TABLE 310.15(B)(2)(a)

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PLR

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Does this look correct or did I miss it::?

450 amp 3 pole breaker
if we run 3 conductors per phase 5'
we need to de rate by 30% per TABLE 310.15(B)(2)(a)
do we use 60c column of table 310.16
450+30% = 585 /3 =195 amp.
4/0 copper
 
PLR said:
Does this look correct or did I miss it::?

450 amp 3 pole breaker
if we run 3 conductors per phase 5'
we need to de rate by 30% per TABLE 310.15(B)(2)(a)
do we use 60c column of table 310.16
450+30% = 585 /3 =195 amp.
4/0 copper
Click to expand...
5' in what? ...raceway, wireway, enclosure, etc.??? If raceway, how long is it?

What type of conductor: 60?C, 75?C, 90?C (TW, THHW, THHN)???
 
Also your math is incorrect. You do not add your derating factor of 30% to the 450 amp load. The minimum ampacity would be 450 * (1/.7)= 643 amps
 
infinity said:
Also your math is incorrect. You do not add your derating factor of 30% to the 450 amp load. The minimum ampacity would be 450 * (1/.7)= 643 amps
Click to expand...
May depend on the load too. Ampacity of the conductor(s) could be as low as 401A and still permitted to be protected by a 450A breaker. And there are even specific cases, such as motor circuits, which the ampacity of the conductor(s) could be even lower yet.
 
Smart $ said:
May depend on the load too. Ampacity of the conductor(s) could be as low as 401A and still permitted to be protected by a 450A breaker. And there are even specific cases, such as motor circuits, which the ampacity of the conductor(s) could be even lower yet.
Click to expand...

True, I was going on the assumption that he was asking about the full 450 amp rating of the CB since he used that figure in his calculation. Could certainly be next size up from 401.
 
Let's start from the beginning and go back to the derating for a minute, what type of insulation are you using?
 
THHN is derated from the 90?C column ampacity value, but cannot be greater than the 75?C column ampacity value (assuming 75?C termination rating).

However, since we are discussing a generator and conductors in PVC, I will surmise these conductors are outside, which is considered either a wet location. THHN itself is not rated for a wet location. It may be dual rated THHN/THWN or THHN/THWN-2. What column you derate from will depend on this dual rating. THWN is 75?C, while THWN-2 is 90?C-rated.
 
PLR said:
daul rated THHN/THWN-2
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Derate from 90?C column.

450A ? 70% = 643A
... ? 3 conductors = 214A min. @ 90?C
3/0 copper
3 ? 200A @ 75?C-rating for terminations = 600A
... > 450A
Good to go with 3/0, assuming no adjustment for ambient.
 
PLR said:
Thank you
so if I run paralleled 4/0 THHN/THWN-2
in 2 conduits I wont have to derate
Click to expand...
I'm assuming this is a 3? 4-wire wye installation. As long as the neutral doesn't qualify as a current-carrying conductor, you wouldn't have to derate for number of conductors in each raceway. You will have to run a full-sized EGC in each raceway, though.

And another thought... at such a short distance, you should make certain the conductors are the same length per phase.
 
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