Design Common neutrals and such things for LEDs, etc

[BillConnerFASTC]

Long understood the requirement for no common neutrals for SCR dimmed circuits, but wondering with constant circuits, switched centrally or locally (ColorSource relays), if it matters. Would for instance three hots, a neutral, and a ground be fine for LEDs, like 10 ColorSource units on each of three circuits? 30 units on a pipe might be adequate (like the old b-way standard of 28 or 29 units on a 42' electric). I'd actually prefer 4 circuits and am considering two drops of two circuits each, each with 2 hots, a neutral, and a ground.

Then I wonder if a traditional 6 circuit multi - I believe usually 6 hots, 6 neutrals, and two grounds, would actually support 12 constant circuits. That would seem plenty on a Prodigy for instance - one flat cable.

Just curious what others are providing and finding with all LED rigs.

 

[Van]

LED Technology really does change the Equation doesn't it? We've debated this topic a few times in the office. I feel that as long as your neutral isn't overloaded then there is no reason not to run a common, they are , for all intents and purposes, standard power outlets. They adhere to the same codes and limitations. It sure would make it easier for Electrical Contractors to wire theatres, Wouldn't it?

 

[JD]

I suspect not.
I am sure ST could speak to this in great detail, but even in house wiring, the concept of having all circuits "home run" their neutral has become the norm. It is more a safety issue than an over-current one.

 

[BillConnerFASTC]

I suspect not.
I am sure ST could speak to this in great detail, but even in house wiring, the concept of having all circuits "home run" their neutral has become the norm. It is more a safety issue than an over-current one.

Really? I have a lot of circuits, or pairs of circuits, in my house - two circuits on a single 2 pole breaker. I'm sure its allowed by code. May have become like grounds in conduit. You can use the conduit for ground but most engineers seem to believe a separate ground wire is better design. That is kind of like saying following the NEC is not safe enough. It is only possible essentially with multi-pole breakers as I recall, I believe to prevent the inadvertent rewiring two branch circuits on same leg with common neutral - a definite over-current problem.

What I'm specifically trying to do is do 4 circuits on a pipe - FOH, electric, etc. - with two 2-circuit plug boxes, on at each quarter point - and then powercon daisy chain in each direction. While a fixed pipe is no big deal to add another conductor, on a winched pipe the idea of one or two fewer conductors is especially appealing, using a cable reel, ideally in the retriever arrangement (reel mounted to batten), which is more possible with a smaller reel and fewer conductors.

In the end copper and pipe has a cost and if its not used, why buy for it?

 

[JD]

Use to be very common in house wiring. Two breakers on opposite legs feeding the old red/black/white romex that later split off into two branch circuits. I don't think there is any problem regarding the older style being grandfathered.
The problem arises when you kill circuit "A" to go work on a box down stream, you twist off a wirenut on and find you white is hot from backfeed from something plugged into circuit "B." I am not sure if it is code required yet (ST would know) but all newer wiring I've seen go in sticks to the home run principal. When you kill circuit "A" there is no way any voltage will be present on any boxes down stream on circuit "A." Still plenty of things using RBW, such as cloths driers, and feeds on three way switches, but not branch circuits. Time to get an answer from @STEVETERRY

 

[sk8rsdad]

@JD, code requires both phases to be fed from a common 2-pole breaker so the scenario you describe should not be possible following LOTO.

The thing that's driven the use of more single phase wiring in the home is the advent of requiring 20a GFCI outlets in the kitchen. Most GFCIs work by sensing differential current and that means they don't work properly with a common neutral.

 

[JD]

@JD, code requires both phases to be fed from a common 2-pole breaker so the scenario you describe should not be possible following LOTO.

Only when a common load (such as a dryer or range) or a common connector is involved.

 

[BillConnerFASTC]

I'm not sure, other than what I thought I read in the NEC and that I asked the inspector about it, but pretty sure if the breakers are locked together (I think the code says something like that but practically its a two pole breaker) you can feed two separate branch circuits with a common neutral. Both 20 amp small appliance circuits in my kitchen are fed from a two pole breaker, but separate neutrals after the GFCI devices. Discussed and approved to by inspector.

@STEVETERRY will hopefully weigh in.

Here is one answer: http://ecmweb.com/content/code-quandaries-48

and another: http://www.doityourself.com/forum/e...20a-circuits-share-same-neutral-ground.html#b

And there are more.

 

[STEVETERRY]

What you guys are describing is a "multiwire" circuit. It is quite permissible with a few caveats. However, it may not be a great design for a theatre.

1. It must be fed by a common means of disconnect ( generally a common-trip breaker--but also see 240.15(B)(1) below) for the two (or three, which is possible with a three-phase service) branch circuits in the multiwire arrangement.

2. It cannot be used with circuits that feed emergency lighting.

3. It won't work with GFCI or AFCI circuits.

4. A fault on one circuit will kill all circuits in the multiwire arrangement. This may not be desirable in a theatre.

5. It needs to be designed to cover the informational note in 210.4:

Informational Note No. 1: A 3-phase, 4-wire, wye-connected
power system used to supply power to nonlinear loads may
necessitate that the power system design allow for the possibility
of high harmonic currents on the neutral conductor.

Herein lies the rub. What will the harmonic content be of the loads serviced by this multiwire circuit? Since they are cord-and-plug-connected, that is difficult or impossible to predict. The result could be neutral overcurrent on the circuit and a safety problem.

