ECN Electrical Forum - Discussion Forums for Electricians, Inspectors and Related Professionals
ECN Shout Chat
ShoutChat
Recent Posts
Childrens safety outlets
by dsk - 06/25/22 04:07 PM
AC-DC stage Ultra-Fast charger, how it's made?
by libellis - 06/25/22 12:17 PM
Your favourite food?
by NORCAL - 06/16/22 12:06 AM
Price action trading with RENKO
by gfretwell - 06/15/22 02:08 PM
Please Help Wiring DCDC Buck Converter
by gfretwell - 06/15/22 01:58 PM
New in the Gallery:
240/208 to a house
240/208 to a house
by wa2ise, October 9
Now you know.
Now you know.
by Tom_Horne, September 7
Who's Online Now
0 members (), 11 guests, and 17 robots.
Key: Admin, Global Mod, Mod
Previous Thread
Next Thread
Print Thread
Rate Thread
#196137 09/15/10 12:01 PM
Joined: Jul 2002
Posts: 241
S
SJT Offline OP
Member
We are planning to run a feeder to a 225 amp 3 phase panel. 208V. The cable is a M/C type. Four conductors plus ground. The question is, I have to follow the 75C column in table 310.16 of the NEC, right? Next, I have to derate to 80% because of 3 or more current carrying conductors. Then the size cable is 300kcmil. I wanted to double check this. The contractor wants to use 4/0 size. If I am correct, it may be changed to conduit instead.
Thanks - Good Day

SJT #196139 09/15/10 12:57 PM
Joined: Jul 2004
Posts: 9,696
Likes: 11
G
Member
You can derate from the 90c column. That puts you in 250MCM for 4 @ 225a @ 80% (290 x .8 = 232)
If you used the round up rule that could be 4/0 since there is no standard breaker between 208a and 225.


Greg Fretwell
Joined: Jul 2002
Posts: 241
S
SJT Offline OP
Member
Thanks, sounds like 4/0 is the cable.

SJT #196149 09/15/10 11:52 PM
Joined: Jul 2002
Posts: 8,419
Likes: 3
Member
Originally Posted by SJT
Thanks, sounds like 4/0 is the cable.

OK,
What is the length of run?
Voltage drop could come into this as well.
Just a thought.

Joined: Jul 2002
Posts: 241
S
SJT Offline OP
Member
Approx. 60' in length. Back to the 310.16 table, I thought you had to use the 75C table. it's tempting to go to the 90c coloumn to use a smaller wire. 75c is what all the treminals are rated for?

SJT #196154 09/16/10 12:58 PM
Joined: Jul 2004
Posts: 9,696
Likes: 11
G
Member
You have to use the 75c (or lug rating) for the basic ampacity but when you derate you can use the 90c column for 90c wire. The thought is the heat will be distributed along the wire where it is bundled, not concentrated at the lug.


Greg Fretwell
Joined: Jun 2004
Posts: 1,273
T
Member
As a practical matter the ONLY reason for the 90c table is for the derating calculations. All ordinary lugs and terminals are rated for 75c or 60c.

One is also permitted to use the full rating on branch conductors for the crowding calculation, #10 @ 40A derated 50% = 20A.

#12 @ 30A derated to 70% = 21A ... still above standard derating of 20A for branch circuit #12 conductors.

When, in a 3-phase system, the fourth conductor is carrying only the unbalanced current AND no significant harmonics are present -- one may omit that fourth conductor from the wire count for the purposes of derating a bundle.

Meaning that four-wire feeder cable counts as just three and is NOT derated for bundling.


Tesla

Link Copied to Clipboard
Featured:

Tools for Electricians
Tools for Electricians
 

* * * * * * *

2020 Master Electrician Exam Preparation Combos
2020 NEC Electrician
Exam Prep Combos:
Master / Journeyman

 

Member Spotlight
Radar
Radar
Los Angeles
Posts: 349
Joined: April 2004
Top Posters(30 Days)
dsk 2
dobler 2
Asset 2
Popular Topics(Views)
293,163 Are you busy
225,241 Re: Forum
210,635 Need opinion
Powered by UBB.threads™ PHP Forum Software 7.7.5