MWBCs in 3-phase 208V Wye

[PetrosA]

Hi all! I've been reading for some time, but this is my first question to you.

I'm working on a job now wiring what will be a 4000 sq. ft. lighting showroom. The service is 3-phase 208V wye. There will be both plug strips (not the wiremold kind, but one designed for lighting displays) and clouds. Needless to say, there are circuit requirements out the wazoo in this place.

Most of my experience has been with residential single phase, hence this question: When running MWBCs in a 3-phase system, is it acceptable to run a common neutral to only two of the phases instead of all three? The reasoning behind the idea is so that the lighting display circuits can be controlled more precisely (two circuits shut off at the panel instead of three). I haven't been able to find enough information about phase canceling in 3-phase systems to answer this myself.

Thanks in advance,

Peter

 

[iwire]

Yes you can use just two ungrounded conductors with one grounded conductor on a 3 phase Wye system.

I personally try to avoid this as doing this means the neutral counts as a current carrying conductor when applying the derating rules.

So doing it as you suggest you end up with two circuits but 3 current carrying conductors. However if you run 3 ungrounded instead of two you end up with three circuits and still only three current carrying conductors.

This can make a big difference on a pipe job.

See 310.15(B)(2)(a) and 310.15(B)(4)(b)

 

[nakulak]

in single phase, the two lines are 180 out of phase, so cancel

in three phase, the three phases are 120 out of phase, so they only cancel if all three are present

note that in any case, the assumption that the neutral has zero current is based on a perfectly balanced load; any difference in the currents to the phases results in an imbalance which gives a nonzero value for the neutral current.

 

[iwire]

the assumption that the neutral has zero current is based on a perfectly balanced load

That is the reality, but it is not how the NEC deals with it.

Code wise, the neutral of a 3 phase Wye MWBC is not a current carrying no matter how out of balance the circuit is.

In other words, the neutral may well be carrying current but is not considered a current carrying conductor. :grin:

If you add in non-linear loads then the NEC might call it a current carrying conductor.

 

[roger]

The reasoning behind the idea is so that the lighting display circuits can be controlled more precisely (two circuits shut off at the panel instead of three).

Thanks in advance,

Peter

Along with the others posts I will add, you don't have to use handle ties on these breakers, so even sharing the neutral with all three phases will not mean you have to turn off more than one circuit at a time if that's what is desired.

Roger

 

[iwire]

I agree with Roger 100%

But unfortunately time is running out for single handles on MWBC.

Under the 2008 NEC handle ties will be required.

 

[Smart $]

I haven't been able to find enough information about phase canceling in 3-phase systems to answer this myself.

You need equal current from all three phases to cancel all neutral current. Equal current on two line conductors will produce an equal current on the associated neutral conductor.

Running two circuit mwbcs on a 3? wye system will have the same I?R (heat) loss in the wiring as running three circuit mwbcs yet it distributes by comparison only two thirds the power. That is to say, if all the circuits are run three-wire, the system will experience 50% greater I?R loss than running the circuits as four-wire.

 

[roger]

I agree with Roger 100%

But unfortunately time is running out for single handles on MWBC.

Under the 2008 NEC handle ties will be required.

I hope NC amends this requirement when they adopt the 08, but I don't have much faith in that happening.

Roger

 

[PetrosA]

Thanks for your help. One way or another, I'll get the breakers in before the end of the month without ties .

For iWire: This is a pipe job, and I'll look better in my boss' eyes for every foot of wire saved . In spite of a 15 yr. hiatus from electrical work, I'm still pretty much the only guy in the company with the experience to do a job like this in pipe.


Smart $: I'm a hands-on learner, so what I did today was hook up some lights to two phases with a shared neutral and took readings. One phase had a 4.1A load, the other a 5.2A load. The neutral showed a 4.8A load. I then clamped the two phases together to read and also read a 4.8A load. Coincidence, or just a goofy mistake on my part?

I can't say enough how this forum has helped me recently. When I was learning the ropes as a teen, I had my master mech. to bug with questions all day long or until he started yelling at me in Armenian . Then came a 15 year stay overseas doing something completely different... At the company where I work now, no one seems interested in talking shop, code, or learning anything new. What I need most of all is to catch up on all the changes and remember what I forgot in the meantime. Maybe it's time to start looking for a night school...

Thanks!

 

[winnie]

T
Smart $: I'm a hands-on learner, so what I did today was hook up some lights to two phases with a shared neutral and took readings. One phase had a 4.1A load, the other a 5.2A load. The neutral showed a 4.8A load. I then clamped the two phases together to read and also read a 4.8A load. Coincidence, or just a goofy mistake on my part?

Neither coincidence nor a mistake; that is _exactly_ how it is supposed to work.

When you put multiple conductors in the current clamp, the reading is the vector sum of the current in all of the conductors.

If you had taken all three conductors (the two hots and the neutral) and put them into the current clamp, then the result would have been zero, _or_ there would have to be a fault, another path for the current flow to follow. This is exactly how a GFCI detects faults.

If you take any two of those conductors and put them in the current clamp, then the result will be equal to the third.

With three hots and a neutral, then the sum for all four conductors is zero, and the sum of any three conductors is equal to the result of the fourth.

With a three phase wye system if you use two hots and a single neutral, then you will get _partial_ cancellation of the current flow on the neutral, such that the neutral will not be overloaded but will essentially carry the same current as the hot conductors.

-Jon

 

[PetrosA]

Thanks Jon. I later found a thread with the formula for calculating the neutral load in 3-phase systems, and saw that it came up with exactly that answer. What I thought may have been goofy was clamping two phases together for a reading, but I see it was a good instinct.