3 phase disconnect

[17Steve69]

Yes I know Dave. And it's a fair point.

I agree, I would still pull separate neutrals. If you don't all three will share the same neutral (current will be flowing on this neutral). If they are all pulling near capacity, the neutral has to carry 12-16A times three. This part of the NEC is weak to me but we have to remember it is minimum requirements not a design guide.

 

[17Steve69]

In fact, performance wise it is better to have a single neutral unless the harmonic current is very large.

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Please elaborate. I don't see it.

 

[kwired]

I agree, I would still pull separate neutrals. If you don't all three will share the same neutral (current will be flowing on this neutral). If they are all pulling near capacity, the neutral has to carry 12-16A times three. This part of the NEC is weak to me but we have to remember it is minimum requirements not a design guide.

Please elaborate. I don't see it.

If the three ungrounded supply conductors is A B and C phase of a wye system, the neutral only carries unbalanced current. If A B and C are equal current that means neutral current is zero. If A would be 5 amps more then B or C then the neutral sees that extra 5 amps.

 

[ActionDave]

I agree, I would still pull separate neutrals. If you don't all three will share the same neutral (current will be flowing on this neutral). If they are all pulling near capacity, the neutral has to carry 12-16A times three. This part of the NEC is weak to me but we have to remember it is minimum requirements not a design guide.

Ummm... that's not true, not one little bit. Have you ever looked at a three phase feeder? It doesn't have a separate neutral for each phase. A Multi Wire Branch Circuit is a miniature version of that same wiring scheme that powers up all of North America. The neutral only carries the imbalance between phases.

 

[ggunn]

Ummm... that's not true, not one little bit. Have you ever looked at a three phase feeder? It doesn't have a separate neutral for each phase. A Multi Wire Branch Circuit is a miniature version of that same wiring scheme that powers up all of North America. The neutral only carries the imbalance between phases.

It's weird, but I run into people occasionally who are convinced that current can run two directions simultaneously in a conductor and that the net current is the sum of the two.

 

[kwired]

It's weird, but I run into people occasionally who are convinced that current can run two directions simultaneously in a conductor and that the net current is the sum of the two.

I even heard three currents driven by voltages out of phase with one another can do similar.

 

[victor.cherkashi]

Ummm... that's not true, not one little bit. Have you ever looked at a three phase feeder? It doesn't have a separate neutral for each phase. A Multi Wire Branch Circuit is a miniature version of that same wiring scheme that powers up all of North America. The neutral only carries the imbalance between phases.

ActionDave, Good point, and not only North America, but actually in system above 600V they don't even use neutral.

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[Abdel Rahman Mohamed]

I agree, I would still pull separate neutrals. If you don't all three will share the same neutral (current will be flowing on this neutral). If they are all pulling near capacity, the neutral has to carry 12-16A times three. This part of the NEC is weak to me but we have to remember it is minimum requirements not a design guide.

I agree with you"This part of the NEC is weak to me but we have to remember it is minimum requirements not a design guide".

 

[Little Bill]

I agree with you"This part of the NEC is weak to me but we have to remember it is minimum requirements not a design guide".

It's already been stated in the above posts. With lines from three different phases, the neutral will have only the imbalanced current of the three lines. Very little current will be on the neutral. Nothing weak about this at all!

 

[kwired]

ActionDave, Good point, and not only North America, but actually in system above 600V they don't even use neutral.

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Utility distribution at 4.16/2.4 and 12.47/7.2 kV is very common and utilizes a neutral

I agree with you"This part of the NEC is weak to me but we have to remember it is minimum requirements not a design guide".

Only way NEC addresses this is in selecting conductor ampacity, otherwise you need to study electrical theory to know how/why it works like it does.

 

[Andy Delle]

There is an exception to the three phase MWBC and I beleive it is in the 2008 code somewhere.

In technical installations, computer centers, broadcast plants, call centers, anywhere there is a large collection of electronic equipment. You always pull a neutral per circuit and in addition I always spec 1.5 to 2x awg for the panel neutral feeder.

The problem is harmonic current caused by rectification in power supplies which does not cancel out. There have been several documented cases in the 70s of red hot neutral bars, literally glowing red, in computer centers and other high density electronics installations.

And where you have sensitive analog electronics, shared neutrals are an excellent to way to transmit ground noise.

 

[kwired]

There is an exception to the three phase MWBC and I beleive it is in the 2008 code somewhere.

In technical installations, computer centers, broadcast plants, call centers, anywhere there is a large collection of electronic equipment. You always pull a neutral per circuit and in addition I always spec 1.5 to 2x awg for the panel neutral feeder.

The problem is harmonic current caused by rectification in power supplies which does not cancel out. There have been several documented cases in the 70s of red hot neutral bars, literally glowing red, in computer centers and other high density electronics installations.

And where you have sensitive analog electronics, shared neutrals are an excellent to way to transmit ground noise.

exception for what involving three phase MWBC? Keep in mind if you run three "hots" and three neutrals you don't have a MWBC, you have three two wire branch circuits.

Simple rectification does not cause harmonics, it is high speed switching by semiconductor devices that results in harmonic currents, and some of those harmonic currents are additive on the neutral of a three phase wye system, and is the reason we need oversized neutrals on the feeders and services where there is high harmonic producing loads.

