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

3-wire, and 4-wire circuits are your friends. You be nice to them, and they be nice to you... You give them a nice balanced load with some tight connections, and they give you a little extra money in your pocket for not running extra cables, conduit, and 1-2 extra conductors. In fact, it needs to be specified for me to do anything else.

Granted harmonic issues are on the rise due to the type of loads that generate them are on the increase. But I don't see that as a reason to stop using them. Lost neutrals can be problematic, but only if you dont follow a few simple rules in usage of them.

• Tight connections that are not totally dependant on the cheapest wire nut known.
  (WW-3 - Twisting does help here!)
• Balanced, or even relitively balanced loading.
• Circuit design, branching away from 3-wire, and 4-wire circuits once you have reached an area, to keep the number of connections on the shared neutral portion down. Instead on daisey chaining through every point. Get rid of one circuit right off by taking a HR to a dedicated load, find a load to match for the next, and so on.

Oh, I heard a wierd copper statisic today. We have now surpassed what is left in the ground, (expected yield) compared to what is already in use. But that could just be a ruse to keep the price up...



[This message has been edited by e57 (edited 01-18-2006).]

To the guys that like the use, do you mark the shared nuetral somehow in j/device boxes?


Dnk..

What to do about harmonics at the panel?

Pretty much nothing. In these set-ups, my panel neutral (and feed) is already a lot larger than double the size of the #12 branch circuit conductors. Then, there is a short run to the transfer switch, with the monster battery-bank UPS and isolated transformer close at hand.
Get harmonics through those, and you probably shoud re-consider your appliances!

:-)

And, yes, I absolutely agree that twisting the wires helps!

If you are really paranoid about open neutrals then you could consider using crimp caps on the multiwire neutral splices. The critical item in using them effectively is to use the manufacturers crimp tool and set it for the sleeve you are applying. As they are not readily opened there would be less chance of an open neutral.
--
Tom Horne

John,
No, all you have done is to mask one symptom of the harmonics issue. You have reduced the excessive heating and voltage drop in the neutral that can because by excessive harmonics. All of the other power quality problems are still there.
Don

Let's think about this.....

He wants to run 3 hots with a nuetral.

ASSUMPTION: he has a 208Y120 service.

So if we have 2 of the circuits at 20a each and the third circuit has nothing plugged into it.

We should have 20a on the nuetral? no biggie so far. However, with laser printers, power supplies, copiers ect. introducing harmonic currents on the nuetral also, you see we could be taking chances with the nuetral becoming overloaded. Now lets for fun do this for a whole commercial building for todays electronic equipment. We start to have problems.

They other problem I have with them, is the 1 loose connection problem.

I'll give you that some multi-wire circuits can be used for lighting, maybe a hair salon and such. But for todays typical commercial applications, I'll choose to stay away from them.

I don't think there is no right or wrong here, just preference.....


Dnk...

Has anyone ever seen an article written about harmonic problems that was not written by someone with an economic interest in solving the problem? I know a number of electrical engineers who do not specify oversized neutrals for large office building where we have all been told that there would be a "harmonic problem". They have never experienced problems. I have read of very very few documented cases of problems caused by harmonics. Most failures are a result of poor installation and not harmonics.
Don

dnk you are right it's just a preference thing and by all means do what makes you happy.


1) What is 12 AWG rated at 75 C?

2) Many times the cables we use are super neutral.


Perhaps, but the use of multiwire or two wire circuits only deals with the branch circuits.

All the panels and feeders will be subjected to the same amount of harmonics using multiwire or two wire circuits.

It is common in the buildings we wire that all the 208 neutrals, including the panelboard neutral bars will be 200% of the phase conductors.

All that said, I agree with Don, harmonic currents are not new, they existed before PCs came along. The only new thing is need to fix the problem.

I am not sure you can fix the problem as you say, the problem in inherent with the devices we use.

Using filters, mitigating transfomers and such help indeed.

Everyone has their own way, some for different reasons or for different results.

Don, I had a problem/project that I am just finishing. After months of measuring and consulting with suppport enginneering from Cutler Hammer, we eliminated excess heat from the system, directlty caused by harmonics.
The savings are qiute pleasing.

Dnk...

"consider using crimp caps on the multiwire neutral splices" Like the idea, but experiance drives me the other way. Did many years of service work where crimps in current carrying conductors would fail. They heat slightly in use, and due to evironmental temp changes, and eventually work loose under load, and start to arc. I don't even like them for grounding. But if twited AND crimped, I might be OK with that...

"So if we have 2 of the circuits at 20a each and the third circuit has nothing plugged into it."

(Assuming they are 20 circuits...) If you have a circuit pushing a full 20A there are going to be problems anyway. Try to keep the loads even and well below 80%... Even if you had that scenario with 2 @ 20A on a wye system the neutral load won't be more than 20A. And if one was slighly higher or lower, it would go down, instead of up. Say A were @ 15A, B @ 10A, C @ 0A, N @ ~13A... #12 THHN is actualy rated for 30A. (Before derating, or 240.4d) With a 2-wire circuit on a wye system @ xA, the neutral is xA, with no chance for the benefit of an unbalanced load to offset it. IMO

"To the guys that like the use, do you mark the shared nuetral somehow in j/device boxes?" We normaly group the conductors of one roundhouse together with some tape, or a different colored lettering on each. So say I had two, 4-wire circuits... One circuit would have all the conductors in that circuit with a wrap of tape normally. We do the same for 2-wire circuits so we don't mix them up... Some of the larger supply houses, if you order in advance, can get you different colors on the lettering, but that is only usefull if you are doing a WHOLE job, not very usefull on remodels. And you can get stripped wire in different colors if you have more than one transformer in the building.