Design Common neutrals and such things for LEDs, etc

[BillConnerFASTC]

I think most ecs use stranded because it pulls easier, and there will be some conduit and some labor. I believe for estimating, 3 hots and 3 neutrals is about 50% more than 3 hots and 1 neutral, so it's reducing total wiring costs a lot.

 

[MNicolai]

I see a lot of twisted THHN, unjacketed. Usually #8-#12. Jacketed isn't such a big deal on the pulls for #14 and down but adds a lot of extra friction to those heavy #10 pulls.

For sound systems, being able to source unjacketed, twisted THHN is a must. The alternatives are untwisted, unjacketed, and you can hear the crosstalk rather noticeably, or twisted, jacketed, which can be much harder to pull.

You wouldn't typically see twisted THHN for power though. Unnecessary expense. You're paying for someone to twist it, you're paying for bigger conduits, and you're paying for more copper. Once you twist 2 500' rolls of THHN, your final roll will be noticeably compressed down from your initial 500' length.

I have seen cases though where a contractor measures all their pulls and gets custom put-up lengths of THHN so they have a lot less copper waste on site and a lot less time with material handling setting pulls up and then deconstructing them to move onto the next pull.

The cost per roll of THHN #12 may be negligible, but you're looking at bigger pipes that take longer to install, with more wire that could end up being #10 or #8 depending on your distance to the panel. Without stepping voltages or putting in enough circuits you can get some efficiencies through diversity, I'd estimate it's likely a similar amount of copper whether you remote a subpanel or not.

Adding a subpanel is more labor in and of itself, particularly if that panel ends up being in a location that is hard to access or requires architectural coordination to give proper clearances and such.

I could see this cost savings measure working out in certain applications, but I think the numbers have to be scaled up. You'd have to be talking about either a geographically complex project where access to pull points or potential conduit runs is restricted, or talking about a greater number of circuits in the catwalks that would otherwise require more pipes to get up to.

The potential for savings probably won't be seen at any magnitude in materials. It'll be in labor.

 

[BillConnerFASTC]

Are you confusing twisted pairs with stranded vs. solid conductors? Jacketed? For power? Sorry, just not grasping the relevance.

 

[SteveB]

One problem I can see with shared neutral circuits is if you desire later to use conventional fixtures with backpack type dimmers, you are inducing potential loading problems on the neutrals.

One thing I feel strongly about is that theaters, especially newly designed ones, should have flexibility built in and shared neutrals is potentially moving in the wrong direction. Yes you save some money in the install, but will it be worth it long term to the end user ?.

 

[BillConnerFASTC]

Well, in fact are shared neutrals a problem for solo dimmers? Feed side not the same as load. Yes, oversized neutrals for dimmer loads on different phases but this is not that.

I agree on your point on designing for changes, to a reasonable point. But feeds will be a bigger problem if a theatre reverts to all quartz on solo dimmers. Plus there is the company switch.

 

[MNicolai]

Are you confusing twisted pairs with stranded vs. solid conductors? Jacketed? For power? Sorry, just not grasping the relevance.

I was discussing twisted cables, not stranded/solid conductors. Brief sidebar someone asked a question about and dropped a link to earlier in the thread.

 

[BillConnerFASTC]

I was discussing twisted cables, not stranded/solid conductors. Brief sidebar someone asked a question about and dropped a link to earlier in the thread.

My apologies - the sequence of posts mislead me. Still not sure what twisted pairs has to do with 120 vac and common neutrals.

 

[JD]

Sorry for the tangent!
I have just been surprised that I've been running into this 12/3 THHN twist on the shelf everywhere I go. I see no logical reason to use it for the very reasons that have been brought up, but was curious if I was missing something.
-Back to the regularly scheduled topic-

 

[AlexDonkle]

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

I'd suspect the cost difference also depends on how far the cabling distance is from the main panel (or dimmer rack) to the FOH catwalk. Depending on that location, #8 AWG may be required for individual circuits due to voltage drop. In that case, 12 x #8 AWG conductors home run on the 1st option for 6 circuits. And option 2 remains the same with #8 AWG running from main panel to catwalk subpanel, then #12 AWG to individual circuits. The savings for a catwalk subpanel would go up for longer catwalk to main panel distances, and also for higher circuit counts where additional conduits would be required for running individual circuit conductors.

