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Posted By: Micmann Conductor sizing - 07/14/09 02:59 AM
Looking for a little help. I need to size conductors for a 400 amp 480v 3wire feed from a buss bar fused tap box to a transformer. The lugs on the tap box are rated for 75 degrees the lugs on the transformer are 75 degrees also. The wire will be thhn. Am I right in figuring I need to run 600ís to satisfy the 75 degree column even though the thhn is rated for 90 degrees and I could size it at 500ís? I think so, but my second question has me questioning myself.

Second question. I will the be going from the transformer to a 400 amp 480v 4 wire panel. Same question as above, what wire size? 75 degree lugs on transformer, however lugs in main breaker in panel maxes out at 500, the 600ís wonít fit. I believe the breaker is also rated for 75 degrees. Because the breaker is 400 amps and only has lugs big enough for 500ís itís leading me to this question of is it 500ís or 600ís I need to run? I think 600ís, am I missing something that lets me run 500ís?

Thanks for your help.
Posted By: Niko Re: Conductor sizing - 07/14/09 05:05 AM
You use the 75 degree column.
If you can change the lugs and make them double so you can install parallel 3/0.

Posted By: gfretwell Re: Conductor sizing - 07/14/09 07:07 AM
am I missing something that lets me run 500ís?

You have this

240.4(B)Devices Rated 800 Amperes or Less. The next higher standard overcurrent device rating (above the ampacity of the conductors being protected) shall be permitted to be used, provided all of the following conditions are met:
(1) The conductors being protected are not part of a multioutlet branch circuit supplying receptacles for cord-and-plug-connected portable loads.
(2) The ampacity of the conductors does not correspond with the standard ampere rating of a fuse or a circuit breaker without overload trip adjustments above its rating (but that shall be permitted to have other trip or rating adjustments).
(3) The next higher standard rating selected does not exceed 800 amperes.

It might not be the right design decision but it is legal to put 500MCM on a 400a breaker using the 75c ampacity of 380a rounded up.
Posted By: Yoopersup Re: Conductor sizing - 07/14/09 01:31 PM
4 wires 310.15(B)2 derating check if this applys on your 4 wire. If it does then # 500 x 430 x 80% = 344 .
240.4(b) as Greg states you can go to next highest size . 240.6 next higest size is 350.
If 310.15(B)2 applys to your run the 350 amp breaker is what 500's are good for in this case.
As far as the raceway with 3 500's in it no derateing so 500's are good togo in that case.

If the derateing on the 4 wire applys in your case then # 600 = 475 x 80% = 380 .
240.6 next highest size breaker 400 amps.
Posted By: Micmann Re: Conductor sizing - 07/14/09 04:08 PM
Thanks for the help.

Yoopersup, regarding your mention of the derateing leads me to another question. The conductors on the primary side will be 500ís according to what Greg pointed me to (forgot about the step up). However the secondary side of the transformer fit into the adjustment factor because there are four conductors Ė three phase conductors and one neutral Ė and my flex is over 24Ēlong. 310.15(B)(4) addresses the neutral conductor. If my loads consist of nonlinear loads I have to count it as a current carrying conductor. If my loads are not non linear than I donít need to count it. How do I know if my loads are nonlinear? 1/3rd from this panel will be 277v lighting and the other 2/3rds will be small 277v blower motors. I have an identical setup next to this one but it will be a 480 to 120/208 transformer feeding two 400 amp panels and all the circuits will be for wall outlets. I know you canít say for sure but whatís your feeling, is this nonlinear loading? If itís nonlinear than I need to derate and Iím up to 600ís to get into the 400amp breaker but the breaker only accepts 500ís, still no good. I guess I may have to see if I can special order parallel lugs like Edward mentioned. Not what I wanted to do for this installation, but may be forced to.

Posted By: gfretwell Re: Conductor sizing - 07/14/09 04:30 PM
If you have a significant inductive load you might want to be using a "super neutral" bigger than the phase conductors.
I don't know if there is a Excel triplin harmonic calculator out there but I bet one of our engineers will have some guidance
Posted By: Yoopersup Re: Conductor sizing - 07/14/09 04:33 PM
read 310.15 B (4) Neutral Conductor read exceptions also. This is veryyyyyyyyyyyyyy common most inspectors don't catch it & let 500's be installed.
Posted By: Micmann Re: Conductor sizing - 07/14/09 11:39 PM
Again, thanks for your help. I think Iíve resolved to run parallel. I have 4Ē conduit and flex between everything so I can fit paralleled conductors in one conduit. Iíll run 500ís from tap box to trans, paralleled 4/0 between trans and panel (because of the derateing of four single or eight paralleled conductors), and paralleled 3/0 again through the close nipple ( no derate necessary Ėless than 24Ē) connecting the tap lugs on the main panel to the main lugs on the second panel for the double tub 400 amp panels. Unless of course you have insight on something I can or need to do otherwise.

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