Are 3-phase residential apartment neutrals a current carrying conductor?

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Tainted

Tainted

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New York
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Engineer (PE)
We want to use 8x8 wireways and feed (10) 120/208V 3-phase apartments using the 8x8 wireway without derating the wires. If you have more than 30 current carrying conductors you would have to derate wires which I'm trying to avoid.

I understand that if you have more than 50% nonlinear loads then you would count a neutral as a current carrying conductor.

But generally, are 3-phase residential apartment neutrals a current carrying conductor? How can I calculate and make sure the apartment does not exceed 50% nonlinear loads?
 
I would imagine you are not going to have to worry about too much in resi for nonlinear loads. I think it pertains more to commercial/data centers with lots of electronics
 
If the HVAC systems for each apartment use a VFD, you might have a not-insignificant amount of non-linear loads. Otherwise, its LED lights and maybe personal computers which plug into general purpose receptacles anyways. Even with VFD-controlled HVAC, the chance you reach 50% is close to 0.
 
Tainted said:
But generally, are 3-phase residential apartment neutrals a current carrying conductor?
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If you mean two lines and a neutral (120/208 1ph) from 208Y/120 3ph, yes, the feeder neutral is a CCC.

Tainted said:
How can I calculate and make sure the apartment does not exceed 50% nonlinear loads?
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Unless each unit has 3ph and a very large computer load, it won't be enough to be a concern.
 
drktmplr12 said:
If the HVAC systems for each apartment use a VFD, you might have a not-insignificant amount of non-linear loads. Otherwise, its LED lights and maybe personal computers which plug into general purpose receptacles anyways. Even with VFD-controlled HVAC, the chance you reach 50% is close to 0.
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And line-to-line-only loads, non-linear or otherwise, don't contribute to neutral current.
 
In most cases each apartment is only supplied with two phases and a neutral, where the complex is supplied by a 3 phase 208Y/120 volt system. In that case the neutrals in the apartment feeders will be current carrying conductors.
 
Tainted said:
We want to use 8x8 wireways and feed (10) 120/208V 3-phase apartments using the 8x8 wireway without derating the wires. If you have more than 30 current carrying conductors you would have to derate wires which I'm trying to avoid.

I understand that if you have more than 50% nonlinear loads then you would count a neutral as a current carrying conductor.

But generally, are 3-phase residential apartment neutrals a current carrying conductor? How can I calculate and make sure the apartment does not exceed 50% nonlinear loads?
Click to expand...
Forget nonlinear anything for apartments. For 3-wire single phase apartment feeders the neutral count as CCC's. For 4-wire feeders the neutral is not a CCC.
 
Are you really feeding the apartments with three phase power, meaning 3 hots and a neutral? Or are you feeding the apartments with 2 hots and a neutral derived from the three phase system? The latter is far more common!

As others have noted, when you have 2 hots and a neutral derived from a three phase system, the neutral is _always_ counted as a CCC.

However you might look at 215.4, and use a common neutral for groups of these feeders. This would let you make the neutral 'balanced' between all three phases, and then it would not count as a CCC.

-Jon
 
Thanks everyone for your inputs. Yes I actually mean 3 ungrounded +1 grounded for each apartment.

This section of code confuses me because most of the loads are generally electronic operated machines. Everything you plug in a wall outlet has some sort of electronic device doesn't in? If I plug a computer, a phone, laundry machine, or dryer in an outlet, it's controlled via electronics, so isn't most of the loads in a dwelling unit electronic nonlinear loads?
 
Tainted said:
If I plug a computer, a phone, laundry machine, or dryer in an outlet, it's controlled via electronics, so isn't most of the loads in a dwelling unit electronic nonlinear loads?
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None of those loads are of any real concern for causing the neutral to be more than 50% nonlinear .
 
Tainted said:
Thanks everyone for your inputs. Yes I actually mean 3 ungrounded +1 grounded for each apartment.

This section of code confuses me because most of the loads are generally electronic operated machines. Everything you plug in a wall outlet has some sort of electronic device doesn't in? If I plug a computer, a phone, laundry machine, or dryer in an outlet, it's controlled via electronics, so isn't most of the loads in a dwelling unit electronic nonlinear loads?
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your washer and dryer are big loads but they are 99% inductive/resistive loads that have nothing to do with electronics along with most big loads in a house.
 
Tainted said:
Thanks everyone for your inputs. Yes I actually mean 3 ungrounded +1 grounded for each apartment.

This section of code confuses me because most of the loads are generally electronic operated machines. Everything you plug in a wall outlet has some sort of electronic device doesn't in? If I plug a computer, a phone, laundry machine, or dryer in an outlet, it's controlled via electronics, so isn't most of the loads in a dwelling unit electronic nonlinear loads?
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These put how much current on the neutral, much less how much is harmonic (which are almost never 100% of the line current).
Phone 500mA
Computer 1.5A
Washer 12A
 
Just re-reading the relevant code, and IMHO it doesn't fit the situation:

"On a 4-wire, 3-phase wye circuit where the major portion of the load consists of nonlinear loads, harmonic currents are present in the neutral conductor; the neutral conductor shall therefore be considered a current-carrying conductor."

The problem with the above code is that if you have a residential situation, the major loads are going to be L-L loads such as HVAC, heating, well pumps, etc. Such loads are very likely non-linear (inverter driven everything being all the rage), but can't impose current on the neutral. The code section should specifically focus on line-neutral loads, but it simply says '3-phase wye circuit'.

I don't believe a residential neutral (even a 3 phase residential neutral) should count as a CCC, but code might say it does.

-Jon
 
Since they are 3 phase feeders to the dwelling units, the wire sizes will probably be a little larger anyways since you cant use 310.12 anymore.
 
winnie said:
Just re-reading the relevant code, and IMHO it doesn't fit the situation:

"On a 4-wire, 3-phase wye circuit where the major portion of the load consists of nonlinear loads, harmonic currents are present in the neutral conductor; the neutral conductor shall therefore be considered a current-carrying conductor."

The problem with the above code is that if you have a residential situation, the major loads are going to be L-L loads such as HVAC, heating, well pumps, etc. Such loads are very likely non-linear (inverter driven everything being all the rage), but can't impose current on the neutral. The code section should specifically focus on line-neutral loads, but it simply says '3-phase wye circuit'.

I don't believe a residential neutral (even a 3 phase residential neutral) should count as a CCC, but code might say it does.

-Jon
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Yes that section really needs a makeover. The other big flaw is it treats loads black and white - either linear or nonlinear with no consideration for "how nonlinear" they are. With modern PFC power supplies, I question what the neutral current would be even with entirely "non linear loads".

I have always wanted to do more tests with this for a larger sample size, but the only test I have done is with 1000w HPS electronic ballasts. 12 units connected on a 480/277 MWBC resulted in a line current of about 16 amps, and a neutral current of about 2 amps.
 
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