200% Neutral - Apartment Building

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Sparky2791

Sparky2791

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Northeast, PA
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Electrical Design
Hello all!

Have a building that has a 277/480V 3 phase, wye service. It was an old commercial building that is being renovated into apartments.
We are feeding the apartments (about 80) using a 120/240V delta 3-phase transformers to a distribution panels that feed each unit from the distribution panels to a 100A, 120/240V single phase panel in each apartment (owner insisted the units be fed with 120/240 not 120/208V but that is another post that could be written as to why)

Now they are questioning if the distribution panels should have 200% neutrals and oversized neutral conductors. Not a whole lot of non-linear loads involved here and even if there were it's not 2001 when computer power supplies were a big factor like in data centers. Not sure what would be the reason for making this an issue. Am I missing something?

Thanks for the replies!
 
Why would you use 3 phase Delta transformers? Just use single phase transformers. You are wasting 2 winding of the Delta transformer and they have a 5% maximum neutral load. Since the transformer is limited to a 5% neutral load why would you want oversize size the conductors?

Sounds like whom ever is trying to tell you how to design this system has zero electrical knowledge. You as the engineer and the EC need to explain to them that feeding each apt with 120/208 makes more sense.
 
Don't waste someone else's money on oversized neutrals for this application. The single phase 120/240 system is dumb too but it seems that the OP feels that way too.
 
LibertyEngineering said:
Now they are questioning if the distribution panels should have 200% neutrals and oversized neutral conductors. Not a whole lot of non-linear loads involved here and even if there were it's not 2001 when computer power supplies were a big factor like in data centers. Not sure what would be the reason for making this an issue. Am I missing something?
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The only case where a ~200% neutral would be needed is if there are only L-N loads that are on all 3 phases, and the current is drawn in pulses which have NO overlap between the different phase current waveforms. That would result in the neutral carrying 1.73 times each L-N current, assuming balanced L-N loads on the phases. There are other theoretical cases where this could also happen with highly reactive loads, but it's not relevant for this discussion.

If you are distributing 120/240V single phase, the voltage will peak on opposite phases at the same time. And so with typical nonlinear loads like rectifiers, any L-N current pulses on opposite phases will tend to overlap in time and cancel on the neutral, instead of adding in an independent manner as in the 3-phase case. HVAC might have VFDs which would have a rectifier but it would likely be a L-L load. I'd expect the TDDi on the neutral of an apartment to be relatively low. If anything, you may be able to reduce the neutral on a 120/240V single phase feeder.
 
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LibertyEngineering said:
...
We are feeding the apartments (about 80) using a 120/240V delta 3-phase transformers to a distribution panels that feed each unit from the distribution panels to a 100A, 120/240V single phase panel in each apartment...
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You're doing what, exactly? How are you deriving the neutrals?

LibertyEngineering said:
Now they are questioning if the distribution panels should have 200% neutrals and oversized neutral conductors. ...
Click to expand...
For residential units? Sounds like crazy talk.
 
I agree with the others. You don't need an oversized neutral.

In a 3 phase system, currents are supposed to be 120 degrees apart on the different phases. Harmonic currents are current flows at multiples of the base frequency. The phase angles of the harmonics also get multiplied. So if you have a L-N load which is rich in 3rd harmonic current, and put three of these loads on each of your three phases, the 3rd harmonic current will have a phase angle difference of 360 degrees. On a 3 phase wye system, multiple of 3 harmonics add on the neutral.

In a residential single phase system, even harmonics would add on the neutral. While even harmonics are possible, they are not often encountered and IMHO not a significant problem. Low power rectifiers with half bridge diodes are an example.

I'm also on the bandwagon about not using a delta transformer to supply these loads. I sure hope that the high leg isn't being supplied to the single phase panels.

The caution about the 5% neutral is real but transformer specific. A high leg delta can be built without the 5% limit but the limit is very common.

I concur with post #2 about using multiple single phase transformers if the customer is dead set on 120/240 single phase to each unit. Another option is hexaphase, but that would require a custom transformer. (Basically 3 phase primary, 3 center tapped secondary coils with a common neutral. )

Jon
 
winnie said:
Another option is hexaphase, but that would require a custom transformer. (Basically 3 phase primary, 3 center tapped secondary coils with a common neutral. )
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Wouldn't that effectively be the result of three 1ph units once the three neutrals are bonded?
 
LarryFine said:
Wouldn't that effectively be the result of three 1ph units once the three neutrals are bonded?
Click to expand...

My _guess_ (I'm not a transformer designer) is that 3 separate single phase transformers of X kVA would be a heavier than a single 3 phase transformer of 3X kVA. The 3 phase version also gives you 208/120 three phase for loads that need it.

But as far as the 120/240V output of a single hexaphase vs 3 single phase? Exactly the same.

-Jon
 
Thanks all for your posts. I straightened this out today with the owner. 120/208V 3 phase Wye system is the way to go to feed the units so the size of the distribution panels are kept to a minimum and all three phases can be used equally. Delta transformer was a bad idea. What was I thinking!!!!!! Complete brian fart!
 
LibertyEngineering said:
Thanks all for your posts. I straightened this out today with the owner. 120/208V 3 phase Wye system is the way to go to feed the units so the size of the distribution panels are kept to a minimum and all three phases can be used equally. Delta transformer was a bad idea. What was I thinking!!!!!! Complete brian fart!
Click to expand...
As far as the transformers, not a complete b.f. I've seen it done fairly often to allow the use of more conventional 240v equipment. As tortuga mentioned, you could stagger the phase between the various transformers and balance the load somewhat .
 
winnie said:
... In a residential single phase system, even harmonics would add on the neutral. While even harmonics are possible, they are not often encountered and IMHO not a significant problem. Low power rectifiers with half bridge diodes are an example.
Click to expand...

I agree with that Jon. Also, a load drawing even harmonic currents such as one with even order transfer function components x2, x4. etc. will typically have content at DC (0 Hz). And your example of a half wave rectifier certainly does. If such DC current is sufficient, it could cause saturation of the transformer core during one half of the signal phase waveform.
But I agree that this would not be an issue with the type of loads an apartment would have,
 
Joethemechanic said:
How many leads come out of the secondary of the three phase transformer's secondary? Is it 12 leads? Can you connect the 3 centerpoints together and leave the phases separated?
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I don't think (as a practical matter) that anyone makes transformers that way.

But it would certainly allow lots of flexibility. Add 120 and 139V taps to each coil and you could get anything from 120V delta up to 480/277V out of a single transformer.

-Jon
 
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