Xfrm. Secondary conductor sizes.

[Mike01]

:-? Under 240.21(C )(2).1.b it states that the conductor ampacity ?is not less than the rating of the device supplied by the secondary conductors or not less than the rating of the overcurrent-protective device at the termination of the secondary conductors? the code also states that 240.4(B) shall not be permitted for transformer secondary conductors.

With all that being stated if you had a delta / wye 112.5 (480Δ-208Y/120V, 3?-4W+GRD) primary fla = 135A, secondary = 312.5 @ 125% = 390 = 400A breaker. So in this case the terminations on the breaker are only rated for 75?C.

Now for the question:
Can I use the ampacity of the 90?C conductor specified seeing that the ampacity of the conductor is rated at 90?C or do I have to use the 75?C column because the terminations are only rated for the 75?. So does this mean that I can use the 90?C ampacity rating so long as my load does not exceed the 75?C rating? For the 400A would I be safe running 500Kcmil (430A@ 90? / 380A@75?) or do I need to run parallel 3/0?s (225A@90? / 200A@75?) only because the standard breaker will not accept anything larger than a 500Kcmil so running one set of 600Kcmil will not work? I have been asking around my office and seem to get differing opinions and would like to hear everyone else?s views on the subject. Maybe I am just being to picky??

 

[don_resqcapt19]

Mike,
You are not permitted to use the 90?C ampacity for conductors that are terminated on breakers. If the OCPD is 400 amps, then you need 400 amps of wire. This has always been the rule for taps and transformer secondary conductors, but that was not clear to many code users in the past so wording was added to make it clear that you can't use the provisions of 240.4(B) for these conductors.
Don

 

[Mike01]

??

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What I do not understand is the 90?C is used all the time for conductor de-rating, is there even any equipment with 90?C lugs? So what I am understanding is that the ampacity of the cable is determined by the temperature rating of the terminals it lands in. So if I use 90?C. wire but land in a 75?C terminal the extra insulation really does nothing if I cannot use that column and de-rate from it. So can?t I use a conductors 90?C rating for the ampacity because that is really the ampacity of the conductor, and provide my load to be less than the 75? column? What about article 110.14.C

 

[charlie b]

The design sequence is, (1) Define the load, (2) Pick a wire that has an ampacity at least as high as that load, and (3) Pick an OCPD that protects that wire size. Sometimes, we get to use the next higher standard rated OCPD.

The difference between using the 90C column and the 75C column, for the purposes of derating, is that this will sometimes prevent us from having to use a larger wire. For example, suppose you put 9 current-carrying #12 conductors in the same raceway. The 90C rating for #12 is 30 amps. The derating factor for 9 CCC?s is 70%. When you take 70% of 30 amps, the result is 21 amps. That value is lower than the value of 25 amps posted in the 75C column, so it becomes the limiting factor. But on top of that requirement, 110.4(C) tells us to use no higher an OCPD than 20 amps. The bottom line is that with 9 CCC?s in a raceway, we can still use #12, and we can still use 20 amp breakers.

If, however, we had to start with the 75C value, we would take 70% of 25 amps, and get 17.5 amps. We would still be able to protect that wire with a 20 amp breaker, but we would not be able to use that wire for a 20 amp load. That is, if 100% of the non-continuous load added to 125% of the continuous load were higher than 17.5 amps, then we would be forced to biggee-size the conductors to #10. The conclusion, then, is that use of the 90C column, for the purposes of derating, allows us to use #12, and not have to go to #10.

By counter example, if you tried to use 9 CCC?s in the same raceway, each sized at #10, and wanted to use the ?usual? 30 amp breaker, it would not be allowed. You can do the math, as a homework assignment.

 

[Mike01]

what about xfmrs.

what about xfmrs.

Thanks Charlie but what about when they are Xfmr secondary conductors the code says that 240.4 shall not apply and is says that the rating of the secondary conductors has to match the ocpd it terminates within, so could I use 500Kcmil@90? for the feeders on the secondary as long as I do not exceed the 75? rating of do the conductors have to be fully rated at 75?C?

 

[infinity]

The secondary conductors must be protected at or below their 75 degree C ampacity. If your secondary conductors terminate in a 400 OCPD you'll need a conductor rated for at least 400 amps at 75 degrees C. Typically this would be one set of 600 kcmil or two sets of 3/0 cu conductors.

 

[bob]

Thanks Charlie but what about when they are Xfmr secondary conductors the code says that 240.4 shall not apply and is says that the rating of the secondary conductors has to match the ocpd it terminates within, so could I use 500Kcmil@90? for the feeders on the secondary as long as I do not exceed the 75? rating of do the conductors have to be fully rated at 75?C?

Mike
If you use the 500 kcm and the load is 380 amps, the conductor will rise
to a temperature of 75C under the conditions listed in table 310.16. If
the load is 400 amps, the conductor will rise to a temperature greater than
75C and heat the breaker to a temperature higher that its design rating.
The code requires that the conductor ampacity be equal to or greater than the rating of overcurrent device so you can not use the 90C rating.