MV Cable and Sizing - Continuous Load

[persian451]

First time for me posting so please bear with me:

I have a 12.47kV circuit that feeds a total of 3.5MVA transformers. My FLA is about 162amps and I'm using #4/0 AWG copper single conductors in a duct bank with other 12kV circuits. The correct table to use is 310.77 (6 circuits).

Now I'm aware that MV systems are 100% rated but I cannot find anything within the NEC that would force me to size my conductors 1.25*FLA.

MV105 is good for 205 amps
MV90 is good for 190 amps.

I need to provide documentation to the AHJ that we do not need to size our conductors 125% of the FLA because continuous load only applies to 600V systems and less.

The transformers are for a PV system

Am I in the right direction?

 

[Dennis Alwon]

Not my area of expertise but I agree that there is nothing in art. 450 that states the circuit conductors must be rated 125% however if this is a continuous load then I believe the 125% would apply even for over 600V--- art. 210.19(B)(1). I guess that would depend on the load on the transformer. Not sure this helps

 

[Smart $]

Are the 12.47kV conductors service conductors or feeders?

If feeders...

(B) Feeders over 600 Volts. The ampacity of conductors
shall be in accordance with 310.15 and 310.60 as applicable.
Where installed, the size of the feeder-circuit grounded conductor
shall not be smaller than that required by 250.122,
except that 250.122(F) shall not apply where grounded conductors
are run in parallel. Feeder conductors over 600 volts
shall be sized in accordance with 215.2(B)(1), (B)(2), or
(B)(3).

(1) Feeders Supplying Transformers. The ampacity of
feeder conductors shall not be less than the sum of the
nameplate ratings of the transformers supplied when only
transformers are supplied.

Derating and/or termination temperature ratings may limit your ampacity to less than table values you have indicated.

 

[Hv&Lv]

I need to provide documentation to the AHJ that we do not need to size our conductors 125% of the FLA because continuous load only applies to 600V systems and less.

Ask the inspector to provide the article number if he thinks you do have to size them that way.

 

[persian451]

Are the 12.47kV conductors service conductors or feeders?

If feeders...



Derating and/or termination temperature ratings may limit your ampacity to less than table values you have indicated.

That's what I'm not sure.. I say they are feeders..

We have 12kV circuit from the Utility feeding a series of padmount transformers, which are fed from a PV farm...

I'm compiling all of my code references and trying to issue a letter to the AHJ

 

[persian451]

Ask the inspector to provide the article number if he thinks you do have to size them that way.

You're right.. I should ask for some documentation from the AHJ

 

[Smart $]

That's what I'm not sure.. I say they are feeders..

We have 12kV circuit from the Utility feeding a series of padmount transformers, which are fed from a PV farm...

I'm compiling all of my code references and trying to issue a letter to the AHJ

You need to determine the service point. The NEC does not apply to conductors on the utility side of the service point. Service drop and underground service [lateral] conductors on the consumer side of the service point (which either is usually not possible) are not required to be sized at 125% of continuous plus the non-continuous load, but service-entrance conductors are, even when over 600V [230.23, 230.30, 230.42, 230.200].