Shared Neutral Question

[erichall1]

I understand the Sharing of a neutral wire with 2 breakers each being on a different leg than the other that the load then will be the difference between the 2 circuits as they are 180 deg. out of phase when on a 2 phase service panel.

But what is the situation when sharing a neutral wire with 2 breakers using a 3 phase service panel when you have the 2 breakers on different legs. Are the 2 circuits are now 120 deg out of phase instead of 180 deg out of phase with each other?

What I have run across is two 20 amp cirtcuits sharing a neutral wire. The two 20 amp breakers are on 2 different legs of a 3 leg/phase service panel. Can the shared neutral become carrying an overlaod (over 20 amps) in this case? what is the formula for determining the load carried by the shared neutral wire on these two 20 amp circuits?

Thanks in advance for any info.

 

[ibew501ed]

the formula
I neutral = SqRt[(A? +B? +C?)] - [(A x B) - (B x C) - (C x A)]
thats the square root of the final answer (sorry)

when you have 2 of the 3 phases the neutral dosen't carry the difference as it does in a single phase application

A PHASE 100 B PHASE 25 C PHASE 0

SQ RT of (100X100)+ (25X25)+(0X0) - (100X25)- (25X0) - (0X100)=

SQ RT of (10,000+625+0) - (2,500-0 -0) =

SQ RT of 10,625 ? 2,500=

SQ RT of 8,125 = 90.13AMPS
I hope that is clear as mud:roll:

 

[erichall1]

I just found this info at Wikipedia:

In a three-phase circuit, a neutral is often shared between two or three phases. In the case of two phases sharing one neutral, the worst-case current draw is one side has zero load and the other has full load, or when both sides have full load. The latter case results in 1 + 1@120deg = 1@60deg, i.e. the magnitude of the current in the neutral equals that of the other two wires.

Ok where they say "or when both sides have full load. The latter case results in 1 + 1@120deg = 1@60deg, i.e. the magnitude of the current in the neutral equals that of the other two wires" apparentley this means if both circuits are using 20 amps then the nuetral is carrying only 20 amps, not more?? The way they word this is not exactly clear.

 

[erichall1]

the formula
I neutral = SqRt[(A? +B? +C?)] - [(A x B) - (B x C) - (C x A)]
thats the square root of the final answer (sorry)

when you have 2 of the 3 phases the neutral dosen't carry the difference as it does in a single phase application

A PHASE 100 B PHASE 25 C PHASE 0

SQ RT of (100X100)+ (25X25)+(0X0) - (100X25)- (25X0) - (0X100)=

SQ RT of (10,000+625+0) - (2,500-0 -0) =

SQ RT of 10,625 – 2,500=

SQ RT of 8,125 = 90.13AMPS
I hope that is clear as mud:roll:

Clear as mud is right. LOL

I believe I get it. by inserting 20 amps for A Phase and 20 amps for B phase I come up with 20 amps load on the neutral. That is sort of counter intuitive.

I guess I am not seeing why the C phase comes into the formula when the C phase is not connected to either of the two circuits the neutral is being shared with.

Evidently the answer to my question is, that no, the shared neutral cannot become carrying a load larger that the highest load on either of the two circuits. Is this correct? the nuetral can never become overloaded?

 

[roger]

Evidently the answer to my question is, that no, the shared neutral cannot become carrying a load larger that the highest load on either of the two circuits. Is this correct? the nuetral can never become overloaded?

Talking purely resitive loads that would be correct, additive harmonics from non-linear loads can change that.

Roger

 

[Mike01]

Afci?

Afci?

Can you use a shared neutral with an AFCI or GFCI breaker??

 

[dkarst]

Being picky I know but the original poster mentioned a "2-phase" system. There are 2-phase systems but I think what was meant was single phase system where the two legs are 180 degrees out of phase (common residential service). In a 2-phase system, the phase voltages are displaced by 90 degrees from each other and it is not common.

You don't need non-linear or harmonic loads to get more current on neutral than the individual phases. Just having loads with significantly different power factors such as a one leg powering a resistive heater drawing full load and the other an induction motor with poor power factor will cause the neutral current to be larger than the individual line currents. The solution can be found by adding the line currents as phasors (i.e. paying attention to phase angle).

