ECN Forum Forums Code Related Discussion NEC & other Code issues sharing neutral

Just so we compare apples, Let's look at it like this.

In Roger's above example, the IR losses would be cut in half. Let's call it 50%...

Now then, Roger's example is based on (4) 15a loads. Total 7200W..

The losses using 2 wire is 5%..
The losses using 3 wire is 2.5%.

So even know 50% in losses, are only really a 2.5% savings, but it is savings.

Now, let's say we use 2 wire, 10ga.

Using the same loading, 2 wire installation equates to 223W. The 12 ga was 376W.

Just being fair here, this experiment was on a 240V residential power. Not 208V 3ph.

I think we all can agree, that 3 ph power and nuetral currents, do not fit the above examples, and I think this is what the original poster was intersted in.

Good discussion....


Dnk....

Dnk, apply any equal numbers (unequal if you want) to two sets of A,B,and C phases of wye connected MWBC's verses (6) two wire circuits and do the math, you will see a savings as well.

When you do this assume linear loads for simplicity.

We all agree that certain loads need special consideration in the way of oversized neutrals or even two wire circuitry, but to think this is a problem with all MWBC's is simply not the case.

As an aside, how many people here can swear that they have seen a melted neutral conductor or connection due to additive hamonics?

Roger

Don't believe one would have 20 amps on the neutral in this case. The phases are out by 120 degrees so it wouldn't it be something less than 20??

Walrus, it would be 20, see the formula below.

Using 20 amps on A and B;

sqrt of (IA^2 + IB^2) - (IA x IB)

(IA^2 + IB^2) - (A x B)
20x20 + 20x20 - 20 x 20
400 + 400 - 400
800 - 400 = 400

sqrt of 400 = 20


Roger

Bob:


But are you referring to connected equipment or the wiring system itself? In my case, only one of many experiences was due to wiring system failure (loose wirenut); all the other problems were despite the fact the wiring itself was o.k.


Sort of. In my case, I never used a 'scope or power quality analyzer on the circuits, I just followed my instinct and pulled in the extra neutral. Wham, bam. Problem gone. Remember, my situation is usually noise, hum or erratic automation issues, along with occasional amplifier repeated failures. My file of old service reports (from only the last five years, though) was really the only logging. And admittedly, the percentage of neutral problem repairs in relation to my total workload is quite low, maybe 2% total. {Most cinema service calls are for mechanical breakdowns or non-power related sound issues.} I have a hunch your experience with this issue reflects about the same overall percentage.


To each other, maybe. I like hearing of other's experiences, that's how I learn new things. To ourselves, I disagree. We are, after all, in two different working environments. But we do have the same objective: a quality, cost-effective and trouble-free installation.


You are the last person I'd ever suspect of trying to pick a fight.
But since I don't like MWBC's, I won't use them. They do have a place and proper application, my work isn't one of them.

As dnk said, good discussion!

edited to fix html

[This message has been edited by mxslick (edited 01-20-2006).]

A total of ONE (for sure) in over 15 years... It was a maxed out panel feed with a reduced neutral. Panel was scoped before and after the new feeder and panel to have high harmonic content, but still can only say it was a contributing factor. The only other PQ problem sever enough for mention was a building that kept killing anything with a transformer... UPS's, E lighting inverters, and ballasts, etc. Loads of 4-wire circuits, but for more than a year on that project, no sign of a single burned neutral on the any new or existing wiring. New POCO transformer, and a seperate one, and service for the elevator (the main culprit) corrected the problem.

Harmonic issues do exist in varying degrees, but are rarely enough to cause issues great enough to think about change of wiring method. In just about all of the burned neutral wire-nut problems I have seen over the years, I can point more toward poor connection practices than anything else! Un-twisted wires in a cheap spring nut...

Roger
Thanks for posting that equation, I'm guessing its some quadratic solution. Can you post a link? where I can find an explanation for how they derive it?? I'm asking as this has puzzled me for some time. I had a location where I was feeding a pump with 2 motors. I shared a neutral for these 2 120 loads. After I had got this pump running I switched on both motors and checked the current in neutral with a clamp on, expecting to see 0. Surprise,I didn't. This was fed from a sub panel installed by others. It was 2 phases of a 208 y service. Anyway the motors name plate showed the motors at 9 amps and they checked correctly on hot conductor but the neutral showed 4 amps when both were running. I finally realized why I didn't see 0 but couldn't explain why I was getting 4 amps. Using your formula I should see nameplate on neutral.?

Hello Walrus, the formula I showed is a simple standard formula for wye neutral current, it was shortened using just two phases from the following.

neutral current = sqrt of
(IA^2+IB^2+IC^2)-(IAxIB)+(IBxIC)+(ICxIA)

To go into more depth of wye neutral current you can go HERE .

Roger

The multiwire paranoia seems pretty silly to me. I've done commercial service work for about 30 yrs of my "electricianing". I have yet to see a problem that could be directly attributed to a shared neutral. Loose connectons, sure. 2 within the last week.
There are an awful lot of modular wiring (like Reloc) systems in use with fluorescent and HID lighting. I've yet to see one with individual neutrals, and still haven't seen one burnt up because of it.

Oversize neutrals are fine if you want, but considering: You can't buy a 3 phase 6 wire device, disconnect, panelboard, transformer, or service, I fail to see the point of running individual neutrals on branch circuits.

It reminds me of the $158 audiophile duplex receptacle that was discussed here once. It had gold plated contacts. It would then be connected to a home's copper wiring, and at the service, to the PoCo's aluminum service drop and distribution system.

Iwire and I were the original guys that "agreed to disagree" on many subjects.
We actually agree on so many things that it's kinda scary. I'm with with Roger and him 100% on this one.

I had some typos



[This message has been edited by electure (edited 01-21-2006).]

Roger
Thanks for the link, not sure my feeble brain 20 something years removed from my last Calculus class can handle it . Its going to take a few reads thats for sure.