Super Neutrals

[KyleFowler]

so for years I've hooked up plenty of cubicle whips and they always had that #10 neutral that was the common for three #12 ungrounded phases. I never knew why they were made this way until today.

Basically I'm working on a remodel job that had tons of cubicles but then when the business shut down all the cubicles were removed. Now a new business is moving in and we have to provide power for the cubicles they bring in. So I look at the existing building wiring and I am shocked to see for the first time in my life #10 neutrals being paired with 20 amp circuits that share a neutral. This job is even more unique as we have the original blueprints for when this job was wired in '95 and that's were I learned of the term "Super Neutral" after seeing it on the blueprint. These #10 neutrals were specified for all cubicle circuits that shared neutrals. I've never seen that before.

So I go online and look up what is the deal with Super Neutrals and I am overwhelmed with technical information way above my knowledge. For years and years we wired lights and plugs "three hots to a neutral" but never upsized the neutral when doing so.

So here are my questions:

1. How do you know when to upsize your neutral? I read this has to do with power supplies from computers, etc. putting extra current on the neutral from the 3rd harmonic or something but I have no idea how serious this issue is? Obviously my understanding on this is fuzzy at best. As a real world example on this job we are supplying power to 6 cubicles off of one cubicle whip with three hots and a neutral. So basically each circuit is feeding two cubicles a piece. Is 6 computers and monitors enough to overload a shared neutral? It doesn't seem like it but again those original blueprints from '95 seems to think so. (the new setup is somewhat similar to the original)

2. Has anyone ever seen a shared #12 neutral burn up because of additive 3rd harmonics? I never have.

3. Am I overthinking this? LOL

Sorry for the long post!

 

[GoldDigger]

so for years I've hooked up plenty of cubicle whips and they always had that #10 neutral that was the common for three #12 ungrounded phases. I never knew why they were made this way until today.

Basically I'm working on a remodel job that had tons of cubicles but then when the business shut down all the cubicles were removed. Now a new business is moving in and we have to provide power for the cubicles they bring in. So I look at the existing building wiring and I am shocked to see for the first time in my life #10 neutrals being paired with 20 amp circuits that share a neutral. This job is even more unique as we have the original blueprints for when this job was wired in '95 and that's were I learned of the term "Super Neutral" after seeing it on the blueprint. These #10 neutrals were specified for all cubicle circuits that shared neutrals. I've never seen that before.

So I go online and look up what is the deal with Super Neutrals and I am overwhelmed with technical information way above my knowledge. For years and years we wired lights and plugs "three hots to a neutral" but never upsized the neutral when doing so.

So here are my questions:

1. How do you know when to upsize your neutral? I read this has to do with power supplies from computers, etc. putting extra current on the neutral from the 3rd harmonic or something but I have no idea how serious this issue is? Obviously my understanding on this is fuzzy at best. As a real world example on this job we are supplying power to 6 cubicles off of one cubicle whip with three hots and a neutral. So basically each circuit is feeding two cubicles a piece. Is 6 computers and monitors enough to overload a shared neutral? It doesn't seem like it but again those original blueprints from '95 seems to think so. (the new setup is somewhat similar to the original)

2. Has anyone ever seen a shared #12 neutral burn up because of additive 3rd harmonics? I never have.

3. Am I overthinking this? LOL

Sorry for the long post!

1. You have to make either a measurement-based calculation or an educated guess as to the degree of non-linearity (in particular third and other 3N harmonic level in the load current). That will tell you when you cannot reduce the neutral and when you have to oversize the neutral.The exact problem is that with any 3N harmonic current, the current from line to neutral loads on the three phases will add on the neutral instead of cancelling. In the extreme of pure 3rd harmonic load current, the neutral current would be three times the individual line current, even with a fully balanced load.

2. I have not seen it personally, but have seen credible reports of just such failures in the neutral of the internal wiring of modular office systems. At the time of those incidents computer power supplies (rectifier input switching power supplies) had a much worse current harmonic level (low distortion PF) than is currently allowed.

3. Maybe. The power supply load situation has changed a lot since '95, and there are not that many really old computers still in use.

 

[dkidd]

In the extreme of pure 3rd harmonic load current, the neutral current would be three times the individual line current, even with a fully balanced load.

Actually it is 1.73 times the line.

http://library.powermonitors.com/understanding-triplen-harmonics

From https://www.copper.org/applications/electrical/pq/issues.html

Oversizing Neutral Conductors
In three phase circuits with shared neutrals, it is common to oversize the neutral conductor up to 200% when the load served consists of non-linear loads. For example, most manufacturers of system furniture provide a #10 AWG conductor with 35 amp terminations for a neutral shared with the three #12 AWG phase conductors. In feeders that have a large amount of non-linear load, the feeder neutral conductor and panelboard bus bar should also be oversized.

