Panel Feeder Sizing Sanity Check

[SteveBB]

Hello,

I currently am sizing a feeder for a 480VAC 3 phase 3-wire service from a gear to a distribution panel.

600A Bus Panel, No Main Breaker
600A 80% rated OCPD
THHN 90C Wire
Assuming 75C Terminal Ampacity

Here is my approach:

-I believe that with a panel without a main you cannot have an OCPD higher than the bus of the panel, therefore the 600A breaker referred to above.
-Taken that 600A breaker, at 80% limit, panel current will not exceed 480A
-Given the size I am jumping straight to parallel feeder calculations as I'm sure I won't want to deal with single.


6-300kcmil THHN w/ 2-#4 THHN ground in a single 3" rigid conduit
3-250kcmil THHN w/ 1 #4 THHN ground in two parallel 2" rigid conduit (This is the preferable option given to me by the contractor)

To get to the above I:

90C 300kcmil wire is good for 320A * 0.8 for 4-6 conductors in a conduit = 256A which is less than the 75C wire rating of 275A (2 sets for 512A > 480A)
90C 250kcmil wire is good for 290A * 1 for no additional derate = 290A which is more than the 75C wire rating but the 75C rating of 255A still covers the load. (2 sets for 510A > 480A)

Do this seem sound? I've ran it through a calculator I found online and it's telling me I need 350kcmil regardless of single or parallel conduit, so that has me second guessing my calculations. To be more clear, this calculator lists both a "grounded conductor size" and "ungrounded conductor size". The "grounded conductors size" matches my calculations above but the "ungrounded conductor size" is where 350kcmil is listed regardless of single or parallel conduit. This tells me that I am either calculating something wrong or I don't understand what the difference is between a grounded and ungrounded conductor.

 

[iwire]

Hello,

I currently am sizing a feeder for a 480VAC 3 phase 3-wire service from a gear to a distribution panel.

600A Bus Panel, No Main Breaker
600A 80% rated OCPD
THHN 90C Wire
Assuming 75C Terminal Ampacity

Here is my approach:

-I believe that with a panel without a main you cannot have an OCPD higher than the bus of the panel, therefore the 600A breaker referred to above.

I agree

-Taken that 600A breaker, at 80% limit, panel current will not exceed 480A

The breaker and panel can carry 600 amps up to 3 hours at a stretch or 480 amps forever.

-Given the size I am jumping straight to parallel feeder calculations as I'm sure I won't want to deal with single.

I would go with at least two sets for sure.

6-300kcmil THHN w/ 2-#4 THHN ground in a single 3" rigid conduit
3-250kcmil THHN w/ 1 #4 THHN ground in two parallel 2" rigid conduit (This is the preferable option given to me by the contractor)

To get to the above I:

90C 300kcmil wire is good for 320A * 0.8 for 4-6 conductors in a conduit = 256A which is less than the 75C wire rating of 275A (2 sets for 512A > 480A)
90C 250kcmil wire is good for 290A * 1 for no additional derate = 290A which is more than the 75C wire rating but the 75C rating of 255A still covers the load. (2 sets for 510A > 480A)

Do this seem sound? I've ran it through a calculator I found online and it's telling me I need 350kcmil regardless of single or parallel conduit, so that has me second guessing my calculations. To be more clear, this calculator lists both a "grounded conductor size" and "ungrounded conductor size". The "grounded conductors size" matches my calculations above but the "ungrounded conductor size" is where 350kcmil is listed regardless of single or parallel conduit. This tells me that I am either calculating something wrong or I don't understand what the difference is between a grounded and ungrounded conductor.

I suspect the online calculators are basing it on the overcurrent device size alone and do not take into account the allowance to move up to the next larger overcurrent device.

To be able to roll up to the next size breaker you will need to have the calculated load in hand.

 

[david luchini]

6-300kcmil THHN w/ 2-#4 THHN ground in a single 3" rigid conduit
3-250kcmil THHN w/ 1 #4 THHN ground in two parallel 2" rigid conduit (This is the preferable option given to me by the contractor)

You will need to look at your EGC sizes. A 600A OCPD requires a #1, rather than a #4 (250.122).

 

[kwired]

Not sure about how the calculator came up with what it did.

300 kcmil has a 75C rating of 285 x 2 = 570. This can be protected with next standard size up which is 600 as long as the load is not over 570. A load of 571 would require larger conductors.

With one raceway, you will have six current carrying conductors so an 80% ampacity adjustment is necessary - but can be made to the 90 deg C rating. 300 has a 90 C ampacity of 320, 80% of that is 256. Two parallel conductors gives us an ampacity of 512. We can still protect that at 600 amps but the connected load can not exceed 512.

If you need full 600 amps for the load then 350kcmil is not large enough if all in one raceway because of temperature adjustments.

Also as mentioned you can not split the EGC into smaller conductors, it must be sized per 600 amp overcurrent protection device in each raceway, but would never need to be larger then the ungrounded conductors should they be smaller for some reason but that situation is not present here.