 

[BillConnerFASTC]

To be honest, I don't worry much about voltage drop these days. Use too with planning for 2 - 1k quartz on a dimmer, the drop across the choke, etc., but doesn't seem do critical now with LEDs and planning for max 1.2 k and robust (tolerant) power supplies.

 

[JD]

Funny part is, a little resistance and voltage drop feeding a switch-mode power supply can be a good thing! When power is first applied, the inrush through the bridge rectifier to fill the capacitor is massive. If something is going to pop, that's when it will happen. The ratio between inrush and running current draw is even worse that a cold tungsten lamp. Anything (within reason) that can reduce that start-up stress is helpful. Early switch-mode supplies included a Glo-bar Thermistor to reduce inrush. As parts evolved and were more tolerant of surge, their use diminished and disappeared.

We had an interesting occurrence with our pipe organ the other year. The old 1980's DC power supply was replaced with a new switch-mode unit. (Pipe organs use the 12 volt DC supply to operate all the mechanical flap valves under each pipe.) The breaker panel was two feet away from the outlet. Every time they would try to turn on the organ, the DC supply would pop the 20 amp breaker in the box, despite the rated current draw on the supply of 6 amps. On a hunch, I plugged the supply into a 50 foot 16/3 extension cord. Worked fine. Operating current draw, 3 amps. Later, I slapped an amprobe on the feed from the breaker, plugged it back in direct and switched on the supply. Breaker popped and I read a 60+ amp inrush. Large supply with huge capacitors. Organ builder contacted the manufacturer and they were told to extend the run, or switch to a 30 amp breaker. Washed my hands of the project and left the rest up to them. (Their final solution was to pull a #12 about 50 feet down a different conduit and then back to the box before feeding the outlet.)

 

[AlexDonkle]

On voltage drop, I just came across an article that was news to me, apparently California and New York now require Voltage Drop of 3% to be followed, instead of just an NEC recommendation, and I'm not sure how they would view LED stage lighting loads, based on this wording from California:
"The voltage drop in branch circuits is limited to 3% of design load."
"For branch circuits, the design load is either (a) the branch circuit rating for receptacle loads (usually 16 amps), or (b) the 100% load of a specific load such as motor or fixed equipment." - California Building Energy Efficiency Standards 8.4.2
Also a little surprising they state the design load as a full 16A, instead of the 180VA per receptacle NEC typically uses.

 

[JD]

On voltage drop, I just came across an article that was news to me, apparently California and New York now require Voltage Drop of 3% to be followed, instead of just an NEC recommendation, and I'm not sure how they would view LED stage lighting loads, based on this wording from California:
"The voltage drop in branch circuits is limited to 3% of design load."
"For branch circuits, the design load is either (a) the branch circuit rating for receptacle loads (usually 16 amps), or (b) the 100% load of a specific load such as motor or fixed equipment." - California Building Energy Efficiency Standards 8.4.2
Also a little surprising they state the design load as a full 16A, instead of the 180VA per receptacle NEC typically uses.

The way I read that is that there should be no more than a 3% drop on your run when fully loaded. So, if you had a 20 amp circuit at 120 volts and you switched on a 20 amp load (2400 watts), you should still read better than 116.4 volts at the load end. If you drop below that, then the wire needs to be up-gauged until you read better than 116.4 volts (assuming the voltage at the panel didn't drop.)

Now, if we could only get the electric company to abide by that!

 

[SteveB]

The way I read that is that there should be no more than a 3% drop on your run when fully loaded. So, if you had a 20 amp circuit at 120 volts and you switched on a 20 amp load (2400 watts), you should still read better than 116.4 volts at the load end. If you drop below that, then the wire needs to be up-gauged until you read better than 116.4 volts (assuming the voltage at the panel didn't drop.)

Now, if we could only get the electric company to abide by that!

Our Brooklyn electric company - ConEd, gets around voltage drop just fine by suppling us with 123-127 volts, single phase. Causes no end of headache aches with our UPS's.