neutral conductors and electronic ballasts

[electrofelon]

I am pricing out a job where I need to feed 104 1000 watt HPS electronic ballasts. I need to design both the branch circuits and the feeder. The system is 277/480 and the lights will be on MWBC's. So you know where this is going In regards to the neutral being a CCC, and for sizing the neutral conductor, It seems like there is no other way than to get a hold of three of the lights and clamp the neutral and scope the line conductors? The system will be balanced so I would like to use the smallest neutral possible, sized to 250.122, but I cant be sure of the harmonic currents. Does a "total harmonic distortion" from the spec sheet give anything meaningful? I am sure this comes up all the time when lighting systems for office buildings are designed/installed, what is typically done?

 

[jumper]

220.61(C)(2) seems to say that you gotta run a full sized noodle for the feeder.

I cannot find anything regarding sizing the noodle for branch circuits other than an informational note on MWBCs. A common spec for your install is a #10 noodle for a 20A circuits.

 

[ActionDave]

All I know is I have done hundreds of light fixtures with electronic ballasts. All of them have been on MWBC, all of them have been on panels fed with a downsized noodle, never was the noodle counted as a CCC, and never been a problem. Not for the last fifteen years anyway.

 

[JoeStillman]

Feed them at 480.

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[electrofelon]

Feed them at 480.

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Unfortunately the lights are not available in 480V plus we want them to be cord and plug connected and 480 poses a big problem for that.

220.61(C)(2) seems to say that you gotta run a full sized noodle for the feeder.

Thanks for that, I didnt see that, I was looking at 310.15(B)(5). But......(C) states "there shall be no reduction of the neutral or grounded conductor....." Reduction from what? From the value in 220.61(A) or 220.61(B)? I think (C) is saying when we cant use the reductions in (B).

All I know is I have done hundreds of light fixtures with electronic ballasts. All of them have been on MWBC, all of them have been on panels fed with a downsized noodle, never was the noodle counted as a CCC, and never been a problem. Not for the last fifteen years anyway.

I hear ya. Same here. This linear load/harmonic/CCC issue is real annoying. It seems like no one ever worries about it, and there is no industry standard or figure for classifying the non-linearity and how it would effect neutral current and at what point it becomes a CCC in the eyes of the NEC.

 

[topgone]

Electronic ballasts have about 15-28% THD. On 277/480V MBWC, harmonics do not increase the neutral current much. As previously suggested by other, size your neutral the same as your line conductors and you will have no problems.
BTW, the poster saying you supply with 480 is right. Your line to line voltage is 480V but you will connect your loads line to neutral (277V).

 

[don_resqcapt19]

I am pricing out a job where I need to feed 104 1000 watt HPS electronic ballasts. I need to design both the branch circuits and the feeder. The system is 277/480 and the lights will be on MWBC's. So you know where this is going In regards to the neutral being a CCC, and for sizing the neutral conductor, It seems like there is no other way than to get a hold of three of the lights and clamp the neutral and scope the line conductors? The system will be balanced so I would like to use the smallest neutral possible, sized to 250.122, but I cant be sure of the harmonic currents. Does a "total harmonic distortion" from the spec sheet give anything meaningful? I am sure this comes up all the time when lighting systems for office buildings are designed/installed, what is typically done?

You can't size the neutral in the feeder based on the loads being balanced. It has to be sized for the maximum possible unbalanced load. That would be all of the single pole breakers on the most loaded phase turned on and all of the other breakers turned off.

220.61 Feeder or Service Neutral Load
(A) Basic Calculation. The feeder or service neutral load shall be the ma ximum unbalance of the load determined by this article. The maximum unbalanced load shall be the maximum net calculated load between the neutral conductor and any one ungrounded conductor.

 

[electrofelon]

You can't size the neutral in the feeder based on the loads being balanced. It has to be sized for the maximum possible unbalanced load. That would be all of the single pole breakers on the most loaded phase turned on and all of the other breakers turned off.

Ok good point that is an easy mistake to make, however in this case, all the branch circuits would be mwbc's and three pole breakers.

 

[ptonsparky]

Ok good point that is an easy mistake to make, however in this case, all the branch circuits would be mwbc's and three pole breakers.

Are they switched via the breaker or other three pole means?

 

[electrofelon]

Are they switched via the breaker or other three pole means?

They have no switch other than the breaker. The lights do have software that allows them to be controlled from a computer and all lights will be on or off at the same time

 

[don_resqcapt19]

Ok good point that is an easy mistake to make, however in this case, all the branch circuits would be mwbc's and three pole breakers.

In my opinion, they are still single phase loads and subject to the rule is 220.61. The fact that you connected 3 single phase loads to a 3 pole breaker does not change anything for me.

 

[ptonsparky]

In my opinion, they are still single phase loads and subject to the rule is 220.61. The fact that you connected 3 single phase loads to a 3 pole breaker does not change anything for me.

That certainly is what 220.61 says.

 

[electrofelon]

In my opinion, they are still single phase loads and subject to the rule is 220.61. The fact that you connected 3 single phase loads to a 3 pole breaker does not change anything for me.

That certainly is what 220.61 says.

After re-reading it, I concur.