Installing busduct or cablebus for padmounted transformer

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rotmgbetter

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Location
Houston, Texas
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I have a customer owned transformer that will be located right outside of the building and the switchgear will be right across the wall in the inside of the builidng. This will be a 480V, 4500 amp service on the secondary.

The main goal is to decrease cost for having to jackhammer the interior of the building and pulling the wire.

First of all, is cablebus and busduct exclusive for the substation style transformers with flanges and throats or is there commercially available solutions to convert a padmounted transformer so it is compatible with cablebus/busduct?

Secondly, will there be any cost advantage at all to having an overhead cablebus/busduct going directly into the switchgear vs just normal underground conduits?

Thirdly, for a run that is less than 10 feet, would cablebus or busduct be more economical?
 
rotmgbetter said:
I guess one more question, is it any cheaper to have the underground conduits pop out while still outside, run along the wall and then enter from overhead.
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This is pretty common.
One job had a cable tray run up the side of the building. The customer had a picture taken of the brick wall which was sent to a 'sign' company to be turned into a decal for the tray cover, 10 yrs later it is still camouflaged.
 
jim dungar said:
This is pretty common.
One job had a cable tray run up the side of the building. The customer had a picture taken of the brick wall which was sent to a 'sign' company to be turned into a decal for the tray cover, 10 yrs later it is still camouflaged.
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In that scenario, how does the derating and ampacity work? Would you follow the derating for 3 or more conductors in a conduit + ampacity of buried insulated conductors or is there a critical length of conduit run where under that length you can follow the ampacity for in free air?
 
rotmgbetter said:
In that scenario, how does the derating and ampacity work? Would you follow the derating for 3 or more conductors in a conduit + ampacity of buried insulated conductors or is there a critical length of conduit run where under that length you can follow the ampacity for in free air?
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Because the run began in underground conduits and finished in indoor conduits a choice was made to not have transitions, the conductors were sized as if the entire run was in conduit not open air.
 
Anyone know if the additional cost of a bus duct will offset the labor cost of installing 12 conduits at 500 kcmil with 4 strands in each if it is a short run.

This doesn't seem to be a common approach but one of the contractors on the project is proposing this to save money.
 
jim dungar said:
Because the run began in underground conduits and finished in indoor conduits a choice was made to not have transitions, the conductors were sized as if the entire run was in conduit not open air.
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rotmgbetter said:
Anyone know if the additional cost of a bus duct will offset the labor cost of installing 12 conduits at 500 kcmil with 4 strands in each if it is a short run.

This doesn't seem to be a common approach but one of the contractors on the project is proposing this to save money.
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Can you use bus duct as service entrance conductors?
I know they have a SCCR, but it that not based on the bus duct being protected with an OCPD with that rating and will clear the circuit in 1-3 cycles?

What is the bus duct X/R test value to evaluate if it over of range?
 
MyCleveland said:
Can the utility trans primary OCP satisfy 368.17(A)?
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I don't know, I haven't looked yet.
But, utility primary side protective devices are typically intended to removed faulted equipment from their grid. They are all but never sized to meet NEC protection requirements.
 
368.1 says service entrance bus duct is allowed.
368.17 outlines OCPD requirements for bus duct used as feeders or branches.
There is nothing about service entrances, so 230.91 should still be applicable.
 
jim dungar said:
368.1 says service entrance bus duct is allowed.
368.17 outlines OCPD requirements for bus duct used as feeders or branches.
There is nothing about service entrances, so 230.91 should still be applicable.
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Not disagreeing...just have never seen this done.
The UL white book has a line that states something to the effect that all components used for SE must carry that label.
There must be some specific testing needed to get this label.

Thanks for looking at the topic.
 
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