I have a 4000 amp 480v service and below is a concrete pit approx 10' wide 5' deep and 25' long. Engineered steel structure for the service to sit upon on top. The idea is to bring feeder conduits into the pit and then free air from where it enters to the associated branch breaker. In some cases this will span 20'. I cannot find in the NEC the support requirements for the cabling within the pit. I see in 328 its 6' but I do not find anything for 600v rated cables. Any help is appreciated.