 

[augie47]

Can you use a shared neutral with an AFCI or GFCI breaker??

On a two pole breaker, yes (don't think anyone has a two pole AFCI yet)

 

[erichall1]

Talking purely resitive loads that would be correct, additive harmonics from non-linear loads can change that.

Roger

These circuits will be powering musical equipmment for band rehearsal rooms. Amplifirers, PA, and such with also portable space heaters and lighting. I am guessing all of these loads would be considered purely resistive? Is that correct? Thanks for all the responses.

 

[charlie b]

Are the 2 circuits now 120 deg out of phase instead of 180 deg out of phase with each other?

Yes. Two "hot" conductors taken from a two-pole breaker on a three phase panel will be 120 degrees out from each other, and the shared neutral will be 120 degrees apart from the two hots.

Regarding the amount of neutral current, the situation described above is no different from that of a three phase load that has no load on one of the phases.

 

[erichall1]

These circuits will be powering musical equipmment for band rehearsal rooms. Amplifirers, PA, and such with also portable space heaters and lighting. I am guessing all of these loads would be considered purely resistive? Is that correct? Thanks for all the responses.

Oh, I. Forgot to mention that some of those small refridgerators will also be on these circuits. Would their motors be purely resistive?

 

[roger]

The amplifiers, PA, and refrigerators would all be non-linear, the lighting may be unless it is incandescent however, if there are dimmers involved this may not hold true.

The reality is, that unless you are loading these circuits heavily, the harmonic situation is probably not a problem.

Roger

 

[erichall1]

The amplifiers, PA, and refrigerators would all be non-linear, the lighting may be unless it is incandescent however, if there are dimmers involved this may not hold true.

The reality is, that unless you are loading these circuits heavily, the harmonic situation is probably not a problem.

Roger

Unfortunately these circiuts may be periodically loaded heavily at times as the contractor did not follow my advice by giving each rehearsal room their own individual 20 amp circiut and is sharing only 1 20 amp for 2 rehearsal rooms. This means a total of 4 rooms are sharing a single neutral wire (2 rooms on one breaker plus 2 rooms on the other breaker.) They are going to be tripping breakers occasionaly for sure when 2 or 3 or even in the unlikely event 4 bands might rehearse at the same time on these 2 20 amp circuits. I am considering recomending they do bot allow the refridgerators the bands use to keep their beer cold with. That may help. But I am suspecting they will come to see after a while each rehearsal room needs to have it's own individual 20 amp circuit abd have to make corrections.

 

[mxslick]

These circuits will be powering musical equipmment for band rehearsal rooms. Amplifirers, PA, and such with also portable space heaters and lighting. I am guessing all of these loads would be considered purely resistive? Is that correct? Thanks for all the responses.

In this case, I would also forget about sharing neutrals. There are potential issues of hum and noise problems, and a lot of newer P.A. and instrument amps are using switching power supplies, so harmonics can enter the picture.

And I strongly suggest that if you can't talk the client into dedicated circuits for each room (I'd personally run TWO to each room, bands use a LOT of power, add a space heater or some fans and look out) then make sure your contract has iron-clad "no warranty of suitability of purpose" language so you're not on the hook to add the circuits later on your dime when they keep overloading the original circuits. :grin:

 

[iwire]

In this case, I would also forget about sharing neutrals.

Of course you would, you have a fear of MWBCs. I am still wondering what sort of magic happens at the panel when all circuits become 'common neutral'.:grin:

There are potential issues of hum and noise problems,

I would like to see proof of that assuming the wiring is done correctly.

and a lot of newer P.A. and instrument amps are using switching power supplies, so harmonics can enter the picture.

Which is another non-issue unless you planing on loading the circuits heavily.

 

[mxslick]

Despite having to get up very early for work tomorrow, I had to respond to this tonight:

(FYI for those who don't know, Bob and I have had a long-standing spirited yet amicable debate over the pros/cons of MWBC's. ) :grin:

Of course you would, you have a fear of MWBCs. I am still wondering what sort of magic happens at the panel when all circuits become 'common neutral'.:grin:

Simple, the magic is the lower impedance of the neutral bus and feeder as opposed to the comparatively "wimpy" #12 or #10 wire. Lower impedance = less resistance to harmonics and noise.

