Sharing neutrals in lighting circuits

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ryan_618 said:
I've never seen a luminaire that has a yoke.
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You have to get out more. :wink:

pool-03.jpg


Single yoke with two lighting outlets.

Multi-pole breaker required? :lol:

Edit in smaller picture.
 
Let me restate some of this. If a multiwire branch circuit is used to feeder a nonlinear load, such as flourescent lighting electronic or magnetic ballast, the neutral that is shared must derated. Most electricians, including myself, I have worked with do not like sharing neutrals. If there is a nonlinear load, the neutral must be derated.


I guess you could say 310.15(B)(4)(a) requires this but if they meant (a) to cover non-linear loads what would be the need for (c)?

Anyone with the 2002 NEC Handbook should take a look at the commentary following 310.15(B)(4). They present a lot of info on Non-Linear loads and the effect on equipment.

Here are a couple of quotes.

The data were obtained from consultants, equipment manufacturers, and testing laboratories, and included hundreds of feeder and branch circuits involving 3-phase, 4-wire, wye-connected systems with nonlinear loads. The data revealed that many circuits had neutral conductor current greater than the phase conductor current, and approximately 5 percent of all circuits reported had neutral conductor current exceeding 125 percent of the highest phase conductor current.

125% sounds extremely damaging until they go on to point out.

quote:.....the total continuous load on any overcurrent device located in a panelboard should not exceed 80 percent of its rating (the exception being assemblies listed for continuous operation at 100 percent of its rating). Because the neutral conductor is usually not connected to an overcurrent device, derating for continuous operation is not necessary. Therefore, neutral conductor ampacity is usually 125 percent of the maximum continuous current allowed by the overcurrent device.

There is much more in the handbook it is worth a look.

http://www.mikeholt.com/cgi-bin/codeforum/ultimatebb.cgi?ubb=print_topic;f=11;t=003000
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210.4 (B)
"Devices or Equipment. Where a multiwire branch circuit supplies more than one device or equipment on the same yoke, a means shall be provided to disconnect simultaneously all ungrounded conductors supplying those devices or equipment at the point where the branch circuit originates."

While this is code for multiwire branch ciruits, it is not enforced. But for the most part, if I'm spliting the yoke on a duplex receptacle for utilization equipment, I'll make sure both conductors turn off when one is disconnected. For the most part, any yoke containing a multiwire branch circuit is going to be disconnected when the breaker is off. Can anyone say 50A 240 volt 4 wire receptacle? Multipole breaker.
 
wavector said:
Most electricians, including myself, I have worked with do not like sharing neutrals.
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Why?

As for 310.15(B)(4), I try to remember that each of these are used as a rule to know when to consider the neutral a current carrying conductor (when applying 310.15(B)(2)(a)). That's it. Since derating will probably affect all conductors evenly, then it simplifies things a lot, IMO.

While this is code for multiwire branch ciruits, it is not enforced.
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It is around here. Why aren't they in Alabama? :)

But for the most part...
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Remember, in the event of a shock or fire, you are responsible for adhering to the NEC, even if the local jurisdiction is lax in their enforcement. Someday, in the event of an incident, the insurance company could be the AHJ...

...so I try real hard, myself, to adhere to the NEC, even if the local AHJ isn't looking for some things.

That's my polite cluck of the tongue for you. I fall short sometimes, but I try hard not to lazy up when I can avoid it. Tempting sometimes, but ultimately not worth it.

JMO. :)
 
wavector said:
Let me restate some of this. If a multiwire branch circuit is used to feeder a nonlinear load, such as flourescent lighting electronic or magnetic ballast, the neutral that is shared must derated.
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Really? :?

Can you cite a code section that requires this?

wavector said:
Most electricians, including myself, I have worked with do not like sharing neutrals.
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Don't speak for others as my friends and I run as many MWBCs as possible. 8)

Doing so saves material and reduces voltage drop issues.

wavector said:
If there is a nonlinear load, the neutral must be derated.
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Again....a code section that backs that up?
 
iwire said:
wavector said:
Let me restate some of this. If a multiwire branch circuit is used to feeder a nonlinear load, such as flourescent lighting electronic or magnetic ballast, the neutral that is shared must derated.
Click to expand...

Really? :?

Can you cite a code section that requires this?

wavector said:
Most electricians, including myself, I have worked with do not like sharing neutrals.
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Don't speak for others as my friends and I run as many MWBCs as possible. 8)

Doing so saves material and reduces voltage drop issues.

wavector said:
If there is a nonlinear load, the neutral must be derated.
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Again....a code section that backs that up?
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310.10(2)
 
Wavector, there is no 310.10(2).

If you are refering to an FPN, note that FPN's are only information and suggestions, they are not code.


Roger
 
Thanks Roger. :)


90.5(C) Explanatory Material. Explanatory material, such as references to other standards, references to related sections of this Code, or information related to a Code rule, is included in this Code in the form of fine print notes (FPNs). Fine print notes are informational only and are not enforceable as requirements of this Code.
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You're welcome Bob, :) and speaking of welcome, Wavector, welcome to the forum.

Roger
 
Consider the following.

3 non-linear lighting circuits fed from a 3 phase 4 wire 20 amp MWBC.

Lets say this circuit was run in 12/4 MC

Assume the circuits are a continuous load and the designer loaded the circuits to 14 amps each. (I find most engineers in this area only load the circuits to about 10 amps)

It is my understanding under the worst conditions the harmonic current will be 1.73 times the phase current.

That being the case the highest possible (very unlikely) load on the neutral will be less than 24 amps.