6. Single-pole breakers can be used to feed multwire circuits under the conditions of 240.15(B)(1):

(1) Multiwire Branch Circuits. Individual single-pole circuit
breakers, with identified handle ties, shall be permitted as the
protection for each ungrounded conductor of multiwire
branch circuits that serve only single-phase line-to-neutral
loads.

More than you wanted to know? I guess so.

ST

 

[BillConnerFASTC]

I still don't know if other than potential multi circuit trip problem if it's an issue for constant circuits for LEDs. Considering the conservatism for inrush - like max 10 Colorsource on a circuit - is it really an issue?

 

[STEVETERRY]

I still don't know if other than potential multi circuit trip problem if it's an issue for constant circuits for LEDs. Considering the conservatism for inrush - like max 10 Colorsource on a circuit - is it really an issue?

Not until it is an issue. There are many causes to tripping of OCPD's--which cannot be predicted. How lucky do you feel?

ST

 

[Scenemaster60]

Like Steve says, it isn't an issue until it is.

Inrush is far less of an issue than the multiplier effect of triplen harmonics on the neutral from switch mode power supplies. That is still a wild card that the industry as a whole is starting to recognize as an issue. The only way to know for sure is to meause the current on the neutral under multiple loading scenarios.

 

[AlexDonkle]

Harmonics from SMPS's are relatively new in the lighting industry, but IT has dealt with this issue for decades and thankfully a lot of their advances now help lighting equipment. Power factor correction (PFC) is built into most modern PC's and other high quality power supplies. There's also regulations in Europe limiting maximum harmonic content of electronic devices including LED's, EN61000-3-2. I don't believe the US has adopted this standard (I'm not aware of it at least, the standards I've seen are voluntary-only), but most manufacturers use universal power supplies worldwide to save cost so PFC gets built into many products sold internationally. At minimum, any product carrying a CE listing meets this requirement.

That all said, there's likely always going to be some cheaper and/or older equipment that doesn't comply with standards and causes some harmonics when you're dealing with theatres and portable lighting that gets changed / upgraded / rented-in over the years, so that concern varies theatre to theatre.

 

[DuckJordan]

Another thing to remember is the code is a minimum, not a guide book.

Sent from my SM-G930V using Tapatalk

 

[BillConnerFASTC]

Another thing to remember is the code is a minimum, not a guide book.

Sent from my SM-G930V using Tapatalk

And a minimum for safety, not functionality.

 

[STEVETERRY]

And a minimum for safety, not functionality.

Nor is the NEC a guide for achieving minimum installed cost, which might be the primary (only?) reason for using a multiwire circuit in this application.

ST

 

[BillConnerFASTC]

I'm probably missing something significant but if I wanted say 6 circuits on a catwalk, I could feed with two 3 pole breakers, and I think the comments here suggest some weaknesses. But what about feeding a small 40 amp sub panel and distributing from that with 6 two wire circuits? Less weaknesses?

It all goes back to "taking cost out of the walls".

 

[STEVETERRY]

I'm probably missing something significant but if I wanted say 6 circuits on a catwalk, I could feed with two 3 pole breakers, and I think the comments here suggest some weaknesses. But what about feeding a small 40 amp sub panel and distributing from that with 6 two wire circuits? Less weaknesses?

It all goes back to "taking cost out of the walls".

I heard that somebody smart said that.

This approach would give a bit more robustness and less chance of killing three branch circuits in a fault. The cost comparison would be:

6 1P 20A breakers
6 pole spaces in the panel
12 x 12AWG conductors home run outlets to main panel
Labor

vs.

1 3P 40A breaker
6 1P 20A breaker
9 pole spaces across two panels
1 MLO subpanel 6 pole spaces
4 x #8AWG conductors (neutral is current-carrying so it's four conductors for ampacity adjustment)
12 x #12AWG conductors between sub panel and outlets
Labor

I'm guessing the second one costs more.

ST

 

[JD]

Assuming you are pulling conduit, a 500 foot roll of #12 THHN is about $46. Not much of a bump in price for pulling those extra neutrals.

Tangent: Somewhat amused by this trend. Anybody try it yet? http://www.homedepot.com/p/Cerrowir...Cabled-Solid-THHN-Cable-112-161253C/205493343

 

[RonHebbard]

Assuming you are pulling conduit, a 500 foot roll of #12 THHN is about $46. Not much of a bump in price for pulling those extra neutrals.

Tangent: Somewhat amused by this trend. Anybody try it yet? http://www.homedepot.com/p/Cerrowir...Cabled-Solid-THHN-Cable-112-161253C/205493343

Being twisted, it'll occupy more pipe space plus be more difficult to pull past other twisted pairs / triples within a given pipe.
I'll shamefacedly admit to learning this the hard way when pulling in red and black twisted pairs of #10 stranded THHN twisted pair low impedance speaker runs vs. pulling them in as single, untwisted, conductors. Arguably less coupling between lines but definitely occupying more space within conduits.
BTW; We had the black and red twisted pairs custom manufactured as none of the major suppliers offered it as a standard 10 gauge product. When you pull in twisted black and white pairs you can run into termination problems when electricians terminate one end and electronics folks terminate the other. I trust I don't need to elaborate on their differences in logic and thinking.
Toodleoo!
Ron Hebbard.