 

[GoldDigger]

...
...
Simple rectification does not cause harmonics, it is high speed switching by semiconductor devices that results in harmonic currents, and some of those harmonic currents are additive on the neutral of a three phase wye system, and is the reason we need oversized neutrals on the feeders and services where there is high harmonic producing loads.

Actually, the rectification itself causes harmonics independent of switching speeds. The latter are responsible for really high frequency (think RF) currents.
If you take any periodic wave form that is not sinusoidal (and the rectifier current is not sinusoidal whether you use a capacitor input or a choke input) you can represent that waveform as the sum of a fundamental sinusoid and higher order sine and cosine harmonics of that frequency.


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[Andy Delle]

exception for what involving three phase MWBC? Keep in mind if you run three "hots" and three neutrals you don't have a MWBC, you have three two wire branch circuits.

Simple rectification does not cause harmonics, it is high speed switching by semiconductor devices that results in harmonic currents, and some of those harmonic currents are additive on the neutral of a three phase wye system, and is the reason we need oversized neutrals on the feeders and services where there is high harmonic producing loads.

By "exception" I meant that three phase MWBC are not recommended for technical power installations. I don't know of any code preventing it although there is a code section relating to oversized neutral feeders IIRC. I always specify 2x in a facility design. Most PE's know this as well.

Simple rectification certainly does cause harmonics. Even when done through a 60hz power transformer although not as severe as a direct line connected rectifier such as in switch mode power supplies. Of course the switching transistors contribute as well.

Excellent engineering explanation in the above post. A diode is not a linear device.

 

[kwired]

By "exception" I meant that three phase MWBC are not recommended for technical power installations. I don't know of any code preventing it although there is a code section relating to oversized neutral feeders IIRC. I always specify 2x in a facility design. Most PE's know this as well.

Simple rectification certainly does cause harmonics. Even when done through a 60hz power transformer although not as severe as a direct line connected rectifier such as in switch mode power supplies. Of course the switching transistors contribute as well.

Excellent engineering explanation in the above post. A diode is not a linear device.

I stand corrected on the simple rectification comment, but I don't know of any NEC requirement to run separate neutrals with each ungrounded conductor just because you have non linear loads. Might be good design practice, but oversizing a common neutral is also an option. I think the handle tie rules have also pushed designers to opt for separate neutrals as they don't want to shut down two other ungrounded conductors anytime they must shut just one down.

 

[Smart $]

I stand corrected on the simple rectification comment, but I don't know of any NEC requirement to run separate neutrals with each ungrounded conductor just because you have non linear loads. Might be good design practice, but oversizing a common neutral is also an option. I think the handle tie rules have also pushed designers to opt for separate neutrals as they don't want to shut down two other ungrounded conductors anytime they must shut just one down.

That point is rather moot when using a 3Ø disconnect... all at once or none at all is the only option.

Empirical evidence of the need to oversize a full-size neutral for harmonic loads is fairly absent...
...but running separate neutrals is in effect accomplishing the same thing.

 

[Andy Delle]

I stand corrected on the simple rectification comment, but I don't know of any NEC requirement to run separate neutrals with each ungrounded conductor just because you have non linear loads. Might be good design practice, but oversizing a common neutral is also an option. I think the handle tie rules have also pushed designers to opt for separate neutrals as they don't want to shut down two other ungrounded conductors anytime they must shut just one down.

I don't know of any NEC article that requires dedicated neutrals, but I think there is something about over-sized neutrals in 2008?

I hate MWBC's in general. My current house is full of them and when I moved in the panel was wired RED, BLACK, RED, BLACK all the way down the breakers. Good right? Well they were all tandem breakers! Funny how some EC's don't even get it. And that passed inspection as well!

 

[kwired]

I don't know of any NEC article that requires dedicated neutrals, but I think there is something about over-sized neutrals in 2008?

I hate MWBC's in general. My current house is full of them and when I moved in the panel was wired RED, BLACK, RED, BLACK all the way down the breakers. Good right? Well they were all tandem breakers! Funny how some EC's don't even get it. And that passed inspection as well!

You sure you aren't thinking of 310.15(B)(5)(c) and the need to count neutral as a current carrying conductor for ampacity adjustment reasons?

I have a school building that is a regular client of mine, in an addition that was done about 20 years ago I noticed they ran three phase "full boat" sets in most home run raceways, Black, red, blue, each landing on three adjacent single pole breakers and a white. The facility only had single phase service at the time that construction was done and the panels they landed in only had single phase bussing:blink:.

 

[ActionDave]

I don't know of any NEC article that requires dedicated neutrals, but I think there is something about over-sized neutrals in 2008?

I hate MWBC's in general. My current house is full of them and when I moved in the panel was wired RED, BLACK, RED, BLACK all the way down the breakers. Good right? Well they were all tandem breakers! Funny how some EC's don't even get it. And that passed inspection as well!

I love MWBC. They are the most elegant wiring there is. They allow a significant reduction in labor and materials and reduce voltage drop. Except for some specific environments, neutral overloading is non-existent.

And I hate 210.4(B) so much it's one of the few code references I know by heart.

 

[GoldDigger]

Regarding separate neutrals versus a single oversized neutral:
Either way handles potential overloading of the shared neutral.
But separate neutrals will double the voltage drop for the balanced fundamental compared to a standard sized shared neutral, while an oversized shared neutral will also be better for unbalanced loads and triplen harmonics as well.

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