Conductor size transition at equipment

[Chrism308]

I have a project where the conductors are on vented cable tray up to 700' long are sized per NEC Table 310.15(B)(20) but I was told by a couple electrical engineers the conductors can hit a terminal box between cable tray and equipment or transition sizes within the electrical equipment to change conductors to 75C ampacity per Table 310.15(B)(16) for terminations onto 2500 amp breaker. I have never used this method on cable tray runs but seems to be an option to take advantage of the 310.15(B)(20) ampacities for long cable tray runs opposed to sizing full conductor runs up to 700' long per 310.15(B)(16). Any help, NEC references, web links or photos of an installation like this would be greatly appreciated.
Thanks

 

[infinity]

Any concerns about voltage drop since you want to use smaller or less conductors in the cable tray?

 

[Chrism308]

Any concerns about voltage drop since you want to use smaller or less conductors in the cable tray?

No, voltage drop has been calculated for the conductor lengths on cable tray.
If a 250' 2500a feeder will need (6) 600 kcmil AL on cable tray per 310.15(B)(20) but per manufacture tested termination temp rating the conductors terminating into equipment have to be sized per 310.15(B)(16) which would be (8) 600 kcmil AL or (6) 1000 kcmil AL

 

[Chrism308]

Any concerns about voltage drop since you want to use smaller or less conductors in the cable tray?

Someone stated that conductors can be sized on cable tray per 310.15(B)(20) but they need to transition in size at a terminal box or within the electrical equipment to ampacities per 310.15(B)(16) for equipment terminations. This is the response - [FONT=Arial, sans-serif](since the cable tray ampacities are allowed in the tray if termination limitations exist in the equipment a transition from the sizes allowed by 392.80 to those specified by 310.15(B)(16) are installed either in the equipment or a junction box to comply with both sets of requirements.)
[/FONT]I have never changed conductor sizes on runs like this so I am not 100% sure how this is accomplished. Looking for feed back about changing large conductors in terminal box or within equipment.
Thanks