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Re: 400 A service entrance neutral wire size? [Re: Grover] #203030 09/05/11 06:54 PM
Joined: Mar 2007
Posts: 101
M
mikethebull Offline
Member
Grover if it's a 240 volt single phase system why not use Table 310.15 B 6 from 2008 and not down size.

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Re: 400 A service entrance neutral wire size? [Re: Grover] #203053 09/06/11 08:11 PM
Joined: Jan 2001
Posts: 1,044
Tom Offline
Member
As pointed out by KJay, 250.66 is the minimum sized grounded conductor (usually a neutral) that is permitted. This size is required to provide adequate capacity to conduct ground fault currents back to the source.

To figure out the the minimum size for a grounded conductor, see 220.61(A) You'll need to determine the net calculated load for each phase and that will be the maximum load on the grounded conductor (pick the largest calculated load of the two phases). Then you can apply 220.61(B)(2) which says you can apply a 70% demand factor to any neutral load in excess of 200 amps. See example D4 in annex D, page 767, left column, near the bottom "feeder neutral" and "further demand factor."

I'm with Hotline, this isn't worth the few $$$ you'll save.


Few things are harder to put up with than the annoyance of a good example.
Re: 400 A service entrance neutral wire size? [Re: Grover] #203056 09/06/11 10:02 PM
Joined: Apr 2002
Posts: 7,113
HotLine1 Offline
Member
Gentlemen:

Not to repeat, but Article 250 is for grounding electrode conductors, and bonding conductors (GECs & EGCs) not the 'Grounded Conductor' (Neutral).



John
Re: 400 A service entrance neutral wire size? [Re: Grover] #203058 09/06/11 10:27 PM
Joined: Jul 2004
Posts: 9,394
G
gfretwell Offline
Member
John, they are probably thinking about second to the the last line of 215.2(A)(1)

... except it is 250.122, not 250.66

Quote
The size of the feeder circuit grounded conductor shall not be smaller than that required by 250.122, except that 250.122(F) shall not apply where grounded conductors are run in parallel.


Greg Fretwell
Re: 400 A service entrance neutral wire size? [Re: Grover] #203073 09/07/11 04:31 PM
Joined: Oct 2000
Posts: 2,720
Scott35 Offline
Broom Pusher and
Member
Grover's Service Grounded Neutral Conductor size is figured by bouncing around Chapter 2, referencing several Articles in Chapter 2, and finally landing at Section 220.61(A).

Here is the path to 220.61(A):

Beginning in Article 230 - Services, seek out Part II. Overhead Service-Drop Conductors, then slide down to Section 230.23 Size and Rating.
At the end of Section 230.23 is Subsection 230.23(C) Grounded Conductors.
This Subsection directs the user to the reference for the _MINIMUM_ Grounded Conductor size.

"The Grounded Conductor shall not be less than the Minimum Size as required by Section 250.24(C)"

So off to Article 250 we go! yay

*** BTW, Section 230.31(C) for Underground Service-Lateral Conductors references 250.24(C) for Minimum Size Grounded Conductor. ***

Arriving at 250.24, we stroll down to 250.24(C) "Grounded Conductor Brought to Service Equipment", and follow it's pathway to Subsection 250.24(C)(1) "Rating and Sizing", which dictates:

"This conductor shall be routed with the phase conductors and shall not be smaller than the required grounding electrode conductor specified in Table 250.66, but shall not be required to be larger than the largest ungrounded service-entrance phase conductor.

In addition, for service-entrance phase conductors larger than 1100 kcmil copper or 1750 kcmil aluminum, the grounded
conductor shall not be smaller than 12-1/2 percent of the area of the largest service-entrance phase conductor.

The grounded conductor of a 3-phase, 3-wire delta service shall have an ampacity not less than that of the ungrounded
conductors."

(Personal Note/Comment to the last paragraph: The Grounded conductor on a Grounded 3 Phase 3 Wire Delta Service will be the "Corner" of the Triangle - or simply one of the Three Phases.
Reduction in Conductor size is not an option, as this Grounded Conductor is not a "Balanced Common Neutral")

With that whole Delta-Thing in the Rear-View Mirror, we now drive our Golf Cart over to Section 250.66 "Size Of Alternating-Current Grounding Electrode Conductor.", at which point, we volunteer for a Breath-ilizer test!
It seems we may be Drunk!!! Why are we at 250.66, which deals with Grounding Electrode System Conductor sizing!!! We are trying to size the System's Balanced Neutral Grounded Conductor! The two are worlds apart!

