any problems as the temp factors should change with the season and conditions.
2. Which option would a better choice-
a. using XHHW-2 #10 for all 4 wires,
b. XHHW-2 for only the two Hot wires and THHN for the ground and neutral or
c. Two #8 for the hot and THHN for the other two
d. Just use common THHN/THWN for all the wires as it won't be a problem?
Thanks
Al
Comment