| Instead of derating, new neutral cable can be installed. Still not | cheap, but at least the outcome of the expense is that the system has | been uprated rather than downrated.
You have to handle this also in the panel itself (200% neutral panels are available) and the transformer (200% neutral transformers are available).
Actually, you don't need that kind of load (hundreds of computers) before it becomes a concern. They manufacture cable (called super neutral?) for branch circuit wiring in offices with 12 gauge phase and 10 gauge neutral conductors. So the excess neutral current is a concern even on 20 amp circuits. Certainly the problem would be magnified at the service, which is what you may have in mind.
By the way, your approach to showing the problem with a diagram is great. You can see where the pulses on each phase would create neutral currents that would not cancel each other out.
| Actually, you don't need that kind of load (hundreds | of computers) before it becomes a concern. They | manufacture cable (called super neutral?) for branch | circuit wiring in offices with 12 gauge phase and 10 | gauge neutral conductors. So the excess neutral | current is a concern even on 20 amp circuits. | Certainly the problem would be magnified at the | service, which is what you may have in mind.
Yes, you are right. Even on a small branch circuit you can have an overcurrent situation on the neutral. I was thinking about 2 wire plus ground 120 volt circuits, but I guess there are 4 wire plus ground circuits around with a shared neutral, such as feeding NEMA 18-15R, 18-20R, L18-20R, and L21-21R receptacles powering a big rack cabinet.
This risk could even occure on single phase circuits with polarized current, such as with a single rectifier, possibly seen in very cheap power supplies for some types of equipment, and in some "light bulb savers" I once saw that used a rectifier to cut out half the AC cycle. Each side of the split single phase will be conducting at opposite times, and if someone adds more of these loads to circuits thinking there is now extra capacity due to the reduced power drawn, they could be in for a rude surprise. I suspect these things are very hard to find anymore.
| By the way, your approach to showing the problem | with a diagram is great. You can see where the | pulses on each phase would create neutral currents | that would not cancel each other out.
Thanks. Picture do often say so much more.
I'm finding this fascinating as I have just enough background in this stuff to be dangerous without being helpful...
so please forgive what may be a dumb question to you both:
In the case of the 300 watt supply provviding 100 watts of power, the diagram, as I read it, demonstrated some hefty power peaks. BUT they only seemed to be for one third the time, with the other 2/3rds showing nothing.
Did I interpret it correctly? If so, shouldn't the wiring capacity be rated for the averaged out load rather than the short term (fractions of seconds) peaks?
Or am I completely off here... (wouldn't be the first time)
Thanks
| I'm finding this fascinating as I have just enough background in this | stuff to be dangerous without being helpful... | | so please forgive what may be a dumb question to you both: | | In the case of the 300 watt supply provviding 100 watts | of power, the diagram, as I read it, demonstrated some | hefty power peaks. BUT they only seemed to be for | one third the time, with the other 2/3rds showing nothing. | | Did I interpret it correctly? If so, shouldn't the wiring | capacity be rated for the averaged out load rather than | the short term (fractions of seconds) peaks? | | Or am I completely off here... (wouldn't be the first time)
This is not a dumb question. It's a perfectly valid question.
Yes, the pulses are on only 1/3 of the time. Switching power supplies can do that depending on load.
If you have a current of 60 amps for 1/3 of the time, then in normal cases you would rate the wiring for only 20 amps average. The wire under 60 amps peaks would heat up as much as 20 amps smoothed out.
This applies to both DC and AC, but with AC under a normal linear load, the peaks on a 20 amp average load goes up to 28.28 amps.
What one needs to look at is the average. If the average is the normal 20 amps RMS equivalent (though not shaped that way), then we have 20 amps of heating.
In a normal 2 pole split single phase circuit, with half the load on each side, the current flow would all be in the same direction for both sides at the same time (assuming similar power supplies and similar load ratios). The peak currents on each side would share the neutral in a way that current would flow between the two wires joining up at the neutral, but not through the neutral back to the main service.
In 3 phase power, the problem changes.
With sine wave currents in 3 phase, there is a more complex sharing of currents on 3 phases joining at the neutral. But they do total up to zero when all 3 phases are in balance with linear (where the current has the same sine wave alignment as the voltage) loads.
With non-linear currents, this changes. What I was showing with the pulses at 1/3 of a cycle time is the worst case. The pulses on each phase never occur at the same time as a pulse on either of the other phases. So during a pulse, the one hot leg plus the neutral carry the current. The next pulse is on a different hot leg, but it's the same neutral carrying the other half of the current circuit.
So the averaging, where you can use 20 amp wiring for 60 amp peaks, only applies to the 3 hot legs. The neutral is going to get 60 amps of average current load.
Imagine you have wiring rated for 20 amps, but you put 60 amps of lights on it. You add a motorized switch to turn the lights on and off so the lights are on only 1/3 of the time. Assuming the rate is fast enough, the wiring will only heat up based on the 20 amps of average. You could get away with 20 amps of wiring. Now imagine you have 3 of those circuits and synchronize the motors so that only one set of lights is on at the same time. Now you feed all three sets of lights with individual wires for the motor controlled "hot" side, but you bring the return current "neutral" back on a shared wire. That shared neutral wire will need to be rated for 60 amps of current even though the three hot wires only need rating for 20 amps.
That's a rough analogy to what 3 phase power is doing WHEN the currents are pulse type currents that don't coincide in time. The neutral has to serve all 3 phases without any benefit of sharing currents out at the load. So the neutral has all 3 loads in full.
Mathematically, any harmonic frequencies which are odd multiples of
3 times the fundamental frequency can introduce this problem. Square waves contain these frequencies, and if narrow enough, pulses can be composed entirely of these frequencies. In this worst case scenario, the neutral will be carrying 180 Hz in US/Canada and other places that use 60 Hz, and carrying 150 Hz in Europe and other places that use 50 Hz.The industry refers to these odd multiples of 3 harmonics as "triplens".
This applies to the more common "wye" configuration of 3 phase. The issues exist with "delta" configuration, too, but not as dramatic because even sine wave currents on delta configurations require a
1.732 times rating which is already figured in, while the worst case non-linear load would push that to 2.0 times rating, which is only a 15.5% increase over the linear case. So in the worst case with a delta configuration, you only need to overrate each conductor by just 15.5%.And worst case situations are not actually common. In real life the non-linear loads are only a portion of the whole load, and the pulses don't usually get so narrow as to be 1/3 of the cycle.
This document provides a more detailed examination:
Also offices full of electronic florescent ballasts also do this.
As for the computers, most are configurable to operate on 208-250VAC and
100-125V. So all the computer power supplies could be set for 208-250V and connected to 208v sources. Thus the neutral never sees any of these loads. But this would require new power cords on all the computers to be fitted with 6-15 plugs. And don't forget to switch all those switches on the supplies (some are hidden behind plastic bezels on the back). Probably cheaper to just install a super neutral.
For whatever it's worth, the computer lab here has a large box connected to the electrical room (with a HUGE conduit) labelled "Third Out Harmonic Neutralizer". Don't know exactly what's inside, other than it has dire warnings to wait 1 minute after shutting off power to it before opening to allow caps to discharge.
Transformers wired delta-wye prevents the third harmonics from getting into the public power supply, something the power companies don't like.
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