200% Neutral

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Ben Trueblood said:
What is the purpose for requiring a 200% neutral rated bus?
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For additive odd order harmonics, but this is kind of like the rumors of Mark Twains death, the need for this has been greatly exagerated.

Roger
 
I was wondering the correct answer to this, my guess was that the maximum fault current that the neutral had to handle was >100%.
 
As crossman stated, the oversized neutral is to handle the excess amperage cause by 3rd harmonics. The 3rd harmonic is typically the largest harmonic and in a 3-phase system it is added from each phase onto the neutral. (Other harmonics can cancel each other out when they're combined on the neutral.) I've seen calcs where it's theoretically possible to get neutral currents 60-70% higher than the hot legs. Most of these harmonics are caused by electronics when they convert AC power into DC. Panelboards with high numbers of computers are sometimes given oversized neutrals. If you have a 200% neutral on a panelboard then the feeder for this panel should also have a 200% neutral. If you oversize the neutral conductor then you are considering the neutral to be a current carrying conductor and all of the conductors in that feeder will need to be derated to 80%. (4-6 current carrying conductors in a single conduit, Table 310.15(B)(2)(a))
 
The 3rd harmonic is typically the largest harmonic and in a 3-phase system it is added from each phase onto the neutral. (Other harmonics can cancel each other out when they're combined
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Generally true and 100% in the OP case, but

Depending on the loads served 6 and 12 pulse rectifiers on 3 phase distribution systems result in higher order harmonics in 5,7, 11,13, 17.19......
 
I have seen neutral current at close to 100% on balanced 3 phase systems, but both cases were on overloaded distribution systems one was a call center, that kept adding tables and PCs, the other was a race track that took on slots and added as many as they could handle, smoking a 500 kva transformer. Neither place gave a thought to what they were doing in the haste to grow.
 
I can't recall where I read it and I would appreciate comments but as I recall if you had a perfectly balanced 3ph system without any harmonics you wound have no unbalance current and as such no neutral current flowing. It that "balanced load" were to consist of a harmonics there would be neutral current that would increase as the harmonics increased. If you had an unbalance load the consisted of no harmonics the neutral would carry the value of the imbalance. If that unbalance load had harmonic content then the neutral current would be greater than the imbalance.
What this indicates is if the current is measured in all three phases were to be measured and if your measurements showed an imbalance, the calculated the imbalance, the imbalance should be equal to that of the neutral. If the current measured in the neutral is greater that that it would indicate harmonic content in the load.
This seemed to make sense.
I don?t know about the requirement of the 200% neutral because that?s one boatload of current. It takes some unusual big time unbalance 3ph loads to for the neutral to carry 100%. Then add a Harmonic current on top of that for up to 100% more current?
It appears as though it is a theoretically calculated worst case scenario.
When it cone to the requirement of K-factor transformers it is important as the core of the transformer must be beefed up to deal with the heating that occurs in the core cause be the harmonics.

Another thought. I was told that the harmonics actually go down at some point as more and more harmonic loads are added as thay start canceling out. It?s an intriguing concept that may have some truth to it depending upon the combination of the harmonic loads involved.

Dave
 
As for the "triplen" current in the neutral, the entire problem is that current does not balance on the phases. The power supply contains electronics which essentially switch on and off... it only conducts during a portion of the cycle, typically near the peak of the sine wave. Because each power supply on each phase is doing the same thing, when one phase is conducting, the other two are not, and the current has no choice but to return on the neutral. So, triplen currents cannot balance.
 
If you haven't seen neutral currents well over 100% of phase current, then you've not poked a clamp meter into enough data centres yet.

Though outside data centres and theatres, heavily loaded neutrals are unusual...
 
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