Run a Neutral for each phase conductor?

[brian john]

How do you over-load a neutral with a mwbc?

I haven't but I have seen a lot of them in or IR reports, We see a hot neutral conductor and trace out the circuits in the panel 1,2,4, or such.

Me I am all for multi wire circuits, save copper. Why change things because of a few stupid electricians. (Unless there is a mitigating reason by the engineer)

We had a job with individual branch circuit and neutrals, the engineers reasoning was to avoid multi-wire BCs, was to not overload the neutral. I questioned why he only had 3 PCs on a circuit printers and scanners had their own circuits?

 

[iwire]

You're so mean. :grin:

Just stating the facts.

 

[chris kennedy]

Put all 3 ungrounded conductors on the same phase.

That by definition is not a MWBC

I'm sure you are well aware I may be familiar with the definition of a MWBC.

Branch Circuit, Multiwire. A branch circuit that consists of two or more ungrounded conductors that have a voltage between them, and a grounded conductor that has equal voltage between it and each ungrounded conductor of the circuit and that is connected to the neutral or grounded conductor of the system.

Just pointing out what we often see in the field.

 

[tkb]

If you pulled multiple circuits, each with their own neutrals, would you have to group them as noted in 210.4(D)?

I know that 210.4(D) is for MWBC and not two wire circuits, but if you had a conduit with four two wire circuits, I don't think you would be required to group them.

 

[LarryFine]

Just stating the facts.

Remember that there are those who have made up their minds and don't want to be confused with the facts.

 

[iwire]

I'm sure you are well aware I may be familiar with the definition of a MWBC.

I am. :smile:

Just pointing out what we often see in the field.

And I am just pointing out those mis-wired circuits are not MWBCs and that is virtually imposable to overload the neutral of a MWBC. :grin:

I am also pointing out it is in my opinion, ridiculous not to use MWBCs just because some pinhead in the future may relocate the circuits. If we have to start wiring for the 'pinhead problem' then we should just cut the service off at the street and become fry cooks. Once a pinhead starts making changes all bets are off.

 

[iwire]

Remember that there are those who have made up their minds and don't want to be confused with the facts.

Well we can all be guilty of that. :grin:

 

[LarryFine]

If you pulled multiple circuits, each with their own neutrals, would you have to group them as noted in 210.4(D)?

I don't have a 210.4(D), just a deleted-text dot on the margin. But, I can surmise what it says.

I know that 210.4(D) is for MWBC and not two wire circuits, but if you had a conduit with four two wire circuits, I don't think you would be required to group them.

Not according to my surmise, but it's certainly permitted, and not a bad idea.

 

[LarryFine]

I am also pointing out it is in my opinion, ridiculous not to use MWBCs just because some pinhead in the future may relocate the circuits. If we have to start wiring for the 'pinhead problem' then we should just cut the service off at the street and become fry cooks. Once a pinhead starts making changes all bets are off.

Maybe someone should ask the EE if he also specs that the 3-phase service should be wired the same way.

The same 'dangers' still exist, and on a larger scale, too.

 

[chris kennedy]

just because some pinhead in the future may relocate the circuits.

I will spare you the pictures I took at the school today, thats getting old.

If we have to start wiring for the 'pinhead problem' then we should just cut the service off at the street and become fry cooks.

Thank you, drive through.

Once a pinhead starts making changes all bets are off.

Only hire electricians with really huge heads, like the members here.:smile:

 

[iwire]

I will spare you the pictures I took at the school today, thats getting old.

Which in my mind proves the point that pinheads could screw up anything.

Only hire electricians with really huge heads, like the members here.:smile:

Or at least ones that are qualified.

 

[Rick Christopherson]

Exactly. You can't over-load the properly sized neutral conductor of a mwbc.

Well, that's why the engineer gets the big-bucks. :grin: If you have single-phase loads with varying powerfactors on a 3-phase system, it is easy, very easy, to exceed the ampacity of the neutral. See my example in this thread http://forums.mikeholt.com/showthread.php?t=110403 where current in the neutral is about 30 amps.

 

[LarryFine]

Well we can all be guilty of that. :grin:

Hey, I practice what I disdain. :wink:

 

[iwire]

Well, that's why the engineer gets the big-bucks. :grin: If you have single-phase loads with varying powerfactors on a 3-phase system, it is easy, very easy, to exceed the ampacity of the neutral. See my example in this thread http://forums.mikeholt.com/showthread.php?t=110403 where current in the neutral is about 30 amps.

What EE that makes the big bucks supplies 20 amps loads with 20 amp circuits?

On the A-phase outlet, you have a frosty machine which is 20 amps at 120 volts, and an inductive powerfactor of 0.8.
On the B-phase outlet you have incandescent tent lighting which is 20 amps at 120 volts and a unity powerfactor.
The C-phase outlet is not being used.

Then I also should ask what the ampacity of a 12 AWG typically used for 20 amp circuits?

 

[Cavie]

That is a good question seeing how he actually worded 210.4 in the spec.

It might be as simple as he simply doesn't understand the article section.

Can you submit a question for clarification before bid time?

Roger

What? an engineer that doesn't understand the NEC?? surely you jest!!:grin:

 

[220/221]

I have a general note from an engineer which says" Run a neutral conductor for each phase conductor. Do not share the neutral conductor in homeruns. (NEC 210.4)

I don't know why he referenced 210.4 but it's not a bad idea IMO.

I have seem my share of damage resulting from MWBC's. A loose wirenut on a regular circuit drops the power. On a MWBC it can smoke things.

In a fresh, clean and proper installation the MWBC is fine but things change over the years. Wiring is added, removed and stuff happens.

I have come very close to making smoke just by opening a jbox and pulling out the wires (troubleshooting). It wasn't my fault but I would have probably been held liable for the damage if the connection had come completely apart.

 

[Rick Christopherson]

What EE that makes the big bucks supplies 20 amps loads with 20 amp circuits?

What electrician worth his hardhat would assume that all loads are continuous?

 

[LarryFine]

I have come very close to making smoke just by opening a jbox and pulling out the wires (troubleshooting). It wasn't my fault but I would have probably been held liable for the damage if the connection had come completely apart.

Thus, the 10-page (and growing) to-twist-or-not-to-twist wirenut thread.

 

[iwire]

What electrician worth his hardhat would assume that all loads are continuous?

Continuous or non continuous you don't supply a 'frosty machine' that draws 20 amps with a 20 amp circuit.

And sure it is possible that 20 amps of 'incandescent tent lighting' would be non-continuous but I would say that is highly unlikely.

 

[ronmath]

The 210.4 reference is in the 2008 NEC which states you have to do 1 of 2 things, either provide a multipole (or handle tied) breaker for each MWBC or provide separate neutral (210.4(B)). Here is what I don't totally understand, questioning Engineers without talking to them to see the reasons behind the decision. Very likely this could be an Owner decision to not have multipole breakers in the panel. Talk to the Engineer and DON'T decide on your own because you think you know the whole story. Also, admittedly, the Engineer could be .... wrong. But you don't know that! Sorry for the rant, just get tired of getting calls such as, "the inspector is here for the final and we installed this different than your plans, can you send us a revised drawing today to match what we did so we can pass inspection?" ARRGGGHHHH! Thanks for listening, I feel better now. Nurse! :smile: