ECN Electrical Forum - Discussion Forums for Electricians, Inspectors and Related Professionals
ECN Shout Chat
Shout Box
Recent Posts
New fan motor for Panasonic
by HotLine1. 06/25/19 03:53 PM
Burning out of return(white) side of 120volt rec
by crselectric. 06/14/19 02:15 AM
Video door bell causing chime activation of coil
by JoeTestingEngr. 06/13/19 12:43 PM
Troubleshooting Voltage Drop Across Relay
by gfretwell. 06/09/19 11:49 AM
Apple Recalls Three-Prong Wall Plug Adapters
by Admin. 06/05/19 09:06 PM
New in the Gallery:
What is this for?
Plug terminals
Who's Online Now
0 registered members (), 7 guests, and 11 spiders.
Key: Admin, Global Mod, Mod
Previous Thread
Next Thread
Print Thread
Rate Thread
Parallel conductors #48940
02/23/05 06:31 PM
02/23/05 06:31 PM
PAUL0510  Offline OP
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.

Work Gear for Electricians and the Trades
Re: Parallel conductors #48941
02/23/05 07:09 PM
02/23/05 07:09 PM
highkvoltage  Offline
Joined: Aug 2004
Posts: 34
Massillon, Ohio, USA
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.

Re: Parallel conductors #48942
02/23/05 07:34 PM
02/23/05 07:34 PM
iwire  Offline
Joined: Jan 2003
Posts: 4,391
North Attleboro, MA USA
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
Re: Parallel conductors #48943
02/23/05 07:51 PM
02/23/05 07:51 PM
PAUL0510  Offline OP
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.

Re: Parallel conductors #48944
02/25/05 08:37 PM
02/25/05 08:37 PM
earlydean  Offline
Joined: Dec 2003
Posts: 751
Griswold, CT, USA

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?

Re: Parallel conductors #48945
02/25/05 10:14 PM
02/25/05 10:14 PM
therain4  Offline
Joined: Dec 2000
Posts: 38
Dunmore,Pa. U>S.A.
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".

Re: Parallel conductors #48946
02/28/05 04:25 PM
02/28/05 04:25 PM
PAUL0510  Offline OP
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).



2017 Master Electrician Exam Preparation Combos
2017 NEC Electrician
Exam Prep Combos:
Master / Journeyman


Member Spotlight
Campbell, CA
Posts: 356
Joined: August 2006
Show All Member Profiles 
Top Posters(30 Days)
Popular Topics(Views)
255,877 Are you busy
191,856 Re: Forum
181,618 Need opinion
Powered by UBB.threads™ PHP Forum Software 7.6.1
(Release build 20180101)
Page Time: 0.025s Queries: 14 (0.004s) Memory: 0.9852 MB (Peak: 1.1337 MB) Zlib enabled. Server Time: 2019-06-26 14:52:52 UTC