Mwbc

[liquidtite]

I'm on a job were the existing 277v lighting system controls mostly 2x4 fixtures t8 bulbs,the 3 ckts all share the same nuetral .I thought that when dealing with non lenior loads computers/floresent bulbs its not ideal to share nuetral.

 

[iwire]

In my opinion it is an overblown issue perpetrated by people selling copper.

 

[GoldDigger]

I'm on a job were the existing 277v lighting system controls mostly 2x4 fixtures t8 bulbs,the 3 ckts all share the same nuetral .I thought that when dealing with non lenior loads computers/floresent bulbs its not ideal to share nuetral.

It is not ideal to share the neutral in the sense that you definitely cannot undersize the neutral relying on cancellation of neutral currents for a balanced load.
And in some cases (more likely, for non-linear power supply loads and CFLs than for conventional magnetic (or power factor corrected electronic) ballast tubular lamps) you may actually have to oversize the neutral because the neutral current is higher than any phase current. But computer supplies and CFLs with distortion power factor correction will reduce the problem or even make it insignificant.
There are no hard and fast guidelines for this. The safest route is to use individual neutrals, although the material and labor costs are higher. But I would not do that blindly when the lights in question are tubular fixtures.

Any time an AFCI is required or it would be desirable to be able to switch off only some of the lighting loads at the breaker (prevented by the presence of a handle tie or multipole breaker) separate neutrals make life easier. It also allows work to be done on individual fixtures without powering off a whole MWBC.

 

[jusme123]

...the requirement of disc within the fixtures make it somewhat easier to work on them.

 

[iwire]

It is not ideal to share the neutral in the sense that you definitely cannot undersize the neutral relying on cancellation of neutral currents for a balanced load.
And in some cases (more likely, for non-linear power supply loads and CFLs than for conventional magnetic (or power factor corrected electronic) ballast tubular lamps) you may actually have to oversize the neutral because the neutral current is higher than any phase current.

You enjoy the kool aid?

I am not saying that harmonic currents do not exist, all I am saying is the need for oversize neutrals is overblown.

In an otherwise well designed lighting circuit the loading on the neutral will not exceed its rating.

I have installed tons of oversize or doubled neutrals as we have done a lot of data center work. But it gets ridiculous when you look at the panel schedule and the circuit loading is only 4 or 5 amps for each 20 amp circuit and feeder loading a very small percentage of the load.

Have you personally seen damage of neutrals from harmonic currents?

 

[roger]

You enjoy the kool aid?

I am not saying that harmonic currents do not exist, all I am saying is the need for oversize neutrals is overblown.

In an otherwise well designed lighting circuit the loading on the neutral will not exceed its rating.

I have installed tons of oversize or doubled neutrals as we have done a lot of data center work. But it gets ridiculous when you look at the panel schedule and the circuit loading is only 4 or 5 amps for each 20 amp circuit and feeder loading a very small percentage of the load.

Have you personally seen damage of neutrals from harmonic currents?

Thank you, that is exactly my experience as well.

Roger

 

[infinity]

I'm on a job were the existing 277v lighting system controls mostly 2x4 fixtures t8 bulbs,the 3 ckts all share the same nuetral .I thought that when dealing with non lenior loads computers/floresent bulbs its not ideal to share nuetral.

In this existing installation is there any sign of the neutrals being overloaded? I like Bob and Roger think that the non-linear aspect of these circuits has been over hyped.

As Bob stated in data and IDF rooms 200% neutrals are commonly installed due to boiler plate specifications that engineers love to follow. On my current job each IDF rooms has a 200 amp, 42 circuit panel with a 200% neutral (parallel #3/0) and the total of all the 120 volt loads in the room equals 0. All power for the equipment is 1?, 208v. There is no neutral load at all. :slaphead:

That's a pretty good waste of someones money.

 

[don_resqcapt19]

I agree with the others. Most every document that you will find out there about the neutral harmonic issue was written by someone who has a vested economic interest in selling you something that will solve the problem.

 

[robbietan]

You enjoy the kool aid?

I am not saying that harmonic currents do not exist, all I am saying is the need for oversize neutrals is overblown.

In an otherwise well designed lighting circuit the loading on the neutral will not exceed its rating.

I have installed tons of oversize or doubled neutrals as we have done a lot of data center work. But it gets ridiculous when you look at the panel schedule and the circuit loading is only 4 or 5 amps for each 20 amp circuit and feeder loading a very small percentage of the load.

Have you personally seen damage of neutrals from harmonic currents?

similar experience here.
and there is always the "k-rated" transformer solution for buildings with non=linear loads