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#75444 02/25/07 01:27 PM
Joined: Oct 2005
Posts: 5
B
bdonus Offline OP
Junior Member
Is it permissible to install a larger size neutral than the hot conductor, such as a #10 neutral with a #12 hot conductor, or do they both have to be upsized. I can not find anything in the NEC.

#75445 02/25/07 01:35 PM
Joined: May 2002
Posts: 1,716
R
Member
It is permissible and is becoming more and more common on MWBC's where additive Harmonic Neutral current is possible.

I have a University project going on that is specked to have a #10 neutral on every 20 amp #12 MWBC to the point where it turns into two wire circuits.

Roger

#75446 02/25/07 01:38 PM
Joined: Jan 2003
Posts: 4,391
I
Moderator
I agree with Roger and end up working under similar requirements at times.

Now can we talk about if it is really needed? [Linked Image]


Bob Badger
Construction & Maintenance Electrician
Massachusetts
#75447 02/25/07 03:51 PM
Joined: Jul 2004
Posts: 9,669
Likes: 6
G
Member
Nah, just go to the AFC site and read their stories on it
Start here http://tycoelectrical.com/pdf/1801a_nations.pdf


Greg Fretwell
#75448 02/25/07 07:23 PM
Joined: Jun 2005
Posts: 821
S
Member
Those MWBC's are dangerous. [Linked Image]

#75449 02/26/07 10:36 AM
Joined: Jan 2007
Posts: 1
L
Junior Member
Hi all Im new here, but have been in the trade for 35 years.
I ran an large total remodel project on a city hall in this area. The engineer speced out parrell neutrals to all 120/208 V 3P, 4W panel boards. These were 200 amp. We had to pull 5 #4/0 to each panel from the distrubation panel.
He even had parrelle neutrals from the 500 KVA transfromer to the 1600 amp Distrubation panel.

#75450 02/26/07 01:35 PM
Joined: Mar 2005
Posts: 1,213
S
Member
With the advent of switched power supplies, the neutral current is no longer just the unbalance of the 3 phases- current on the neutral ends up being, in the worst case, essentially a square wave with an RMS value close to to the peak current on the primary conductors. So, if you've got a 100A 3-phase 60Hz circuit, you neutral could actually be seeing 173A 180Hz current with the 3rd harmonic alone. In the real world, it should never actually be this high, but it's very hard to predict, and because the neutral is unprotected from harmonic current, it's critical to err on the side of caution; I've seen 150% neutrals specced out on several installations.

The same is true of transformers- the delta primary in a delta/wye tends to trap circulating current, and thus must also be rated (or derated) to handle the harmonic current.

I would not be the least bit surprised if future revisions of the NEC require OCP on the neutral as well as the ungrounded conductors.

I'm not aware of any single-phase switched power supplies that would require an oversized nuetral.

Edit: I neglected to consider that the distorted wave on the 100A current conductors would allow higher peak current through OCP than if it were undistorted. 200% is generally regarded as the worst-case scenario, and I'm inclined to believe it! 245% is the theoretical maximum, but you're never going to see that in the real world. Neutrals are mostly impacted by odd triplen harmonics- 3, 9 and 15.

[This message has been edited by SteveFehr (edited 02-26-2007).]

[This message has been edited by SteveFehr (edited 02-26-2007).]

#75451 02/26/07 02:12 PM
Joined: Nov 2006
Posts: 348
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ITO Offline
Member
I am doing a university job now that has 200% neutrals spec-ed.


101° Rx = + /_\
#75452 02/26/07 03:08 PM
Joined: Mar 2005
Posts: 1,213
S
Member
I found an excellent pdf:
http://ieeexplore.ieee.org/iel5/9671/30555/01409326.pdf
Quote
In installations where there are a large number of single-phase, electronic non-linear loads connected to the same neutral, a high load factor can be found in that neutral. In these installations, the neutral current may exceed the phase current and a special attention must be given to sizing the neutral.

The diversified power absorbed by such a group of loads is generally limited, and even if the neutral current exceeds the line current, then the neutral conductor capacity is only exceeded in extreme circumstances if its size is equal to the line conductor's.

A common practice in these conditions is to use a 200% neutral conductor. This possibility is admitted by the National Electrical Code (USA), but does not form part of the electrical/ building regulations.

In high power installations (order of magnitude: P>lOOkVA or I>lSOA), various factors contribute to reduce
the neutral conductor load factor:

- More and more high quality IT equipment (work stations, servers, routers, PC, UPS, etc) include Power Factor Correction circuits, reducing considerably the generation of 3d harmonic currents

- Heating, Ventilation and Air-conditioning (HVAC) equipment in large buildings are supplied by a threephase network, and as such do not produce 3d
harmonic currents

- Fluorescent lighting equipment (with magnetic or electronic ballast) are generating 3" harmonic currents which are phase shifted with harmonic currents generated by PCs, giving a partial vector cancellation.

This attenuation effect is all the more as the installation power is high.
Except in exceptional circumstances, the 3d harmonic level in these installations does not exceed 33%, so the neutral current does not exceed the line currents. It is not therefore necessary to use an oversized neutral conductor.

#75453 02/26/07 03:55 PM
Joined: Jul 2006
Posts: 272
L
Member
Quote
The same is true of transformers- the delta primary in a delta/wye tends to trap circulating current, and thus must also be rated (or derated) to handle the harmonic current.

Rated mean K-Factor rated correct? Either use a K-Factor rated transformer or derate in kva to make sure that the the rated-temperature rise stays within limits.


Luke Clarke
Electrical Planner for TVA.

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