150 kva transformer 3phase 480 volt to 120/208 2005 nec

[grumpy11]

i have to give a customer a full 400amps at 208 volts 3 phase 4 wire.
the design is to install a 150kva step down transformer.
now since a 150kva tx. on the primary is 180amps i intend on running 3#3/0 fused at 180 amps
off the secondary side i believe {using 90degree wire }i should use 750 mcm
since i am going into a 400amp switch
750mcm at 90degree the amp rating is 535
since it 4 wire and derating at 80%
i believe it should be 535 x 0.80 x0.91{derating of 90 degree wire} i come up with 390 amps.
am i on the right track.
yes the gec is based on 250.66 and egc is based on 250.122

 

[iwire]

Just some quick thoughts

180 is too low on the primary, you may have trouble energizing the transformer due to inrush. Use at least a 225 and as high as 450, the conductors would have to exceed the overcurrent device rating you chose.

On the secondary I would likely go with two sets of 3/0s for 400 amps but if you want to provide a 'full 400 amps' continuously you need to use a 500 amp breaker / panel / conductors instead of the 400 amp set up.

 

[infinity]

i have to give a customer a full 400amps at 208 volts 3 phase 4 wire.
the design is to install a 150kva step down transformer.
now since a 150kva tx. on the primary is 180amps i intend on running 3#3/0 fused at 180 amps
off the secondary side i believe {using 90degree wire }i should use 750 mcm
since i am going into a 400amp switch
750mcm at 90degree the amp rating is 535
since it 4 wire and derating at 80%
i believe it should be 535 x 0.80 x0.91{derating of 90 degree wire} i come up with 390 amps.
am i on the right track.
yes the gec is based on 250.66 and egc is based on 250.122

With a 390 amp ampacity you cannot use a 400 amp OCPD on the secondary condcutors. Why are you considering the neutral as a CCC is the transformer load more than 50% non-linear?

 

[grumpy11]

150kva transformer

150kva transformer

ok i could see the error in the primary i forgot to look at table 450.3{b}

on the secondary i think 3/0 is to small with derating and all that

i dont think i want to run pararell sets

 

[grumpy11]

150 kva transformer

150 kva transformer

the load i understand is for a resturant

 

[augie47]

as Rob noted, I see no reason for you to consider the neutral as a current carrying conductor. Are you looking at the 400 amp load as continuous ?

 

[grumpy11]

150 kva transformer

150 kva transformer

i dont know if the load is going to be continuous for more them 3 hours .

lets say it is

but doesnt article 310.15 4 c tell you to consider a neutral a current carrying conductor

 

[augie47]

More specifically it states:

(c) On a 4-wire, 3-phase wye circuit where the major portion of the load consists of nonlinear loads, harmonic currents are present in the neutral conductor; the neutral conductor shall therefore be considered a current-carrying conductor.

It is highly unlikely that the MAJOR portion of the load is non-linear

That said, your 150 kva transformer has an output of 416 amps with 25% allowing 520 amps so you could easily go to a 500 amp secondary (legally higher) and use 900 kcmil but that would be a monumental task. Most everyone would parallel 250 kcmil.

 

[david luchini]

180 is too low on the primary, you may have trouble energizing the transformer due to inrush. Use at least a 225 and as high as 450, the conductors would have to exceed the overcurrent device rating you chose.

I would agree with the primary OCPD sizing, but I see no reason the conductors would have to exceed the ocpd device rating. If a 450A primary OCPD was used, the conductors could be 600MCM, or use 500MCM with a 400A primary OCPD, etc. The "next standard size up" would be applicable for primary OCPDs rated 800A or less.