Sharing neutrals and ballasts

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Eddy Current

Senior Member
I worked with a guy a several years ago that had a theory that ballasts will last longer if the circuit they are in is not shared by a nuetral. Since then i have went into industrial work and that just seems to be the standard, no sharing neutrals.


Anybody have any knowledge on his theory?
 

iwire

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99.9999% of ballasts in the US are supplied with shared neutrals as the feeders and services are all shared neutrals. There is no magic that happens at the panel that changes how electricity works in regards to shared neutrals.
 

Little Bill

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99.9999% of ballasts in the US are supplied with shared neutrals as the feeders and services are all shared neutrals. There is no magic that happens at the panel that changes how electricity works in regards to shared neutrals.
Sometimes there is "magic" smoke from overloaded/undersized neutrals.:p
 

Eddy Current

Senior Member
99.9999% of ballasts in the US are supplied with shared neutrals as the feeders and services are all shared neutrals. There is no magic that happens at the panel that changes how electricity works in regards to shared neutrals.

Do you mean inside the ballast its self? I was referring to supplying lighting circuits with one nuetral. Are you talking about down the line on the main feed?
 

ActionDave

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99.9999% of ballasts in the US are supplied with shared neutrals as the feeders and services are all shared neutrals. There is no magic that happens at the panel that changes how electricity works in regards to shared neutrals.
Do you mean inside the ballast its self? I was referring to supplying lighting circuits with one nuetral. Are you talking about down the line on the main feed?
Yes he is talking about main and sub-panel feeds. That is the point.

Is there something inside the ballast that knows that it is getting power from a shared neutral at the panel, but the branch circuit from the panel to light has no shared neutral? It knows which wires the circuit is being completed on? That would be a pretty smart ballast.
 

fmtjfw

Senior Member
Try the following experiment:
Take three fixtures, connect them to a shared neutral and to three separate phase wires.
Using a true-RMS DVM measure the current in the common neutral.
Disconnect one fixture, take the measurement again.
Disconnect another fixture, take the measurement again.

The true-RMS meter should take into account any harmonics produced by the ballasts and show you the worst case loading.

With that information you can determine if the neutral needs to be over-sized (depending on the number of fixtures and the worst case balance).

:happyyes:It is my belief that in all cases a properly sized shared neutral will be less expensive to install and less expensive to operate than separate neutrals.

:happysad:The only negative is that electricians who install them and later maintain them need to understand them.
 

mbrooke

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There is no magic that happens at the panel that changes how electricity works in regards to shared neutrals.
Maybe yes maybe no, In a panel the neutral tends to handle the system as a whole. If the majority of the load is phase phase or linear balanced than lighting would be only be a small percentage. So a 4/0 neutral with lighting being lets say 40 amps in a 225 amp panel would mean only a few harmonics going through that neutral along with a small imbalance from lighting. Now compare this to 300ft runs where all neutrals are shared and the same size guage as the phases is used serving only electronic ballasts. The neutral is almost always loaded well above what youd see from a linear load, thus more heating, thus a higher impendance return.

Experience has shown me that a supersized neutral solves issues with harmonics and voltage rise on non linear loads. Think #8 or #6 neutral on a #10 3 phase run. So in certain situations a shared neutral and non linear load may be an issue. I may be wrong but thats what Ive seen.
 

jumper

Senior Member
While there is a time and place for over sized or separate neutrals, general MWBCs serving commercial fluorescent lighting is not one of them.

Considering that most circuits are 20A and wired in #12, a burned up neutral is more likely due to a poor termination.

The circuit is continuous, so we derate to 80% loading or 16A.

THHN and NM are 90C conductors good for 30 amps.

You would need a full 200% harmonic load to overload the neutral.
 

iwire

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Location
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While there is a time and place for over sized or separate neutrals, general MWBCs serving commercial fluorescent lighting is not one of them.

Considering that most circuits are 20A and wired in #12, a burned up neutral is more likely due to a poor termination.

The circuit is continuous, so we derate to 80% loading or 16A.

THHN and NM are 90C conductors good for 30 amps.

You would need a full 200% harmonic load to overload the neutral.
Very well said and I agree 100%.

Additionally when I look at the panel schedules we get from engineers the lighting circuits are often only loaded to between 10 and 12 amps. When it is up to me I also shoot for 10 to 12 amps as that leaves some room for additional fixtures if needed without going over the 16 amp mark.
 

iwire

Moderator
Staff member
Location
Massachusetts
Maybe yes maybe no, In a panel the neutral tends to handle the system as a whole. If the majority of the load is phase phase or linear balanced than lighting would be only be a small percentage. So a 4/0 neutral with lighting being lets say 40 amps in a 225 amp panel would mean only a few harmonics going through that neutral along with a small imbalance from lighting.
Of course.



Now compare this to 300ft runs where all neutrals are shared and the same size guage as the phases is used serving only electronic ballasts. The neutral is almost always loaded well above what youd see from a linear load, thus more heating, thus a higher impendance return.
Yes, in that case the load on the neutral will be higher. Will it be that much higher to cause an issue? Unlikely unless it is a poorly designed circuit to begin with ..... to long .. to much load on each phase etc.

Experience has shown me that a supersized neutral solves issues with harmonics and voltage rise on non linear loads.
What experience has shown you this?




Think #8 or #6 neutral on a #10 3 phase run. So in certain situations a shared neutral and non linear load may be an issue. I may be wrong but thats what Ive seen.
I have seen that many times, right now in our warehouse we have a few 1000' of MC with 10 AWG ungrounded and 8 AWG grounded conductors.

The engineers call for it and we provide it but if their design is other wise good a super neutral is a waste of time in most cases.

My point is just because it is often done does not prove it is really needed.
 

jumper

Senior Member
Very well said and I agree 100%.

Additionally when I look at the panel schedules we get from engineers the lighting circuits are often only loaded to between 10 and 12 amps. When it is up to me I also shoot for 10 to 12 amps as that leaves some room for additional fixtures if needed without going over the 16 amp mark.
Most of the EEs I have worked for use 1200W as the initial total draw on a 20A circuit.
 
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