This section has always been confusing and doesn't make much logical sense. Maybe someone can shed some light on this particular section.
392.10(A)(1) (2005 NEC) tells us cables 1000 kcmil and larger cannot exceed the cable tray width and shall be installed in a single layer.
392.10(A)(2) tells us cables 250 to 1000 kcmil shall not exceed the maximum fill area of column 1 table 392.10(A) for the appropriate width. No mention of installing a single layer and the table does not take into account the rail height of the tray.
392.10(A)(4) tells us that if any of the conductors are 1/0 through 4/0 the ALL the conductors can't exceed the tray width.
What is special about 1/0 through 4/0 conductors that differs from 250 - 1000 kcmil??
392.10(A)(2) tells me that if you conductors are 250 up to 1000 you can stack the cables in the tray until you have reached the make fill listed in the table but the minute you put a 1/0 - 4/0 in you are limited to the sum of the diameters of all the cables. What is the reasoning behind that??
Now when figuring the ampacities of these cables 392.11(B) gives you criteria from 600 kcmil and larger & 1/0 through 500 kcmil. Why the different breaking points in the conductor size?? I can have a tray full to max fill of 250 kcmil and rate them at 65% of table 310.17 but if I use 4/0 I can only put as many as will fit in a single layer and I have to use that same 65% of 310.17. Does that make any sense to anyone??
392.10(A)(1) (2005 NEC) tells us cables 1000 kcmil and larger cannot exceed the cable tray width and shall be installed in a single layer.
392.10(A)(2) tells us cables 250 to 1000 kcmil shall not exceed the maximum fill area of column 1 table 392.10(A) for the appropriate width. No mention of installing a single layer and the table does not take into account the rail height of the tray.
392.10(A)(4) tells us that if any of the conductors are 1/0 through 4/0 the ALL the conductors can't exceed the tray width.
What is special about 1/0 through 4/0 conductors that differs from 250 - 1000 kcmil??
392.10(A)(2) tells me that if you conductors are 250 up to 1000 you can stack the cables in the tray until you have reached the make fill listed in the table but the minute you put a 1/0 - 4/0 in you are limited to the sum of the diameters of all the cables. What is the reasoning behind that??
Now when figuring the ampacities of these cables 392.11(B) gives you criteria from 600 kcmil and larger & 1/0 through 500 kcmil. Why the different breaking points in the conductor size?? I can have a tray full to max fill of 250 kcmil and rate them at 65% of table 310.17 but if I use 4/0 I can only put as many as will fit in a single layer and I have to use that same 65% of 310.17. Does that make any sense to anyone??
