XLPE Cable vs. Bus Duct
XLPE Cable vs. Bus Duct
(OP)
Hello.
Can anyone make a comment or an opinion with regards to using XLPE Cables on secondary of Power Transformer to Switchgear instead of using Bus Duct. We have a 20/27/33 MVA substation project, switchgear/outdoor type. Our contractor proposed the used of XLPE Cables installed in a concrete encased conduit which, according to them, flooding or water intrusion is very remote.
Thanks in advance. Any reply would be highly appreciated.
Can anyone make a comment or an opinion with regards to using XLPE Cables on secondary of Power Transformer to Switchgear instead of using Bus Duct. We have a 20/27/33 MVA substation project, switchgear/outdoor type. Our contractor proposed the used of XLPE Cables installed in a concrete encased conduit which, according to them, flooding or water intrusion is very remote.
Thanks in advance. Any reply would be highly appreciated.






RE: XLPE Cable vs. Bus Duct
Using cables (underground encased in concrete) to connect the two units would eliminate the entry problem. One issue with cables is maintainig conductor lengths.
I would recomend that if you go with the conduit/wire have the contractor install at least two full size spare conduits with a pull rope. This will enable you to install a replacement cable in the event of a cable or termination failure.
Lastly, I am a professional engineer who work for many contractors in the past (40 years in electrical designe, construction and maintenance) and I would suspect the cable substitution is most likey driven by economics.
Famous last words in the construction business "you get what you inspect not what you expect"
RE: XLPE Cable vs. Bus Duct
I believe the busduct is more reliable since is factory rated and tested while the solid dielectric cable has to be engineered for the application, exposed to mechanical damage during installation and more sensitive to insulation degradation and thermal stresses during normal operation.
If the cost difference propose by contractor is not significant, I suggest to continue with the original busduct design.
RE: XLPE Cable vs. Bus Duct
XLPE cable by itself is generally not a long-lived cable in medium voltage applications. Moisture is bad news for it, even if you "keep it dry". Use TRXLPE cable or EPR. EPR can handle higher temperatures which is what usually drives the current handling limitations in a duct bank.
Don't worry about getting the cables "exactly the same length" in a duct bank. It's generally a non-issue. 5 conduits run in parallel for 100' will make the cable within 5% of each other or less. The cable ampacity calculations have enough slop in them the way it is.... Since when in this business can we predict something better that within 5%? Not that often.
Mark in Utah
RE: XLPE Cable vs. Bus Duct
Much cheaper in cost
Easy to install
Easy to replace in the event of a failure
Repairs can be carried out quickly by any experienced cable jointer whilst with bus duct its an overseas call to a specialist and mega bucks in cost not to mention the delay.
Historically the cable connection is less likely to suffer from a service fault then bus duct.
RE: XLPE Cable vs. Bus Duct
You don' mention the voltage, but assuming it is 15 kv class, there will be MANY conductors per phase in a cable installation. This complicate the terminations. Also, for an underground installation, the ampacity of each conductor must be de-rated to account for the mutual heating in the duct bank. This will be significant.
If you don't want to use rigid bus, consider use of cable bus such as manufactured by Calvert. This is a good compromise and is an engineered product.
If you have already bid this project, then the contractor's agenda is not the same as yours. Even if there are substantial cost savings, the contractor will get most of it if you have to execute a change order.
Also, I agree with Mark that use of EPR would be much preferred over standard XLPE in this short run of cable.
There is another old construction industry axiom:
"Everyone has within them the capacity to make a really terrible decision."
Good luck!
RE: XLPE Cable vs. Bus Duct
RE: XLPE Cable vs. Bus Duct