Staggering light fixture circuits?

Status
Not open for further replies.
D

dicklaxt

Guest
This question has come up frequently in design.

How do you ckt fixtures and their location when served by a MWBC?

ie: 3phase,120/208V,using 120v lighting loads protected with single pole breakers,,,,,The Designer often asks or the Client often dictates " circuit grouping or circuit staggering witin an area." The argument for grouping fixtures by one circuit in an area would be>>> if that circuit were interupted for what ever reason then this would naturally create a dark spot and force plant maintenance to act on this. On the other hand if mutiple circuits were used in an area and one of those circuits were interupted the loss of light would be reduced only partially and maintenace could fix at a more convenient time.I always chose the later when client spec gave me the option,basically for possible safety and egress issues.

Now with the advent of NEC 2008,,,,,210.4 requiring that all ungrounded conductors of a MWBC be simultaneously disconnected at the same time then the argument of the two scenarios above becomes mute but brings to mind other scenarios/questions.

If any of the 3 phases in the 120/208 MWBC were to interupt then all 3 phases are lost and a much greater area is in darkness.Does this change lighting design thinking? #1,,, a second MWBC might feed this area as well,#2,,, more emergency fixtures be used,#3,,,additional MWBC in the same area would reduce lighting loads per circuit,#4,,, to use circuit ampacities to their max now has to be rethought,#5 more lightly loaded circuits may lead to more thought out placement of panelboards,maybe even smaller paneboards and transformers.I guess one could go on and on with these "what if's"

I'm not complaining here on this issue or saying it should not be this way but only saying change breeds more changes be it good or bad. Its a pretty good impact to the thinking. I'm sure cost studies will be done by engineering companies to fine the best or thought to be best solutions in design changes.

Do you as the man with the tools see as many impacts on the construction phase with such a small change in phylosophy as seems to be looming in the future of design.

I personally like the thought processes getting shaken up from time to time to keep the boredom down,LOL.

dick
 
Yes Jim, what do you think should have been done?

Roger
 
dicklaxt said:
This question has come up frequently in design.

How do you ckt fixtures and their location when served by a MWBC?

ie: 3phase,120/208V,using 120v lighting loads protected with single pole breakers,,,,,The Designer often asks or the Client often dictates " circuit grouping or circuit staggering witin an area." The argument for grouping fixtures by one circuit in an area would be>>> if that circuit were interupted for what ever reason then this would naturally create a dark spot and force plant maintenance to act on this. On the other hand if mutiple circuits were used in an area and one of those circuits were interupted the loss of light would be reduced only partially and maintenace could fix at a more convenient time.I always chose the later when client spec gave me the option,basically for possible safety and egress issues.

Now with the advent of NEC 2008,,,,,210.4 requiring that all ungrounded conductors of a MWBC be simultaneously disconnected at the same time then the argument of the two scenarios above becomes mute but brings to mind other scenarios/questions.

If any of the 3 phases in the 120/208 MWBC were to interupt then all 3 phases are lost and a much greater area is in darkness.Does this change lighting design thinking? #1,,, a second MWBC might feed this area as well,#2,,, more emergency fixtures be used,#3,,,additional MWBC in the same area would reduce lighting loads per circuit,#4,,, to use circuit ampacities to their max now has to be rethought,#5 more lightly loaded circuits may lead to more thought out placement of panelboards,maybe even smaller paneboards and transformers.I guess one could go on and on with these "what if's"

I'm not complaining here on this issue or saying it should not be this way but only saying change breeds more changes be it good or bad. Its a pretty good impact to the thinking. I'm sure cost studies will be done by engineering companies to fine the best or thought to be best solutions in design changes.

Do you as the man with the tools see as many impacts on the construction phase with such a small change in phylosophy as seems to be looming in the future of design.

I personally like the thought processes getting shaken up from time to time to keep the boredom down,LOL.

dick
Click to expand...

The reasons for staggered circuit loads are:
1./ Especially fluorescent lights and other arc type lamps it helps withe the stroboscopic effect. The stroboscopic effect is where the light dims and brightens 60 times per cycle. Two lamps with different phases reduce that effect. The problem with stroboscopic effect is judging if a rotating machinery turns or not and eye fatique.
2./ If the alteration of the phases done correctly and consistently, your panel will be balanced as much as possible without much other effort.
 
weressl said:
The stroboscopic effect is where the light dims and brightens 60 times per cycle.
Click to expand...
Actually, twice per cycle, or 120 times per second, but who's counting? ;)
 
I think this was only an issue with magnetic ballast fluorescent fixtures.

No electronic ballast fluorescent fixture is going cause a stroboscopic effect. These operate around 20k Hz.

I don't think the HID fixtures cause this either because if the arc would become extinguished after each half cycle then there would be the cool down and warm up time required to get an arc reestablished.

Incandescent fixtures still have the filament glowing in between the cycles because it can't cool down instantly.
 
Mr. Bill said:
I think this was only an issue with magnetic ballast fluorescent fixtures.

No electronic ballast fluorescent fixture is going cause a stroboscopic effect. These operate around 20k Hz.

I don't think the HID fixtures cause this either because if the arc would become extinguished after each half cycle then there would be the cool down and warm up time required to get an arc reestablished.

Incandescent fixtures still have the filament glowing in between the cycles because it can't cool down instantly.
Click to expand...

Electronic ballasts still have the same effect at a higher frequency.

HID with the ceramic tubes still will have some light coming from the heated tubes but the main energy is not there, since according to the energy affinity laws at ) current there is no energy flowing in the cuicuit so no light is produced. Since the plasma ionized gas can not cool down there is no problem for it to conduct the arc. The do produce observabe stroboscopic effect.

For the above reason and to the fact that the majority of the energy is convered into heat the incandescent light will provide a (nearly) continous light output. The latent heat in the fillament that continues to turn into light.
 
The OP was not how or why stagger but how/if it effected construction as much as design. In giving it some additional thought after Crossman's answer,,,,I guess the change is not relevant to the man with the tools as its still just lights and wires,the works the same.

dick
 
dicklaxt said:
This question has come up frequently in design.

How do you ckt fixtures and their location when served by a MWBC?
Click to expand...

I see a few options and i'm sure there will be many more. Depending on the area, keep the MWBC and add another single circuit to cover the area.Or use a multi neutral cable.Or if using raceway, add additional neutrals.

Rick
 
Status
Not open for further replies.
Top