Underground electrical installation

Status
Not open for further replies.
6

601John

Member
Location
kermit, TX
I am at a sand mine, contractor has installed 4 parallel runs of #350 from 2000 amp main, located outside of MCC, underground, transitioning to cable tray under building. Building is 5 ft off ground. PVC extends about 18" above sand. it goes in free air to tray (aprox 2ft) grouped by pipe, supported on tray, then all wires tied together rising another 18" into building. The plan to seal penetration ( 6x6" square chase) with Duct Seal. Wire is rated for cable tray. No clamps or supports other than tray. This looks wrong in many ways. Superintendent is grouchy old man, instead of having a discussion about it he claims it is right and challenged me to prove him wrong. I need a little help or explanation why it is right.
 
Is your concern the supports or the ampacity of 350's in sand?

I'll leave the supports to someone else.

Sand is a poor conductor of heat, I would guess a rho of 120, so depending how the ducts where configured, the amapacity of the 350's may be about 209A each set, so an overall amapacity of 836A. What is the upstream breaker or fuse size?
 
601John said:
I am at a sand mine, contractor has installed 4 parallel runs of #350 from 2000 amp main, located outside of MCC, underground, transitioning to cable tray under building. Building is 5 ft off ground. PVC extends about 18" above sand. it goes in free air to tray (aprox 2ft) grouped by pipe, supported on tray, then all wires tied together rising another 18" into building. The plan to seal penetration ( 6x6" square chase) with Duct Seal. Wire is rated for cable tray. No clamps or supports other than tray. This looks wrong in many ways. Superintendent is grouchy old man, instead of having a discussion about it he claims it is right and challenged me to prove him wrong. I need a little help or explanation why it is right.
Click to expand...

Tray Rated cable is allowed a 6'-0" free air drop.
 
300.50 (C) "shall be protected by an approved enclosure or raceway from the minimum cover depth to the point of entrance" It changes from underground PVC to "free air" to cable tray then enters building through chase. In my opinion if they were going to do it without pipe the whole way, changing over to sealtite and attaching to building would be best option. I know most of this is debatable, but when it leaves PVC to cable tray at that point the raceway is not continuous, so therefore does not meet requirement. And thank y'all for the help
 
601John said:
300.50 (C) "shall be protected by an approved enclosure or raceway from the minimum cover depth to the point of entrance" It changes from underground PVC to "free air" to cable tray then enters building through chase. In my opinion if they were going to do it without pipe the whole way, changing over to sealtite and attaching to building would be best option. I know most of this is debatable, but when it leaves PVC to cable tray at that point the raceway is not continuous, so therefore does not meet requirement. And thank y'all for the help
Click to expand...

How do you plan to install in PVC, Sealtite whatever if cables have already been pulled? Extending the tray would be a better option.


Sent from my iPhone using Tapatalk
 
Dry Sand Thermal Resistivity

Dry Sand Thermal Resistivity

ron said:
Is your concern the supports or the ampacity of 350's in sand? Sand is a poor conductor of heat, I would guess a rho of 120, so depending how the ducts where configured, the ampacity of the 350's may be about 209A each set, so an overall ampacity of 836A. What is the upstream breaker or fuse size?
Click to expand...

Dry sand will have a thermal resistivity in the 300-400 rho range.
 
Status
Not open for further replies.
Top