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Joined: Jan 2005
Posts: 109
Grover Offline OP
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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

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Joined: Jan 2005
Posts: 109
Grover Offline OP
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It appears that, since 1996 or thereabouts, the NEC "discovered" harmonics and non-linear loads, and it's been a quagmire ever since.

Joined: Apr 2002
Posts: 7,297
Likes: 6
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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.


John
Joined: Nov 2007
Posts: 763
K
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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.

Joined: Apr 2002
Posts: 7,297
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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.


John
Joined: Nov 2007
Posts: 763
K
Member
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.

Joined: Jan 2005
Posts: 109
Grover Offline OP
Member
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

Joined: Jun 2004
Posts: 1,273
T
Member
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.


Tesla
Joined: Jul 2005
Posts: 183
J
Member
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

Joined: Apr 2002
Posts: 7,297
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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?



John
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