I would like to see proof of that assuming the wiring is done correctly.

As per our earlier debates, I offer over 20+ years of experience in audio systems wiring, and so far IN EVERY CASE I've encountered with MWBC's there were problems of noise, early equipment failures and other issues which were SOLVED by pulling in dedicated neutrals. In only two cases I can recall was poor workmanship by the EC a possible cause factor. (One was a three-phase equipped booth where 5(!) circuits to the sound rack were served by a SINGLE #10 noodle.)

As I have also conceded before, there are times and places that MWBC's are perfectly acceptable..but audio installations are NOT one of them!! :grin:

Which is another non-issue unless you planing on loading the circuits heavily.

LOL did you read what the OP is up against? I have spent a lot of time working with bands and they NEVER have enough power available, if there is an empty socket somewhere they're gonna stick something in it. :grin:
And guitar and bass amps are power hogs.

 

[erichall1]

First I would mention I did not perform the work in question here. I was too busy to meet their schedule. And gave them my advice (as they asked) as to what would be the requirements for their planned usage. Then they went ahead and disregaurded my recomendations. Only when comming in for some additional work did I find what they had done. As they have been an important client over the years I make a habit of looking out for their interest.

I am now seeing other issues with the decisions that were made by the contractor on this Job and will address those questions here in this thread rather than create new threads for these other issues I am seeking input on they being related to same job.

First, in these 4 rehearsal rooms being served by an MWBC. 2 rooms are served by each 20 amp breaker. Here's the deal each of these rooms has 2 quad receptical boxes that makes for 4 quads on each 20 amp breaker. OK, 4x4 is 16 recepticals. I believe by NEC code only 13 by the chart (in section 201.21 ?) are allowed. This appears to be a violation unless a dual receptical only counts as 1. Any opinions on this?

Second question is, with the application of the derating principle/factor. It would be an unbeleivable claim to state the 20 amp circiuts were only planned for 16 amps usage (80%). I mean, give me a break. 2 rooms for band equipment with a total of 4 quad receptical boxes on 1 20 amp breaker? I am wondering exactly how the derating factor applies here and if there is a violation of the NEC here. In any case it is not a good idea at all what they have done here, but is it a violation of code?

Thanks for all the previous, replies.

 

[augie47]

You are probably correct that there will be load associated problems, however, two of the items you mentioned in the last post are "grasping a straws". Without getting into the discussion of Art 220 vs Art 210, the 180 va per receptacles outlet refers to outlets per yoke, so a duplex is one receptacle outlet. In addition, derating would apply to the number of conductors regardless of continuous and non-continuous and I doubt you will find any of the loads to be continuous as in full load at three hours or more.
Again, not to diminish your stance that this is a disaster waiting to happen, those two areas of concern are not Code violations.

 

[raider1]

220.14(I) states that we calculate the branch circuit, feeder and service load at 180 VA for each single or each multiple receptacle on one yoke.

So for a quad box you would use 360 watts (180 va for each duplex receptacle) so for the entire circuit I come up with a calculated load of 1440 VA.

Chris

 

[LarryFine]

So for a quad box you would use 360 watts (180 va for each duplex receptacle) so for the entire circuit I come up with a calculated load of 1440 VA.

Which is one reason I say load calculations are all but useless when it comes to layout design in non-residential applications. Every receptacle is installed for an expected load. Some 20a circuits will never see 20a, and others will be overloaded on a regular basis.

With the wide-spread use of power strips, including UPS's and the like with multiple receptacles, the load of one receptacle can load a circuit just as readily as the combined load of a dozen or more. Each receptacle's supply should be based on a realistic, expected load.

Here, I thing each quad should have been supplied by its own circuit, or MWBC. That's easy to say in retrospect; it should all have been discussed between the customer and the electrician. Two poeple may have entirely different mental images of what a band is.

It sounds like the goal of the design was to supply what was asked for at minimal cost, instead of to provide the best performance for the needs. An electrical installation is dynamic. It's just one reason I like separate receptacle and lighting circuits, even in homes.