Breath-ilizer Test says we are 100% Sober, so we hop back in our Golf Cart and follow the path of 250.66, running into Table 250.66.

Grover's Ungrounded Service Entrance Conductors are specified as 500 MCM Copper.
Per Table 250.66, the Minimum size GEC applied to 500 MCM Cu. Ungrounded Conductors is either:

  • 1/0 Copper,
    or
  • 3/0 Aluminum.


So now we know the _SMALLEST_ size our Service Grounded Conductor may be - 1/0 Cu. or 3/0 Al.

For uniformity, let's choose the 1/0 Cu., since the Ungrounded Conductors are Copper.

OK, jump back in the Golf Cart, and drive directly to Article 220 - Branch Circuit, Feeder, and Service Calculations.

Make a left turn at Section 220.61, then toss out the Anchor - as this is the end of the Trek!!!
cheers thumbs

Section 220.61 "Feeder or Service Neutral Load" contains the Subsections "(A), (B), and (C)", which regard the calculation of the Balanced Grounded Neutral Conductor.

*** Note: Since the System has been referenced as "120/240V 1 Phase 3 Wire", the Grounded Conductor will be derived from a Center-Tap on a Single 240V Secondary Winding - therefore making this a "Balancing Noodle" type of System Grounded Conductor (carries the unbalanced Load from L-N / N-L, and L-N-L connected circuitry).

This would qualify it for "Permitted reductions" called out in Subsection (B)

IF THE SYSTEM IS DERIVED FROM A 208Y/120V 3 PHASE 4 WIRE NETWORK... The above does not apply, and the Grounded Conductor is considered "Non-Balancing" (maybe a Circus reject...???).

More on this in another long-winded thread... wink

So, the shortcut method would be to drive the Golf Cart directly into Article 220, and hoist Anchor at 220.61.

Hope you enjoyed the Trip!!! clap

-- Scott


Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!
Re: 400 A service entrance neutral wire size? [Re: Grover] #203077 09/07/11 05:22 PM
Joined: Jul 2004
Posts: 9,394
G
gfretwell Offline
Member
Right you are Scott. I grabbed the feeder calculation by mistake, hence the 250.122 reference.

Sorry for any confusion.


Greg Fretwell
Re: 400 A service entrance neutral wire size? [Re: Grover] #203089 09/07/11 09:48 PM
Joined: Apr 2002
Posts: 7,113
HotLine1 Offline
Member
Scott:

With all due respect, and a hearty 'Thank You' for the full tour and details, how often do you design or work on a single phase, 400 amp, 120/240 service and install an undersized neutral?


John
Re: 400 A service entrance neutral wire size? [Re: Grover] #203099 09/07/11 11:04 PM
Joined: Oct 2000
Posts: 2,720
Scott35 Offline
Broom Pusher and
Member
Greg and John;

Thanks for the replies!!!

Per John's last message:

Quote

how often do you design or work on a single phase, 400 amp, 120/240 service and install an undersized neutral?


The last 1 Phase 3 Wire 400 Amp Residential Service, where I performed any design or installation tasks upon, was in Late September of 2000; and that was an Underground Service Entrance!
We supplied and installed the Duct;
PoCo supplied, installed and terminated the Service Lateral Conductors.
PoCo (SCE) installed (2) 250 MCM XHHW Al., and (1) 2/0 XHHW Al.
IIRC, the Demand Load was around 250 Amps. That Load Calc is floating somewhere in my Archives, and likely is covered by a 1 Inch layer of Dust! eek

More to the point, I have never installed, nor designed, any 120/240V 1 Phase 3 Wire Service Entrance Conductors with a "Reduced Neutral".
Never saw a feasible benefit to going with that option, especially when an Overhead Service Entrance Feeder Set is - at most, 20 Feet in length!

-- Scott


Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!
Re: 400 A service entrance neutral wire size? [Re: Grover] #203105 09/08/11 10:11 AM
Joined: Apr 2002
Posts: 7,113
HotLine1 Offline
Member
Scott:
Thanks for the reply!

On comm jobs here our POCO does not allow a reduction in size of the neutral conductor(s).

Reasoning is for future change of use.



John
Re: 400 A service entrance neutral wire size? [Re: HotLine1] #203126 09/09/11 09:09 AM
Joined: Oct 2000
Posts: 5,373
S
sparky Offline
Member
b-b-b-b-b-ut.....the triplex urd comes with a reduced noodle....~S~

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