neutral conductor sizing for 3 phase panelboard

[billete]

Where can I find the neutral conductor sizing for a 3 phase panelboard 208/120? I would like to run 1 neutral for each 3, 120v. circuits. I would imagine it would only carry the imbalance of the 3 loads? I am looking in 250 of the NEC, but not finding anything. I must be way off base

[ February 03, 2004, 10:14 AM: Message edited by: billete ]

[roger]

Re: neutral conductor sizing for 3 phase panelboard

220.22

Roger

[ February 03, 2004, 09:15 AM: Message edited by: roger ]

[ryan_618]

Re: neutral conductor sizing for 3 phase panelboard

See 250.24(B) as well.

Notice, however, that this section is limitted in its application to service equipment.

[ February 03, 2004, 09:26 AM: Message edited by: ryan_618 ]

[websparky]

Re: neutral conductor sizing for 3 phase panelboard

Please see 215.2 Minimum Rating and Size.

[daveinca]

Re: neutral conductor sizing for 3 phase panelboard

This may not be in the Code, but my opinions. If you have anything in the way of harmonic-producing equipment on these circuits, computers, etc., I would oversize the neutral to handle possible triplen harmonics which could exceed the current in any one hot. You will find this in office cubicle wiring harnesses, usually 3-12 ga. hots with a 10 ga neutral. I prefer (data center) to run separate neutrals and grounds for every circuit, minimizes noise between those big computers!

[ryan_618]

Re: neutral conductor sizing for 3 phase panelboard

Daveinca: I certainly don't disagree with your post, but I want to add that I find it very interesting that when talking about non-linear loads in an office environment, the flourescant lighting is seldom disussed. It is my understanding that these lights are just as big of a problem as computers for creating harmonics from their non-linear nature.

[bennie]

Re: neutral conductor sizing for 3 phase panelboard

I have heard the gloom and doom about harmonic current for 50 years. I always understood the appearance of the (in phase neutral) current points out the need for using full size neutral conductors instead of reducing the size.

I never considered using a larger size neutral than the active line conductors.

Does anyone have the calculations for the highest possible current on a system with a high THD?

[Ed MacLaren]

Re: neutral conductor sizing for 3 phase panelboard

AFC markets a line of "Super Neutral" cables, with both an oversized common neutral version, and one with a neutral for each phase.

AFC cables

Ed

[tom baker]

Re: neutral conductor sizing for 3 phase panelboard

I have had students in my classes from a school district that have had neutrals burn up from harmonic currents.

[ryan_618]

Re: neutral conductor sizing for 3 phase panelboard

One of the things that I find interesting on the plans that I review is that the EE will often spec a 200% nuetral on feeder circuits to panelboards, but not on the branch circuits that supply the non-linear loads. I wonder why that is?

[iwire]

Re: neutral conductor sizing for 3 phase panelboard

Ryan I have three replies for that.

1)Oversizeing the neutral would not do anything for a standard two wire circuit.

2)At least in the circuits I deal with that feed non-linear loads, the ciruit is very lightly loaded. 4 or 5 amps.

3)Take a multiwire branch circuit fed with 12/4 MC, each of the three legs even if loaded with 10 amps of non linear loads will not overload the 12 THHN neutral with a rating of 25 amps.

JMO, I would like to here Derecks take on this.

[bennie]

Re: neutral conductor sizing for 3 phase panelboard

Would reducing the load on each branch circuit and feeder be a more positive method to reduce current?

[dereckbc]

[iwire]

Re: neutral conductor sizing for 3 phase panelboard

Dereck thanks for the info, I have a question.

I thought I read that the harmonic currents could only add up to 1.73 of the phase current.

Is that a fact, or is there no limit to the amount of harmonic current.

Bob

[dereckbc]

Re: neutral conductor sizing for 3 phase panelboard

Theoretical limit is 173%, but I have never witnessed it.