derating question more then 3 conductors in conduit

[grumpy11]

4 # 1/0 to 150 amp circuit breaker using 90 degree wire 1/0 rateing is 170 amps.

if i derate 80% the rating is now 136 amps. to small a rating for 150 amp,

does this sound right.

i think i should use 2/0 90 degree at 195 amps x 80% is 156 amps.

any comments.

 

[jumper]

4 # 1/0 to 150 amp circuit breaker using 90 degree wire 1/0 rateing is 170 amps.

if i derate 80% the rating is now 136 amps. to small a rating for 150 amp,

does this sound right.

i think i should use 2/0 90 degree at 195 amps x 80% is 156 amps.

any comments.

What is the calculated load?

Is the neutral a CCC?

 

[grumpy11]

135 amps yes on the neutral

 

[jumper]

135 amps yes on the neutral

I believe that 240.4(B) allows up sizing the breaker, barring any info that is missing.

Why is the neutral a CCC, assuming a Wye circuit, harmonics?

 

[don_resqcapt19]

135 amps yes on the neutral

If the calculated load is 135 and the conductor ampacity is 136, the code permits the use of a 150 amp OCPD to protect the conductor.

 

[grumpy11]

ok what if this is from the secondary side of a 45kva transformer.

no next size up rule there.

 

[jumper]

ok what if this is from the secondary side of a 45kva transformer.

no next size up rule there.

Correct, no upsizing secondary protection of conductors off a transformer. 240.21(C).

 

[grumpy11]

SO THE WIRE SIZE SHOULD BE 2/0 correct

 

[A/A Fuel GTX]

ok what if this is from the secondary side of a 45kva transformer.

no next size up rule there.

Looks

 

[david luchini]

Looks to me like you'd be fine with the next size up OCPD based on 240.4(B) and 450.4.

450.4 is for "Autotransformers" :?

 

[A/A Fuel GTX]

450.4 is for "Autotransformers" :?

My bad.....Sorry.

 

[jumper]

SO THE WIRE SIZE SHOULD BE 2/0 correct

I think 3/0. Not sure.

http://ecmweb.com/content/transformer-installation-made-easysort

 

[david luchini]

I think 3/0. Not sure.http://ecmweb.com/content/transformer-installation-made-easysort

Der?k, the OP is showing a 150A ocpd on the secondary, not a 175 as shown in your illustration.

 

[jumper]

Der?k, the OP is showing a 150A ocpd on the secondary, not a 175 as shown in your illustration.

Oops once again.:ashamed1:

 

[grumpy11]

ok if i were ocpd at 175 .
but at 150 ocpd it should be 2/0

 

[infinity]

ok if i were ocpd at 175 .
but at 150 ocpd it should be 2/0

#1/0 copper, THHN, 150 amps T310.15(B)(16) without derating, #2/0 at 80% deration.

 

[grumpy11]

so the 4 conductors have to be derated

 

[don_resqcapt19]

so the 4 conductors have to be derated

We don't know that yet, because you have not told us what type of load is being served. The grounded conductor would be counted as a current carrying conductor only if the major portion of the load is non-linear.

 

[grumpy11]

lets say it does it feeds a panel that feed outlets

 

[kwired]

lets say it does it feeds a panel that feed outlets

That does not help much, what kind of loads connected to the outlets? When talking NEC terms an outlet is "A point on the wiring system at which current is taken to supply utilization equipment". Pretty much all branch circuits supply at least one "outlet"