 

[GoldDigger]

Actually it is 1.73 times the line.

http://library.powermonitors.com/understanding-triplen-harmonics

It shouldn't be, since in-phase waveforms such as pure triplen harmonics, will add as scalars, no angle offsets to introduce scaling factors. If you are talking about neutral current versus phase current, there may be a sqrt3 factor, but I am simply comparing neutral current to line current.

 

[infinity]

2. Has anyone ever seen a shared #12 neutral burn up because of additive 3rd harmonics? I never have.

Nope. IMO this is one of theoretical problems that barely exists in the real world.

 

[SceneryDriver]

Nope. IMO this is one of theoretical problems that barely exists in the real world.

It was (and still is to a certain extent) a problem in the Entertainment industry. Multiple hundreds of KW of incandescent dimming used to be the norm in theaters and rock and roll shows. All those tungsten halogen lamps were controlled with SCR (and later) TRIAC dimmers. The harmonics were atrocious. It was common practice to run 4/O 5-wire 400A Cam feeder to the dimmer racks on touring shows, but it was actually 6-wire, because the neutral Cam was always doubled. The last decade or so it's changed with LED sources with built-in dimming, and HID light sources in moving lights.


SceneryDriver

 

[ActionDave]

Nope. IMO this is one of theoretical problems that barely exists in the real world.

That's my opinion as well. The super neutral current is not a mythical beast like the Pegasus but a rare one like the giant squid.

 

[romex jockey]

That's my opinion as well. The super neutral current is not a mythical beast like the Pegasus but a rare one like the giant squid.

Yet resulted in CMP action, a good dz 'non-linear' code ref's , along with a gaggle of trade rag articles ,much like it's prior iso-ground jihad

~RJ~

 

[Tony S]

It was (and still is to a certain extent) a problem in the Entertainment industry. Multiple hundreds of KW of incandescent dimming used to be the norm in theaters and rock and roll shows. All those tungsten halogen lamps were controlled with SCR (and later) TRIAC dimmers. The harmonics were atrocious. It was common practice to run 4/O 5-wire 400A Cam feeder to the dimmer racks on touring shows, but it was actually 6-wire, because the neutral Cam was always doubled. The last decade or so it's changed with LED sources with built-in dimming, and HID light sources in moving lights.


SceneryDriver

A theatre was the first time I came across the problems with neutrals.
Problems = a total rewire.
The place was so dangerous it couldn’t get an insurance “fire ticket”. Fortunately I didn’t have to do the work.

 

[zbang]

It was (and still is to a certain extent) a problem in the Entertainment industry. Multiple hundreds of KW of incandescent dimming used to be the norm in theaters and rock and roll shows. All those tungsten halogen lamps were controlled with SCR (and later) TRIAC dimmers. The harmonics were atrocious. It was common practice to run 4/O 5-wire 400A Cam feeder to the dimmer racks on touring shows, but it was actually 6-wire, because the neutral Cam was always doubled.

Interesting... I know about up-sizing the neutral but don't think I've ever seen it doubled, not in the temp wiring nor in the building wiring. Maybe upsized one step, but not doubled. YMMV on this.

Won't try to figure out the theoretical harmonics/currents, but I'm guessing that to get to a neutral current over 100% would required all dimmers to be operating at exactly the same phase-angle near the center of range (maybe 85-95 deg). Yes/No? The quality of the chokes in the dimmers will make a difference, too. Some dimmers had almost no harmonic filtering.

 

[SceneryDriver]

Interesting... I know about up-sizing the neutral but don't think I've ever seen it doubled, not in the temp wiring nor in the building wiring. Maybe upsized one step, but not doubled. YMMV on this.

Won't try to figure out the theoretical harmonics/currents, but I'm guessing that to get to a neutral current over 100% would required all dimmers to be operating at exactly the same phase-angle near the center of range (maybe 85-95 deg). Yes/No? The quality of the chokes in the dimmers will make a difference, too. Some dimmers had almost no harmonic filtering.

Doubling the size of the neutral on touring rigs has a lot to do with only having to think about one size of conductor, I suspect. Everything else in the rig is 4/O, so just toss another piece in the roadbox

As to exactly where it was the worst, I'm not really sure. I just know we always doubled the neutral. The one time I remember it not being done, something expensive burned up in the power distro rack.


SceneryDriver

 

[zbang]

Everything else in the rig is 4/O, so just toss another piece in the roadbox

The idea of "tossing" a 50' hunk of 4/0 is, um, intriguing, but I know what you mean.

As to exactly where it was the worst, I'm not really sure. I just know we always doubled the neutral. The one time I remember it not being done, something expensive burned up in the power distro rack.

Not doubting that happened, but for the life of me I can't figure out how unless all of the planets lined up and the lead singer helped unload the truck . Or it wasn't related to a double neutral at all.