NS90 Triplex derating for distance

powerplay

Senior Member
Hello,

For an 240 volt 600 amp Service (480 amp load) we've got almost an 90m span of triplex that the Pole Contractor suggests will be no problem for an double run of NS90 4/0 Triplex...(although I asked for 266kcmil it is apparently very difficult to get.)

Table 36A for maximum allowable ampacity for aluminum neutral supported cable has no derating references.... is there an formula for derating open wiring ?

Thanks for any feedback!
 

powerplay

Senior Member
Utility usually has their own rules.

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This is an private installation after the Service Entrance Hydro supplies.

Our Table 36A has allowances for greater ampacity due to open air rating allowing heat to dissipate, but has no reference to derating. Table D3 derates raceway enclosed conductors having values in Tables 1-4, but the numbers are not the same as Neutral Supported NS90 Triplex.

I spoke with an Electrical Engineer I am working with on a few other Projects, but he was going to charge $400 to make an Table based on Manufacturers Impedance Data. The Lineman we work with seems comfortable with the application, but he is more familiar with Hydro Standards. We will likely go DIrect Burial to save the Masts on both ends and onnections because the Owner is taking care of the trench, but it would be nice to have an Code response to confirming the right size Conductor.

Thanks for the response!
 

ramsy

Owner/Operator
Location
LA basin, CA
Occupation
Service Electrician 2017 NEC
For an 240 volt 600 amp Service (480 amp load) we've got almost an 90m span of triplex that the Pole Contractor suggests will be no problem for an double run of NS90 4/0 Triplex...(although I asked for 266kcmil it is apparently very difficult to get.)
NFPA-70 Table 310.20 has been used for triplex cables in free air. (No derating)
NFPA-70 Table 310.16 has been used for conduit / direct burial. (Must derate for > 3ccc)

With NFPA table 310.20, lineman's parallel 4/0 aluminum triplex gets 6.4% voltage drop, or 225vac at load.

If load pf=1, with copper triplex in free air, maybe 3.5% VD, or 232vac with parallel 4/0. That assumes this pf=1 impedance calculation errors on the side of caution, when used with NFPA table-conductor properties based on pf=0.85.

Regardless, it will cost far less to pay the engineer to get it right for the lineman using Tbl 310.20, than larger cable & parallel raceways per Tbl 310.16.
 
Last edited:
NFPA-70 Table 310.20 has been used for triplex cables in free air. (No derating)
NFPA-70 Table 310.16 has been used for conduit / direct burial. (Must derate for > 3ccc)

With NFPA table 310.20, lineman's parallel 4/0 aluminum triplex gets 6.4% voltage drop, or 225vac at load.

If load pf=1, with copper triplex in free air, maybe 3.5% VD, or 232vac with parallel 4/0. That assumes this pf=1 impedance calculation errors on the side of caution, when used with NFPA table-conductor properties based on pf=0.85.

Regardless, it will cost far less to pay the engineer to get it right for the lineman using Tbl 310.20, than larger cable & parallel raceways per Tbl 310.16.
OP is Canadian so CEC applies not NEC.

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kwired

Electron manager
Location
NE Nebraska
Keep in mind a major reason for "derating" is about protecting the insulation, with a long run voltage drop will remain pretty much the same regardless of insulation type.

If you are trying to make that long of a run and expect the conductors to be loaded to their insulation based ampacity there is a good chance you might need to at least consider what effects of voltage drop are going to be.
 

powerplay

Senior Member
Thank you for your feedback!

I had been told we in Canada are working towards matching NEC... The theory is always interesting to hear regardless of where it comes from!

We've decided to go with an cleaner look and direct bury, which by the time Masts and Poles are set works out close to the same anyways it appears.
 
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