A friend called and asked some questions about a job he may do.

A residence with a remote building where the remote building has a 200 amp single phase service. The owner wants to install many, many computers in the building at 800 watts each. He is not sure how many but it may be 50 or so.

The question is if you have a load on the neutral of 1600 watts for one circuit (2 machines), and 1600 watts on the neutral for the other phase, then is load on the neutral 1600 watts. If I think about the service as a multiwire branch circuit then the neutral load would be "zero" but I don't think it works that way.

Secondly, once the neutral is calculated and the non-linear loads are more than 50% then how do we size that neutral ?

Thirdly, do we need to even worry about this on single phase since the code mentions harmonics for a 3 phase wye and not single phase.

2. Originally Posted by Dennis Alwon
A friend called and asked some questions about a job he may do.

A residence with a remote building where the remote building has a 200 amp single phase service. The owner wants to install many, many computers in the building at 800 watts each. He is not sure how many but it may be 50 or so.

The question is if you have a load on the neutral of 1600 watts for one circuit (2 machines), and 1600 watts on the neutral for the other phase, then is load on the neutral 1600 watts. If I think about the service as a multiwire branch circuit then the neutral load would be "zero" but I don't think it works that way.

Secondly, once the neutral is calculated and the non-linear loads are more than 50% then how do we size that neutral ?

Thirdly, do we need to even worry about this on single phase since the code mentions harmonics for a 3 phase wye and not single phase.
I won't claim to be an expert on harmonics, but we are always told they are additive in the neutral. So something to chew on a little before someone that knows harmonics better chimes in - for single phase multiwire applications the effects would not likely be as bad as they are for three phase multiwire applications as the additive effects of the harmonic currents is only coming from two ungrounded conductors instead of three. What I don't know is if the harmonics are still additive when there is 180 degree phase angle or if it is only additive because of the 120 degree angles of a wye system.

3. It is just a multiwire circuit, so not considering the harmonics, the neutral conductor will have 0A.

Harmonics are load based and will differ depending on the kind of power supplies are in the computers.

3rd harmonic and all of its integer multiples (collectively called triplen harmonics) generated by 120ophase-shifted fundamental waveforms are actually in phase with each other, that is why on a 4 wire feeder, there is a concern of excessively high current that may be more than the individual phase conductors. This led to years of oversized and double neutrals.

The triplens are not additive for 120/240V with the 180ophase-shifted fundamental waveforms

Most will just run a single full sized neutral to accommodate either one circuit full on and the other off, or both on and significant harmonics on the neutral.

4. Ron, I don't think a calculation of a neutral conductor is calculated that way. The branch circuits are not multiwire branch circuit so if you calculated the load of the neutral as a service with just those 2 circuits I don't believe the load would be 0

I guess you would have to size the neutral to the largest load on one phase as you could have one phase off

5. Originally Posted by Dennis Alwon
Ron, I don't think a calculation of a neutral conductor is calculated that way. The branch circuits are not multiwire branch circuit so if you calculated the load of the neutral as a service with just those 2 circuits I don't believe the load would be 0

I guess you would have to size the neutral to the largest load on one phase as you could have one phase off
In a two wire (120 volt) branch circuit the current (linear as well as non linear) is same in both circuit conductors. You are concerned about additive effects of harmonic currents in a neutral conductor of a multiwire circuit - which your feeder and service conductors would have this apply to them. But as Ron mentioned the linear load on the neutral of the single phase source is going to be zero (the linear balanced load) So you are starting out with less current on the neutral to begin with before any harmonics start to come into play on such a system, is kind of what I think he is saying. Then as I said earlier you don't have a third phase to add even more harmonics to the neutral.

6. Grow lights

7. Originally Posted by Ingenieur
Grow lights

No pc's-- lots of them. My friend didn't know why he had so many but he thought it had something to do with bitcoins

So it sounds like we really don't have to worry about the neutral load at all... He will be happy to hear this.

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Originally Posted by Ingenieur
Grow lights
Many computers usually means Bitcoin mining, so minimizing power cost is critical.
For a single phase MWBC, all even order harmonics would be additive.
But to get even harmonics would require a current asymmetry between positive and negative half cycles, such as a single half wave rectifier input.

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9. "Bit coin mining" sounded like something fun and interesting... until I looked at the details. BOOOOORING! It's basically an incestuous process of setting up computers to help process and verify other bitcoin transactions from all over the world as part of a decentralized network based system, for which you are rewarded with... bitcoins.

The thing about the Power Supplies will boil down to this: Are the Switch Mode Power Supplies "power factor corrected" or not? If they are, then the harmonics is insignificant. Everywhere else in the world EXCEPT the US require SMPS to have PFC, we do not. But indirectly we generally end up with the PFC versions anyway, because manufacturers don't really want to bother making different versions and our domestic marketplace is not big enough for a US mfr to stand alone and compete against suppliers selling all over the world.

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There is probably a delta-Y somewhere in the system.

Circa 1992, company installed about 200 new computers in the offices. Not PFC corrected inputs, straight rectifiers in those older power supplies.

About a week later the poco transformer on the roof smoked due to all the triplett harmonics.

OP should check on the type power supplying the new computers, they likely have a PFC circuit in the front end, so no problem. Easy to check, slap on a CT and look at the current waveform.

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