single phase/3 phase neutrals

[benmin]

Are the electrical characteristics of the neutrals in a 120/240 single phase system and a 120/208 3 phase system different?

 

[Rockyd]

Not in regard to purely resistive loads. However, once the time placement of the wave is modified by electronics (VFD's, electronic ballsts, machinery that needs a square wave, etc). Neutral current is no longer self canseling,but moves to another level with multipliers applied (3rd harmonic, 5th, 7th) that don't neccesarily follow a simple formula as to the total current impressed upon the neutral.

Suffice to say, return currents may exceed the ratings applied typically to conductors. A neutral per phase may become neccesary, or a "super neutral" (upsized) may be the corrective action if you have a neutral bar full of brown neutrals (formally white, but have had enough load on them to discolor).

There will be some experts through, and they can explain in detail, mathmatically, where the teturn wave is. Will be back to see . Got to go to school right now.

 

[Mr. Bill]

Since most buildings have large numbers of electronic loads there are harmonics in the building's electrical system. Just to try to expand on what Rockyd said.

In a 120/240V single phase system, most of these harmonics will cancel each other out and the neutral can still be considered a non current carrying conductor.

In a 3-phase system, the odd number harmonics (3rd, 5th, 7th, ...) become additive. In a computer lab with a high density of electronic loads it is preferable to have a dedicated neutral for each hot conductor.

If you still want to wire the system with 3-hot, 1-neutral, then the neutral needs to be oversized because it can see currents higher than the hot conductors will see. This "super neutral" is when the neutral is sized 150% to 200% of the hot conductor size. This neutral is now considered current carrying and so all of the conductors in the conduit need to be derated to 80%.

I would only worry about the neutral in a 3-phase system when there are large densities of electronic loads. Or loads with high THD.

 

[megloff11x]

Balance your power as best you can and the neutral current will be minimized.

Matt

 

[Smart $]

Are the electrical characteristics of the neutrals in a 120/240 single phase system and a 120/208 3 phase system different?

Personally, I have to ask you to elaborate a bit on your question. Rockyd and Mr. Bill have jumped straight to harmonics and skipped completely over fundamental differences.

 

[Rockyd]

[

Rockyd and Mr. Bill have jumped straight to harmonics and skipped completely over fundamental differences.

In the general way it was asked, I'd beg to differ...

Not in regard to purely resistive loads. However, once the time placement of the wave is modified by electronics

Great thing about the forum is how many different angles people see things from.

 

[benmin]

Personally, I have to ask you to elaborate a bit on your question. Rockyd and Mr. Bill have jumped straight to harmonics and skipped completely over fundamental differences.

I'm looking for any and all differences. Big and small. I work on mostly high end residential which is typically single phase but on occasion have to work on 3 phase systems and am looking for a refresher

 

[LarryFine]

I'm looking for any and all differences. Big and small. I work on mostly high end residential which is typically single phase but on occasion have to work on 3 phase systems and am looking for a refresher

Then I'd tell you this:

Whether 1 or 3 phases, any neutral that carries current for more than one hot wire must be joined and pigtailed, and never rely on a device's terminations for continuity.

 

[Smart $]

I'm looking for any and all differences. Big and small.

In addition to what others have noted...

A 3? wye neutral for the purpose of ampacity derating is counted as a current carrying conductor in multiwire branch circuits except for the so-called full boat: a 4-wire circuit. However, if the circuits power utilization equipment relying for example solely on electronic power supplies (i.e. non-linear circuits) this exception is nullified.

Current on a 3? wye neutral can be approximated with the formula:

In = √(Ia? + Ib? + Ic? ? IaIb ? IbIc ? IaIc)​

However, except for certain situations regarding feeders, the neutral has to be sized for the worst case scenario, which is to say it must be sized to carry the full amperage of a maximum imbalance state on the circuit (i.e. an ampacity sufficient for the highest rated OCPD serving the circuit).