ECN Electrical Forum - Discussion Forums for Electricians, Inspectors and Related Professionals
ECN Shout Chat
ShoutChat Box
Recent Posts
Unusual Well Pump Switch Failure
by sabrown - 10/28/20 04:31 PM
Look at this mess...
by InspectorE - 10/27/20 09:09 PM
New tool
by gfretwell - 10/24/20 11:09 AM
Where is Everyone?
by Bill Addiss - 10/17/20 07:04 PM
New in the Gallery:
Facebook follies, bad wiring
FPE in Germany pt.2
Who's Online Now
0 registered members (), 17 guests, and 17 spiders.
Key: Admin, Global Mod, Mod
Previous Thread
Next Thread
Print Thread
Rate Thread
Page 1 of 3 1 2 3
200a conductor sizing #29519 09/18/03 03:54 PM
Joined: Aug 2001
Posts: 545
A
aldav53 Offline OP
Member
Installing a 400a service with a 200a subpanel and tring to find the conductor size. Main conductors should be 3/0 (good for 215 amps). Not sure if I can down size the neutral one size or go with 3/0 also?
I believe the ground can be #4 according to table 250-66. Am I correct on this?


The Golden Rule - "The man with the gold makes the rule"
Work Gear for Electricians and the Trades
Re: 200a conductor sizing #29520 09/18/03 04:01 PM
Joined: Nov 2000
Posts: 246
R
rmiell Offline
Member
We will need a little more info. Is this for a residence? What type of wire? What does this "subpanel" feed? What is the supply voltages? Where do you get 215 amps for 3/0? What year of codebook are you using?

Rick

Re: 200a conductor sizing #29521 09/18/03 04:27 PM
Joined: Apr 2002
Posts: 189
T
triple Offline
Member
What do you mean by "main conductor"?

Cu or Al?

Unless you calculated the maximum possible neutral load, how can you reduce the neutral size? And, even though reducing the neutral may be possible now, a good electrician will plan ahead for future neutral loads. There are, of course, plenty of cases where a reduced neutral is perfectly fine (a motor control center for example).

Re: 200a conductor sizing #29522 09/18/03 05:00 PM
Joined: Aug 2001
Posts: 545
A
aldav53 Offline OP
Member
Yes it is a residence. I am installing a 400 amp panel feeding a 200 amp load center. It is single phase 240 volt. Sizing the feeders for the 200a load center, using THHN wire.
Got the sizing for the feeders from table B.310.1 in the 2002 NEC.
Will go the the same size on the neutral, as for the ground I believe it is #4.


The Golden Rule - "The man with the gold makes the rule"
Re: 200a conductor sizing #29523 09/18/03 05:21 PM
Joined: Apr 2002
Posts: 189
T
triple Offline
Member
3/0 Cu is what you want to use. Even though you are installing this wire in a residence, table 310.15(B)(6) cannot be used for a subpanel (otherwise 2/0 Cu would be sufficient). This was a matter of debate at one of my previous places of work so I bought two different books to clear up the confusion. Both books state that 310.15(b)(6) is not for subpanels since they don't carry the whole load of the house. In my opinion, neither the codebook nor the handbook do a very good job of describing the use of that table.

Re: 200a conductor sizing #29524 09/18/03 06:14 PM
Joined: Nov 2000
Posts: 246
R
rmiell Offline
Member
OK, but table B.310.1 is part of Annex B, and therefore not part of the requirements of the NEC.

Table 310.16 is the one to use. 3/0 cu in the 75 degree column is rated for 200amp, so 3/0 it is.

I seem to remember that the rule of thumb for reducing the neutral conductor is two sizes smaller, which would result in a 1/0 cu conductor. Take extra time in calculating this, as there are a number of rules to follow to do this.

As triple has mentioned, the use of the table 310.15(B)(6) would result in a smaller conductor, but it has some believers that it can not be used for a subpanel. Since you are installing a 400amp service, where will the other part of the load be going? If it will be for outbuildings, then perhaps the smaller wire will be justifiable, since the subpanel will indeed be the "main power feeder" to the residence.

Hope my, and the other's, information has helped.

Rick

Re: 200a conductor sizing #29525 09/18/03 07:36 PM
Joined: Apr 2002
Posts: 914
E
Electric Eagle Offline
Member
310.15(b) does apply here. It is for service entrance conductors AND feeders. So 2/0 CU or 4/0 AL is acceptable. It is to(or can) be used anytime the conductor serves more than 1 load, such as a sub panel.

Technically even the main panel in a house is a sub panel if the disconnect is not in the panel. The reason you can use a smaller conductor for a subpanel is because you aren't using all the loads continuously as you might be with a dedicated load.

[This message has been edited by Electric Eagle (edited 09-18-2003).]

Re: 200a conductor sizing #29526 09/18/03 10:08 PM
Joined: Apr 2002
Posts: 558
C
caselec Offline
Member
310.15(B)(6) says that the conductors listed in Table 310.15(B)(6) can be used for the MAIN power feed to dwelling. If a single family dwelling has multiple feeders I don't feel any of them can be considered the main feeder.

Curt


Curt Swartz
Re: 200a conductor sizing #29527 09/18/03 10:43 PM
A
Anonymous
Unregistered
What about the heading for table 310-15(b)(6) which says "120/240 Volt, 3-WIRE, single phase Dwelling Services and Feeders"

Wouldn't the part that says "3-Wire" rule out subpanels?

Re: 200a conductor sizing #29528 09/19/03 12:11 AM
Joined: Sep 2003
Posts: 200
U
u2slow Offline
Member
208/240V motor and heating loads will contribute the most to reducing the neutral conductor size. The CEC allows us to deduct these loads for determining neutral conductor ampacity - perhaps the NEC does too.

Page 1 of 3 1 2 3

Featured:

2020 National Electrical Code
2020 National Electrical
Code (NEC)

* * * * * * *

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

 

Member Spotlight
Niko
Niko
Campbell, CA
Posts: 356
Joined: August 2006
Show All Member Profiles 
Top Posters(30 Days)
Popular Topics(Views)
271,055 Are you busy
205,040 Re: Forum
193,165 Need opinion
Powered by UBB.threads™ PHP Forum Software 7.7.3