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#48940 02/23/05 05:31 PM
Joined: Jul 2004
Posts: 13

Location-industrial bldg

I need to feed a 400a 3 phase panel from an existing 400a 208v 3phase main breaker panel. I'm going to conect directly to the bus and use a mlo panel because the existing panel is full but only draws 50a each phase during peak houres. My question is abuot the size conductors to use. If i parallel the conductors what size do i need?
3/0 thhn =200a *2=400a?
500kcmil thhn= 430a 500/2=250kcmil
Do you go by the circular mil area?
This is in a small electrical rm and the panels will be about 4 ft apart conected with EMT or Trough i'm not sure yet.

#48941 02/23/05 06:09 PM
Joined: Aug 2004
Posts: 34
This I believe would be a code violation. I will have to look it up. If I remember correctly the largest breaker you can use is 225 amps. I'll have to check.

#48942 02/23/05 06:34 PM
Joined: Jan 2003
Posts: 4,391
Two sets of 3/0 if run in separate raceways will be code compliant. You would also need to use a 3 AWG EGC in each raceway if you choose to use one

If you run the two sets together you will need 4/0 minimum, but only one 3 AWG EGC.

Bob Badger
Construction & Maintenance Electrician
#48943 02/23/05 06:51 PM
Joined: Jul 2004
Posts: 13
Iwire thanks for the info. Do you have any code articles? I've looked and i couldn't find any info that tells you how to caculate a wire size when in parallel. Thanks again.

#48944 02/25/05 07:37 PM
Joined: Dec 2003
Posts: 751

Start with 310.15 Ampacities of Conductors. Section (B) tells us to use Table 310.16 for standard building wire with not more than 3 current-carrying conductors in a raceway. 310.4 covers Conductors in Parallel. Note the last sentence, which tells us to derate for more than 3 current-carrying conductors in a conduit.
Although the NEC never says so, we always simply added the allowed ampacity of the paralleled conductors to determine the total allowed ampacity of the paralleled assembly.
In other words, if I paralleled 3, 250 kcmil in separate conduits, 255 + 255 + 255 = 765 amps.
Reading the commentary, however, I see that conductors connected in parallel are considered a single conductor with a total cross-sectional area of all the conductors in parallel. Would that mean that my 3, 250 kcmil conductors in parallel should have their ampacity determined as if they were a single 750 kcmil conductor?, with an ampacity of only 475 amperes? Doesn't make a lot of sense when run in separate conduits.
Why is the allowed ampacity of a 750 kcmil conductor only 475 amps? Skin effect? I squared heating from all that current?
If I ran all my paralleled conductors in one conduit, I would need to derate to 70%. 70% of 765 is 535.5 amps. Must be a combination of the two. Or maybe something I am missing. Is the surface area of smaller conductors greater than that of one large conductor, and can therefore dissapate heat better? Anyone?

#48945 02/25/05 09:14 PM
Joined: Dec 2000
Posts: 38
In tne 2005 NEC article 310.4 has deleted the words "(electrically joined at both ends to form one a single conductor)".In articl 310.15 (B) (2) they have added "Each current carrying conductor of a parrelled set of conductors shall be counted as a current carrying condutor".

#48946 02/28/05 03:25 PM
Joined: Jul 2004
Posts: 13
Earlydean, heat dissapation and surface area are the ancers that i came up with also. You would think that there would be a table or a formula to calculate amperage and conductor size for parallel runs. As thick as the book is some info just isn't in (black and white).


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