Neutral Sharing 3 phase 120 volts

[Fordean]

In area we been working. I tend to need only two circuits. And low and behold after done they would like to add another. Which turns out to be a chore.
Would like to know the drawbacks to running 4 wire, 3 hot +1 Neutral +1 g. What is drawback on using only two circuits for time being and 3rd having no load (spare) but all sharing. I know I must use a 3 pole Tied breaker.
To comply with NEC. Which finally I Like. But I have never been a neutral sharing contractor. I would avoid at all costs. This is only because of this Scenario. Cant run pipe. And pain to keep running multiple cable runs.

Would like to Run 1 - 4 wire. I would probably run # 10 and Still rated on 20 amp breakers. But is 10 overkill. Would you run # 10 or stay 12 under 100 feet.

 

[iwire]

There is nothing wrong with what you describe, we do it often

It is worth mentioning that you do not need a more expensive 3 pole common trip breaker, all you need is 3 single poles and an approved handle tie for them.

There is no reason to use 10 unless you have voltage drop concerns.

 

[jumper]

12/4 w/ground MC is a staple of mine. Makes life real easy.

 

[iwire]

12/4 w/ground MC is a staple of mine. Makes life real easy.

I agree 100%

 

[Fordean]

How about the issue of using only just two of the phases leaving one as a future spare. I don't see no problems. Even though it not balanced now. Their should be no problems. Question is can you overload neutral some how.
With the use of only two of the phases.

I no the math. But just double checking. I would like someone to post a scenario as what would could do this type of overload. Linear and non linear loading.

 

[iwire]

Question is can you overload neutral some how.
With the use of only two of the phases.

No, no chance assuming wired correctly.

I would like someone to post a scenario as what would could do this type of overload. Linear and non linear loading.

If you loaded up all three circuits to 20 amps of non-linear load you might have more than 20 amps on the neutral.

But would you design a circuit like that?

 

[jumper]

If you loaded up all three circuits to 20 amps of non-linear load you might have more than 20 amps on the neutral.

My brain must be fuzzy this morning, wouldn't that still be 0 amps on the neutral?

I thought non-linear loading of the neutral would be the highest at full loading of 2 phases and none on the third.

 

[iwire]

My brain must be fuzzy this morning, wouldn't that still be 0 amps on the neutral?

I thought non-linear loading of the neutral would be the highest at full loading of 2 phases and none on the third.

On a ballanced linear circutuits the neutral load would be 0

With non-linear loads the neutral load would be the triplen harmonics.

I really don't know if that would be worse or not with 2 loaded legs or 3.

 

[jumper]

On a ballanced linear circutuits the neutral load would be 0

With non-linear loads the neutral load would be the triplen harmonics.

I really don't know if that would be worse or not with 2 loaded legs or 3.

I gotta admit, I never really understood this, before I ask anymore questions I will see if OP wants to pursue this avenue of thought.

I do not want to threadjack and I can start a new one.

 

[T.M.Haja Sahib]

I really don't know if that would be worse or not with 2 loaded legs or 3.

Surely,with 3 loaded legs,the neutral load of 3rd harmonics is worse than with 2 loaded legs because the 3rd harmonics in each phase are in phase.

 

[Rick Christopherson]

No, no chance assuming wired correctly.

This is not correct, and is actually quite common with single phase loads on a 3-phase system. If you have a motor load on the B-phase and a resistive load on the C-phase, then your neutral current will exceed the ampacity of the neutral. (Forgive me if I have my leading/lagging backward, but the principle is the same.) This is the biggest reason for burning up neutrals in power distribution systems and MWBC's.

What is even harder to comprehend for many, is that if you simply reverse the phases, then the current in the neutral will be reduced for the same loads. This is a situation where phase rotation is important.

On a ballanced linear circutuits the neutral load would be 0

With non-linear loads the neutral load would be the triplen harmonics.

No. Anything that is balanced will still cancel, including the harmonics. The wording in the NEC is rather poor in this area because it causes people to overlook powerfactor. I don't recall the actual wording at this time, but my recollection of it is that it is misleading because it "suggests" that powerfactor is not part of the issue, when in fact, it is the major contributor to the problem.

P.S. Just for reference, here is the same diagram I used recently for another discussion. It is the same diagram, except it is based on resistive loads.

