Does a 208v distro need a neutral?

[Lighting Newb]

Hi guys, can you please correct me if I'm wrong? So yesterday I learned that an L-6 20 only has two hots, a ground, and no neutral. The hots come together to compete the circuit ( I'd always assumed that the hots went in and the output went through the neutral).

Where I'm a little confused is that someone taught me since all three phases are connected to the neutral and if they're balanced correctly, the neutral should be drawing very little since all three phases add up to zero. Is that only true when the output is at 120v since the 208v doesn't use the neutral? Does balancing 208v just mean making sure one leg isn't drawing too much power?

Also where does the current that is now going through the hot going? Does a 208v distro need a neutral line from the company switch assuming you're not passing power through? And is it true that AB, BC, and CA are evenly distributed across all six circuits of a saco?
Thank you for any insights.

 

[DuckJordan]

Just had to deal with this today... Really neutral gets bonded to ground at some point in the chain anyway. So 2 hots and a ground isn't that uncommon. I deal with it when broadcasting trucks show up.

 

[Footer]

A straight up motor distro will not have a neutral. However, if you want to run any drive electronics you need the neutral to make 120v, so you will always see the neutral there. You should not use the ground as your neutral even if they are bonded down the line. In our world you will almost always see the neutral there even if it is not connected to anything inside the box. As far as how a distro is wired, that should be marked. They all are different. Some will do entire phases per soca, some will break it up in groups of 3... there is no standard.

And yes, if you put 3 loads that are exactly the same across 3 phases @ 120v you will see no current on your neutral.

 

[JD]

Generally speaking, devices that run on 208 only need the two hots and a ground. (there are exceptions, see Footer's post)
Most of the services we see are known as 3 phase "Wye" service, where three transformer windings merge at the neutral. Classic 120 from each hot to neutral, and 208 between hot legs.
The other type of 3 phase service is Delta, where the three windings are in series. (Mostly used for HVAC and large motors and usually 240 between each hot. *)
If your loads are all 208 (A-B, B-C, C-A) then your load is Delta. Problem running off a Wye service? No! Basically a Delta style distro just does not make use of the Neutral.
Now, for the * :
The bane of many road warriors is something known as the "tapped Delta", where one of the three windings has a center-tap. So, measured to Neutral, the legs are 120, 120, 208 (240 between any two hots.) This is so called "wild leg" service and is why you ALWAYS want to meter before tying in your equipment. Basically, this is a legacy service from big industry, where most of the equipment runs off 240 3 phase Delta, but you still need some 120's for the front end office.
Think that's weird? Try working on a ship, where they have grounded leg delta! (What looks like two hots and a Neutral/ground, but it's 240 between each of the hots, AND 240 from the hots to ground!)

 

[WayfarerAM]

Also on a 208v distro it is possible to create a phase imbalance or end up dealing with inductive/ capacitive loads causing current to flow on the neutral. Many times there are also 120v convince outlets which require a neutral. So while the individual 208v branches don't need the neutral the overall sum of the distro often does.

 

[Lighting Newb]

Thank you everyone for your answers.
JD, that reminded me of a few things I forgot. Thanks for the info.

Thanks Footer for confirming that. I think the Harry Box book said that neutral was bonded to the ground so that the voltage at neutral will be a zero giving a potential difference between hot and neutral, letting electricity to flow. I think that just means it gives electricity a path to ground through neutral and why. How would that work out on 208v that has no neutral?

And WayfarerAm, how does current end up in the neutral by phase imbalances or capacitance/induction? I thought capacitance/induction caused the voltage and current to be out of phase decreasing the power factor. And yeah, I would always run the neutral for the courtesy outlets and for running power through.

 

[WayfarerAM]

Any time you mess with the sine wave of the phases you end up creating the potential for the there to be current on the neutral. Imperfect loads do this by changing the waveform and creating harmonics while imbalanced loads will alter the size preventing the waveforms from balancing out. Everything attempts to balance out by summing the vectors of each leg onto the nuetral conductor creating the current flow.

Here's an interesting article with illistrations that can give an better explination than I can:
https://www.quora.com/What-is-the-s...What-happens-if-the-neutral-line-doesnt-exist

Another interesting thing to look at would be looking at white papers about K-rated transformers. An internet search should provide some interesting reading into this due to the increasing amount of non-power factor corrected loads.

 

[JohnD]

Since harmonics have been mentioned, you might also be interested in Triplen harmonics.
http://ecmweb.com/content/fundamentals-harmonics
Which is why you can run into company switches, distros and dimmer racks with cams for two neutrals.

 

[JD]

A quick reminder about Neutral and Ground:
Although they (should) have 0 volts between them, they serve very different purposes. Any current flow should occur in the neutral. No current flow should occur in the ground. It is there for fault protection only and cannot serve as a return for anything. Inside your distro, no connection should be made between neutral connector pins and ground connection pins. The only point where neutral and ground should connect is the primary service entrance to the building. By the time it reaches a company switch or sub-panel, connection is off limits.

Think about the reasoning for a second and you will see why- If the ground was part of the circuit and the ground path became open, every frame of every appliance downstream would become live.

 

[JD]

Also on a 208v distro it is possible to create a phase imbalance or end up dealing with inductive/ capacitive loads causing current to flow on the neutral.

Although phase balanced can occur, there is no neutral connection on 208 circuits, therefore no physical way for current to flow. The only purpose for a neutral would be for 120 convenience outlets. This is assuming all loads are hot-to-hot with a standard 120/120/120 Wye service.