12 AWG is rated 25 amps @ 60 C. meaning it is not overloaded under the worst scenario.

Also consider that two wire circuits do no eliminate harmonic currents they just limit the issues to the panel board and feeder.

Frankly IMO this whole issue is much ado about nothing.

If you read around almost all info that speaks of the dangers of harmonic currents are from people or organizations that stand to make a profit by running more copper.

I have hung around electrical forums for a few years and can not remember anyone actually attesting to the fact they have seen equipment damage from harmonic current.
 
wavector wrote:
Most electricians, including myself, I have worked with do not like sharing neutrals.


Don't speak for others as my friends and I run as many MWBCs as possible.

Doing so saves material and reduces voltage drop issues.
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We run as many as possible also, for precisely the reasons that Bob's mentioned. One other reason that I would add would be labor savings. In a commercial application where many multiple homerun are installed, running one cable for every 3 circuits saves a lot of labor. Think about a 42 circuit panel with 2-wire homeruns. This requires 42 KO's, 42 connectors, etc. Now use 4-wire homeruns, that number is magically reduced to 14. Also factor in the termination of 28 less neutral conductors at the panel, well, you can see where I'm going here. MWBC can save a lot of installations costs over 2-wire circuits.
 
I did use the wrong terminology by stating dedicated, but I was in hurry and needed some answers. I have witnessed wannabe electricians, cowboy-type, breaking the neutrals shared by 120/208 wye system before breaking the hots. Why they didn't turn power off is your guess, I donno. Heck, they even broke the EGC first, then the grounded conductor. I went to one room, first floor, as the were about to take the neutral loose, and stopped them. After investigating, I found they had done this two other MWBC before I witnessed this one. Burned GFI recepts, lights, televisions, alarm clocks and a laptop. It was an occupied hotel remodel 4 floors. They were instructed not to do this. They ended up in unemployment line.
 
Wavector, if you're concerned about this and would still like to make use of the economic benefits of multiwire circuits, you can always elect to install handle-ties on the single-pole breakers supplying the MWBCs. It's currently optional if the circuits do not serve the same yoke (210.4).

In fact, if you feel really strongly about it, when the ROP's come out, write in a comment to support this proposal to the 2008 NEC:
2-10 Log #2679 NEC-P02 Action: Accept in Principle
(210.4)

Submitter: Dorothy Kellogg, American Chemistry Council
Panel Meeting Action: Accept in Principle
Revise the last sentence of 210.4(A) in the present code to read: "All conductors of a multiwire branch circuit shall originate from the same panelboard or similar distribution equipment."
Revise 210.4(B) in the present code to read: "Disconnecting Means. Each multiwire branch circuit shall be provided with a means that will simultaneously disconnect all ungrounded conductors at the point where the branch circuit originates."
Panel Statement: The panel has accepted the submitter's recommendation, but has revised the text in 210.4(A) and (B) to accomplish the intent of the submitter.
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I personally disagree with this proposal, and plan on commenting in that CMP-2 should reconsider their decision. But if you feel the technique is dangerous, you should write in to support them.

If they were busting open circuits without even shutting off the one they were dealing with, then they are putting more than just some equipment at risk. No matter how stupid those fellas were, they were more expensive if damaged (or dead) than a computer or a GFI. Their eventual trip to the unemployment line is well deserved. :?

You can't blame the wiring for the monkeys that play with it. 8)
 
Bob,

Do you have a reference for the "1.73 times the phase current" being the worst case? I would have said that if ALL the current were at the 3rd harmonic of the voltage frequency, then the neutral current is 3 times the phast current.

The more I thought about this, I might be thinking wye and you thinking delta. Never mind.

Mark
 
busman said:
Bob,

Do you have a reference for the "1.73 times the phase current" being the worst case? I would have said that if ALL the current were at the 3rd harmonic of the voltage frequency, then the neutral current is 3 times the phast current.

The more I thought about this, I might be thinking wye and you thinking delta. Never mind.

Mark
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Bob is correct, it is 1.73 time the phase current. I'll see if I can find the reference.
 
I got the 1.73 from one of the engineers on this forum.

They had stated that 'theoretically' the highest would be 1.73 times the phase current.

Are harmonics an issue at all with delta? :?
 
Mike Holt's published opinion is Nonlinear loads cause the neutral to carry as much as twice the phase current Scroll way down to the Note: under the "Art. 210 Branch Circuits" section just after, Q. Explain the disadvantages and dangers of multiwire branch circuits.
 
What is your point Roger?

It is hardly news to anyone that an open neutral on a MWBC or any feeder can cause wild voltage swings.

The rest of Mikes text deals with non-linear loads which again is not earth shattering news.

The fact is neutrals are not overheating on a regular basis on non-linear loads.

Yes you may reduce some heating in the branch circuit with the use of more copper.....that is always true no matter what the circuit characteristics.

No mater if you use two wire or MWBCs the harmonic issue (if there is one) will still be an issue for the panel, feeder and transformer.
 
iwire said:
What is your point Roger?
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Bob, try searching the page for the text "twice" with your browser's "FInd" text feature. You will see Mike Holts statement, "Nonlinear loads cause the neutral to carry as much as twice the phase current."

Without a reference supporting your 1.732 figure for max harmonics, Mike Holts published opinion includes one for busman, who asked for a reference.

I would't feal too much grief over this, 1.732 is close enough to 2.0. Besides, Mike Holt does not support this published opinion with any references either, so ultimately your figure and thoughtful advice could be more practical.
 
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