 

[iwire]

This is not correct, and is actually quite common with single phase loads on a 3-phase system. If you have a motor load on the B-phase and a resistive load on the C-phase, then your neutral current will exceed the ampacity of the neutral.

If you say so I will believe it to be possible.

This is the biggest reason for burning up neutrals in power distribution systems and MWBC's.

This I have an issue with as I work in a world where MWBCs are used and abused to excess yet I still don't see branch circuit neutrals of NEC compliant MWBC branch circuits burning up.

I also don't see it happening in feeders either.

 

[Rick Christopherson]

If you say so I will believe it to be possible.

Oh that makes me feel so much better. I was afraid you were going to use your moderator powers to make me disappear.

This I have an issue with as I work in a world where MWBCs are used and abused to excess yet I still don't see branch circuit neutrals of NEC compliant MWBC branch circuits burning up.

I also don't see it happening in feeders either.

You're absolutely right. An electrician running around with a volt meter certainly must know more than an engineer on this matter. When your neutrals burn up, it must just be those pesky gremlins we all hear about.

Oh, iIf you're saying that you've never seen a neutral burn up, then I guess that must be kind of like, "well I've never seen it, so therefore it can never happen?" I didn't say it will happen. I said when it does happen, this is the most common reason. And it does happen.

 

[Hv&Lv]

No. Anything that is balanced will still cancel, including the harmonics.

View attachment 6789

So triplens aren't additive?

 

[Rick Christopherson]

So triplens aren't additive?

Not if their magnitudes are similar and phase angles consistent with the 3-phase primary, no. Harmonics are no different that any other signal. They will cancel if they are all the same, and fall within the 0, 120, 240-degree triangle. If they are unbalanced, then they will not cancel. The only time you will have a problem is when there is a signal manipulation on 1 phase but not the others. If all three phases are manipulated in the same manner, then all three manipulations will negate one another.

 

[iwire]

Oh that makes me feel so much better. I was afraid you were going to use your moderator powers to make me disappear.

I agreed with you and you respond like that?

What would have been the proper way for me to say I agree with you?

You're absolutely right. An electrician running around with a volt meter certainly must know more than an engineer on this matter. When your neutrals burn up, it must just be those pesky gremlins we all hear about.

Oh, iIf you're saying that you've never seen a neutral burn up, then I guess that must be kind of like, "well I've never seen it, so therefore it can never happen?" I didn't say it will happen. I said when it does happen, this is the most common reason. And it does happen.

Your post made it sound like it was a frequent thing and I disagree with that.

 

[Rick Christopherson]

I agreed with you and you are a wise guy.

What would have been the proper way for me to say I agree with you?

The funny thing about vBulletin (and you know that I am an owner of vBulletin myself) is that when post notifications are sent out via email, it is the original posting that is sent, and not the edited posting that appears on the forum.

Your original posting was "Point Taken", but you went back in and changed this to the snarky comment of "If you say so I will believe it to be possible."

It was your snarky comment that garnered my reprisal.

 

[iwire]

The funny thing about vBulletin (and you know that I am an owner of vBulletin myself) is that when post notifications are sent out via email, it is the original posting that is sent, and not the edited posting that appears on the forum.

Your original posting was "Point Taken", but you went back in and changed this to the snarky comment of "If you say so I will believe it to be possible."

It was your snarky comment that garnered my reprisal.

Yeah, that was my first comment, it was not arcuate. I could not really say 'the point was taken' because quite honestly your post was above my head.

Now that said, can we move forward without you trying to belittle me?

 

[roger]

I was afraid you were going to use your moderator powers to make me disappear.

I'm' sure Bob wouldn't but, I advise you to get off your high horse. It is apparent that you are daring us as well as continuing to be abusive to a number of members.


Roger

 

[Rick Christopherson]

I'm' sure Bob wouldn't but, I advise you to get off your high horse. It is apparent that you are daring us as well as continuing to be abusive to a number of members.


Roger

I am sorry Roger, but I have privately requested that Bob be removed as a moderator of this forum for ongoing actions similar to this one. This isn't a one-time occurrence. It's a trend. I am not challenging the forum or any of its members. I'm questioning the validly of Bob as a moderator.