400 A single phase 240 volt service, outside building run in 3" PVC with weatherhead.

400 A copper from 2008 Table 310.16 calls for 500 MCM THHN, but what about the size of the neutral? I've been told I can use 250 MCM, but can't seem to find a code reference.

Anybody?

Thanks!

Grov

It appears that, since 1996 or thereabouts, the NEC "discovered" harmonics and non-linear loads, and it's been a quagmire ever since.

Grover:

220.61 (from '11)
"(C) Prohibited Reductions. There shall be no reduction of
the neutral or grounded conductor capacity applied to the
amount in 220.61(C)(1), or portion of the amount in (C)(2),
from that determined by the basic calculation:
(1) Any portion of a 3-wire circuit consisting of 2 ungrounded
conductors and the neutral conductor of a
4-wire, 3-phase, wye-connected system
(2) That portion consisting of nonlinear loads supplied
from a 4-wire, wye-connected, 3-phase system
Informational Note No. 1: See Examples D1(a), D1(b),
D2(b), D4(a), and D5(a) in Informative Annex D.
Informational Note No. 2: A 3-phase, 4-wire, wye-connected
power system used to supply power to nonlinear loads may
necessitate that the power system design allow for the possibility
of high harmonic neutral-conductor currents."

In addition to the above, a local POCO does NOT allow reduced neutrals for comm/ind. services that have a neutral.

If its single phase and you’re using 500 Kcmil in a single raceway, couldn’t you size your grounded/neutral conductor using 230.42[C], which references 250.24[C] in the 2011 NEC?
250.24C[1] says to use Table 250.66. Sounds crazy, but following this, it looks like 1/0 Cu would be the minimum size grounded/neutral conductor.

Art 250 is the grounding (earthing) article and deals with GECs and bonding conductor sizes.

Art 220 calculations allow for 'undersize' neutral conductors, based on load calculations, in some crcumstances. Usually substantial 3 phase loads, but the POCO winds up trumping the load calcs with their requirements.

IMHO, it's not $$ worth it.

Yes, but did they change the meaning of grounded conductor in the NEC?
230.42[C] says minimum size and rating of the grounded conductor.

It looks like what they are getting at is that if the unbalanced load calculated in 220.61 results in a ampacity of less than the conductor sized in 250.66, then that then becomes the minimum size for the grounded service conductor. In essense it appears they are saying that the grounded/neutral conductor can't be smaller than the grounding conductor.

Thanks for all the comments guys! I'll dig some more, but, as I said before, it's a single phase 240 V system, not 3 phase, and it's a small variety store with pizza ovens etc, and we're still on the 2008 code back here in the woods!

Grov

PG&E routinely uses undersized neutrals:

1000 KCmil hots (3)
350 KCmil neutral (1)
no ground bond
3 phase system.
per pipe... each 5" (EUSERC) last job only 3 out of 8 pipes had wire.
The MAIN was set at 2,000 Amps
This enters a EUSERC big box from below slab... with dependent Services at 400, 400, and a string of 200's

-------

I've installed 3 phase panels with massive 3 phase rotary loads.
1100 Amps in a 1200 Frame chassis -- four raceways with a massive Al bonding wire ( 1100 Amps ) while the largest possible unbalanced load was 90A resulting in a dinky neutral.

Reducing the Service neutral is entirely determined by the Poco -- it's outside the scope of the NEC. That hunk of wire is going to be deeded over to the Poco and become their liability for performance.

Because of those dynamics, it's almost never worth the hassle of even raising the topic -- unless it's been established on the approved one-line submitted at the beginning.

The answer to this issue is never to be found in the NEC. It's a Poco thing. They have their standards -- and they don't give a dang what the NEC says.

FYI, in BC we have a directive that provides interpretation about the neutral derating that is permitted under certain circumstances under Canadian Code. The rule recognizes effects of third harmonics from specific loads.

http://safetyauthority.ca/sites/def...4%20Section%204%20-%20Conductors%202.pdf

Grover:
NJ is still '08 NEC as of today, just FYI.

The main reasoning for NOT undersizing the neutral is what happens upon a change of occupancy/use of the structure.

As I said, IMHO, it's not worth the brain cells, and BTW, what is your POCO requirements? Also, have you ran this by the local AHJ?

Grover if it's a 240 volt single phase system why not use Table 310.15 B 6 from 2008 and not down size.

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.

Gentlemen:

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

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

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!

*** 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!!!


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...

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!!!

-- Scott

Right you are Scott. I grabbed the feeder calculation by mistake, hence the 250.122 reference.

Sorry for any confusion.

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?

Greg and John;

Thanks for the replies!!!

Per John's last message:



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!

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:
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.

b-b-b-b-b-ut.....the triplex urd comes with a reduced noodle....~S~

I was going to bring that up myself

It can be ordered either way tho.

The wiring on the load side of the service point has always had a full sized neutral in my experience.
I never gave much thought to what the PoCo was doing since I didn't have much ability to change it.

Greg:

Yes, triplex is reduced, but that is on their side!

My references are to typical NJ (PSE&G) secondaries that are 'customer installed' from pad mount xfrs. The rule also applies to O/H.

I agree, everything on "my side" has had full sized neutrals.

God willing, and the creek don't rise (again!) I'll be installing 11 runs of 750KCMIL w/ 250 KCmil grounds monday

at first sniff that 250 seemed a tad small

but they're not calling it a neutral

they're calling it a groundING conductor (there's your 12.5%)

and 'earthing' a noodle from the 4000A 480 switchgear

this was engineered , and done so with both the poco and architectual engineers working in unision btw

all code ref's aside, i just don't like creating something that is readilly available, especially in close proximty to so many potential returns to the serving xfomer (or the other 2000A xfomer bank aboput 40' away)

~S~